letrological aspects of some volcanic rocks on aji by aoter ...
248
letrological aspects of some volcanic rocks on aji by aoter Ibboteon, B.Sc., A :1; -4Co6s A thesis ;_resentod for the degree of ! ' h. D. in the »;niversity of London Dept. of Creology. Imperial College s London. July 1963
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Transcript of letrological aspects of some volcanic rocks on aji by aoter ...
letrological aspects
of some volcanic rocks on aji
by
aoter Ibboteon, B.Sc., A :1;-4Co6s
A thesis ;_resentod for the degree of !'h. D.
in the »;niversity of London
Dept. of Creology. Imperial Colleges London. July 1963
A. Ti T
Volcanicity in six stages is described from tee PlioAleistocene volcanoes on the north coast of Viti Levu, Fiji; the larger one at Vutukoula shown eruption in five stages of roan of the alkaline olivine basalt — traehyte association and the smaller one at Vatia, six miles every, is composed of augita and hornblende andesites. litholegy of the lavas and structures of the volcanoes, are deseribed and sone emphasis is placed on the cause and effect of at least two periods of caldera collapse at Vatukoitla; quite intense dyke swarms representing the root zones of volcanoes are analysed and multiple dykes and"layeredu intrusions are desoribed. There is some discussion of the factors controlling zeolite distribution and deposition* Lopolithe of andesite intruded into pyroolastic rocks are exposed at Vatia and structures in the youngest and largest of these intrusions are described at erne length.
The mineralogy and petrology of the lavas are described. Fifteen new rook analyses are presented and the highly alkaline reeks of Vatukoula are contrasted with the potash poor andesiton of Yetis. Some data on minor elements is presented. The evolution of the reeks in discussed and the thesis is subsequently propounded that the rocks are derived from an alkaline olivine basalt parent magma by crystallisation differentiation aided by gravity differentiation possibly. with. some magma mixing assimilation of the oountry rook..
Finally the rocks are compared with lavas from other Pacific Isl.tnds and with alkaline basalt suites from Australasia and the Atlantic area..
GOZRAL
1.1 Introduction 1.2 ?revil,us 'oork 1.3 General Geoloa of Fiji 1.4 :Aammary of Geology 1.5 Geomorvhology 1.6 Rook Classifleation
1.6.1 Voleanie f.eries 1.6.2 Pyroslastic Series
THE VVEUKOULA VOLCILNO 2 2.1 Utholozy and t,tratigraphy 2.2 Ainor Intrusions 2.3 Uajor Intrusions 2.4 structure 2.5 T4ithermal Deposits 2.6 Hot Oprings
3 3.1 Lithology and igrlphy
i:2 Major Intrusions 3 Minor intrusions 3.4 Structure
M1N, RAIOGY 4 4.1 Feldspars 4.2 Pyroxene 4.3 Olivine 4.4 Hornblende 4.5 Biotite 4.6 Iron Oxides 4,7 Apatite 4,8 °:ocondary Ainerals
PETROLOGY MID CH.:,;=;TRY 5 5.1 The olivine basalts 5.2 The traehybaselte 5.3 The augite tredhyandesitee 5.4 The biotite traohyandeeites 5.5 The green andeeitee 5.6 The monsonitee 5.7 The hornblende andesites 5.8 Chemical Composition of the
Major Rock Typos 5,9 3etrogenesis
5.9.1 General Statement 5.9.2 Nature of the Parent
*Ws
THE VATIA VOLCANO
ON
).1.3 ?motional CrystilliselMis and Assimilation
6 6.1 Comparison with volostale soaks
of other Pasifie Wiwi, and Continental Rimless
6.2 Discussion 6.3 summary 2.nd %nolasionn 6.4 Acknowittdgemanto 6.5 Referencss
Plates 1. - 24
1.1 ntroduokion
The British Crown Colony of Fiji, located in the south-wort Pacific, in comprised of more than three hundred islands evenly displaced about longitude 178° and latitude 1841/81 it is a British territory by virtue of a Deed of Oession signed by King Cakebau, the Parlay t Chief, in f;;vour of Queen Victoria in 1874. Suva, the oapitea city, is' situated on the south-east coast of Viti Levu, the largest island of the Group with an area of 4,010 square miles. Of the throe hundred islands only five are larger th,In twenty square miles and the total land area Of Fiji is 7,022 square miles.
Suva is the commercial oeantrio and main port, though Lautoka, a town on the north-met asset of VW Levu, ie the centre of the sugar cane industry. Cane is grown mainly by lediau farmers, the descendants of those who at the turn of the century were indentured to Fiji and later elected to remain there; the Indian population is now larger than that of the indigenous Fijians. Sugar forms the country's main export; copra, Gold, nengenese and a small volume of manufactured goods and other agricultural eroduote are also exported.
The south-eLtet Unite winds bring Mom rain to the e, tarn half of Viti Lovu and consequently the vegetation is thick tropioal ruin forest; rain is seasonal and most of it fella during the hottest months. During the hot eeeren, rain is brought to the western half of the island by variable northerly winds, but the cool season is usually a period of prolonged drought; the vegetation is grass and reed with some varieties of elephant grace. The two halves of the island with their widely differing climatic characters
New Zealand
411 FIJI
78°W
Lautoka
*C>
_l8°s
a Vatia
Vatukoula Nadariva u
avua •
rJ
Areas described in this thesis
ip o 39 Miles
Figure 1 : Sketch Map of Viti Levu showing areas covered by the Vatia and Vatukoula volcanoes.
e
-17°S
1179 °W
0
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are delineated by -t pencil sharp vegetation line which strikes north-south through contra Viti Levu. There is a eireum-insular road from which 'feeder' roads strike off towards the centre of the island though to date no road ham been made across the rugged and aount,inous interior. A ecapleto geogral'hioal account of the Colony is given by Derrick (1957).
The writer first commenced investigations into the basaltic rocks of Fiji when a member of the Colony's Geological Survey, and this present work is A continued and sore det,iled study of rocks exposed near the important aiming town of Vatukoula on the north cost of Viti Levu. After an initial tour of three years, seVea Menthe were spent on field work in Fiji during 1962; all laboratory investigations were sampletaa in London.
The accompanying skotek map, figure 1, showe the tee areas which are dereribed in this report. Both lie on the Lx side of the island and the vegetation is mainly grass with some light bush in the valleys. Rock exposures are usually confined to creek beds, though some cliff sections near Vatukoula were examined; the VrAi't peninsula is lees well exposed.
1.2 "Iravious Work
:Fiji became knom to the western world only in the mid yecre of the nineteenth century and it wIls virtually the beginning of the twentieth century before the islands were easily and e:;.fely aoceseibles Defcre 1000 few geologists visited the Colony, but thnse Mio did so invariably left some record of their visit. Tenieoh-7oods 8979), 3rady (1888) and Gardiner (1901) were the foremost amongst these, though it was left to a handful of organised expeditions to penetrate
3 -
intca the bush from the coastal toene; Dana (1849) i.actionald (1857) and ie,ssis (1898-9) have reported en t 444 Ay 1880, the capital had been moved to Suva and masy marts are oonoerned only with the geology of that area.
It was after the turn of the century however, that the solid basis of Fijian geology was laid. Geologists such no Foy* (1917„ 1,918 and 1921). ';i0olnough (1907). Shackleton (1936) and Ladd (1936) formulated u stratigraphy and produced the first geological sketch maps of Viti. Levu. (4oehrtme (1911) and White (1040-44), Inspectors of alma in Fiji, both added to the collected data. Geolo4cal v'ork gum accelerated during the 1950e when the Geological -lirvey 13epartment we.e formed.
The: e publioations, together with many more give a good indication of the general geology of Fiji, and it is this t,ckground against which tho geology of any smaller area must be studied. Foye (1918) did not visit the Vatukoula roe no but he investigated the bamalts of the De Serlev In their outcrops between the towns of 3a and Lautoka. His remarks &bre voted below;
"Olivine 3aealt frpm 14autoka. Mae of greenish - blaCk olivine banalt occur an reeidual boulders on the late mature hills about Lautoka, on the western side of Viti Levu'
"The rook is holoorystalline and very fine/grained, The phenoorysts in the rock consist of stubby, eahodsal augite (4 to 5 mm long) and rounded grains of mod plagioclepe, Abe An75 to Ab7Ang3 (2 to 3 mm long) with many olivine crystalee
"The ophitic groundmase iv composed of olivine, augite, feldspar and abundant minute flakec of Magnetite. Serpentine in peeoadary after the olivine."
Rot ulte of aerlier petrological work on rock of the same corioe were publieLed by Zaklo (1898-1). Two of his ioneriptiono are on rook from the Na&xivatu region; he deeoriben both as augite andesiter, one from Mt. "iotori.
- 4 - hevine, a 'oolourlaes to yellow glans', and the other
coetelning oliviee 'in tars e and small grains, but is not
abundant'. From 2400.0f3 deserietion of the mineral assemblage --.sad taeture it oases peeeible that the rock from Xt. Victoria mey be p-.lagonitie.
W.G. Woolnow;h (1903 and 1907) made a eerie of
north—eouth treverrer acrors Viti Levu and spent so time
in the liadarivatu aree. Be described the earth coat of Fiji er bciee comeoeed of a flat sheet, 2,000 feet thick, of black
volelnic rock which he variourly termed andesitic—
olivine—dolerite and extremely basic limburgitic lava. Ee also recognised the tuff outcrop's in. what is now known as the Tavua berinS
vZhin plain is dotted with conical hills whose fern ie exceedingly suggestive of partially ruined tuff cores. Where elections are obtainable, tuffs are exposed whom) dipo run *t corparativoly high envier (up to ?-i.=? much as 34u) and are very vexieble in direction. These facts seem in favour of time theory that the seall coastal hills art lergely cretere.*
1=ooluough noted hot spring action and concluded fielrly
recent voloanicity.
In 1934 Parry ,• Ladd published hie compreheneive
'iJieoloey of Viti Levu'. of Ladd's efforts were
concentrated in 4i:,e kalaeontologioal field, but a ?etrographio
utudy of the rock e he collected was made Ly Arthur A. Peg ,u. add'e work followed clezoly that of Brock (1924), though
Ladd claeoified the Euva marl. e and Ba volcanics in one large
eyatem, tee Suva 4eriec, wnilet maroon applied the term
'.decent voicanics' to tee 3a rocke. Ladd mapped the 'elsie
lameoe across ti:.x-n alC1. Levu out omitted tee basin tuffs at Vatukoula. Pegau of the opinion that
....the voicaoio rooks of the 3uva formation have been derive(' from a basic magma. They vary
in texture from glasu to r%tLar coarne Ciabanie....."
All the rocks described by Pegttu ars porphyrttic and soot contain come glass; labradorite, augite, altered olivine
magnetic are th,u ezLiantil coLet-ituants. thoueiJ, he also records Chlorite, calcite, iddingelte and serpentine. R.A. EIackleton spent several months in Piji it 1936 and compiled a wp of Viti Levu %thick charts the Tvtrua bain but him' report and notc, lihich at prosott are tot available so the wri';or, have not been publishA.
It was around the ot.xly 1930e that prospecting activities in the Vatukoula area were aceulerted, and an account of tLece dayt; it zivou in the autobiography of one of the Auntralian prospecturn -7orking or.. thu Prager (1957). Mixing beo,Fin in 1935 vra since that time there have been several rw.bliections desoribih underground structume fuld ore trirttnent, upd fern concerned with the structure, rpm rKi petrogr4ih,, of the mine and basin rooks. Those uuthort:, Loftus Hills (unpubi), Garrett: (unpub.) tind Tilfttchford (1953) have ltttle to vet" on the subject of thn retkion”.1 utructures or detailed petrology. A recent paper by Cohen (196), hoever, iv more eamprehenrive on lte 'twiy of the rtlationshivm of local strueturer to regional onus. Tinpul.)114hed Goolajca rurvey reports by elritm (1953a, 1954) and Gut rtnd Ibbotnon (195t) and nest (1957) nontain flarter information on tho petrology of the blealto and their intrlAnivoo.
their, raLmitted to the University of Tondon n.T. 4ongtx (1)4S) lomorihen 711a0kleton's collection of
rockr from luny partn of Viti LOW.* ThiN report jr entiroly petroloi;iof,1 and rill he ruforred to IT: the ensuing chapters.
0 .0'
1.3 f;',4*N_G4.01.04RY OX ,tU I4evu Bysr. after much reconnalsance systematic
mnpving, the geoloeicl history of Fiji is still isperfe ig known. however, pro-Tortiarg rooks !lave not been identified, though from, the oldest roam en far dated (Tertiary. tBooene) Drobahly until quite into in the ,'tone,11to n,c4;ivitYp toterl by two periods of sedimsntetion, wrA continuous.
There aro, nt i)resent nc active volcanoes in the island 
7
Probb1y roapcnsib,,Lc fcr A4Ile regional tiorpbte of e040 Waihimala eortainly produoin auroclen in the suxroundina mob!. aro the ?lutoact which are• a group oZ plug-liko intr,telors emplaoed in a bolt Which otrikee north-entlocuth-wc,73 74.oroos ttoe inland, LeeentlullY granitiet *wee Tizere basic intruoions have been, =looked but these are pretubly hybrid rocks rather than emplacements of a *rue 40sic agas, The plutoril,,,r are Tortiary lL in ago. It has ben suggested that to =placement of the :granite
in the troll:A-tilts forr. of the Wainimala and agutoka rooks indicates oiloriclirell condition,. This may.1).e a valid hypotbecie and petrologically the Iile of redimentn and
11 t'aective of geovynclinal do'pocition. There is however no evidence for the landmacr of pre-wainimala time ,xilioh the crjxmc,ut pre-muppose mile which hae been referred to by other workers at the Australasian mEhinert.
The intrusion uf the Colo autonicz rr..m followed by o long period of intermittont volcanicul in a eerier of Ortiters tlrouzhout ii5i. By fur the re*t oor.rot product ie andooltio a.rclomerutet generally =cite bearing but sometimes with either hornblerds or biotite ev. the I ferro-=.gmainn mintrfas The extrusiwn of these reeks accounted
f© deposition in Lorer to Upper. Vietvere tires- tut oa far it hce not 'JceN7 folAnd potsible to corrzItte Any ?Articular grou i
of rooks ofthstr eoriod throuct different parts of theland. Theekleter (193) dividei. theme i3.# irte tLres the ftme174, 7oroimTu,7', sod F.41bsto volcantop!, As far as can 'be deterniret thoNatosi rocks have not since beer eisimined; the term )Koroialvue4 hgn !7, certly otin applted by 71okard to rler!.em of endetio aggiorate lying totretro tbe as And
Suva rocket too ocriew wtieh Stmekleten inverts in hie otratizrahy. Laid divided the lavas iAio the auto and
- 3 -
Viti formations, but the latter term is now used to denote a diagnostic limestone xind within thi. .7igatoka !;eriess
The terminoloy of these rooks t thus somewhat confused and in spite of much work no satisfactory grounds have been determined on wnich correlatilmo could be attempted. The conclusion of undeeitie activity is denoted by a widespread and diagnostic conglomerate whit* heralded deposition of a series of mudetones with oelo bands - the Suva Series - deduced by Ibbotson (1960) to be of Upper Aooeneilower Pliocene age. These rooks of 600+ feet tnicknese are elevated and occur on man/ parts of Viti Levu up to 3,500 feet above rasa level. The most covpiete post 'iAlva succession is in nortn,Irn Viti here, Richard has identified a group of .1;ndesitic agglomerates (Koroimavue) separating the (-uva mudstones and the later basalts. Details of the thiekness and contacts of this agglomerate sequence are not yet fully known, but it Is certain that the conclusion of thin ar„Tarently looal andesitio activity wua followed by widepread basaltic volcanicity, which probably commenced in 21looene times and continued through into the PleiStooene.
incs ':leistooenie limos, 1131 has been volcanically qui,scent lnd deeposition has been confined to delta formation at vu 3 ut Labast.,. Coral reef growth, Nuns of which prohably oommenced in Niimtene times, has continued.
The petrology and structlara relationships of the 3a basalts and their andecitic derivatives are described in the thecis whit* follows. Similar young basaltic rocks
have ')cen recognised in other parts of Fiji; their relatiozmb to the rocks about to be descried is unknown and remains to be inve3ti6ated.
1.4 s a of A GeologY 1.4.1. - at Yatukoula
It was stated earlier that volcanic activity and rceek deposition in early and mid-Tertiary times in Fiji. had produoed a eerie of andesitic rooks; investigation hams shown much interdigitation of extrusives, with deposition of those dominantly pyroolastic rocks in environments varying from shallow freshwater baeinf, to deep ocean floors and continental plateaux. The resulting structural pattern, where almost every flow and bed is a local unoonformity, is oomplex and volcanism Was continuous at least from Tertiary j to g by Van der Vlork's classification. Craters and vents in several parts of the island were intermittently, but con-temporaneously erupting andesites oontaining large phenocrysts of augite, hornblende or biotite. The heterogeneous lava pile that comprises the pre•Pl.iacerae rocks of 7iji vms formed in this my.
At a st9.0 there vas is oessation of volcv.4,niont though the ensuing period of quiescence must have been quite short. It is represented in the ettigraphic ao1w by material stated by Houtz (1958) to be this rewe*ed marls of the f,uva Seriem; these rocks he found in trite 'realm* district of north-east Viii Levu. SUAY3 sediment'Ition yowl proceeding at the outset of basaltic) activity; this is the opinion of Rickard, Rodda and Hurst (pers. comm.) who have recently been
pping over the Cava / 3a boundr.try. The eruption of the 3a salts heralded a pronounced avow) in the volcanism of Fiji'
thou.; h the eArliest representativee of this Series are a group possibly up to 300 feet thick - of fine pyroclastic rocks which are in the field, almost indistinguishable from the of vetie rocksw, of the Upper Suv,.1 4eries. It is therefore possible that Suva sedimentaion w:4s stitl proceeding when
-10e the :max pyroclestica wore deposited, end it in ooneidered by the workers mentioned above that the lower boundary of the is basalts, whioh maree this very eignifiemnt and chemical Change, is dichronous.
c4:ether the vent at Vatukoula wen active from the commencement of basaltic eruption in unknown, but as will WI detailed later, there is evidence of the fiesure zone OM which Uount VatUkonla grew; there is also confirmatOry evidence that the volcano was built on an andesite platform, though inevitably much of the early history of the structure is unknown. However, the volume of rock extruded through thie creter is sufficiently large to sagest that it 18 one of the principal, if not the Urgent, of t?ke centres which produced the total 400 square miles of bas It and andesite. The observed ratio of effusive to pyroclantic rock is generally about 1 to 5 and it is likely that the volcano grew through series of violent and cataclyamle eruptions as lee testified
by the extronely coarse volcanic breocies and agglameratoo, and, in some instances, chattered blocks of dyk* complete with their chilled margins. -1:xplosions were usually followed by quiet extrusion of lava through the main vent (any trecen of parasitic vents have since been removed) and dorm the sides of the cone crystallising ar a cement for the shattered blowouts; congealing of the lava in the vent led to the next **plosive phese. Celdera formation resulted from alternatine destruction (by explosion) eni reconstruction (by deposition) of the vent area; it is probable that at the completion of phase I activity the caldera was five or six miles in diameter. .eillieme (1941) ham demonstrated the invalidity or postulating explosion as the main OAUSII of caldera formation, particularly when the oaldera is more than two ales in diameter. At Vatukoula, the caldera wan enlarged
by collapse, and this process of sliding of the ealdera walls into the lava lake of ever increasing diameter wee concurrent with eruption. The elevation of the lava lake in uncertain, but the evidence of the minor intrusions euggeeto that during tee final etagee of eruption the lava was extruded from a level which aeproximates to the present 700 rest contour+
Though the eenalts were later erupted into e marine environment, there is no doubt that the earliest flows and pyroclaetice (which in the vicinity of the Goldmine, both on the serfage and underground have yielded fossil plants) were deposited in continental condition. The regional dip of the bamalte alto dates from the time of uplift, some tilting to the north occurring during eustatic moveeente. Throuehout the central part of northern Viti Lova, the dip Je4 generally 100 to the north or nor het, but aodda (eore+ come.) has not found such a ooneistent dip further et. The tilt in; may have caused elock faulting, the blocks vo formed having different amounts end directions of dip, thoueh tee block boundaries have not yet eeen recognised.
The explosions continued to the and of thin phaee of volcanicity and were followed oy a period of quieraenoe; ourface activity at thin time was confined to fumarollo effects, erosion of the oaldera and craters within it and some reworking of the deposited pyroclestice. A violent outburst heralded the enelaught of another phase of activity, though as in the earlier extrusions the location of the vent is unknown; the form of acme of the rocks produced however, suggests that it lay to the east of the first crater. Throaeh this voleeno were extruded, again 1/y a series of explocive and intervening quiescent periods,easeltio rocks, dark end dense and containing mane large faleer phenocrysts theoe ere the treoeybasalte of phae II. sort of these
rooks have Teen affected ey post extrusion deformation, though in contrst to the phase basnats, the feldspathic roes have horimntal dip.
Ath continuing eruption the tradhybasalts gradually filled the existing caldera; the volume of rock so contained is estimn.ted at 13 cubic miles. The then overspilled on to the olivine bas:,1to “rid some of it flowed down the north—western flanks of the volcano and was eventually ponded up in the area of V4.tia peninsula. Meet of this overspilled material has been removed by erosion and only the deeper, now disconnected pools remain.
Except for compositions a changes there are few differences in the ch.:,racteriutice of the phase I and II rock; s,ssentially explosive with intermittent effusive periods, they intruded by dykes and sills of similar material. The size of the phase II crater ip unknown, but since it lay Wholly within the phase I structure it w(x obviously such smaller; the volume of products extruded from it was alto considerably smaller than from phase I. The Attern of quiescence, fumarolic activity and compositi)ni changes in the magma chamber WAS repeated before tLe onset of phre III, which involved another VOlOA,10, in much he same situation as the earlier two, end omitting :-J41site trNmhyandesite. Phase III material outerci,f! over several square miles, but sinee depoaition it has all been disturbed by collapse tend brecoiation; the .fesult is a group of breccis and tuffs. Some finer Kroolastic rocks have been displaced and tilted, but their orii4nal textur ard mineralogy remain lid in some instances, notably near Davotu, they indicate a marine environment of deposition.
Marine 'eands in the phase I rocks are known east and south--o .1st of 'aikublikubu. aodda (pers. com ,) has
13 ••
traced marine tuffs ad thick pillow ].av eatwrds, to Rakiraki and seUthwards to the Nadrau Plateau whore they had previousiy been described by Ekiba (1953b). A marine incursion from the south and east, which had no noticeable affect on deposition west of waikubukubu lz reproented by a 200 feet thick fossiliferous marina tuft, overlain and underlain by normal continental basalt flows. 4arine structures are also found in the phase III rocko, but there is a complete absence of life which may be attributed to sulphurous or anaerobic oonditionsi alternatively, marine faunas may not have had access to a freshwater or orator lake.
The origin,AI extent of phase III ib unknown; there is no evidence that they overflowed the basalt structure as had the phase II rocks and their present exposures, three miles long by four miles wide, are structurally thaineated av the VatUkcul basin. Before the completion of phase III eruptions, the erth MOVOMMAA *11.1,0): to thin time had been associat d with, bit subordinnta to, volcanicity now assUmed dominance and movements on a large scale bsdan. :engulfment distorted all the rooks of phase III, most of these of phase II .4.d a few o those of phase I an well afT the ensuing phase IV rocks. IAsturbanees in the phase I reeks possibly resulted. in block 1ulting
lingulfment was a slow procevs but It is possible that phase III activity and subsidence wore concurrent. Phase III WUP concluded with the intrusion of any thick augite traohyandeeit cone sheets; these structures had originally been mapped as flows, but Cohen (1962) has verified their intrusive natures
The final extrusive phase (IV) produced a group of biotite trachyandesites and trachytes lsVas And pyroolastiosi
of the four phases thin onu produced t%s let cmount of lava aad exposures are confined to tha centr of t'710 structural basin. The position of the eruptive' ceratre in uncertain, but fused ai;z1omarates, intrusive into the lava, probably reprec:ant the blocked conduit. The eruption was taitially explazive but later beome quieter ,:vith effusion of biotite trachyandesite, and exosures show a sharp break between tuffs and overlying lavas. There racks lie =—conformably on thooe of phase III and although rlightly warped held distended, they- do not exhibit the collapse featurar of earlier rocks. rellowinG the biotite trachy—andesite eruption there was further slight settlin and this has imparted to the has IV rocks:, a very shallow saucer—like fern; maximum dip is about 100.
Ilease V wus Wholly intrusive and produced trachy—=docita and trc:%yte dykes, a number of w17,1ch.are tangenttU to . 1 'banin cont::.ct. This L4ctivity was most intense in the ea western pcxt of the tructure vtcre ,101 the minor intrusions have a consistent strike of 300 — 3300. Aso exposed in this
a few small plui; like Intrusions of vuriable wmPositioni they were possibly emplaced during phase IV and V time.
Volcanicity ca»Ao to an and in 71eistooene times Ath hydrotilerml acttvity and mineralisation; alteration is noted in many areas, thou6h the bit, •4.xic usu11.y more Intensely affected thai the pyrociaotic and fragmental anderites. Hot sprin4?s in waikatakata creek near Vatukoula and circulating Lot ground water in tile deeper workings of the 'Emperor mine testify to the generl youth of the volcano.
1.4.2 at Vatia The extrusion of tha phase I rocks from Vat pule
5
anti subsequent ac Inarai on upon them of phase II trashy batialto at Vatia peninsula has been described; sublliquentlY voloanicity communised in the Vatia region 1314 forthwith naralleled that at Vatukoula, although the two vents emitted rn.viier different pro4ucts. In pi:lases emitted only at Vatia ‘no suifix ovi le applied to the phase numbers
Phase Inv is composed largely of augite andesitee, hornblende andesites and hornblende its andesites with intrusions of slightly more acid material; it has a steep, oZten faulted ()cutout with phase II rocks, Tan phase wee camp:Lately explosive; no flown have Alen recognised uad there is no cuggention that origim*1 lave may have eel subsequently brecciatea. There is nome justification for dividin6 4he phase into two parts on the grounds of the ferromagnesian content, augite andeeite proceeding the hornblenuic rocks, thou nn in wall are.ns the oontot cannot be mapped witn any degree of accuracy. lne result of volcuiclty was the constraction of a pile of augite and hornblende berth pyxocl,A.oticst tuffs and anglomeratee with a radial dip pattern. Thin series of pyroclaetics of maxim= observed taickness 1,100 feet is uniform in straelare from to to uottog4 it was deposited as a result of explosions in two large caideron on the western side Of tne peninsula. Volcanicity wan not accompanied by any minor intrusions thou6h explosion may have seen respenalble for the disintegration of G etruotures as soon as they were emplaood•
einai1y, plugs of nornolande andeeite and related roans were emplaced within tae calderae. The majority of tnenna reedit from quiet influx of lava into the heterogen pyrooiastio material 'onion formed the floors of tine ealderos; -Wei were acoompanied by a few mirror lutrdsions moat of wh'oh
— 16 a.
are also concentrated in the save areas. The youngest Intrusion on Vatia is also situated on the western side; though essentially andesitie it contains much variable material from amphibolite to quartz—andesine—aegirine segregations. Associated with breccia, also of variable lithology, the intrusion represents a section across a conduit. Subsequently to intrusion, volcanicity was not renewed.
The fact that the phase lily rocks are exposed in a depression within phase II material is indicative of either much erosion of the basalts or subsequent collapse of both phase II and III rocks.
1.5 Ceomorphology The Vatukoula area is geomorphologically young
and the intimate relationship between rock types and land form is emphasised by differential erosion' Physiographically the region shows a three fold division; the youngest feature con-orises the alluvia] plains which fringe the coast and often extend inland for five or six miles along the major rivers. Alluvial plains of the Nasivi extend upstream as far as the mine and smaller patches of alluvium border the river in the higher reaches; they are invariably cultivated, though the black clays and silts have been examined in one or two places in connection with placer gold investigations.
The alluvial indentations extend inland to the northern edge of a natural topographic basin which forms the second physical feature. It has a maximum diameter of eight miles and to the east, west cud south is bounded by hills from one to three thousand feet his h. The basin forms gently- undulatin country, usually riot uore than 400 feet high though one or two hills rise to 600 feet. Steep slopes
- 17 -
are confined to the more incised. creek v2,,Aleys and to the 'ewer sloven of the mIrps. kwolnough (1903) considered tht the lower hills were remnants of cinder or ash cones, thoue,h this ide is not born out by structurn1 investigations. Un much of this land is devoted to the cultivation of sugar
cane though the higher southern part of the basin lrea is rolling grassland with occasional cattle stations.
The surrounding hills form very distinctive third feature. To the south a'steep raTkee of mountains strikes east—west and rises to 3,500 feet at the south— eastern extent of mapping; two lower rffmges strike at right an,,-:.Les to this main one forming the western and eastern boundaries of the basin; they deorcrtse in elevation northwards.
The high. .r ground is formed of olivine basalt, the whole of the southern and western ranges 7 -nd the eummit of the eaptern range being formed of this rock. The lower ground of the basin area and lower slopes of the hill ranges are out in the softer and fragmental rocks of phases II IV. The vieuAl effect of a large olivine baeviit caldera containing a floor of andeeitic rocks is beet observed from the ridge to the en.st, where the crater like form of the rocks and concentric structure of the "hole is evident. The steeper basaltic slopes have been differentially eroded so that the more resistant beds are visible both on air photographs and in the field.
Of the 56 square miles oomprising the basin some 80% is drained by one river, the Wasivi, whieh flows out to sea at Tavua. The remaining 20% in the north—east part of the basin in drained by two mis—fits which reach the sea at Yorovou. The natur!Al strike of the creeks is north—south but from Vatukoula to 7aikubukubu the Nasivi has out back and
',captured the his r> or all other sensequont crook* producing a radial drainage paltern. Cutting bAok continuos AA the upper reaches of all the tributari*, of the Nanivi and the onwhows and field terrace*. pion' of a yonng rive" bound.
Geological *Pont l of morphology is lees well developed at Yalta, which in creed of irregularly staid mad shaped hillnt most of tbO*i boar no apparent rastitimshIP to geoloa though ono or two ar* controlled by intrusive 'bodies. Dip olo)o, of hornblende animate ars sett identifiable, in the area two ails* east of the Wharf though these are frequently deeply dissect** as a malt of flash flooding in water courses.
1.6 ClasoificatAgn o roolgo 1.6.1 Voloanio series
The mars rocks doeori, ed in this thesis bslang to two principal clams of volcanic rooks, basa.Ite and andoeitee. Moss rooks are slassified primarily or WI* beats of the modal feldspar. The secondary is of alkuisitiettlea ire degree it saturation and the sum of modal reale minsvalo (olivino, pyrozons, horublond0 and biotite). Reeks in shift the -ATOW00 same ass teldspar ie labradorite or bitommits aro termed basalts and thioe with antesine or °nepotists* are classed as andeeitinno
Modal analyses wore mad* by the point-sountor set 1. Rookr for which no ohomien1 or modal :n€ 1. data were klvailavie are olssifiod en the 4nels of comparison with the analysed rooks. All the vole; is rooks are undernaturated or saturated with silica and *was of the .ode tea contnin normative quarts but little or no modal quarts.
The following wah—livision of the volc
- 19 -
hre h,”or adopted fnr re* in strnttilmphiosl nappinf; And subsaTient petrtIolical ae;udy:
ersontially .orphyritic timmlt cofitaining olivine :!n6 pyroxnno 7,honecryrtn; co=louly with 7loolitem %nd clottr in anie,d4141,• “Ap-ite basalts diffore from the olivine t'arrats only in their cm&lier percentage of ollvirc with tha upper limit of 5%. Psldrar trachybaralt with less than 31c', of- olivine. :bundllnt rhonceryets ef plajocIans end pyroxand; commonly with zoolit4 ,11(1 ralvite in amygdalos, Trachmntneitol an.O. ands-rites* are ensigns-tea into sub—types o t-he bnei of their dominant maiio pherierynts. Aornblande undesitmr, gueito tmehvxdocites nnd biotite trrohyandesites ars retie prinetrr,1 rreerntetivs roc* types. Clusen andenitest a sarepicuous awl. readily mlpped group of fine-0%171ei voicrnio rooks ,shirh texturally and chemically eorrespond to trAohyter.
Tbe clas.tifiation outlined in this report makes a eatint!%ctery 1.Jasie, for rogional oloid. ii7v7rion Comparison of rock iL that it defincu the position of the mambere in the reLionta serioo zn1 confoms to the olasrifioatien already in use for tha Pacific roLion (Atcrinall 1?491)).
Ppr detiled compsr1n loJtveen the rooks the olarsifiention astblisted is far froN. in sltisfactry. As a rule, the groundmacs hi ' r of the serial aro no% determinable c-I2ticalty. 'iwerover microliter could ts dstrmired it W311 still a difficult ta7k te calculate the averaza nstortnite content of the ,;rI.Th ''Artharmoro, the 7icts.sh content is moll too hi for norm,11 tts and andanitan.
The mon% comprehensive ..atro134oa1 invaltigntion of tho roots of . .j/ le thlit °)3r tyangar (1949) wto Aiecussed
i• 20 -
the merits of several elemeifieetione and detailed the one best snited to M,s reeks, whloh differed widely in textures an(!. mineralogy* Iyenger used a cIessifietion by Tlhand (1947)1 thir system had previouely been adoted by larth (1931) when describing basalte, paeificites, traohyter7 and phonolitee of a Pacific emite.
nh^nd'e clae9ifietion. eam2r7thensiva base or terture, degree or saturation, mineralogy, types of f*14sps and colour index. One limitaUon of the application of this eystem holp,over, iv in its ure ,f colour index see a moans of distinguishing groepn, not eyjecies, of rocket Iyergar said of htn collection that imomt of the voloanic rotate are hardly distinguinhable in hand specimen one from another' and this eorment was pertinent to a root collection with a wider variety of rock type than the present euite. Fr4crospecimens of aufsite-olivine basalt and feldsbar -11,_selt may be equally dark and modal determinations are neveeeary before oolour index eorrelotionn oan he made even though the irdioee of the two roote prove to be very different; such determinations sire improtic;11 in the finent drained rook,. Tyongar (1949) found rirlilar limitations and ntated that
nmodril determinatione have beer made in some canes where it has 7ieen highly denirable to dietincuish border lire cases 'aetveen andesiten and bamalts",
Mother dieadvantw:e of Shand's classifieation is that it sub-divided rooks into four °lassie or the alumina content after a primary diviein on the degree of sn.teratien* This is virtually a subdivielon by the dominant ferromagnesian mineral of the root which Shand apparently- considers more important than a. grouping on the type of feldear; most other workers would consider subdivisio 1:.)y forromagnesian subsidia.ry to that by feldspar.
The properties on which the oleesifioation is made have been enumerated. Most of the roots to 'com deocriAld fall into t ,ie olaseificAtion - olivine basalts, au6ite and biotite traehyandeeites, traohytes and hornblende andosites wId some monsonites - but one grcu of reeks* mopped as feldsxAr b-isalt, is apparantly anomalous*
The die inotion between andesite and basalt poses one of the greatest problems of rook nomemelature and classification, but it is imperative that in a Volcanic* area with a thick pile of inter-bedded basalts and andesites, the grounds on whieh the two are distinguiehed should be well defined. There are seversJ charaotors by whiff& the clans enn be separated, but no one criterion c=am be applied universally. 0,h characters are presence or -Abeanae of olivine, the colour
index and the type and amount of feldspar. if the first property in accepted, and all basalts contain olivine and all andesites de not, then rooks with a basaltic texture and oracle plailioelme may bosom' andesites* This is obviously unsatisfactory. lysmgar differentiated then as did ?Jarth and Shand on the basin of oolour index, the basalt* containing more than 30 of daft minerals; SP stated this is also oonsidered useatisfootory.
amorally the feldspar co position is the most reliable distinetion* the composition of the groundmase feldspar being the critical factor; using this *rite rlos basalts carry groundmass Labrador/to end brtemmite, and amdesites have ground mass *Morin* and eligoslase, though Maellemald (1949a and b) used the average eomposition of the feldspar* This system anist be qualified in two ways; firstly it admits to the widespread development of rooks which are olivine andesites and seoondly rooks of the mugearite and hawaiite type are anomalous to the ells ification. Thus,
-22-
using texture and colour index AC criteria the phase II rocks basaltic- and usin the plagioclase composition they are
ndesitio. If the chemistry of the rooks be considered, the
situation beeomes further complicated. The rocks contain too mach II 0 and normative Orthoclase to be basalts and • 2 the silica content is conziderahly lower than in normal andesitest xnde its 9.1so contain more soda than potash, though the present rocks contain approximately equal %mounts* The rock° also contain normative nepheline and on this property larth would call them ,pacificiteso. and Lacroix .4asinitoids,, though these terms are nit really descriptive of mineralogy, texture or typo of feldspar.
The phase. II rocks therefore ars not members of either basalt or andeeite clans. Rocks of similar affinites which have been found in other parts of the world are termed writes and hamaiites; these rooks have recently 'leen re-defined by Muir and ?illoy (1961) who concluded e
"In the light of this investigation into the ebemietry and mineralogy of the hawalites and megearitee vs may describe hawaiitee mineralegically as 'sing ebareeterieed by strongly zoned blade ranging from andemino to a feldspar of It:: .anorthoolame typo. the two type of feldspar being. present in Melly Sepal proportions or with
gllactase denimamt. In nugearites, the ple6loolase ore sodic and the alkali feldspar (lime-
anarthoclase) in present in greater ammo* than the plagioclase, less commonly mmgeerites say contain interstitial eeda Sanidine. Chemioally the hawaiite has normativelilegioolase in the andeeine range and carries signifleant normative orthoolase while smgierite has noMMative plegioolase in the ellgoolase range with still higher amounts of normative orthellaee•
Neither of these terms are applicable in the present instance, since as indleated in the moalyeee of table 1 magearitee contain
-23-
abut 20 lash rInd this in always subordinate to the amount of soda. 'Mir and Tilley (es. eit,) discussing rookr of 3kye stateds
trushyandesitee fall in an intermediate position ')etween amgearita proper i_ind trashyto, and rocks resolved te traehybasalt aad trmetkr... dolerits. though falling in the same silica range, ars distinguishad fres mmgmexitos ehemically bar their mere markedly pet'ssio eharastee.
Table 1 s Chemlstry of feldspar basalts. mugs4rites and triohybmealte•
IL 3 Si 5102 51.78 50.59 48.76 51.35 51.18 1.,
A1203 19.38 17.98 15.82 16.34 21.41 16.61
Pc103 3.11 4.71 4.10 4.64 4.1 5.61
Fe0 3.23 4.36 7.53 6.1f; 3.32 5.59
2.26 3.69 4.77 3.73 1.75 MS 0.0 6.51 8.13 7.99 6.61 6.56 4.44 Ns20 4.11 3.83 4.50 5.01 4.72 3.94 K20 4.94 3.81 1.58 1.94 3.53 3.45 112O. 2.50 0.79 1.08
H20- 0.70 1.06
TiO2 0.56 0.66 3.19 2.74 1,34 1.81
P205 0.46 0.41 4672 1400 0.48 1.11
Mn0 0.17 0.21 0017 020 0.27
Others 0.19 2
Tot :1 99.91 100.23 99.9$
-24 A.11, I Treeksybasalte, Tainkells. O. s Average of 21 analyses of C113ro 141:14 .1:o
(hmicalits) uccalWte- t94.91.
D. s Average of 11 analyse,n of olit6oc14!„-) widest*, (110114,aritill) 1.40 cvxld
B. Trachydolerite, Ascension island s D. “udp. MO. II p.652 1912.
f. s Average of 3 oliceelsoowlimftimmats am4040111") of Conti-Al Visteria. Edward* Q.017.6 Es W. IOU 1936. p.309
The tern trasbybaselt has been ecopted for t iese rostko though not completely sstiosfactory in the mace teat the rocky do not have the bAsis requirements of the basalt clan, chemically and texturally the term is acceptable.
During the field work and in the -early chapters of this thesis rooks have beers classified into five Or iiss, as4er groups of Jades/tee, basalts and traohytees as their dommity, apparent deffroe of satti ration and to some Wm* aeleur index and also on their phenoorystic content, a scheme successfully applied to U.mGregor and WaoGroor (1136) to the Carboniferous volcanios of the Midland Valley of Footland. The variation of rock types within those groups Is fully described in part 5.
1.6.2 c0-01:0 Y0.040 The present seotion deals with the neaterciature
of pyroolastio and sellmentary• pyroolastio materi Meet of the following are adapted from Wentworth and 1/11isess (1932).
1. Aggloper,;tel contempersneoum, coarse pyroolastio reeks oont,lining rounded or eub-rounded fragments greater than 32 ma motor lyinc in ash, or lava matrix,
are in di
std efly
and blanks They may be pertly ly pyroelestlep on-eontemperanesue
activitY indurzited pyroalastie *Wire of finer grain size than 4 mmw According to the pridox inane* of glass, crystals or fregmeStP of holoorystelline reeks, vitric tuffs, Orpotel-tuffe and lithio tufts are distimaished. Tuffs asy contain 0 - 10% Of sedimentary materiel
s diluent:1:mo coerce yroolastic material ut stating of large, chi ‘fly remind exsi water worn fragments.
- 26 -
Part 2 Y The Vatukoula Volcano
2.1 hAthology *Ad IratUraphk
2.1.1 The olivir,e baoalts of lahare ; The sarliewt rooks of the Vatukoula volcano are
olivine basalt pyroolautic rooks with subordinate lavas; these rooks, which attain a known maximum thickness of about 4000 feet are referred to in this account as the rooks of phase I. The pyroolastio rocks include agglomerates, breocias an tuffaeeous rocks and these are interbedded with laves and conglomerate bands; subsequently this lava pile has been intruded by basalt and trachyandesite dykes.
This earliez4 volcanic phase producod large quantitie of basalt; this rock, which now outcrops over much of north and central Viti Levu is considerably more abundant than products of the ensuing phases and it is doubly important since it probably corresponds -to the primary magma of the entire volcanic sequence. The ',)asalt is iark grey to black and usually massive in form; density determinations by the walker Steelyard have indicated a range of 2.76 to 3.01. The rooks are occasionally vesicular and oontaia secondary calcite or seolite. They have olivine and pyroxene phenoorysts, usually idiomorphic frequently four or five millimetres in dialartsr; a glomsroporphyritic texture ir not developed. The range of modal olivine in estimated at 0 - 40% and pyroxene trot 15 - 50%, though no rocks are sufficiently basic to be oceanitic or ankarumitic. Olivine is sometimes fresh and visible as green-yello , often granular material; more cormonly, however, the minerLa is pseudemerphed. t;mall amounts of iddinGrite have been formed by magmatic alteration and iha remainder of the olivine has been altered to produce iron oxides; the mineral is then recognised by its rusty brown colour and rather platey habit. Pyroxeno is alw.ys freoh .Ind well cleaved and dark
2T
zrefin to black in colour; it is genemlly more euhedral than olivine. The groundmass is bleak or very dark grey and fine-grained.
The rooks are coaratively little affected by weathering; occasionally the basalt io completely oseudamOrphed by clay minerals, but more usually a thin blue-grey or green-. grey sin develops on a core of fresh rook. Pink, blue, purple, white 7ond bi.ok :Lo well opiçreen nd yellow clays are also charooteristio of weatherino in arid ro,,,00. asewhere the skin is yellow or yellow-brown and shows a pitted appearanoe Where phenocrystio ferremagneciano have weolthered out; these crystals ore sometimes locally concentrat A on
e surface as a coarse sand. Olivine basalt is usually structured...lee thou eb it
it cometimes Jointed and lens often hap a flow alignment of vesicles or pin rule. The basalt flows normally have a thin brecoiated top ond bottom, each a few iocites deep, with a rhick middle neotion of masoive basolt having two eeto of Joints developed at right angles to the cooling faces. A third set of joint, parallel to the oedding conetimee for on a result of compression, but this ie usually weak and a ouOic fracture ie never well developed. A eimilar three fold division of flews was descri1Jed by Wentworth and 4aollonald (1953) as being typical of the blooky flows of Raman.
blost of the basalt occurs in , Jyroolastio form and a relatively minor amount is effusive material, fig. 2, the overall proportion beinz aoout 7 : 3. The lava flows are variable in thickness but only rarely are greater than 30 host and probably average a out 5 feet; Blatohford (1947) coneiderod that they aro 100 feet thick in the vicinity of the sins.
Wailotu II Malusu Matekenaka
1100 Basala Wailotu I Nasaranga Nakailevu
1000
900
800
Elevation in feet
700
600
500
400
Figure 2 : Stratigraphy of some phase I basalts as exposed in creeks.
7 lia
The oxtraviree are interbedded with four typos of frovnental raft; the oommonest of these it an agglOMOrAte in which rounded boulders of various sizes are oommetid by basalt lava.oe of the boulder 'r'a alluvial, but most ''Ire pyroolartio• A few boulders have a baked skin az A roeult of contact with hot Lava and often quite large masses of agglemerate are heamatitieede apparently by bakink;. Thou6h very imam and rounded agglomsrates ,4re known from the Yatukoula (Maw, they do riot appetir to be the counterparts of the ball 'avail of 'entworth and cDonald (1953).
Volcanic breccia is aloo foxed of olivine basalt boulders lying in, an olivine basalt matrix but the boulders aro of irregUlar shao) id size; breooiss are greatll subordinate in amount to agglomeratie. TUffaceous agg1oaertse are rather rara but the haws _.iefan, mapped interbedded with the lavac to the south of Vatukoulat they (*mint of basalt fragments, usually rounded, embed4sd In buff coloured tuffaceous matrix. A,611 and turf bands are more lirriable than the precasdin6 pyroclastic types but 7:1,rs, with few xceptions, of subnacrosco4c texture and minerulocy. They vary in colour from greyish-whito to buff and white, though anh hands tond to be white (greenish when propylitined an at vatukoUla) nd bolos are always rust red; the latter rocks often contain bends of idiatorphic tlnd broken crystals In tgroundmass if fine rod dust. The tuffs are finer in 'Mixture than the bo1e6 but coarser than the rather floury ashea, which show few stracturesi tuff* on the othsr hand usually show a good rectangular fractur pattern, possibly in ?art jointing induced by compression, but more likely a reoult of dryin:
smoking of the rock. There is some indication that certain IrteAlt .flow
ars extensive and may NI mapped an lithologioal unite l by contrast individua3 ash bands' are sonatinas discontinuous
-276- or lentioular. tructuron of tuff zones from both *moo I and II are illustrated X: fig. 3.
In. the present mapping an attos.ot has boom me* to show the fore and attitude of the pile of bastalt lavas And pyroolastios by indicating the known amd probable extent of the thicker flew. Since there is ao variation in rook type, marker horizons cannot le 4oloroinsa and correlation has to le effected by comparison of established stratigraphie, suocenelons i.e in fig. 2. lAnce aoh is normally more widely deposited thin lava, anh bands probably form the more reliable horizons for correlation, but because of their partial or complete erosion it was found. impracticable to use them. hilst therefore, there are distinct limitations to the
mnpping of individual lavas, they indicate the general stratiraphy and attitude of the lava pile. The general dip is to the north or north-east at about le; most of the dips indicated on the map show the em,n11 irregularition and crumples in the pile rather than the. general dip.
The other important rock type mapped withi s the pboso 1 sequence is oanglemerate. Two bode nave':seers mapped, One lying 200 feet above of per in the Nasoqo section, two alien south-east of Nadole. The lower is comparatively thin (about 50 feet) and can )e traced laterally for two or three miles; the upper in considerably thicker but aoparently diets out very quickly since it wan not identified in adjacent creeks. The lower conglomerate cooiste of roOk fragments set in a red matrix containing some crystal debris. The fragments are variable in shape, size and composition; froga I inch to i feet in diameter, they 'Aro very round to very angular and petrologically variable from the predominant shogIto basalts and non-phenooryetio bas alts:, often containing sesIites, to calcite and zeolite rich andultes. dedding is
Figure 3 : Structures of Tuff Bands of Phases I and II.
A A a a a a A
a A A
TA Location: Maroleke creek, 100ft.
lft.
TA
Thickness of tuff 2-6 ins.
Location: Basala creek, 650ft.
2ft. Locations Aasaroqo creek, 320ft.
2ft.
Band of Tuff in Tuff Agglomerate
Plan
Location) Maroleke creek, 600ft.
-T- TA
TA TA
T a
TA
-T A
6ft.
IIIIIININSIMONSIMUMIllo. no Ilmillosailsonimapagmna M11111111111mR111110mmillo01111111 UMW low
A M ar.--...•
Ilir illi III, U I' II
II A ii
AI ____.alle o-- -
II Plan
A
A
A
Section
TA TA T
Basalt Tuff
Tuff — TA agglOM.
TA T
indicated eel the horizontal alienmeet of boulder ch in. ewe) placeo, are graded from fino to coarees
The upper conglomerate is more than 100 feet thick and in many rompeots is identical with the lower, thoueh mixed with it merry basaltic boulders are some altered biotite andesite fragments; ,fresh andeeite was net recorded. In size the boulders are again variable, though the lareer onee are. useally rounded or sub—rounded whilst the smaller onos are very angular; the matrix is en&y and contains idioMerphic crystal material. A dietinct Jeddine is indicated by ehangee in either the number or size of the bouldern and the deposit it graded. Although no foeeilo wero found, it is prohable that the rooke were deposited in a lacustrine environment such an one caused by the demeing up of a river by volcanic action or lend slidine. This is in part euggeeted by the fact that the bedded materiel roots on a very irreeular surfece of laree 'loo of sndite andeelte, the interstices being tuffeceouel this detrital materiel, forming the floor of the lake, rests on a fine augile basalt agglomerate. Fetbsequently tb0 conglomerate, which ie slightly unconformable with earlier laves, has been incorporated into the basalt eueoession and intruded by a basalt dyke.
Lome of the blocks in the conglomerate are of hiehly altered biotite andel/lite. Telotite andeeite flow, are nowhere seen situ and hence it VI- coneluded thet fragmente represent material brought up -from depth by tee exploding basalt. (Since the thickness of basalt at that time wee probably sroatI7 in excess of 2000 feet, it is unlikely that the andesite is alluvial). The altered nature of the andesites in paggoolivo of a very low grads thermal metamorphism induced by the bet basaltic material in which the aneeeites were traeped at depth. Some confirmation is therefore provided
of the sueeented andeeite platform cee which the volcano wes built.
n.erne band is exposed in the hea wsterrs of the i River throe sties east of Waikutaskub* village; the seed thins out quickly to the west and at its nest of rly exposure it is sere then 200 feet thick; this Dot is allay continuos; with marine ghee napped by Rodda further east In Ra. Aounded Wader*. rook fr116ments ,q14 monomineralio pebbles ore embedded in a grit wAtrix of fine
moderate grain 11,801 the rock is well sorted and graded bedding is often developed. Each oyele of grading is comp' ed through eix inches of mkt the :ledJirk; is h-risontal
The matrix is allow in colour, usuqly quito deeply weathered ..nd covered with green algae; mmmb o-is subsertarescopio but fragments of feldspar and pyroxeme elY0 determinate. The tuff oontains boulders and. framonto of lugit and elivine-aueite basalt. many of them rounded and up to a triximum diameter of to or three ,a other inclusions represent shelly fragments coral roof.
collection was made of molluscse and eo and submitted for mozination to Dr. ferry Ladd of 171.. National Museum; in a personal coMmunieation Dr. Ladd has rsoontIy reported that because of the poor state of preserwItion of the ma and tUe rnthornosore faun, ti a fossils cannot ',)e used to date the bssalimp
2.1.2 ThO tra The ro
py ol,c do forme, but sin oollapsed .nnd brevet fed, structures which they 8 0
largely tr ohybAsalts in Est of then have seen euboolpion$11
an be stoned on the **naps* Undisturbed rooks of phase 12
oentain a few flows and some persistent tuff bands. The Nasitiu voloanice a Localised series of traohybaealts, trao4yanderitesland traehytes probably re ii ti from velesitiMM
30
in a p: io vont rrs correlted Ai. the phase II rooks. MAcroscoAcally the trac4y'44ealts frequently
rociemble the olivizAe bas'itn of phase 11 dark in ooloar they have a density range of 2.26 to 2.80. Phesoerysts an miorophonoorysts of feldspar are always pfesent and are usually associated with pyroxene pboosoryots1 in some rocks the phonooryste are absent and floe and even grained roast which thou6h foldspathio are =fine in appearsmoot sri nectars of this group. (Alvin* is present in nosy phase ii rooks but ia usually miorosoopic; sometime it is the only far Tian mineral, but normally it comprises ler,ft than $ of the rook.
Fresh trachyblaalt is t4e mwt important rook of this phase and has produced the deriv:;tives with which it is occasionally interbedded. Effects of weathering are slight and the large phonoaryatic feldspars are normally very fra,7,71; as in phase I the result of weathering is the production or a grey skin.
Regionally, rooks, f piles+, II may 34 COMplet;ely, or almost completely altered, feldspar and gyroxene phenocrysts are peeudenorpho4 by calcite, zeolite *,nd chlorite and the finesigrainedgroundmant is heamatitios Zeolites qnd °calcite are also found in vesicles. Zleewhere, easIsserstes. often heavily seolitised, have a Matrix of either haliestite or jaspers native caner is to-:and in some of these highly altered rocks. This latense and ;cite widespread alteration over the
iknbuksbu zd -ainivoo. areas is possibly the counterpart of the Vatukoula minortalsokiens geaostite, however, is a common constituent of sew of the rooks from the volcano and is often produced by re-heating or batting of earlier laves. Alteration to he4metite involving an apparent change in the structure of rocks boss else seen observed. Outcrops of
as '..ndelFite show hemintite forma doming of these money, presumably by normal mothering mores, eventually prodeees an apparent agglomerate with
rounded lobules of andesite in a hematite matrix. 3oth boulders and matrix however, shoe the SA VA amount and distribution of pyroxene phenoorysts.
In the oreqk reations two or the eainivee. bridge, the horizont. 1 basalts a disturb but towards the basin contd,ct they bemire d for sad and breoelated. Like the phase I rooks, the hori4ontal traehy basalts show a great prependeranee et explosive material over effusive the proportion being about 3 to 7, though there is much less fine areal/ens material and eemparatiweir few tuff or tuffigglemerato bands are exposed; where observed they are thin but apparently quits sentinuous. Although broadly near horizontal in attitude, the measured dips vary eniewhat and this variation reflects irregularities in the aortae* rather than tits generel attitudes
westwards towards the basin contact r is es the phase boundary innida whisk all the rocks have a haeln-like
form), the dip is indeterminate, all traces of the oriinal orm of the basalt have Wien removed ae a result of collapse and the disterbed reeks have been mapped in three zones. The Mos t ints**i3 disturbed some is a young feature and though nee y onmeeted with phase II volcanism, is structurallY related to t the ether eallepeed baslts; it is Mapplit as best 0114 occurs as a diseentinumes belt of shattered as emeticee up to throe or four hundred yards wide on the phase VALI eentaet. Maaimum development of thie zone is in the esinivoce area akar, it di3O 11,40e Y to the west benath the ehallew dtppimg primilleatic of phass MI to the eAst its bemedery with the phase II reeks is sharp and steeply dipping Further
32
north, in the region of the two bid es on the ulariVa 'PA road are o torepping reeks whieh thOugh slightly different in compo on, are correlated with the breccia; the outorod in this s not confined to the bin contet but it sharply wed.-into the phase II bas alto. Elsewhere the brecois is poorly developed and very thin; on the western aide of the basin, south of waikatekatat it is silicified and miner:limed with much pyrite awl soLie eub..mieroseopic gold. xecola 'LAI; also formed on the southern contact 'After, it is sometimes chloritieed and be pyrite.
This breccia is possibly the reeedeet rook form*" in the volcvolcano and cowls:Intently it contains avow% variety of reek fragments from augite and olivine heselle to traOhybasalts and augite and biotite trash yandewitels and 101.0,0011Yritio very fino..grained* green rocks deecri,Jed later as smut andcAtee as well as some pieces of aanded tuff. A random .selection of 30 pebbles from an exposure 300 yards north-est of the Weiniveme road bridge shows that andesite and trIchy- bAsalt pebble/ predominate. :Wet of tue reeks are alterA and show development of heamatito or chlorite, the chloritieation leiag acre col mon in the andesitio rooks and most of the haematite -40 oonfined to feldspathic rooka. The monts in the breocia are all extremely angular and avera6e about
inch in diameter though 000asional boulders are up to 9 In AMR
in disaster. The matrix, comprising 50 - 70% of the rook, is fine reddish rit; no jointing, lineations or olip structures re noted. In i s o e northerly exposures the breocias display wide cozpoeitional, ra and though the fr,:4ments are somewhat larger they- are not ap angular as at waimiveme.
The breccias were formed by extensive slip und conste att grinding along the *see Mtn boundary; hence,
T breocta confined to the contact exo.'ct at %wo %midges where
breccia and contact are separated by t ac 'basalt. The relationship here ie illustrated in fig. 4.
The second or inner of the three zones is developed only in the Wainivoce arca; its wotterly hotAndsry against the contact breccia is sharp but eastwards it srader into the third zone* of collapse. The maximum width: of the sone in half a mile, but it thin quickly PO
an 'H north of the d
`fliniV11100 it is not recognised at all ties southw:usds it cannot be tracod beyond the Narlvi river. Lithologically the rooks of this zone aro normal phase II trachybanalts sometimes showing alteration to the heamatite—chlorite— .Aeolito—oalcite assemblage. 1:AruoturtIlly however, they diffa: from the normal trashy—basalte in that oollapse has caused. them to be brokon into large blocks u-], to 10 foot square though more usually only 3 or 4 feet square. The edges of the blocks oan be recognised by the fraoture planes and by the attitude of the pre—collapse dykes, Mrtny green andosito dykoo with chilled marLAnt beirc terminated against the fraotures. some blocks also show euperposition of flow and pyroolastio; those oontacts rkre not extensive sand the oriijnal horizontal dips o•ro often much !.lstur'oeti. The blocks ro wall exposed in the northern and southern branches of ti.o Aalivoce system, tow hundred yards east of the bridge4
when; tear are intruled by a very intense i 4:0 swrm which has not been affeated by collapse.
It is considered that these rocks have not been moved very far from their original position and though some of the blocks have been tiltA they have not been rotated --4n completely radepoeitA. There aro several facts in eupport of this conclusion; firrtly the rock is very compaot and noittler spaces nor breccia or tuff occur Altman the blocks. f7occndly, though the blocks appear to .e re: lar and rectangular
F
• . .F" •
• •
• ft Phase I
SS
Basin Contact
Contact Breccia
Phase A boundary
Contact Breccia
Phase II
F Fault
• .......... • • . • • NW
E
/ a a
A
Phase I th. a
A
a erosion
Level
Figure 4
A ; Sketch section across the basin contact at Two Bridges.
Phase III
i 1 Cone sheet
B : Sketch section across basin showing relationship of Phase I rocks
to Phase II volcano and lavas.
Lavas and pyroclastics of Phase II A
Pyroclastic rocks originally part of phase II crater
Position of collapse fractures representing position of
basin contact
-34-
in *haps, the edges fit together reaarkably well, and thirdly, dtkes effectee ey collapse show a distinct orientetion. It in probable therefore that these rooks represent part of an onveloee of a magma reservoir which when drained oeused vettliee of the roof rodeo; this settling WAV poeeiely of the order of 20 or 30 feet.
The third sone is the most extensive of the three and comprises in eatimated 5C of tne phase II baealt outoropt it is up to half a mile wide wed is adjacent to the previous zone. All the rooks, again freepeental, are traohybasalto and the boulders aru variable from very angelar to very round; they are not well eorted or ereaed tmil no dips were determined* There its little tuff in the eeeuence and mien prement the matrix 12 basalt lava.
These hetereeeneo croolaio rooks oan be interpreted in taro waye. Thy may Yee an original deposit or fregmental maeerial eueh at woeld foru to wells of a teratere or they may aeale reereseat collaesed maks of phage II. The former theory 'would euegeot a ehaeo Ii crater lying *ant of the centre of tee eresen'z '31101.13 with relationehles ao illustrated in fig. 43. Qu the other hand, collapse with laeenze brecolatioe would J4 meceseere te reduce the roeke o a series of fine breocias and aeelomeratee; even nearer
the region of mexieee collapse the **coed one Wee not been eomeleAly fraementede though it is poeeible that there were two ?.ride of cellapee et this locality, the later one leas latenee than the eariier* However, the similar form of these rooke over dietancor of four or five 'Aloe is more indicative of orielnel Wroclaetie deposits., and it is concluded that recce of this zone ori6inated V explosive volca4ic activity rather thAn ey breceietion during aellapve.
in addition to the troy 1G raw otherlava
known from this phase Rooks of the per Waikalnikubm which differ markedly from the usualtpbasse II material,
have been included with the triohybassits• goo rook types of the Rasing series are mapped; andesitoo, which are often agzlomeratie, lie beneath dark, fine- rained, fragmental bas,: tier rocks. The andesite, occurring as intrusive and extrusive lava a well as matrix to agglimeNeSit is light apple green and very fresh; it is =solve and nen-vesioulur though !sometimes it contains patches of sosondery oalcite. It is fine-grained, but often contains a few idiomorphic phenoorysts of black-green pyroxene up to 2 or 3 as long and somprising lees than 10% of the rook; occasional white feldspar phsnoorysts are generally smaller than the pyrszenen and somprise lase than 7% of the rook. Rarely the sodomite is grey and very occasionally agglemovetle; boulders within the agglomerate are ripped from bedrock at depth by the aseending lava umd are often conoentrated together* The ',waders are of various types, but augits-rich basalts and sndesites of phave 1, without any olivine, predominate; secondary minerals Ire usually present and phenocrystio folds; r is absent, the pebble() have a light coloured grey groundsmen and whilst having sharp boundaries with the matrix, they aro frequently lined with a band of haematite up be a quarter of an inch wide, a result of baking and battings
The basaltic rook is bleak or 6:ray-black mid very fins and even grAnied, it breaks with an irregular blo fracture :and in not vitreous,. in stromg light Lathz of feldspar are reflected from the ~fame and display a distinct flew orientation. It is usually pwreolastic in form, the boulders showing a dei;rel of roundness and radial fracturing dissent of pillow b Dalt* However, though some of the boulders are spherical, to rataainder show a flattening, as of a pemeake,
fod by the ejection and spattering of a Of plastic lava, Sons of the boulders or beets are extremely arse and a few are more than 20 feat in diameter; in each boulders the rook has a well dweloped blocky jointing,' Thee. rooks were deposited in the walkdbukuhu area from at group of spatter cones (Wentworth .ad ) aeDonald 1153), though there are a few differences in the nature and form of the Hex-miles mad njlen prodneto•
The Rasitiu basalts have a maxisnun thieknosa of 200 feet and thin quiokly to the east And west; on the Isla1r4vetu road they are lees than 100 feot thick end Aro exposed westwards in the Vunivesi secti*n, though here a sural exposure higher up in the sucessiot my indicate renewed activity on a very small and local Pc,Iles Ma main eriotior:r;, cover an area of about half a equare mile, The andesites lie beneath the banalts 1314 extend c)eyond t-:r1 a little outcropping as tar west as dkubukubu village; their o'2ta a to the south emi east is limited by the phase T/II contact kidesites and basalts are intruded by dykes, moot of that multiple structures to be deseribed in greater detail later; they are generally of andAritio composition but have the sAms trend an earlier Jasalt dYkes. The older basalt intrusiONS can be followed up through the Nesitiu %ndesite into the Nanitiu '_oasalts thogh they are more numerous in the modest eet they are usually not :Apra than two or three feet thick, are alwc.qe vertical and strike north t, There is a curious regularity in those structures and mew exposures show two basalt dykes ewes two feet think separated by agglomerate about 30 feet thick; the Agglomerate is fresh and non—jointed whereas the basalt in rather weAthered, extremely well jointed and looking much the older. It is considered that the basalt is intrusive into the agglomerate, though the evidence is
37
rather oonflicting; chilled, sometimes reddened and sometimes leaohed, margin, a flow lineation mere typical of intrusions than extrunins and the fact that the basalt outs through sequences showing some bedding tiro all taken as evidenoe of the intrusive nature of the basalt, The faot
the aglomerate oontainn boulders of similar composition to the dykes indicates two or more periods of olivine basalt activity; tha association of ban:ilt boulJere along dyke mar mesa may denote inherent lineationn in the agglomerate which the basalts stOrevently followed.
It is clear that olivine basalt was being emplaced after the esesation of phase I activity. The Nanitiu rocks havo been mopped se vrt of phase Il since they wer erupted in their present situation from ventn which were assv4iated with the wain phsee II activity' though it is not lispeeible that they are oontemperuneous with later phases.
2 1.3 The iaugite trv,mhyandesitqs of phane 111. The very obvious chine in rock type tween
phases II and III emphaeisee the co=eneement of truchy-andssitic activity proper; the boundary separating the two phases was the first to be mappel at Vatukoula and is teetonically controlled.
The rooks outerop only withi the structural ')mein and are partly covered by a later series of trachynndesitee; the phase III rooks have a volume in ezeess of six cubic milee. Originally they were a merlon of fine augito tradnpm aldeeite pyroclastios with oeemmion11 agglomerates and voleanie breeciant some of which were deposited in a lacustrine smiwonment• There .re no indication? of any lava flow*, though if they were formed they may hlve seen fragmented by the eubsidenes which has subseluently modified most of the
rooks of this phase. The bedded rook zewlly dip7P1rds the centre of the eosin, though often the ';oedding is tilted, disoriented or distorted; the dip io liriable in amount thowiti low angles. predominate.
The rocks are divisible into the finer, more abundant tuffs and ashes and the coarser brevoias and eg4lomerates; thouJI mainly pyroclftetic in origin, fragmentai. tion or the andecites has, in part, be:,,n cauøed. by collapse, The finer products aro well bedded sediments usually white, green, gray or buff in colour and ran in particle size from grits to Ashes and tuffs. Outcrops often display incipient jointing and wall developed bedding or botndin in stored associated with small scale marine stractres...
The coarser rock o are equally bet o&eiieoun and convict of fraoments of augite trachyondealt0 sad olluvial boldere of olivine basalt and traehy-basalt embedded in a tuffaceous g!otrix. Fragments aro very angatar to very round 17, shape, an1 free fraotions of an inch to ono or two. feet in diameter; the proportion of matrix to frogments is aoout 3 or 4 to 1.
*tructares in the rocks indicate that most of thes ware roraid under water; OX,60081Arevi at Duvetu show graded baddin and small turbulence etructures in moderately ecorc0 tuffs which dip to the contra of the ain at angles upto 200, whilst near VatUkoula thinly bedded ashes show turbulence structures without grading. It is probable that all tees socks wore deposited In a crater lake. Zruption, deposition sad sob0i4eince were in part conourrent ElAa it is oossible that water poroolatine; downwards from the crater lake w responsible for some of the explosions, Pros dip direotions and amounts in tto mine workings, Cohen (1962) computed the thickneso of the phase III products to be 5-7000 feet; the surface rocke
39
thew relatively Minor eollopee whilst the material at depth must be much more Wowed* I4igulfment woo probably continuoun during phase III eruptions.
The aedimonts• aro out y a oerieo of inclined sheets of augito trachyondeoite, whioh although examined by the writer, have been mapped in detail '15,. Cohen (9n• Ste) who has demonetrated their intrusive contaote. In one or
arao the sheets can. be traced actress the pee II boundary, but they do not intrude phase IV rooks, Up to five or six of these eheete have been mapped in the cost and west of the basin though mapping of the north and south is *till incomplete; the intruniens are between 20 and 50 feet thick and ocoasionally diverge ana ooalesee. The augite traohyandezitoo are non--vitreous and from grey to light green and blue grey; they are porphyritio, though the variation in type, size and numeer of phanoorysts is considerable. Pyre:keno is the rot common porphyritio taineral; always fresh it ror Olaok Groan pricmatio crystals wtloh are ookeetImeo elone,otea bat more usually stubby. Uodal pyrozene varies from 10 to 35%. Porphyritio feldspar Le prevent in only a few of the. Sheets; in an expoeore or tho seoond mosi weetarly intrusion a the Nanivi, the trochyandesite contains many idio4orphic and egoidimenoional plajoelase phenoaryets up to one or two illimetres square.
The groundmann ie usually sub,...aioroscopio and grey. or green in colour, though sometimeo it hat a bluieh vitreous lustre; other rocks hove a comparative4 coarse...grained groundmaca of feldspar and pyroxene with maoletite. There are no texturao or otructure* which indicate crystallisation. in watr sodden rock o and it is probable that the orator lake had, at the tiro of intrusion, dried up.
A will he described later, the fracturee • .long
—40— which the rose, dip in towards the centre of the structure at angles up to 40 and 'DO, and in the deep levels of the mine, they steepen to 70 or 806. Sines they are not tluoer shaped they oan 5e identified an cone sheets, descrilled by alley ja.a (1901).
a n es-4re_
2.1.4 Ths.bietite tTa4pmfadteMee. ptittorit
An unoonformity, the result of oontinued stelsidealc during the period of quiescence, separates the phase II rock from a group of biotite trAchyandesite and biotite-augite LcccJ
flows and pyrootvxticep They have 'aeon slightly distended and have the form of a very shallow basin, three miles long by two miles wide, and are estimated to a up to 500 feet thick; maximum dip in 100. Pho;pe IT is the latest extrusive phase at Vatukoula.
The major:,ty of rocks u400 fragmental and pyroclastie in origin, though there' wpe no indicatitme of marine deposition; they can be divided into ,4n earlier thicker pyreclantie group and a 1F ter effusive groll„). The .t.go forms are distinct and there is little interbeddifIg of explosive and effusive products as in former phase,. The volume of biotite tmohyandeeite produced is *intimated at lens than one cubic mile - considerably. lose than the volume of rock extruded in any other phase; the volume ratio of lava to pyrecletic is about 243.
Litholoioally the trachyandeelte is a fine-grained, buff, brown or white rock of porphyritic textured more weathered tban other trachyandesites and basalts; the rook surface* become grey brown and the vesicles are enlarged to produce a very uneven surface. In fresh rocks, porphyritic bioti4o4 sometimes with phenocryets of feldspar and pyroxene, Is set i a fine holocryetalline groundmass. 3lotite is uoiquitous in Vlore rocks but phenocrystic and micro
41 oft
phenocrystio pyroxeme semetimes occurs with it; feldspar is essentially a groundmaSe oesstituent• notite le dmrk brown black, very resistant to alteration, and has an admantine lustre; it forms equidimensional plates up to 2 as dissetor or lee commonly stuby prismatic crystals. The biotite content Varies from 5 — l0V but pyroxene in always lose than 5* $ the groundmass is submicrosoopice Some outcrops show a rather different rook t, ;;sae; phenocrysts of biotite :111.4 feldspar, with a few irreular ArroxIne are embedded in a grey to purple dark pink vitreous groundmass, the redder colouro oeing due to hemmatite.
The pyroclastio group crises tuffaceous rockr up to 2/300 feet thick; rock fragments are included in the tuft in only a few localities, though flab's of biotite are commonly found. The lava flown ars well Jointed and separated from each other by thin sore of bracciated material; they are quite often altered or ca platen silicified and mineralised.
The biotite trIcliyandesite conduit outcrop, as an agglomerf.tte of rounded and subrounded boulders of baked biotite trachysadepite, in the centre of the basin, just south of W.I. Nanivi riv1r; it is intrusive into the lavas and pyroclastios. North—south llmeati,:me in this mmterial, noted on air photogrmpho, could not be identified in the field; they may represent a. met of master joints or they may be the ttpwArd extension. of the deeper fissures which fed the orupti:w7,. they are parallel to, and lie in the same plane as, the fissure sone of phase I.
An important aspect of thin Otase is the extensive swarm of dykes, bearinz biotite and sometimes auelte, which 1v1.1 intruded all the earlier rocks. It in possible that the dykes are part of a much wider series of intrusive biotite
42 -
tro,ohyandenites whlah are known from ,abeto in the to Ra in the eant and the Bo valley in the south.
2.1.5 The latrusive rookm:9f vieV Phaoe V rooks volumetrioally
ixaport-int than any of the previous groups; there no indication of saw extrusion though erosion mar have sins* removed all the products. Voloanism of this period is represented 'ay many intrusive rooks, mainly traOhyondesitic and traohytic in composition, which are distingeiched fr.= phase 1Y trashyandesites by the lact of fervelagmosian phenooryets and their very low colour index, Feldrpar is the onsential minoral and is usually asnooisted with some primary calcitol ferromagnesion minerals are groundmass constituentr. They are often oryptooryntalline and have a colour rqn6o from a light goon to qray and somotimoo white' The phamo comprises Some of the 'grim andenite, intrusins nipped in the Wainivoco areal no equivalents are known on VA, north, west and olauth asides of the ._)asin.,
2.2 Mil pr IntrUsions
2.2.1 Akvips )!=koalt Most olivine Jasalt dykes were intruded during
phass I time., thou ,h somo are younger and nay date from phaas 11 or III. The dykes, which are often trachylitio, can be divided into two groups by their geographical distribu- tion and structural control.
a. VA, South* two Zqm. In the middle suctions of the peke which _flow northwards tmtwoen ttse v011ages of Models and Na emu, a swarm of north-south etri.kinc, bas pit dykes, variable in density but locally sort than 1O, in oxposed
4
over an ere= of two or three square mile. to tLe north it is terminated against the phase IA1 boundAry, but to the south the ewarm dies out in the lower slopes of the Korebeia Bills. ;;astwards and weetwards it deoreases in density and thou6h many younger andesite dykes are exposed in Lololevu and Nasoqo *reeks, few basalt dykes are noted. The ,I.*salt swarm is not exposed at the northern end of the basin but several miler south of Rada., Riekard (vArlo _anap.) has mapped some scattered bas31t dykes though he found no preferred orientation.
The 4ftesp intruded into the phase lavas and pyroclastics, are from a few inches up to twenty or thirty feet in thiekness and oVera4,1:e about 4 feet; they are not curved or warped as some amdesite dykes, and except stare deflected by a fracture or other lineation, they may persist with oonstant strike for a mile or so. They dip at angles of 70-90° but the majority are almost vertioal. The war strike', north-south, thow;h from west to oast it swings a little frciill north-east to north-went, all the dykes being oonvergent on a point; whilst radial to the present struoture, they were possibly convergent on the smaller, initial vent of phase I. ?e baeAt dykes diverge more than 30° from the regional treed.
Stractures of these intrusions are illustrated in no. 5 and 61 converging and splitting, of dikes are common And they frequently enclose wedges, of country reek. Indieaatiano of staving and assimilation of country rook are Were. The lava rase along side fractures formed as a result of tensional stresses causing a crustal extension: 'swells' on dykes may have oxAmed very local distortions of the is pile producing core of the high dip ayes shown on the nap.
Lithologioally the intrusive basalts ere very
Figure 5 1 Structures of Basalt dykes.
Basalt flow overlies breccia
Fault strikes north 1/ / A
A
A A i Rft
At Tachylite dyke with 'en echelon-structure.
Location: Basaroqo creek 540ft.
Strike of dyke is 15°
Bs Basalt drke in agglomerate.
M61usu creek 570ft.
Ds Bifurcating basalt dyke.
Location: Nasaroqo creek 720ft. Two limbs of dyke intruded into agglomerate are separated by
breccia.
F: Basalt dyke and tachylite stringers
Matekenaka creek 490ft.
Plan and section
8ft .
3ft. I
20 feet
Plan
A
4ft.
Cs Faulted and dragged basalt dyke.
Wailotu creek 760ft.
1 50ft.
A A
E: Basalt Basalt intrusion in tuff —agglomerate
Matekenaka creek 600ft. A
A
A
Section
Figure 6 ; Structures of Basalt dykes.
A : Brecciated basalt dykes.
Location: Wailotu creek 1025ft.
The oore of the dyke is breccia-ted whilst the zeolitised margin remains intact
A a A
A • •
A
Plan 3ft.
B : Block diagram of a basalt dyke Locations Matekenaka creek, 700ft.
Basalt dyke and lenses are intrusive into westerly dipping agglomerates.
D s Multiple basalt dykes
Location: Nasarange creek 360ft. All the dykes are basalt and intrusion breccia has been formed across the earliest intrusions
4A )1111: A
E ' Faulting of basalt dykes
Location: Nakailevu creek 420ft.
70ft.
A
A
C s Cross-cutting basalt dykes
Location: Nasararga creek 400ft.
Plan 5ft.
Plan
A
A
5ft
similAr to the pharte I lav, thou4h there i u wider variation in texture L,nd structure than in the flow x'uoke. The dominant type is a strongly porphyritio basalt with many phenocryets of an,;15its and olivine; olivine never pradominatea alleatly over pyroxeue, thou44 mom dyko rooks aro so rich in mato plhosom. (trysts an to be 000poroblo to the uogite rich piorite basalts or toe ankarimitie basalts of Nemaii (kaeonnld 190). TM groundtaass in slows eubmicroscopiee TWO finer reek types are noted; one is a black fine-.grained lava oomparable with the Rao/nu basalt and the other is a vitreous blue-106Y techylites
The dyke contaete all chow some chilling, though in many care it ie not obvious$ intratellurie phenocrysto ars present throwt the intrucl4n, and uometimes thez4, Aro adgromted on the oent,ista by VIOCOUA flow* The grondos in usually finer nt the argin than in the centre of tile dyke, and it often vitreour; otie it shc)4r: development of
ru he-Amat.ite• Intemnive JaLatit m.ltAetc often show a Loodkios of the outer half inch of .- ttel krice, VI; rook heir soMourod white or pale #;;revn. This phitnomenon Is not an oriA.nAl feature and the expoeurtte where it hair been observed lie ne.-r, or under, watass knassite dykes commonly nhow ouch cont:tote.
5andir.6 of iqtrusione by flow movements in mot rtaredo Lone* showiug concentrations r intrntollurio t 1 or vesicles, usuzzaly infilled with evi.lelte or zeolite, or b.ride of oriented minerals are often developed pqrallel to tiA dyke contest; they are 6-12 inanes wide and especially caner in the larger Aykoo.
The 40so OW* is moM donee in Matekenaka *rook; nines exposures *ten a4j4cent crooks aro no poor, it is impractio,:t1 to estimate the ;,lorcent dyke eir.,ra, tho,t,t it is
?eriphoral pyke J,17L < 400 ft 39
4.5
400-700 72 4.5 >700 52 50
247'
- 45
unlikely that the density is regionally greater thx, 2. outh of the lutoin the dennity of the swarm v-tries with altitude ari the figures in table 2 show.
T4b1e 2 Olivine basalt dykes of pkeee I
Average % Plesuro % Total HeightIf. Width Strike % Zeno Swarm Basalt Oka'
A. assure gone 1. Olivine and ,xtagite basalt
400 ft 6 4 ft 3 2 400.-700 163 3.5 73 58 37
700-1000 41 4,0 18 15 9 -1000 14 3.5 6 5 3
224 100 sMsAftragrAosIONOVAAAAA.A.....unAAIA,OAAIAAA...00.-,A.A.IMAAAVA.,AtosAmmOSAA.A.AAAA.A.,AaaablaSAVAAP.110.10111011110
2. taehylite dyes < 400 ft 11 '
400.700 32 700-1000 12
10000 2
57 100 100
o....i.e.aw.11111.40,11.6.11.16.10....1.4.01.14V.MMUVICWOMMINMOIMINOMPIONOtt
19 56 21 4
A
11 4
3 7 3
arme0.116.00...... AV(
163 100 100 0.11.0sainlOw......nown.somn0010111011.
.101W.I.0111.11.11.11.
From below 400 feet to above 1000 fist the numor or dykes decreases armatly. In the former ease the 400 feet contour line approximates to ths elevation of the phar5e I oontaot
aze.inet which all olivine tiaaalt dykes are truncated. In the latter case exposuree show that most of the dykes which can ee traced up throuh the eucoseelon are repreeented at 700 feet by thin ntringere of tachylito which coon die out completely; they are not beheaded by unconformitiey, but possibly Indioate the level of lava in the crater durin the final tit= gee of phase I activity. Only at times when the vent %re', blocked would dykes, he intruded to a greeter elevation. Relatively few intrueiono continue through in exoens of 1000 feet.
b. The eeripheral Dyke Smms Though geographically more extensive, this ewmrs in far lees intones than the fissure zone. Formed of the same varieties of basaltic rock the dykes intrude phase I reeks in the north-weet and north-east, and phane 1 and II rocks in the ease and south-east. They are all radial to the baein but their point of convergent,* is rather more open than that of the fissure sone; • lithology, texturen and structures are the seine in both swarms,
The significance of the peripheral dykes lies in the pattern of fractures along which lava rose, the cracks forming as e rw,ult of crustal dietentien due to 1116h level magma intrusion. Theueh they docreeee i deneity with elevation, they have no preferred level of intrusion and in the east and south-east can he followed to 2000 feet above sea level. In this area basalt dyke e were later re-intruded to form multiple atructures.
Some of these dykes represent at,re of volcanicity anomalous to the trend fro c to more e.cid lavas which is indicated by the gOOOMma s e ce of rocks in this volcano.
-47-
207.2 LrcL ybite!ll AY494 ar1404,AYX0,4me The truchybasalt and Green aud-ie dykee ere
disc uese together because although of different egos, they ure sometimes indistinguiehable in the field. Furthermore, in certeia areao they occur together and form a oomplex dyke paotern of conelderable strueturul importance.
The treche,easelt dykes are llied to levee of phase II; generally gm in maim?, they often contain phenocrystic and microphenooryetic feldepare end lest; fre4uen 1 some pyroxonen. Other varieties however, aro fine-grained and indeterminate macrosooeically and it is tneee rooks which most closely oorrespond to sore of the ereen rInderites. Those intrunivee are ueualle pale green or grey in colour and micro-oryotulline, though some have a few phenocryets of augite, l'eldsear and lose often biotite. The rocks are etructurelees and flow lineations ere not well developed.
The green andesitee are generally youngerthe traohy-bazal t rl • (oc exposuree I the nouthern tributary of 17ainivoce creek indicate that a fee tradhyebasalt dykes may heva 'Jaen emplaced after the andesites; these sonnet have boon derived free. the phase 11 megam)., Multiple dyken of the two rocie typee are Uneeemon. lit of the intrueions are eXpesed oe the caetern tide of tee rtructure and havo 4 very eoneisteat north-west otrio and occasionally reach 20 feet in width. The green andeeites however, ere more restricted in their distribution and are exposed only in the Wainivooe creek zystex eelee 1000 feet; westwards tuey are tel !)y the contest breeciee. Some of thee ere farefeclimSO4 and this waggerte contemperaneoue engulfment and intrusion though sloes dykes are coneidera'ely younger than the eubsideneee In part, therefore, they are contemporaneous with the main period of collapse end can possibly be correlated eith phaee XXI suite trachyandssite
1.va. The trnehy-basalt dfteel intrude rooks of the first two phoes and are most dense in the wainivooe area; around the sollthern side of the volcano only about twenty such dyke.; were recorded, and round the northern side the nueber wee lees than this, but in the vAinivoce. area they owlet/MO a donne swarm striking about 3000.
The pattern of trachy-basalt dykes; suggests that the phase II volcano wat eituatod in the vicinity of the ainivoce bride; the dykes OX70304 in this area oon %Ito:ft a root sone which was subeequently reactivated by green nde74.40 activity. The relationship of the green andotAtee
to tha andesite plugs of Coat Hill will be described later. The rooks of this suite dhow a variation in
petroloa which reflects on a small flea. the Mangos throuh the comvlete volcanic Cycle. The earliest member contain ausite an the ferromagneeian component; sileAtly later dykes boar biotite and the Iatos* dykes of all, whial are also the most common, huvu no forrowgnesiars. 7.:xtruelve rocks of the j.reen aedeolte type are unknown, thouz:h thlo lava type may ')e related to to Nanitiu andeoltee of phase II; the chomiotry of these rocke will be disc..Lased later.
2.2.3 7...C.UWAII112.dikazilltnWlf.roAlitliirg- The 'tine' cot of 'dykes correspond to the extrusive
augite and biotit traedvandeeltee of phaseeliZ and IV; geographioally their distribution is irregular but they fall naturally into throo sub-groups. Utholo4cally the dyke rocks afo; trwehyanderite, biotite traohyandoeito and more rarely biotite-au6ite traohyandeeite and foldopar rich traehyandesiteo; the dykee aro. variable in thicknooe and lento aA4 whilrit soma CrAll he traced for thre or core aloe, others are exposed only in on creek' The geographical
— 49 —
oubgroups are the area to the south of the basin, the eeikubUkubu and . Wainiveee reeion and finally the basin nrea.
The dykes of the first subgroup include all non—basaltic intrusions exposed to the north, west and south of the basin, though the majority °wimpy an. area of twelve or thirteen square miles between latukoula and Nadele. Trachy—andesite dyke e strike pareliel to the basin contact and form a sone one and a half miles widee thoueh L few dykes are exposed about four milers south of the basin contact. Many of the intrusions are sheet like in form.
A large proportion of the dykes oontain biotite, thow:h augite iv, present in only A few; they aro often altered and sometimes relict biotite occurs in a rook which is otherwise etmplea'lly silicified And mineralised. Some sheets of eiotitee4n leeite have a thin and often discontinuous mnrelnal zone o caloite and siderite, though secondary in this form calcite is interstitiü. and primary in some Oki, rocks.
The intrusions are too inooneiotent in fora to 1).• a eyetem of rim; dykes; they are teneential to the volcanis centre and eere probably emplaced contemporaneously with, and elightly later than the cone sheets of phase III, Lava, was intruded along steeply dipping fractures, formed by subsides and shoots have formed on faults on which lexce blocks of basalt may have moved in toward Lho centre of collapee.
The (Amend set of trechyendesite dyes forms a much more intone° nwarut and In expoued On the. ty.:';x4torn. side of the structure frem Two nridgee in tta north to Ne,darivatu in the ecuthl the more exteneive outorope are in the inivese and 'elikubukubu creek systems,. The aegite and biotite traohy.-endeoltes are steeply dipptn ad sills are rare. In this area a third rock type is associated with the two more oommes
ones; feldspar traohyandesits is apale green rook and usually very finely crystalline or Isierocrystalline but ocoasionally containing a little pyromeao; biotite lv absent but the truchytio nature of the rook is often apparent maereseepioally. (It is very eimilar in appearance to the green ,Andeeit e *bleb frequently ciittd biotite and rarely augite; often the two oannot be distinguished. The green andesites 4re a more local and younger series of rockn than those at prevent under disoussion).
Dykes of this group, which intrude phase I and II country rooks, aro radial to the main struoture, but in the wAnivoce area they b000me tangential and. parallel the intrusions at v4Aikubukubul this consistent strike is the result of rei*Latrusioo on the radial dykoo of phase I und the root :one Of phase II. Plow etrueture is well developed; band of calcite and awaits, parallel to the margin of the dyke, and the orientation of felispar, biotite and prismatic pyreiten* cryetals are essmon4 seen. dyke lineations of thin IYInds of grain material are illustrated in fig. 7; similar Sands are noted in some plug structures. These senes are resistant to weathering stand out as ridges which are parallel to dye contacts and flew lineations* Irregularities in the bands are due to turbulenee in the lava duri injeetion.
Tradhyandesite dykes often follow pre-existing basalt dykes to 51vs multiple intrusions; sometimes a single fissure has been intruded three or four times as illustrated in fig. 6. Uultiple intrusions are espeeielly common in the waikubUkubu aria east and south of the road bridge. Often thirty or forty feet wide, they can frequently be followed for several mile's, though they vary in. form along the strike and begone less somplex with altitude.
Figure 7 : Flow lineations in a trachyandesite dyke.
Strike of dyke contact
2 feet
Location: tributary of Waikubukubu creek lying a
quarter of a mile west of the village.
The lineation is produced by bands of salic minerals
parallel to the contacts of the dyke.
\1/ A
/ \
\ k
1/A\
8ft.
-3 •
• t
Green andesite
Biotite andesite
Augite — trachyandesite
Olivine basalt
Trachyandesite
Augite —andesite
Agglomerate
Tachylite
Chilled margin
I 1
1$
Strike of dyke = 304°
25ft.
A A
Figure 8 : Multiple dykes of the Wainivoce and Waikubukubu areas.
A : Multiple dykes, Nasitiu creek 750ft.
Strike of structure is 303°
B s Green andesite dykes.
Location: Wainivoce 580ft.
Strike about 300°
A AlA
\‘ A
\ A
20
C Multiple dyke in agglomerate country rock.
Location: Nasitiu creek 800ft.
The multi le eykee ru geet that tor,',1 WAr, little if any ohenge in the stress irattern durifiL: prolon:ad volcanic) activity; in the laikubukueu are e there are no intrusions rhioh diverge free. the regional strike t un extent greater than 300. Several of the intrusiona of eAnivece creek are zoned; they are exposed abet 1200 feet aeove sea level rind though recorded from different creeks and with different
tics, they are prebaely all of the eame eysteme The rock typee are variaelo; a description of one of the intrusion will indicate the general pattern. At 1150 feet on t creek two and a half miles santesoutheeast of the eiirlivoce bridge, the ravine like valley displays sections
of weathered rock up to 30 feet high. A eill, 20 feet or more thick, which in intruded into the Phase II pyrociaetice, is composed of peveral rook typeo which ere into eeoh other; tee di, of the lower contact is twelve deereez to the south—south...wait. The intrusion is divisible Into :our zones es ineicated in fig. 9; the lowest Le six incher wide, the eeteom half inch or which is a ehilled margin of black glass conteining crystale of eugite, feldspar ad magnetite. Thin eless has a share contact with the lower zone proper which is vitreous and bluish in appearance; phenocrynte of waxene and feldspar are still aeundant. The smaller cryntala have a flew orleatatioq parallel to the lower contact; secondary minowels are absent, eut a few small irregularly seepA vesicles are noted. This rock gredee into the second sone which in also about six inches thick. Pale yellowish ;rey in colour and porphyritic, it eonteine more aueits and less feldsear than the lower zone; the :sideear phonoeryste deoreane in number upwards thoeet the intrusion. The groundoess is not vItreome, eut ntales ROM calcite. losidos pyroxene, a few platoy phonooryn n of very leek brown biotite era noted though
Figure 9 The structure and relationships of a "layered" intrusion.
Location: northern tributary of the Wainivoce system 1400ft.
Section Upper intrusion of micromonzonite chilled against the lower intru3ion : Zone 4
11 feet of biotite trachyte : Zone 3
9 ins, augite biotite trachyandesite : Zone 2 6 ins. augite trachyandesite : Zone 1
The sheet sheet is intruded into massive a6glomerates of phase II trachybasalts.
the former ferromagnesian le much more abundant. The upper zone is about 11 feet thick and very
different is apparanoo; it le sweatherod fine.-gmined lizht brown or buff coloured rock which oontains many small irregular venial's" The roak ha* a porphyritic texture; abundant biotite and laths of tel are. the essential minerals. Tice rook is well foliated as a result of mineral orientation by flow.
The to of the fourth some is not exposed bUS its base is chilled agAxvt zone three. It is a baste band 'old comparable litholozically with the micromonzonitee. It is dark grey in colour and has a porphyritio texture, though the groundmaso is more correlr 'rained than in the lower zones. Pyroxenes are tha main phonooryolot though small amounts of microphenoerystio and pseudomerphed olivine; aro also present; biotite ie a miner grounde*** constituent but there ure many phenoorysts of plagioclase.
Microscopic examinstion of these rooks rveUs %bare important faetes 1. Sanidine is present in. small amounts I some of the rocks.
It is developed as phonooryste in zone three, is not identified in Sen. one but Is a groundmass constituent of zones two and our.
2. Though plagioclase phenecrysts are developed through all zones, they aro more numerOma tomArds the *moo of the intrusion where they are more calAo than those at 11,) top. Zone four, howeV40r, also contains banic pla6ioolase.
I. The early frromagneelans, pyroxene and ?artioularly olivine, are well developed agairwt the chilled mArgins of zones one and four; biotite is only abundant in the higher zones'
The chilled chilled maroins indicate that the intrusion is formed from rocks of two successive ruled of injection. One possible cause of layering is crystal settling durini intrusion and cryetallisation. As shown in fig. 9 the shoot also shows a vertical chilled contact against pyroclastics, though the second injection of sone four has planed oft the top of the underlying zones. Therefore, whereas zone four injection was unimpeded* the lava, forming zones one to three war apparently ponded. It ie possible that this goading facilitated crystal einkino and so produced the layering.
boyering may oe due entirely to flow differentiation, e process desoribed by Mayo (in Nevin 1949), though the features daeorioed by Mayo are reoular with the denser material in the zonee of minimum sallowest and the acid materiol forming a zone of foot flow doon the centre. Leming ocours in a biotite trachyandeoloof dyke at Wainivooe; the bonds ore vertical and pavalel to the dyke margin, but•io plan the fifteen feet wide dyke shows a chilled maroin, az/collated with a zone of augite traohyandeelte* which over a dietanee of two feet grades into tOo biotite traehyandeeite which forme the core of the dyke. This reoular and vertical sonar arrangement could possibly be attributed to differential flew.
These structures however could oleo be produced v the mixiMg Of two megmae, the lower zones of the sheet revolting from an initial injeotion of olivine basalt lava followed lomediately toy or injected sioultaneouely with, tracnyte lova, itself a derivative of olivine basalt. The upper zone tour Is a basic layer which Is probably not uoseoiated with ano tradhytes Similar proce000s could produce the apparent comoonite nature of some dykes. The large mmount of potash feldspar and biotite in the intrusions it ,rester than would normally )0 expected ia a body of differentiated bocie lave and toie prGJosinance of minerols more typical of
trichytes than basalts supports the idea of magma mixing, 4 phenomenon which will he more fully discussed in a later section,
The third group of trachyandeeitm dykos lie within the structural baein and intruded phase III and IV rocks; they aro few in number thou eh eoee may have been destroyed during eneulfeent. The auk treenyAn.;esiLe cone sheets of phase III have been discussed; the only other minor intrurionr are u tTh biotite trachyandesite dykts. They have no distinctive features and were emplaced in cracks formed by (tooling of phase III acne sheets and phase IV lavas as well it collapse
fracturee. Many of them are radial to the phase IV went.
2.3. Major 4Atreeions Volcanicity at Vatukoula har not been acoo aniod
by intrusion of largo plutonic mleree; the p14.; structures shown on the ionp are eectioes crones conduits or small bodies of rook formed by swelling of dyke. kreally and volmmelmistalli the majority of the plugs are of insignificant sises With two oxooptions they all occur in an area of two or throe square miles at Wainivoce; it is possible that they more emplaced here because of ornate' weaknesses demonstratod by the collapse structures of the phase II evoke. There ere four types of intrusions; the )oat Hill and associated trachyandesite plugs, the micromonsonite intrusions, the blocked phase IV conduit and the Vatukoula monzonitee
Goat Hill is the largest plug i the are.; it ix situated a quarter of q mile to the eouth of t Wainivoee ridge and three hundred yurde mart of the road, It is circular in outcrop and has a diameter of about 400 yards though at the northern and two dyke—like protrusions strike north..emetwards from the main plug towards Devotuv Goat Hill rises. to 600 feet above sea level and forms a prominent hill
rising froi ph%He III z,,,diman 6 and cone sheets; it emplaeement probeeqy followed thI main period of obeeelfraent since it ie undistured by any regicnal collapse. Goat Hill is formed of au6ite anderite which is a dense light grey to green, freeh and holocrystalline rook with phenecryete of pyroxene sot in a groundm so of intermediate &rain size. The pyroxene, which prises 151 of the roc*, ie black eed freed forms prismatic aad acioular Crystals up to a quarter of an inch long, though frequently they thew a sub-hedral for. with crystals aggre6ated in small clustere; they are locally flow oriented. A few, platey, phanoorystic feldspars ars cleer, fresh and up to an eight of an inch in diameter; the groundmase is feldspathic end contains mall amounts of pyroxene ead primery calcite.
Rooks of nimilar mineraleee but finer grain size outcrop on the north !end north-eastern boundnries of tle Aug; they may represent a chilled margin, though more pre'eably they ate part of the system of minor intrusions associeted with the tongues striking north-east. Goat Hill ia the only plug of this type; it is pow:Jelly related to the intense green andecite dyke swarm in the eainivoce are- .
Intrusioes of micromonsonite outcrop neer Goat Hill in the upper parte of the Waikubukublu creek eyrtem and from wee or two loan in the Wairivose area; occasional4 they form irregular pluge, thh elnewhere they have distinct linear trends. The nonzonitee are intruded into the reeks of phase Ii and were emplaced in phase III or phare IV tinsel they are frequently intruded ,y trechyandenite dykes. In ppearence the monsonite is grey strongly porphyritic rook
little affected by weathering, with very many idiomorphic oryetls of black gyroxeee randomly oriented, set in a light grey roundmass mottled with luite large patches of darker
t* the rook; froquontly refloating o phonoeryotio material, T4o
y dewelopmirlt urfa00 but Jo apes peninsula ana Ill part minor,
cue tits in phemooryntio or siert arood"ted with other kkt -trk. ferromsgn o In n4rmslly microarystAllinc.
t O of plu6 has r000ntly 4008 &cm) work in thcemime; it doge not outerni? on intru404 into thephamo I and li rook. o eisilor to mortAin intrusions or the Ys tia s more acid than other rook from Vatukoula• /RA und bosrir4 pyrite tw the dominant aro
56 yroxems sC ryottlo are up to 00a,toiOnnlly black
4.00 of but more
AMMO' brightly
morel. tit rack in pink 6rey igre in e*tour aad ittrrss4iat. and uniform
in sise• nrAt .4thP it up to
half am Leah, And irregul Atones nkinh foldo;Ar
oomprine of 10*** 4O' of th rook; mn or Is colAponed of ttreoto folds r and ahleri 41 with gri,nu. ea and lot oa of pyroxene 4nd niotitte. Thu rook is *nsoni io And the proven40 of taio Ito suggeolts emplamement durizi ok IV nativity,
urt t iv* h hate anoorlbody f the vent ngglome the phsn Ito outerop Le VS mil, slues, and is sttuAtimi serth-inor wont of ItatIolo villeite• It is intrusi into the lavas
ao IV.
approximately to 10 unJ thy., miles wide el
ong axis arsine
south to the aorthemiattl no ovoid shape is modified by a flats ulna„ oftwa no the 1414o In tUn ahnotori development of tho struoPuro in disounned and oort4A fs tures
— 57 — produced by rock deformatio eecribed.
VOIcanism in five phases and aubsidence have been responeible for the formation )11̀ the hapin. The volcanic produote which, besides being of different oompositione diminish in volume with irtah succeeding phase, are aleo rtruotinsaly related and roots of sae phlec were deposited within the oaldere of the proeeeding phaes. This is particularly true of phases II and III, thoueh phase IV rocks. were deposited on a land surface which had been produced. by engulfment. Lava mo,y ivwe intermittently overflowed the fradern thoun only in phone II is there any definite indioatiON that this.happenedi Suet deposition explaine the usually steep and always unoonfOrmobie contate of sucoessive phases; horizontally 'Indeed outliers of phase IT rocke on the wasters side of the basin are related to the main outcrop as ellQW11 in ft. 10.
It hen been demonetrated in the foregoing p-.t.ges that there were two. 'eerlods of collopee; the first accompanied phmea I '.4:eivity d roduced a *alders five or six miler in diameter. The secend oollaw:k oonmenced during phase III volcanism leel continued duriyw the intruoion of the cone ghettorand the extinto..1.1 of ph9.06 IV lav; ffullooquentay there wae only 0, little eubeldeneek.
Williame (.1141) olaesified ealderav by their form and formation in an attempt to ally these structures with subeidence; by hill eIaleification ‘)oth periods of subsidence at Vatukoul produced. 'Allow,' ealderas of the Krakatau type', though *Meas. Williams stales that
weessltseat follows repeated nil illy short.. lived explosiono of rides as* 3uvenile ash*,
the volcanic producte in Fiji aro sonesbat ooarer•
3,
/4-- form of fractures produced
4-- Point of breakthrough
Figure 10
As Diagram showing the relationship of outliers of trachybasalt
to the main outcrop in the Wainivoce and Two Bridges areas.
I I
II
4__Topography
Bs gypothetical case of cone sheet formation as a result of subsidenoe.
Magma chamber
— 58 —
"A survey of the oireump-Paeific calderas shown that eruptions of large volumes Of pumice and ash, eepecially if accompanied by dyke intrusion at depth, is the most common reason for collapse of eomposite volcanoes".
As considered by williams, extrusion created a void in the upper parts of the maw, otElmber and the overlying crurt subsequently collapsed. This is oxemplified by the second period of collapse in which the evacuation of the mama chilmber ham resulted i<< a gradual sagging of ti Foe roof and inward sliding of the rocks forming the coduit walls to produce a basin structure. This prooeso in colled acroulement by Williams (2p. eit. p.335).
It is not possible to give a reliable indication of the volune of rook involved in the earlier collapse, though the dimensions of the resulting caldera euggeat that it Wie not inconsiderable. A smaller area was subjected to subsidence of the second period and if it is assumed that subsidence has affected rocks to the bottom of the basin (estimated 5000 feet deep) the volume of rock involved ie about 10 — 1ti cubic miles. 'Albsidence took ?lase on a serion of inward dipping faults and slide planes, Any of which can be reco,:mised in the field, and though some are steeply dippin, one roup of fraoturos it:. more horizontal..
400t of the steeply dipping fretures were forvIod as a reoult of collapse and are probably focussed on the are of greatect weakneec and most intense disturbance. Sine the applied strain us) downwIrds in verticAl direction, lines of equal pressure relief on a land surface at right angles to the strain direction will lie concentric to the point of greatest strain. The inward dippinG rv,ture ,of those rieg structures is indicated in the hypothetical owl of fig. 10. The length of the arrows indicate intensity Of pressure at the
tame of of i titial collapse and show the oonieal natura of the fractures formed. Mudh liding and brecoiation submequently took place on these fracture o but some of them were intruded
trao .ndeolte sod traohyte lava. These ortiol rine; faults 1%-i now expooed an the basin contact, the concentric intrusions
of phaoes 17e14 II and the erne mheeto of phaos The basin
contact is in many places exposed al,,) a fivit plane to some
extant, is the phase II/III boundary on wh{ oh DO M? et t 13 rx aoq A.so have tavr.en ?Irmo*
Cone sheet format on has bovn dinoweed at moms 10L th by Anderson. (1936) who coasidered that most concertrie and inward dim): sheets form am a reoult of intrusion.. or indu.oed planes of oreknoss which are canted by localised up—ressure on a point or email area produced by the upward .motion of bodies of lava; in expreosing dout that any melon can have- preceeded formation. of cone oheeto, he
conclude** It appearo therefore that there wee no actual
tension but Only a diminution of pressure aoroes directions wihich have baan followed by the cone heats".
The Fiji structures differ from those described by Anderton in th3,t lava intrusion wan se000dary to fracturing and hence they ere essentially collapse structures. nowever, knderoon also demonstraten that if the erosion level in sufficiently l.ow, cone sheets steepen with depth; under&round at Vatukoula the intrunione Uwe° almoo: t vertioal. It ie curious that the fracture on which most ooliapoe has taken place — the basin oontaot — is not intruded at all and that ring dokev are completely absent. The concentric intrusions of phaoe I and II are larely biotite tr:chy ndenite and biotite trachyte in compoeition; they forma discontinuous und dineonnected swarm of intrusions parallel to the bin contact* Amounts of slip
-60 on these Zreeteras WAS probably very mell and it in likely thet intrusion followed soon efter freoture formation.
The 11 borizontel" fracteree heve long b en known from the emperor Mine workinge, eut they are dificult to identify in the pile of levee eed eyroclestics. They Occur in phase I and ponsibly 11 rook; end dip at angles of aeout
400 thereby being rather diotinot from the other feulte. Cohen (pare. comet.) har sueeeeted that they are frectures on weicn elocks of basalt may hove moved towards the erea of maximum distureenco; farther work is reeuired to demonstrate the validity of this seeeeetion thouet the elocks, ponribly a mile or tee acreeef eey only hove movA a few feet. The relationveip of the vertical and "horizontal" frecturen has not "Jaen investieete3, tut the "horizontal" ones mey be the continuations of tee verticel planes away from the subeided area. This flattening of fractures away from the basin (metre is demonstrated by the cone sheets and parts of the phaoe II/11I boundary; In some areas on the eartern ride of the basin, tee contact i. horisontal.
The relatively unbroken appearance of some of the subsided material eugeetts that latterly at leant the subridence wee more of a nettling than a sudden collapse, and movement possibly continued intermittently for a long period. This rate of settling can only apply to the racks et present exposed and the material which wan subject to most movement is buried to a depth of five thomeand feet. it is therefore pocsible that the earliest movemente were cataelyemlc, tee rate of collapse slowine ouch test the youngest rocke ara only slightly deformed. The oriein of the basin structure is summarised in fig. 4; fig. 11 shows the location of ell„, pla.r and their rolatienehit to the present eeologicel form.
The lower contacts of the cone sheets dip consistently towards the centre, but the structures outside the basin are usually vertical.
Scale approx. 1 — 50,000
Basin Contact
Figuro 11: Schematic representation of ring dykes, sheets and fractures and their relationships with the basin contact.
2,5 4,41; t n aolut ono Itoraoatod into 44ny ',mete
Or the voloAno attar the omplacomont of plisse V dykes; tits' c4uood widooproad altoratioa of tho rocka and productin of diegnostio mineral assemblages. The minoraltstion was evithormal and oemparatively youthful sines it was oortainly later than the phase IV intrusions And probably later Ups* those of phase TI it also had en offset on tho aprigalo minerals in the rooks of phase) 1. loth soolitisation and minoralisAtion are _roirded as low tosperature plUsenmenn bit v3sy will 1 U44%04414 indivitually•
ElAt seelitess usually assooiated with ealeito and found ti ei osoenblegoot bare eaen r000gniose tics Vatukoula. The mineralogy of the *splits* is discussed later 4mt the amitembisgem axial al
themoonito.sesselite-olasite ohabasito.i.matrolits-caloite analoito-Olhabasitoollts-sooleeitc-cAloiti emeolite-onleite oha4asitoinomolitoweith rare antrelite Ohabasito-thomoenito-eoalcite These as000iations are bared emfdale adAmmas
mod do not tJ.ko inte scoount any *oolitic maloriaI whist' any be a groundmass con)omeat of tile rod:kg Zeoliten 000ur in nest plias* I asd some phase II rooks but the lator interwo diner has, in some plaeoss caused de?ooltion of quartz inntsA of the, original seolieg ovi2olz4 sometimes occurs to ths exclusion of soolites gar.tioularly in WA area of ti,s biaein contact, end in the collapsed mike of phass II though in Dome rftiono the emYgdalim rut* Shpy. Thera rare indioations that eons er the Joundarlas of the *ones aro slightly gradational«
QUARTZ
CALCITE
Distribution of secondary minerals and dykes south of the basin contact
scale - 1:20,000
/BASIN CONTACT
THOMSONITE - MESOLITE -CALCITE
CHABAZITE - NATROLITE -CALCITE
ANALCITE - CHABAZITE -NATROLITE - SCOLECITE -CALCITE
NATROLITE - MESOLITE -CALCITE
CHABAZITE - MESOLITE with rare NATROLITE - CALCITE
• /
-.r — •-• •••-• •
•
...
it /
/ •
• • • • •
•
• • •
• *\ • • .\
• •
-„„
•
•
. . ..••••
• • .1
. . . 1 • ........ . .
• • •
▪ •••. en. ••••• MEM* •••••
•••••• ••••••
•
II
III
'11\111.11 •
•
•
fig. 12
II
- 62
Aolitezi wore determined in tzu eld by a study of their form at l reereeentntives of each forriA were Later identified by optical proportion in the laboratory* Renee, there =4y 3e zove errors of field identification and in Sore
case where the zeolite did not have any eharaeteristic form, it war impossible to mate any determination at all. 7owever, sufficient evidence hac been collected to deseribe tile general pattern of zeolite dietri3etion, which is shown in fig* 12* This map sh,:/es that tho eeoliLe artemblagee are .i.rranged in tha form of a wedge which hae ite apex trunceted to the north by the basins contact; it is euperimponed on an externUve development of the analcite-chabazitesnatrolite-eooleoite- calcite assemblage possibly be. split into a few lees complex ones, though in the area south-meet of Vatukoula, whore it is best developed, the attern has been distorted by mineralleatien* The asseiblagen of the. pease I rocks do not correspond with. those of phase II.
The zeolite zones bear a striking and important relationehip to the fist2ir zone dyke ewers; where the swarm is most intense the thomsonite-sesolite-caleite assemblage is developed, where it ie ?ens intense chabasite-natrolite•- caleito are our any beyond the limits of the swarm the analcite-chabezite-natrolite-ecolecite-caleite aeroziation is found. This latter sent may be a regional zeolite development in the lava pile or possibly an aureole around the phaee I vent; more investigation mey show that it cao be further split into zonen associated rith the dyke swarm, but it seems likely that the former zones have 'een produoed in tie sae its by the higher temporaturet =mad by dyke intrueione. Discussing the formation of zeolitee, v;alker (196(') etatert
"The heat to support zeolitinatioe is supplied by dyke, by the slow cceseion of heat fr below and
63 by exothermic rations in ti lavas. The lavas sorely supply the wAter filleQ1 cavities and the required ohemical raw meterials.* There ie insufficient evidence available to comment
Cori the zeolite distribution of phase Ii though the undisturbed reeks have a meseliteevalcite Association and in the WaikubUku creek arose where another dfte swarm becomes quite intense., natrolitueosselite-celcite and chabazite-meselite-rare natraite- calcite assealations are noted; thee(' hnvo the attitude of a fold plunging to the eouth-eaet and say again be due to tomperaturS0 developed on the dyke swarm. Chnbazite and thomsonite occur near tae phase II contact as riles north-wort of Vatukoula.
Zoolitee have oleo been found ap replacement minera131 in one or two localities they are ale° eseociated with granules of native oopper and the copper it youneer than the zeolite% In one exposure the copper appears to 'ee posudomorphachabasite, though thin sections frem other areas show no relationships between the minerals.
The results of mineralisation at Vate.koula are well aiamented but until recently there seen no study of gangue minerals ad wali rook alteration. ilwall (1949) exemined flotation oonoentrates of Ilmperor ore in 1138 and crushed Delphin ore in 1945 and found the following. ninernist I;mperor Rine; pyrite, marcaeite, arsenepyriLite, minor amounts
of Chalcopyrite, ophalerite, pyrthotits, AAna, rare bornite, eylvanite, hossits, nrttiv tellurium and free gold.
Doh ii pyrite, marcasite, areenopyrite, ohnleopyrite, a
epnalerite, tetrehedrite, 'hornite, galena, eevellitet enalcooite, A trace of pyrrhotite, native copper, sylvanite, hessite native tellurium.
Other samara -3hiCh 4avo in identified. el the mines are etibsite and ooleradeite; the important gange minerals . re calcite and quartt.
The co/lap: e fractures and dykes have acted an mineralielng channele and though the ores outcrop both inside and outside The basin, it is the basin contact whieh is the more intensely affected. Marie Cohen (1962) has 3,000ntly made a study of the alteration of the phase I Mike by mineralising fluids; shn waa able to relate the intern ity of alterstion to poeudonorphing effects of the pyroxenes and found six recognisable stai,;cm as below:
St0.141 ConAitio,cf Lite 1. jiAwito iG fret n though crooked. 2. Atu4ta Is Lltered zroand eagew and along
irregular *racks to calcite and a little serpentine/Chlorite.
3. %dvanced but lacomplete alteration of augite to *lowly calcite.
4. AuglUe 13 coApleely altered to calcite. 5. Quartz introduced into peeudomorphed augite. 6. Augito phenocryQta are replAced by quartz and
miner4iiGation. Cohen concludes her report;
"uartz in .the predominant gangue in the lode. Calcite in subordinate* and. mush of it, apparently dolomite, appers to - A.z oT later on filling gapc or4oko in the rook. °Although otir)onatc 1.t the principal replacement
mineral in the wall rock '3asalt, quartz appears to be the most significant, in that it is with the introdaetion of quarts that pyrite makes Ito appearance. This is not seen $L readily in the dyke rooks, Most of which show a little pyrite. Quartz could be a primary conetitIleat in some of there and eo the presence of a later quartz is not apparent unless it occurs in veins or amyadales.
"The formats >, o secondary biotite in chlorite and surroundinz magnetite grains se to indicate tha a the basalts were subjected to low grade thermal metamorphism after a certain. amoUnt of alteration had taken places. The :e;:0 may apply to some of the epidote In the baoaltn send dyke rooks."
Centres of mineralinatiou can. be lsadily located by aee of- the alteration described by CChem and also by the amygdale minerals. The zeolite distribution and subsequent modification of it by mineralisation have already ,:)esn described, and it follows that the soner of uygdalo minerals from the oentr of mineralisation outwards. will be:
quartz quartz. -calcite calcite calcite-zeolite zeolite-calcite
Geolog s u at the mine however, have pointed out that in one plum amygdaloidal quartz arid zeolite aro separated from each other only by a matter of three or four feet. This phenomenon is apparently rare, and in the Cardion open cut where it occurs, the mineralisation is confined to a dyke, and the basalt country rodkr are comparatively little altered; quartz is deposited in the margin of --he dyke mod zeolite a few feet awmy in country rook. In this inetance it is considered that the thermal gradient from dyke to country reek was extremely steep and that mineralisin fluids were confined to the dyke, outside whit* some calcite was deposited with the zeoli-A.
In other parts of the voicunot lim,gnostic mineral assemblages have been produced; these are due to mineralisation rather than metamorphism by burial or collapse. The assemble's,s listed below, are described in detail elsewhere.
heamatite-cloite heamatite-chlorite-calcite
-66-
/loamtite-chloritc-zeolite-calcite zoizito-chlorite-o4lolte.
2.6 1!Q swIngp Two groups of hot springs, one within the basin
area and the other a few miles to the east at Rubulu, reprasant the final stages of volonnicity. The bas,,n t;rouo oomprises four springs emitting email quantities of hot water (38-47°C) along a fracture in Waikatakata creek; an associatea but smaller, cooler group dims:Wargo" water a mile to the south. these springs have been described by Nealy (1960) who records a total discharge of about 1.75 litres/secs, thol.vh (1956) had recorded 3-5 litrasfsee. On recent dry season visits by the writer there has bent' no activity at all.
Discussing .010 origin of the water, Healy states* "The available evidenee indicates that the springs.
are fed for the most part by meteoric water that penetrates to shallow depth, possibly no roro than 2,000 fest, becomes heated by contact with hot rock aud rises to the surface aloe: z faults. Durin6 its paLlsage linderround, the watc:ir is augNented by mlneral matter leached from the rooks, and by varying. contributions of saline water that originates in soT5o e.,,baas from the sea, but possibly also from connate w,tr that has remained entrapped in the rocks for a long period, and rises from lower levels. There is evidence indioating that in some localities at least, the thermal gradient is higtier than elsewhere.
*The origin of these localised 'hot spots' is not known. Some may mark the location of. pockets of magma and some may indicate the generation of heat by seismic activity in an area whose structure is dominated by major faults. There is no evidence as yet for any magmatic water in any of the springs...*
Healy also investic,ted the thermal gradient in several of the deeper :Aaperor ohurndrili holes and from the data he obtained he postulated an increasing temperature
47 —
northwards from the nine to the spring Irea which is one of the 'hot spots' of which he speaks; hot rooks at 2,000 feet in this area are unlikely, but frictional heat due to oollapee may possibly have been stored for transfer to eirculairing groundw,tar.
During r o ent yeurs, development of the loser levels of the mins (1500-1900 feet below the surfa00) has been severely hampered by eiroulatinx hot water, snMetimes at pressure up to 750 lbc/eq.
Part 3 i he Vatia Volcano
3.1 54440eitur an& z!tra:44granlii,
3.1.1 Pr !WI ttanaltilt The Vatia veloano is formed of rook of three
phasou, the earliewt two being comparable to the olivine basalt and trachybasalt phaSOS Of the VatukouIm volcano, frea which they were pro':)a'Jli erupted. "iuttsequently andesites were extruded from a vent on 'Iatia und hornblende andseite pyroclastie reeks were deposited. The pyroclastio pile was later intruded by bodier of '.ndesite lava,
Though they are 21,),t well exposed, there is no indioation that the olivine !mealte were erupted at Txtia, and since they do not have any minor cracking er distortion me mown in the vent area at Vatukoula, it is probable that they were erupted either froga the Vatukoula vsat or from a °rater lying to the west of Vatia, possibly in the 3a area. Lithologicall,y the rocks are indistinuishable trot the phase 1 rooks at Vatukoulae
3.1.2 TheArachybasalte As with the phase I rocks, there is a similarity
between the trachybsalte of Vatia and Vatukoula and whilst it is possible th„lt some phase II rooks were erupted from a vent at Vette, it i mucii more likely that lava overspilled at V.Atukoula caldera, flowed down the north—western flank and spread out over the area of the peninsula. The area covered by phase II rooks in appreciable -tad since their thickness mix milem north—west of the vent is more than 400 feet, the ,imount ot natrial extruded during use II is considerably greater than suggested by the vatukoula etruoture. There are no indications of any flank eruptions which may have produced
these rocks, and the lava must have ot rpiU . th phase eldera net once, but mAny time.
Fowever, the trachybasaltr at Vtio. arc :1.! ve ways ratlaer different frow those exposedat Vatukoula. The Vatia rock's aro much MOr0 homogenous and show no deformation by collapse or fAAIting; pyroclaetio rooks are lass abundant, though frequent tnffeceous bane are apilarvatly perristsnt. 0o&leionhl/y the 4ands Arr, up to five or six feet in thiokners„ though many of Cam .:,re only z, few inches. 4,4- lomerate bards are thicker averagtne throe or four feet, and are oomewLat more pemlotent thAn t tuffs and ashes. The agglomerates consist of very round to roma pebbles of trghehraasalt lav 2 - 3 ineheo diaaeter, embedded in a matrix of the naterial, or 1e frequently in tuff and ads The rooks az.c o aroolasUo origin and contuln rock fragmente, volcanic bo. Alted names alluvial material.
A typioal sequence throuh the basalts is exposed in Ta itagi quarry on Kings Road at the southeastern corner of the peninsula, where the horizontal attitude of the fiow.
vroelar,tics ie well displayed; There are however, some minor distortions due to faulting but though the moral faults often have d 1aee.ent u to 10 feet or more, they cause little rsiona1 2.;isturbanee of the lav%:1 pile. The fractures do not have a conclutent strike and most seem to be local impendatent cracks; they are infrequently trosod and rarely visible o air phetographz.
The trachybJialt is a dense, black rook which is y mrely vesicular und apparently never zeolitirod; in
the few vemioular basalts the aAvitiso are usually lined by i film of pole blue natorial (shown by X-ray to be mont-morillonite) or an aniaentified yellow clay mineral. The rooks do not show any flow etructures. The baMaits all have
-7o-
a porphyritio text'', and conta,in plagioclfoge, of with a platoy or eIuidielmsional form up to 3 or 4 mm in diameter, and lose frequently with a lath-shaped habit; on nverage the rooks contain 20 - 31.0A of phemoorystio plagioclase, though a few rooks contain 40 4.•. 90%. 2henocrynts of eagle minerals occur in Dome baoalts$ black-green pyroxene and olivine, pseudomorphed and brown, are alwL.yo lens abundant than feldspar and the ferromagmesian content is not often greater than 10X. Few rooks contain both olivine and pyroxene and more usually the aessublage is plagioclase and pyroxene or plagioclaee and olivine. Though it is impractical to subdivide the reeks of phase II on their dominant ferro-magnosin mineral, toward' the south (and the phase contact) olivins is much more abundant than pyroxene.
Tufts and ashes are vaAable in colour and t xture; e soatain a little pyroxems, olivine or plagioolao oryota deer though a few of the coarser bands contain angular basalt f_migments and sometimes grade upwards into tuffaceou2 aclomeratess Crystal debris also occurs in the red boles as expooed at Tagitagi where the exposures are fresh, but boles have not been found in deeply weathered oreak sections; it in possible that continuous wea thering bleaches the original red bole to the *or o common yellow cream and whit* tuffs. Thies*, howevor, are usually cryptocryotalline and have a fOmeture pattern similar to that of the tuffs of Vatukoula.
To the east and south, the traohybasalto lie unoonformably on the phase I rocks and in same places the contact dips steep/7 to the north, possibly as a result of faulting. Further east, the boundry has not been followed, but westwards in the 3a area, the phase 11 rooks are apparently overlain by a series of marine tuffs (Rickard pers. 00=)
Te
whieh may be azeociated wite the later Vatia eruptions; the distanc. ki Ba tirand Vatia is six or nevem miles. The contact Oetween the trachybaselt and later hornblende endeeite lr in part also faulted, thoneh elsewhere it is unconformable 'ti variable reel horizontal to vertical*
3.1,3 T4g A9vAplpm#4 4atimpeo These rocks of phave IIIY are exposed only on
Vatia penineula and nearee offshore Islande and the two emaller poninsulaz to tee went; they outcrop over about 12 seuare ails e and teeeir maximum thickneen of slightly more.
than 1,100 feet is ured ae Drautana trig. The rocks are erecciao, meeeloseratee mei turfs surrounding the vent through whiah they were extruded; eubseluently they itAVO been intruded
by many bodies of andeolte* Tha length of time aetween the eiseation of phase II ectivity and development of the phase IlIv voleano is conjeoturel, but whilst phase li rocks were produced at Vatukouia, the hornblende andesites were extruded from a vent on tLe peninvula*
in the preeent mapping en attempt has eeee made to esparato the Illy rocks into a lower Vatia group and an upper Vatia group* Thsubdivision is not oompletely satisfactory since the grouee evmetimas grade into each other and many pyroelaetic rocks contain representativen of both groups* Though the boundary between thou is not always determinate, this elaseification ban eeen used whern pose:tele and the two rock typeu will ee desorleed individually*
The lower Vatia evoke ere, in appeerance, inter— reA
meditate between the older meueeeee, basalts 4nd the youneer hornblende andesites* As well ao lying below the upper Vatia andenitee, they are expo eee around them and outcrop on the
eest and lost coasts of the penineala 40 well as on the two
T2e
small prox o to the west and oee exposure in the middle of the hn hle de andesitee. Thoueh they cover riuite a mall area, it in as imated that volumetrically, almout half of the phase MY reeks belone to this group, They occur as breceiase agglemerstes and nowt, but tuff bands re unknewn• In. the few plaoee where bedding le seen, the rooks are horizontal particularly on the eaetern side of Vatia *bort they outcrop extensively a. mill or so to the north of leal Reale of ti te group on Vatin Lanai island. ars also horieentale
These andesitee are leaden to dark grey in colour, never esionlar end rarely eontaining evoondary minerelet when freeh they have a dietinotive dull and semi-vitreous lustre which occasionally becomes steely. The rocks, which are strueturclens, occur in effusive and fragmental form in approximately oval proportions. Porphyritio in texture, they contain about 20% (or more infrequently 35 e 40A) of eluidimeneional pherocryste of elagioclanecand, oocesionally, porphyritic pyroxene which is !Always eubordinate in amount to the plagioclase and never greater than 100* Pyroxone ie black and oompletele anhearel; olivine and hernblenie are absents
The lower 7att rocke aro ineuffioiently well or widely exposed to indicate any largo soale structures. With a contact which ie in some plaees almost verticel, the andesitee lie unconformably nn the trechybaseltc. 'the elect pOsition of the orator from which they wore erapted in unknoen but most volcanicity at Vatia hes !Jaen centred on the north-western corner of the penineula and lower Vetia rocks haea been depoeited up to teo miler away from a vent at this. location. The eruption wac not PO violent OV that which produced the. Inter hereblonde andesitan and of the several pertodt of voloenioity described from Vatukeule and Vatia
- 73 - this: initial eruption on the peninsula Tres probebly the quietest and the shortest. The maximum thickness of the andediet, le about 200 fest in the region of thee supposed *rater, though many- rocks and structuree of this group stay have been destroyed by later voleenicity.
Rocks of the upper Vatic% group are blue erey to very light grey in colour and alweyn holocrystalline; secondary minerals are absent. Usually the rocks are structureless and not Jointed though a fe' epeoiuene display mineral aligneent and flow structure. The andeeitee hove a microcrystalline groundmaen and phenocryste, ueunily of both pleeioclase and amphibole are Sn. Plagieeleeo forms equidimanylonal platee of 2 — 3 Min diameter, though microphenooryste are more numerout here than in the lower eroup. Hornblende in the essential ferromarnesian mineral and occurs in priematic or ecioular ford Whereas the pyroxenes of the lower group are ntubby. nodal analyser ehow thnt amphibole if; seldom more abundant than 10'i and the ?average is nearer 54. Occavionalle a little pyroxene is_ associated with the amphibole.
There are no andeeite lavas and, most oft the exposures of maseive hornblende andeeite on the peninsula are intrusive. The fragmental country rocks coy posed of angular o round boulders of hornblende andevite in a tleffneeeue or,
very infrequently, a leva matrix; in some erposuree the Wilemereto it unconeolidated and has no matrix at all. The make ere interbedded with thick bands of ash and tuff. A well exposed section chewing the nature of the upper Vatia pyre—elastics forms the lower part of the northern face of Drantana trig. two mile east—south—east of the wharf. Coarse rounded eoulders of hornblende andesite in a tuff matrix alternate with well—bedded yellow and creamy white ash bands which are
41. 14 di,
up to six feet thick, Cryetat tuffs are rare. nit; syttem of pyroclastios is the extrusive stage
of the upper 'Jatta rocks and the centre from which they were erupted J computed from dip directions to lie two miles south-east of the wharf, auw;rJ ot thie point, the pyroclastios are poorly exposed but they apparently constitute a aeries of horizontal rocks overlying lower Vatia andoeitse which are also flat lying; the coEnplote SUCOOUSiO4 contains very little ash. These eruptio;Je may be the source of he tuffs which Wave been mapped 47 aiokard in tie vey. rho olOivalent of the hornblende andesiies in ether parts of Viii 14vu is unknown, jut it 1 possible that Hodda soon.) has found a similar sequence of rooku on ti a coast at aakiraki pout eighteen miles to the east of Vatia.
The tuffs and tuffacsous aggloserteu often show a igroovint4' or cracking of the tuff matrix tato rhomboid patterns; the structure is seen in horizontul and voitioal sections. The cause is unoertain, but i« *.j be the result of compaction pros an incipient jointing. Mare is no indication of ally marine deposition.
During the eruption of the andesites4 vary heterogeneous agglomerates were dopooited within the caldera; continued volcanicity caused raY,porxin of pyroolastica in the
**or area and thin WaS mixed with much alluvial auttQrs The lomeratoes now exposed in thee* re4ons straw u i,;reater
compositional variation than elsewhere.
3.2 or 44Tuskom. 12/clattered in the region of the original waderas
are a eerie.? of intrusions, all apparently saplaoed during the final etages of volo;Inicity; they 414'4 structurally au well as sincraloically related to each other. The most
northerly forms the inlet of Vatubuli, a roek Ii flf1; feet high and three or four hundred feet in circumference at tt bee, lying off the uorth...weet oact of Vatia.. The nearest country rocks are on the mainland and an intru5ive contact is not exposed. The rook le a ivpersthee andet;ite; it lft grey in colour, the darker groundmace oontrasting with the large ahe numerous foLiz4par phonocryte which ar very fresh and variable from white to pinky-cream; they oomprise about 40% of the rook. Though all idiomorphic, they varw considerably it size, the larger ones bolag 12 x 6 mu or 9 mm in diameter, and the sMalIest ones, 2 mm in diameter; on averLze they are 3 - 6 mm diamotei' aad the majority of crystals arc clul- dimounional, few black pyroxene phen6oryste comprise Use than of the rook; though fresh they are aahedral and attain a maximum lehgth of 6 mm, These phenocrystc are zet in a groundmase of intermediate grain size which, though .i.pparently felcispathic, diL.Lay L steely ldstre.
The rock itself 1.e structuraloss, but the exi,cslares anew very distinctive. join•ln; pattern; a series of strong joints re radial to the centre of the plug and (mare froturing of the rock tato slabs each 3 to 6 inches wide. Throughout the intrusion the rock contine many xenoliths of hr,rnblurae andesite; these are u to 1 dm. long an4 generally ha.ve An irre8u1ar outlias which chews Incomplete fuzing. They tend to occur in poekets and may loeally compose 2 - 3% of the whole intrusion,
t is interesting to contrast this rock with alluvial material frond he 1(aticutukubu. Lre t Vatukoula. This latter rock, which has not user round situ, but Alich occurs prolifically on either side of the main ridge to the north-north-et of tkuOUkubu village, is rather more coarsely grained; phenocryetio feldspars attain 20 x 2 mm
-76-
or 12 x 10 mm and 37 x 6 um. These feldepare are very fresh and oolouriess but only Account for 10 - 12% of the rock; pyroxene however is more abundant than at Vatubuli and though still occuring in black eubhedral granules it probably comprises 8 - 10% of the whole, The two rocks have a stellar groundmAss. The outcrops ire about 18 miles apart and there is no possibility that they were both derived free the sets VIIMU However, the two rook types may imply a diroot rolatio Alsip between the parent magma and derivatives of the tw volosnoes.
The ether aa3or intrusiens of Vatia all lie within the two sUppOeed calderas; those within the most north' rly one can be subdivided into tso groups on the grounds of petrolosy. Ono group in typified by an intrusion eoverin most of Yanuca island and this la associated with three intrusions exposed near the coast on the mainland, south and south-south-eazt of Tanuca. They are intruded into a, series of hornblende andesits breeolas and agglomerates. Gontsots are poorly exposed but on the swell mainland prementory lying to the south of Talmo, a contact was excavated. At this locality the intrusive rock shows a thin (* in0h) selvedge of glass where it has been Chilled agsins$ the pyroolsstic root The base of the intrusion dips quite steeply (320) to the morthsinorth-east, that is towards the centre of the body; this suggests that it MO/ have a lopolithio form. The el form is possibly Moored by other intrusions of this group though they all may be part of a larger body, the upper levels of which have been exposed by erosion.
Those intrusive andesites are light grey and have mottled appearanoe due to large, well shaped sad fresh
idiomorphic phenooryste of plagioclase and smaller needles and prisms of black hornblende, The rook is massive, bar a.
-77.. low colour ndex and is apparently holoorys Aline. The grain eine im tine and oomparable with that of extrusive l...Item and pyroolastioe, Though the plagioclase phenooryats are more abundant and often oomprise 40 of the root, the hornblende crystals (comprising up to 7%) are up be 12 x 6 mm. in wise; in this way the rook is distinctive fee* th . t of the other group of intrusion of the northern caldera. The andesite on Yanuoa has well developed cocaine joints along which eome minor intruoton has taken place; ooastal exposuren of the mainland bodies* ale° show thin jointing. exte it strikes censistently north-west as if regionally controlled. inland exposures do not show Jointing and neither flow structures nor xenoliths have been identified in these rooks.
AB euggested, these plugs are probeely all part of one larger intrusion, thouja the form of ouch a body remains undefined, Axpoeuxes of agglomerate on the eouth-eastere. 4:meet et Yeomen are roof rooks possibly in the form of a roof pendant, but eimilear rooks on the mainland s zoo
ei 300 yards slimy lie beneath the intruelon. Exposure of this intrusive andemite cover an area slightly in mese of One square mile.
The other major intrusions of the northern caldera lie further to the west and are exposed on or near the coast between the Wharf and a point a mile to the east of it, and never more thu 50 feet above sea level. This group of three bodiesis not well egpeeeds no Goat:sets hAvo been seen and sometimes the existeece of an intrusion is denoted only by may boulders on the surface). One suCh structure forms the lower part of the hill 170 yards east of the wharf; hers the
IF These are shown on the nap as two bodies, the more westerly Of which is possibly composed of a few separate intrusions.
78
ground iv covered ey very many and sharp boulders, frequently three or four feet in diameter, of black re** Purther east, there is a north.eaetAsouth-weet alignment of exposuree of this material, though s at Yanuca, Very similar hornblende andesi e agglomerate* are found below and above the intrusion.
Tholk;h andesitic in composition, theee intrusive rocks are very different in appearance from the hornblende andeeites and are more comparable with the trachybaselts of phase II. Ttey are black and massive though rather brittle rocks, breaking with nharp but irreGular fracture, and giving a oharacterietic ring with the hammer. Large aqui-dimensional phenooryete of clear black and idiomorphic feldspar comprise 15 20% of the rook and there are no phonocrysticferromagnesiane; the groundmass Is very fine-grained and structureless. Again, though disconnected at the surface, these bodies are probably part of ore larger intrusion which m ay be rather different from that at Tanuca. ihereae the latter is a small but compact body, the former is Sufficiently elong3ted to suggest intrusion alone, a fracture, possibly with come "swelline to produce globular and irregular Mamma alon the length of the crack.
The numerons intrusions of the southern caldera are younger, larger and more complex than any of the foregoing. They are considered to represent the final stages of volcanicity at Vatia and no of them show lete stage differentiation proceeding within a body of lava. The oldest intrusion exposed it this area is the hornblende andesite vent agelelerate. The vent is situated four hundred yardr south of the Asyr and forme a steeps sided hill, 400 feet high; thouji exposures on the lower slopes are poor, it is likely that the actual vent was not more than about 300 feet in diameter. The rook
is very angular and i1 .sorted ho b ends andesite breccia which sl=owla little fusion.
The ether intrusions of the southern cnldere can also be divided into an earlier group, the rocks of which ar **potted in the north and west, and a later more acid group formin6 tho Delelkoule ring structure which Use two miles south of the wharf. The relationship between the teo grouom of intrusions is exposed in a quarter mile long section on a track to the west of toe road and 2.2 miles eouth of i<e wharf.
The westernmost (it is also the largest) of the earlier group of intrusions lies one and tbeem quarter miles due west of Drantana trig. This intrusion, which has no visible base, is exposed between 300 - 600 feet above 00a level and forms the core of a hill; it ie globular in shape and its maximum diameter le a quarter of a mile though the area is poorly exposed and the boundaries cannot be mapped with any precision. The intrueion however, doer out across the oontect of the lower and upper Matta rooks. The intrusive lava ie andeeite and is bladk when fresh but bluish and vitreous when weathered; though usually massive in form, some exposures show fragmentation, by e system of very strong joint planes, often no sore than 2 or 3 inches apart. The andemite contains much phenoorystic feldspar, usually black and clear, but in the blue grey rocks it is sometimes white or pink. Modal plagioclase is up to 15X in both blue and black reeks, which frequently grade into each other. These ;make bear small amounts of black hornblende, but pyroxene hoe only been recorded from a few places; the groundmase is very fins-grained.
La will be shown 1.ter, the eastern contacts of this intrusion are steeply dipping; in general, whereas the
intrusives of the northern caldera are rnirly shallow and flat lying those of the southern caldera are very steep and possibly represent stocks and bosses of a ohonolith. Another large intrusion, also of this eerier, lies a few hundred yards to the north-oast of that just described and two other small bodies occur in this same region. However, Some or all of these intrusions may, at depth, be intruded by the latent rooks exgosed at Delaikoula.
Tb. youngest Intrusion of the southern caldera is centred on a conical hill which rloes to 600 feet above sea level and lies one mile south-south-east of the wharf; this, the Delaikoula intrusion, is formed of several sonen con-centrically arranged. The distribution and lithologY of each eon* will be described and then the relationships diocun ed. The outer sone is a coarse and angular breccia in which the boulders are frnm 3 - 5 inches In diameter and usually fresh. They :1'1.e sometimes aosociated with small amounts of andenite lava matrix but tuft and aah are absent. In appearance the bonlders are rather more acid than the normal andenites; they are vary light grey rocks and contain many large euhedral and pinkish feldepar phenooryste. 31aek eUhedral prisms of hornblende occur, but pyroxene ie absent. The rods is coarser grained than the andesite Levan. Oecasional boulders of granodiorite, mlorogranodiorite and other grnnitoid rocks also occur in the breocia; thane rooks are of similar mineralogy to the andesites but with modal quartz and more potaah feldspar.
There are also some rounded boulders of olivine busf.at up to 2 feet in diameter p,rtioularly in the south-wanton:, and south eastern negments of the intrusion, though in the former area it is possinle that some of the material may 'ae part of an original road foundation. The basalt has
1 - probably been brought up from depth by the andosite lava, though it may be washed down from the olivine basalt hill to the south. The rock is blaCkp fresh and porphyritie 3rid the fine-grained black groundmsze is studied with euhedral black-greon pyroxenen and light greenibrown olivinen. There is no hematite formatian to suggest baking' and no alteration of the primnry indicative of hydrothermi activity. if those boulders were brought up to the ourfaoe by the 414400Itse, they remained Unaffected by the hot lava.
The oomplete intrusion ie about a mile in diameter, and is almost circular in outer°, imeept for small indentations on the south-west 7,,And south.east; the north-west corner is covered by alluvium. The outer 100 - 300 yards of the body. is broceia and whilst the maim mass of the intrwi6n. in 600 feet above sea level, the bx'scoia le confined to the lower ground. and in only rarely exposed above 100 feet. Were viniblet the inner margin of the breccia is sharp and sometimes faulted, but the outer margin with lower and upper VAtia rooks ie sometimes diffuses particularly 03 the western and south-'(rtern edges. At thee* plaoes, the breccia grades into the heterogenoun agglomerate of the caldera floor. On te eaotern margin of the intrusion, the breccia contact i fulted.
In the second stage of tbe intrusion, the most impeortot rook, volumetrically, le fine-grained hornblende andeelt.4, which occurs round the margins of the body. The reek is often fragmented by very strong joint plane which are leeally &mapped; in the south-east part of the plug the jointinit strikon north to north-west. When fresh, the andesite is holoorystalline qnd moderately coarsely grained; It in usually a blue-grey colour. Most of toe rooks Irmo a porphy-ritie texture though ,lome are equigranular; hornblende is often phonoorystio or microphenoorritie and most rocks contain
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cryvtle of both hortiblenAle sad plagioclase. Whether large or small the plagioclaso crystals are always idiomor?hic but variable from pink, whito and cream to colourlose* Hornblende orystals are black and lustrous and show brown internal retleotinsi they are usuaily idlomorphio fAnd the maximum size of i4dividual crystal ie 9 x 3 me, though they often occur in austere* The hornblende ocAiieat of these rooks io not greater than 1W. Quarts, taou4n sometimes visibli macroscopioaliy, ie oomplt,rativoly rare and rooks do not eentain more th,Jz 3%. Notaoh foldopar le a minor around-mass constitusat and the pink and aroma: coloure of ,7;oue fadepers are typical of the plagiooluse of Vaida rooks.
Noarsr the centre Of the intrusion the topoeraphr mnch steeper and a trIveres towardste centre shows sea*
compositional changoo of the rook. The feldspar content of the andezito rieresses and the mineral b000mee white instsad of yellow or pink; the rook is oomsidorably lightor in colour and more "acidic" in charaotar* in the rooks at ths core of the intrusion, the hornblende is replaoltd by a few irregular bleak sraaull(y of pyroxene, though these rooks octl.tain lees than 5% of forromagnesian minerals. There 41,0 no phonocryste of feldspar anJ the perphyritia ferronagnesisms are sot in a white grey fine-rained groundmases The andesitec contain appraciable amounts of kaolin and other clay material, due to the alteration of feldspars, prooably by metasomatio prossomme
4.1 the expeoures on the core of the intrusion shoe the same rook type Whereas those on the bargin *how hornblende widssite host rook in whioh many pe101es and boulders of other rock t,ypa WO prenent as xenoliths and tae6rotions. Descriptions follow of eons of tIle rook types found In the rim of hornblende andwaitel ariL..r ran,:;le of tepee era known from the outer stags I breocia• One of the tiara unusual
types is a dark grey or black vitreous lava (dark brown when wwvthered), which is sometimes a pitchstone and sometimes sufficiently well crystallised to be an andesitel it is usually microscopic in grain sissy but in a few rocks crystal boundaries can be identified. Inthe coarser grained rooks there is littl.e black Lind clear granular feldspar; biotite, however, is more abundant 1.nd form up to 10% or more of the rook. The oryntals are 13.ney coloured and well cleaved and so4s of the Larger phenorytvte (the largest is 9 mm diameter and the average 6 mm) are idiomorphic. The groundmass ir oamposed of pir ant white flair and opaque magnetite grains; in appearanoo it contrasts strongly with rooks of similar mineralogy and texture at Vatukoula. Xenoliths of this material are oommon in the Delaikoula intrunicm, but those whioh contain biotite are rare and only three specimens were found.
One xenolith of a dark grey—green rook with a very well defined bandlo is comparable in texture and struotur wi4h u fine—grained gneiss. Composed essentially of pink feldepar and dull green amphibole, the rook has been deformed to produce irregular, bat elongated pods of feldepart sur—rounded 5y equally irreuar patches of amphibole. The original rook was possibly a hornblende andeolte or diorite, the deformation of whict may nave been caused by intrusion or faulting; alzernatively it way be a xenolith derived from metamorphio rooks lying deep in he crust. The specimen, which is not very fresh, im2 A 2 x 1 inches.
Granitoid rocks occur both in the breocias and in tha intrusion but urs often weathered and sandy. The essential mineralogy is Amphibole ;.1.nd feldspar though quartz in present in a few specimens. The amount of hornblende present is differnt in different specie and there iP often
oonoiderable variation in on,a boalflAr; spin 601 Chows a concentration or hornblende round its mareln ;;Id almost pure feldspar In the centre. The ro contain 15 - 200 hornblende which iv dull blot& -%nd jenerally idiomozhic; in rlomo rockn it fUZILV3 atubby prisms 6 x 3 mm, Isxt elsewhere eryLitle aro elonzutod and -1;10o prisemo 1,1 25 r 3 MM.
SORG of these boulde's ara zenollths of Iranodiorite othara finer LI tA. tn.re and eonlain much leno potash
feldspar than the graaodioriten. They have a ninoralogy equivalent to . that of the VatiA andenites, but show greater mod;a1 vimintino than ia usual ± the lave, and on these grounds it is conidnred that they reprenent eecreatione within the latmeion; the7 ara tome andeniten.
The overall rane of reek type is indioated by the followi7Ag daeoriptions of two rookn from the western nide of the intrusion. 1.n the firot clots of hornblende crystals, which are granallr in form, are u.0 to several irkenes across and contin =all amounts (10%) of interstitial plagioellAse; these aro araphiboliteki. The secrrnd type was found in only ona expeourc; white, siliceous Taaterial, which is .!ragmented to piocee eci tneh in dieter is otruoturaless 'ant usually ooins tiny i4rAlne of a black forrommgnosian rirerii1. In thin soction, roundll phenoerjats of quart, each uy;, to 1 mm in diamator, are cluaterad toeAtAer and set in a fle mortal() of twined lebradoritat bytowaitu and uatwinned There Is little potash feldspar in the rock, but a few tiny crystale of brownish sphene and green pleoehroie grannies of asgirine augite cr:)L.)lets tao tha granules of s.egirin 2ie lie aloug well defined fractares or lineation in rook. They haia probably ronnited from the brealrlown of hornblende oryatals (Iyengr 1949) and were eubsequently redistributed throug6 the rock; hence their alignment on one
. 85
fracture. The rock ic quartz deeine—aegirine augite segregation.
Relationehips between the inoludA material and the host hornblende andeeite are variable; a few of the coarser granitoid rooks have fine margins and appear to have been chilled against ;114, aodssits but in general, sharp contacts are infrequently noted except *hers boundaries have been emphasised by later ealeite deposition. Gradational sontacte 're more oommon and crystal eagregation, ae a process of differentiation, 1.4 probably the it important oauee of litholoei-al variation.
The final etage (3) of tIle Delaikoula intrueion has affected !;he northern margin of the plug where the brocades are oat by two *eta of intrusions; five small pluge, of which four are aligned on an unexposed fraot'*r striking 3200, vary from 50 to 320 feat in diameter. Zech intrusion Is separated from its neighbour by 200 — 300 feet of explosion breocia. 3stween the two northernmost bodies there are small outcrops of stage 2 hornblende andesite. The fifth plug lies a hundred yarda or so to the east intruded into rock e of the upper Vatia series.
The plugs are formed of black, denss fin...grained rocks which have a metallie ring and a ebarp irregular fracture; they contain black And fresh lath shags4 phenocryets of feldepar up to 3 no long and anhedral patches up to 1 mm in diameter of pyroxene, whith comprioes lose than 5 of the rook. The groundmass is fleet but felder, sometimes pink, visible in a unAform dark may vitreouo groundmasso The roc contain xenolithe but they are very weathered and sandy. This andesite is of a similar compesition to the &antral augite andesite of Delaikoula.
The other intresiso if this stags Is a dyke; four'
feet wide it strikes north-north-west and le tengential to the main intrusion. It is formed of hornblende andesite ehich is grey in colour; it is heleeryotalline end oontaime prismatic phenooryete of horeblende up to 9 um in length as sell as irreguler black patches of pyroxene. Feldspar occurs may in the groendeass which is of intermediate grein size and granular texture.
The eentaote of the Delelkoule intrusion are boot exposed WI the track eeetion; this quarter mile section, shows hornblende andenite country rocks separating two intrusive maasea; the sore westerly of these has been mapped is one body though it shows litbological variatione. The ether intrusive mass is Delaikoulap the brecciated outer margin of which is partially overlain by agglomerate. There is no yuggention that the breocia is intrunive and the fact that the pyroclaetic rocks over]L the breccia may be dee in part to local collapse of the °alder& walls; alternetively it may be due to exploe8eon. The exposures of country rook ere only a few hundred feet wide and the western intruei entereps a little higher up the eection. emeemmeee It has a steep easterly dipping contact and exhibits a vin in litholoey previously described.
3w3 Mbeteeketrusione There are only a few intrueione on Vatia;
they are all aseecinted with the Augite and hornblende andesites and have a general north to north eaeterly strike. Seme dykes and milli may have been destroyed by explosive voleaniem anl Ohms may be buried beneath the pyroolastic rocks; lack of development of dykes hewever ie possibly due to the fact that therl are no imeortent frecture systems exposed at Vatia.
3.4 truclture
The deposition of phase I and II rocks has been discussed in a previous ehapter; at Vella, as at Vatekemla, the eontaet ix unconfOrmable and whilst the tmehybasalte ere usually horizontal, the phase I rooks dip to the/ north at low angles ao a result of port depositional tilting.
The oontot of the phase II and III rocker is in part faulted ::Ind quite steep; it is possible that the form of thin contact has been determined by very young poet volcanic faulting, but nowhere it there any suggestion of movement or rock alterltion such ass could be expiated where the vertical dieplaCement war of the or of 300 feet or more. Alternatively, the movement or thie fracture a have preeseded hornbleMie andesite empties and We is oossideria more likely. The boun&try it 'oculist Is the atearetion of the. Vatukoula bas in 3104 this deMiment lineattee raw hairs formed as a relief of pressure for a nowthoeept/Pouth-eAst tension. Otherwise general subsidence of the area to the north of the
fault 107 114,10 Wm Place; this would indioate volcanic fv.beidenee, as at Vatukoula, pawned by eruption of large quantities of lava. This hypothesis provides the only grounds for postulating a voloano at Vatia whieb erupted tredhromagat* Wmesa the 'base II reeks near Vette are too horisontal and undisturbed to be neAr a vent, it is mere likely that the phase II/III bomdary to oontrolled by faulting 1144.4nociated with some of the latest movements at Vatukoule and prior to phaee LTly depoeition. Therefore, it le apparent that eruption of _hornblende andesitee may quite well vont date all activity at Vatukoulas
The vents through which the anderites were erupted nubseluently formed on anotner north-east/south-west !rapture. The products of the original explosions are not exposed, but
oft
woboequently orOptions were offueive oad viscoum ond not of large velum* sines the rockc of the lower Vatia roue ore not very videepreA. Eruptions, which continued until a croter was built up to at lest 400 feet, were followed, after a period of quiescence, by further explosive ootivity which cowed depoeitioL of much tuff aud agglomerate and growth of the crater to at leoat 1200 feet above sea level, The oostoet between the upper and lower Vette groups is Uncenformable and irregular and, ire 70= pyroclestic selueneas, indistinct.
Xost of the upper Vatia rooks have been erupted from the southern coldera %ad ooatpsrntivsly few from the northern one; certainly during the latest eruptions the southern vent was much more aetive• Ths dote of formation of the northern structure is unknown and thouoh it eloy antedate the eouthern one, it is likely that it formed during the eruption of the upper Vatia reeks, A ridge, 150 •• 200 feet high, and striking eastwards from the wheat separates the two colJeras; on the flanks of the ridge hornblende andemite 4ggiomerl.te outcrops, but along the emmmit, augite andeeite is expoeed.
?her 4 3re no oontaoto to alest that these rooks
ounetilute a dyke and they are cooleidered to be original traohyboealt or lower Vatia ondesitee. This 'wall' of andesitio rooks, surrounded by hornblende andonite, in poreibly a surviving pieoe of original rock, though it ie not *leer whether the remainder of this rook was destroyed by explosions of the litatia lodge or collapse in the djoining oaldoras or both.
There are no dykes swarms o.ssociuted with the volcanoeo and after the original north—east/Southoweet fracture formation, there io no evidence for further disturbance.
The north0000tblouthowast lineation in strongly de loped in the Vatia rooks. As well as the phase II/1/1
boundary and than as ie of the Atkoula bowls huvin thie strike truniou of Vatia foss a son' striking in th diraa tiers wid there is also a stellar oloom*ti ou of the i dykes near Tatie Loilol also ho Ve 000rthoosotisouth—woet strike. There to so todloattos that the tomOton 4irectio*1 (uorth...ww,tisomlbesset) *doh formed those tSacturee had Any corttrol over the roan and wirly struoturee at Vatekkoula• Thi fracturo /rattail% 44,101.101 only after completion of volcanicity at Vatukoula; as faros is known it is only looally deTeloped and in hie omolgoto of odykoot faults, 16441*. *make a lo'Alse at the Illopotor mire, Ilaishford (1953) deal not rsoord any with a fists ►et sto t,th to^tt strike.
The outcrops of hornblende ndeoi iatxmoloa *bow a Toror strong north-Fweet fr it;1Soso oxpooureo this takes the form of s jointinek, but sloaohopot dgkes have e.n intruded Ato l.,!?: the cracks so Aykos are X11 and imporsirtent and vi,lry in thickr.os from a Vaiiihun of two inches to An fvernge 40f * inch. The
on is not eomplotely regular and tho (smoke frequantl dive * a little trim the riv-tonal striko• Sisilnr cracks smartly c the island awm no intrnstion•
The otiooturs of of lit v. is the Delalkoula body eh reprooert,3 A arose soot ion of
uxesr levole Of
an intrusion or conduit. Roomer, i oboes three chrlructeristies Mash distinguish it from the malrdec. of
Intrunione —
SO Wessiats4 min* a battle Santermad
n imnor amid Imo with many xonolithe and negro,!Ations
le intormodiato a fewer in the inn0r son**
The outer breacto to conuidered to be explosion breooiu formed ,),r the lava ergrAing through ti's crust; sohomposatly the lava was intruded into the Woman as *hems
• 90 "
by exposures of the breccia intrusion contact on the eastern side. ;.ome of the breecias my be formed iltr intrusion, but it is thought unlikely thwt a boy of this use could produce at intrusion breocia up to four hundred yards wide. 7/uch of the breccia is oparr. And homogeneous; intrusion
by brecci is usually characterisedo.J. great variation in else of boulders.
The intrusion ilroper is hornblende augite andesite contelning many xenoliths of granodiorite and other rocks. The coning is probably dux to convention plus g,ravity carrying the acid minerals from the bottom of the intrusion up through the middle into the higher levels and oarrying the xenoliths and more bqsic minarnls from the higher levels down the side of the intrusion to the bottom. Seerentions and mono—mineralic clusters form during such lava movements.
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Part 4
4.3. INAMPOir Feldspar le the most abandon* niner441 o'er one
of rooks of this suite occurring both as a phenocryst and and ground es constituent* In all reek* fildsprkr aecounts for at least 5055 of the total miser elegy though the proportion imereoees until Lb 300* young istermedlate intrusive* it eemprisee more than 80%. The plegioolaos has boom determined
sally by extinction angles on twin planers and by reireetive Wiese. A number of eeetions were studied on the umivereal stage though the majority wire examimed without it; in both instonoee extinction an, were measured en trier normal to (010) which showed both earlobe& sod albite twinning and the eomposition of the plagioclase revue thin estimated from earns as 74* molight• Feldspar surstals in ail thin qactiong were investigated in this way bat in only a few saves were they separated out for retractive index determinations; phenooryste ant sometimes the coarsergreendoome crystals were fropated and immersed in oils it hem index* The inlet 06 wee determined on the (010) tee* aged the 080901414e* of the mineral estimated from starves by Umbel* In many rooks however, the greundmase sryetals are too small tee be determined by either of these tee methlis and as will be 'hewn later the type and ammo* of feldspar in these reeks has bees deduced in other esp. Detailed work on the Otos extremely well developed sonlmg bees not been done; this subject wee dealt with tally by Iyengar (1949) whose remelts will be 41e0m000d later.
In come of %no trachytie roam the feldspar is extremely 11.194, and miorolitie but In meet rook;, it (mows in three distinct forms - phenoerysts, mierophenocrysts and groundiaLse oryst;Aln* Throughout the volcanic cycle the
Metals show extreme well Wiped twinning on the law lees frequent eariebad wine and eectasional crucifo interpenetration tying, *ou 1 the latter are usually restricted to large ary Albite twinning is more common in the *alai(' pi-` e and in not se well developed in *rye als of .1bite and • Sem erysle have pelyepothetic periolins twisting Aictbbe Observed in some soot:ions parallel to (01O), Zones is usually mersmi out frequently oscillatory as skean by lyengar (1949) 11140 demonstrates a *alas core and a M0,0 sodic margin. Compoe tional soning le particularly well developed in the Valais. rocker 1.nd zoned cry itals are sometimes surrounded by a is of clear oligocla or, the (meg of the nonsemites, potw5h feldeparm Many "Anemia Oontain zones of included partieles.
Phenocryste in a few lavae show small structures interpreted as exeolutiou phoussomal thol01 tee *termite optically, small elengate& pa toheo of
feldspar, which is probably potassic, halm axsolved from a parent plagioclase. More froluently veins of pooh feldspar form a network through plagioclase cry= tale; sometime* zeolites are also aesociated with feldspar though this association usually results from hydrothermal activity.
The mike of phase 1, which contain only small am unto of potash feldspar, have a plagioclase range from
&NM to An54, byteemite to labrAdorite though determinations on two rooks sumps* that **cantonal grounduriss crystals may 5e as sodic as An The average feldspar, is the me in both the intrusive and extrusive rooks of pz s i obi* have the following r.ngees
Intrusive rocks Anse 0. Am% bstrusive rocks Ass
A Rini Moms). Variation is noted Su this pha
-93-
rooks, a bein An35 bytownite and An55 lab doritet though in Any one thin oeaation the plagioelase ran is often quite nmallyas for example An62 An57 :end, inone came Ansa - An66.. 14xtinetion angles have not indicated the presence of andesne, but refrnetive index determinations, to be oonsidered later, dhow that in some rocks the froundimtno feldspar is in part andeaine or more rarely oligoalase.
The extrYmely well developed zoning exhibited by the plagioclase in the phase MI, rooks indicator that the overall range in An93-An34 (onorthite to ant esine) though the great majority of orystale lie between An78 and An54 (bytownite:And labradorite). It is a feature of these h=mblende and augite andesitee that the groundmass is either oryptocry5talline or hyaline and the feldspar is indeterminate. The most ?odic groundmass crystals are probably andesins as identified in the outer zones of one or tic phenocryste.
The trachyandesitesand traehytes of phases III and Wagiodose
IV show a much more restricted range efAcomposition and the most basic crystals measured are Ands (labradorite), Again, in all but two or three of these rooks, the groundmans is too fine to he determinte; in the exceptions the groundmase material is usually oAlcic andenine (in one instance it is a eodio andesine An3s), though refractive index determinations show that these rooks contain untwinned grOunftass aligoolase and rare albite as well as potash feldspar. The monzonites contain large amounts of potash feldspar but the plagloela is quite oaleio and showo a range from An87 to ins 4. The results of Iyengar'n work on the feldspars of bogie and Intermediate lavae from several parte of Viti Levu are summarised in table 3; in general they are similar to those presented here.
;Icoept in phases III and 1V, by to aita (And An86
Good: An94 - An 60 Good
Good: 76 An52
Faint
Poor
Poor
Good: An - 57 • km 4
none
None
None
TItle 3 1 A1MMrY Of gelljD3r C94114 t investigated 4y ixengar 1949
Rook 1.:'.yDe Plailpz-le range Olivine augite 3ytownita bavio labridorite
basalt Olivia te Dasio byt,ranite 4old labradorite
basalt analeite
A ,-ite basalt lia'arAdorite-andesine: An - 60 "-45 Olivine feldspar Bytowaite iradorite:
its 4nd ,indesites
Alkali feldspar oor
None
Good
acmetimes rims Ame.y An57 plagioclase
Faint Faint None
basalt
Feldspar basalt
Biotite augite andesite
Hornblende augite andesite
Hornblende andesite
An7o - !1250 bytownite and
labradorite to ;,ndesine LabradoriteiVtownits -
oliE;oclse/andesine (spilitie) Labradorito/bytownite -
acid labrAdorite
Labradorite (Ann) - oligoclase/andetine green hornblende - labradorite
An ,65 - An. 58 brown hornblende - labradorite oligoclase
average composition Ano
Augite undesite cm Asia andesite
is the earliest plagioclase in all rock groups. 3arth (1931b) 10 considering that the plagioclase ranges in the Pacific area were very great, stated that the most oaloic plagioclase he liadi observed was bytownite An., but Lacroix (1925a) has recorded manxIelavas with An80 AnA5. The writer has not seen, in any,prt of Fiji, olaglocllee ae cabbie a that at Vatukoula, but Crook (1962) hap described anorthlte from othe;r parts of the Ba fitrier.
Table 4 s owiu4; au0144:41us bv,Sco1or777,t)-
Plagioolase Fro. I determined by twinninp, Range of rOx
.....manioNar•Ona.•••••
rtax. M:1X. %An
An
506 73 An 508 74 546 75 579 82 503 71 504 505 78
71% An 1.530 71 68 1.544 22 64 1.546 26 55 1.57'2 74 1.548 31 63 1.562 56 1.538 13 67 1.572 74 1.544 22 68 1.566 64 1.544 22
elorepaney has been found in the rger of plagioclase compositien diotined from the twinning and that determined fro % the refinisawil index. Thus the variation ifl pilae,I meted by twinning im Anu An56 and by refractive index methods An8G Similarly, there is a discrepancy, which is even more pronounced, in the phase II rocks, the twinning of which indictuter a range of
94 ,. - Any) whilst the refractive indioits suggest An 6 An13.
Jetereinntions illustrating than* aalonalios are given in table 4. The differenoes ean is part 5e aludlained 5y the friollt that aecurato datermination of *mall groundmaes cryrIs by twirninc methods is in all sass. difficult and in soma cuss imrossible particularly as the twinning on the 4151.14 law icy not wall developed in crystals of albito and alimonies,. Henna it in probabis thNt andsnine and ellgeelese are often graundmlne components or phase i and II Veoka, thougA4 40 detailed below the properties of the** mineral, are soma 4 % variable.
In riny rooks rofrastive indiess indicate the prose nee of a plAcioolace feldegmr with nolf between 1.V, and 1.55 And raraly as low as 1.53. 'these proportion suggest
Lhoz fzl!vtplx may be mlbite ^r oligoelas, but Piwilar orystale i a the coarser grained red kekare Uemial peettivo Orton with a low 2V. nagiaotava 01701910 aggli sinitur proportion. have 41011m• romerdod free am of the 't ,.f10 Inlvxds an well, no frns part* of the semWeetal margin.
Werth (1931b) coneidired that anemousita in an im?ort%nt constituent of Pacific) lava, He had deeori5ed soma of Ova proportion of this mAteril earlier. (1930) and stnite'd thlt it war biaxial positive with the axial is varying from 10 u, 85° and the refractive index from 1.53
te to 1.55. Anagramming, as first described by Washington and :right (1910) to a normal plagioclasa containing, in solid solution, varying amounts of the nophelite molecule in will& sone of the soda may be replaeed by potash.
Ltaaroix (1927) in his study of Polynesian lavas also found material which, though optically positivir, ho oonsiderod to 5e potash feldspar. Of Lacroix's work, Barth lwrotet
_97 ..
roix nparmt1..y regards the isorthoclase as ily identical with an ordinary potash feldspar. ca tho isorthoolaso occurs in a - highly sairovAted
rock directly in contact with feldspathoide and in one optical property corresponds to the Linens l found by mo, it is reasonable to think that this itorthoolase Is of the very same alkali rich variety that, occurs interstiSially in many paste of the Pacific rocks.*
The preoenoe of potash plagioolaso in Pacific lofts was discussed by MAcaonald (1942) when he found in Hamitan lavas feldspar with similar properties to those attribtt104 by 1arth to anemousite. He found that the material ,xas largely
confined to interstices of the groundmass, 'rya modal amounts of the mineral varylms from 2 or 3 to 20% but being greater in basaltic andesitaa than in -b...salts. 'MacDonald also found that the percentage of modal orthoclase was proportional to the amount of abnormal feldspar. He concludes:
"....the apparent reciprocal re1W41(niohl of anti.. perthite in the gabbrov and the abnormal foldepar in the extrusive rocks supports the euvo)sitiz;n that- th0 latter may be a solid solution of potash feldspar in soda lime feldspar, stable at high tomperaturas, but metastable at low temperatureo."
Benson and Turner (1940), leeored rimil featurao from the mugoarites of New Zealand.
:Awards (1938b) disousaing rocks from Victoria
aA.d of the groundmass feld,loars of basalts, andeeltes and
try chyaxsdesitess ...the potash content of many of the rooks is
proeably in voila solutlo71 in tie pLAzio11,4set, increasingly As the plc, ioclano becomes more goodie.
He. demoribed potash oligoclas phosocrysts from the name rocks: "The feldspar phonocryste of the basic trachytee
and of some of the anorthoclase—traohytas and anorthooll%se—bas,%lts have a co: :position about Or5. Ab51.An25 on the borderline between potash
andesine and potash oligo lases The high percentage of potash
-98-
bringe them vtlthit the anorthoclaso group, and under the microscope they show little difference from the Jotter*
remits, quoted by MT4tDonald (1942) alro found evidence of potash rich plagioolc;zor in Japan and the Asiatic rainland. Both enorthsclaes and pctarh oligovlse are reported in the. .rounemaze of andeeites cf ffakone volearo of Japan (Kuno 1950. Kuno believes that the andezine, potash oligoelase and anorthoolase in the rockr of Ha` one constitute a simple solid solution series_ which forms either a continItout zoning with calcio plagioelase or a discontinTiour zoning without the formation of potash olisoclase.
lyengar (1941) diemseed the results of eon e Chemical analyses and stated:
".".both the anm4see show a of normative orthoclase which is mode. On aocaunt of the highly rocs, it :nay not en apparent or ctitially - but not visibly - in ol Anorth oeito".
fair4 high pro ortion not apparent in the 1k4sey nature of the t ms1 oecur inter- tle form of anemeusite
More reeently (1961), Muir and Tilley have axamined the feldspars of the mugearites of Hawaii; they coneidered that the apparently anomalous properties desorib4d ty ;acDonald were due to ohs presence of gloom potash inin the plaqicclaeo and they considered thot t4ore wie a range in feldspar compositiou from potash oligoelAms to lime-al:orthoclase. There is no evidence for the existence of anemouniti as nrunretood b,1 :;ashiogton Ind *right and BArth.
The proportial of sow. plasioolase (Irils of te Vatukouli. rooks are similar to those deocrtaed 44; tAosio other workers. Also, tree rociv5 contain rubstantial amounts of normative orthoclase which Is not apparont in the rode. It is coneiderod that potaoh is precool; in the plagioolaei molecule, though as demonstrated by the Bowen and Valle
2Si308
Figure 13s Normative feldspars of the
Vatukoula and Vatia rocks.
0 Vatukoula rook
• Vatia rock
59872 5
///604 0 280 0267
0 456 0574
259 0 0260
5040 0 532
0 508
0 505 0515
0 185
492 0
NaAlSi308 KAlSi 0 3 8
— 99
line in fl 13 the amount of potash feldspar which can be locked is the placioclase in, exelept in tha albite and oligoolase field, severely limited.
Poto,sh fellspar is prAsent 4o IMMO extent in most ro - p, of the 41AVAI. It is, with few exceptions, a troundmass constituent though only in tie coarser rocks are indiVidual feldspar granular; daterminatel phenoorystic poteh feldspar occurs in some andesites and trachyten. It is not usually possible to determine by micrometric analysis the modal amounts of potash feldspar and in prqiy canes it im difficult to demonstrate the presence of the mineral in the fine—gr,Ained basalts at all. However, the normative feldspars of 1, analysed roc (ose pert 5.8) were plotted on the anorthite—satite—orthoclae diagr4m (fig. 11); all b,t one of the points lie within the two feldspar field. The normative feldspar compozitions of the analysed nderites send basalts ose. 1 significantly Pittner in potaW3, than those wr the average hlts and andef7ites of the wer14. The normative feldspars of the volcanic rooks of the Vatta And Vatukoula esatresthw form a pot ricl eerier tb.At le oorrelated with the eon—pnratively it oortent of potash in. the bulk oo.position of there rocks.
Some rocs. were subsequently treated with sodium Oobltinitrite to stain the groundmass potash feldspar; the staining WAD suz,...ow;sful in the intermediate rooks, but in the fine basalts it w hot completely satisfactory*
3anidine is preeent in some of the green andesites or phase V and the aueite and biotite tv;Aphyandesites of phr4;es III and IV; lkixas never cont4in. more than 5% pheno—crystic sanidine. Orttv,olase is present in rocks of the monzonita group asd frequfeliily grows as a rim around a core of plagioclase. The coarser rocks of pha:es III, IV and V
*contain grentimsse later of untwinned Or twinned orthoolame sed the mr1 often quite atemndant. Potaeh feldspar is alas abundant in Oortain eonea of the Weinivooe iLtrueive shootin. Individual drnina and !rrogular infilIinas of 1kAll fildsysr home L1:4'z 1.54 ;IT 14 low birofringenee. Beither *operation of alkali feldspar car oalculation of modal amounts wus peeeibloi However, the low rerustise index and birefringence and *lees asseciation with plogioolase mugger:Am that it is anorthortlass or perhaps potash rich plagloolame• nom mho oontaLe small amemmto of interntitial eeolites though with the exeeption et seecedary amaleites foldspathoida are very rare oonetitmente of these rooks*
170nAnr (1949) found potash feldspar to be unfit in the Fiji lavas; in some baealtio rooks he noted that a foe plata plaglooluse crystals b4 a rim of petuth feldspars he* in *thin, owes the potassie mmteriul was confined to vitas setting the plagioclase. Skits (1954) found intorstitial alkali foldepar in come basalt. from Tatekeula though the wuaunt* were apparently small,
4.2 :lEamlaga Lyroxene is the meet abundant of the inatierminerale
;11c1 with few sloe :bone is found in rocks tram phase I to lamas V; in all, mexene La setteated to ho an i*psitsat nonotitwent in or than 95% of the rooltr.1 under dissuasion It ie moat prolific in the slier rooks And he amount of modal pyroxeno diminishes through suomoding ph,l,ses of volounioity from )01) $ meet is some of the younsoPt reeks, where biotite i the essential forromagnesian Mnsral it is oompletely aboent,
kuorosoopially. pyroxene forms black stubby priamati or 't sometIme'; rather dull fn appourAnee;
-101-
it is quite resietInt to t,abaeri'll ?rscas of aroni= and weatheri freolently concentrates on the surface as
coarse pyroxine stand. In thin saotion it is variable in appearanos; 4ew11y it is vale dresa and soxatihoo *ow elidhtly pleothroic, but less commonly it in colourless or vary pale pinkish briormn a%d non—pleoohroic. There ia apparently no jraat compositi4na1 lv,riation between the so two types, though the browner varieties may 3 richar in titanic and the green varieties enriched in iron. Thew show little variation in form or habit; ootagonal in bastat neotion and prizmatio in sections ohrllel to the o— cis they form phenocrysts up to 4 or u Toni, thouth with dscrsing size thoy become loess euhedral. Pyroxeneo are very resistant to altora.tion and only in caa o-Z intenao hydrotherml activity has the mineral been peoudomorphod by Oslate, zeolit:e and come chlorite. A few rocks !which ocintain fresh phenooryets of pyroxcno havo groundmas chlorite insteg4 of. Pucite; this ie probably an original crystallisation feature s.nd the ehlorite is primary not secondary.
?yroxene crystals are frequently twin2ed and the extinotion angle s'‘c her been measured on romo. of the (100) twin planes; the reeults are given in fic. 14. Zoning however ie more comcon than tAnning; many phenocrysts show a coepositiol zoning with very narrow toner: surrounding a large core of uniform compouition. Though throughout the volcanic cycle their is a trend from more to leer oalcie pyroxeneet the zonilg within individual crystat compositional oscillatiene within narrow limits; single minerals do not how the trend of diminishing calcium content. A study of a few basalts show that there were not lens than three periods or iron impoverishment during crystallisation of some pyroxtne phsneorysts. Iyengur (1949) found that
Figure 14 : Histograms of the properties of pyroxene
a. optic axial angle
b. refractive index (B)
c. extinction angle z"c
crystals:
• • ■ • ■
me ms ms mm mom Eon miss sums mom Ems Ems mom mom Ems' moim Ems Eon simm Ems moms um' =Ems
sous emus moms mum mom mos mum msomms Emmons mumoss
ommoo• moms Earn■ mmmemm isoommo mourn moms momossio
mosmEms Emmen mom moms mmusmom Esommoss Eommosom
■ mums.= m omoommossams • momommosi • wommommasmos ■ msomemisommommummo msmommemommommemm
47 51 55 59 63
2V
r 45 49 1.716 1,708 1.700
1.712 1.704 1.696
R. I. (B)
A Z 0
Hedenbergite Pyroxenes of
• Phase I + Phase II A Phase lily Phases III and IV
°Phase V
Hypersthene
Figure 15 : Composition of pyroxene crystals.
A I i .1.. PeGel'Ll‘ 1 r , 60 .". • s.+ I **1
A • •• ++• -..,
^—t-r
-1-4•-- -.......
1 ---1f--,1:- 5_ 5 2V
1.690 1 I I 1---h_i_:: -)0
1.700 1.710 45---- (B)
1.720
Diopside
Enstatite
58
102
a zoned crystal of pyroxene fr a g1,1zy olivine bs.alt showed a variation in 2V from 620 in the gore to se at the margin - thougn most of his pyroxenes, which were taken from all tht major rock groups of Piji, lay in the range 52 56°.
ltour 6tass zonimi has aeon seen in only a few rooks* The nature of the pyroxehe ham 'y)een determined
optically by study of the optic xi2. angle tl.nd the refractive index. The results of the doterminationu aro plotted on fig. 15. The plot shoos a tight buoh of pointv ;Al of which, according to Hess (1941) corrospond to the composition of ealite/ferroeallte and augite/ferroaugite, thou6h other workers would probably refer to them as diopeidic augitee. The graph also reveels that the pyroxenes of the various phases have 4 constant composition and that pyroxene oryntallisetion does not exhibit a very marked evolution 17 trend; howew!r the pyroxeneT of be ire are slightly more magnesian rich and calcium deficient than these of the other phacee. The o,lspositional varia%dah within shah phone is equivalent to the total range In Vie volcanic vole; the variation withIn etch rook however is small when compared with the pyroxenes in other basaltic areas.
Pigeonite it completely •been from the sequen Of the lamest ?V recorded (490) canl acoordinG to He (1g41), to indicative of pigeonitic augites but ih general the rocks contain only pyroxe of diopsidio*Gugite composition. Hence ths mmpositional range of the pyroxone zoning is only very
In addition to the angites there are two other pyrox ne. which occur only to e very limited extent in phase II/v rocks. Aecirine-eugiLe is confined to a segrega. tiou collected from the outer toss of the Dalaikoula plug; the pyroxene is granular in form, deep ap?le green and quite
stronvly pleochroio fros pale to deep men» ?he granules are too sucal for optioa *ruination but the oolour and strengh of pleoohroism indicative of moirine-cmgite rather th,u1 ferrotaugite. The minimal is not known from rocks of other phases.
Pleechroio pyrovene iA nY od from several rooks of the upper Vati pup but it is r3f4.t developod in the aniositl of the Vatubuli intrusion were it occurs with norwa diopsidlo*augite. In other Static rooks it foam mmall prisms, usually associated with magnetite :ranulec and is cometim*s mant13d by resorbing hornblende. The pyrex*** varies from colourless and non-pleoohroic to very pals groom and ploochrolo to a pal* pink brown. (Apatite sometime* shows similar char,cterioticE though its pl000hroism is from colourless to quite don brown), The minstrel has Tory low eztinetion gikaes itld quite often *home straight extinction; It him an oi)tio axictl angle of about 68.. 'fhs abssorvtion nohemes of Vao two vnxisties are indicate slow:
pink very pale brown very pale en very pals pink-green very pale groan pale €rsen
The!), properties are su,;sestivie of orthorhombic pyroxen*, and it is likely th3t this is the minor al *hick ireMgar identified a hypersthene In hornblende andesitos and daoitsed, thou g4 rarely present in excess of 10%, hyperetheme mousliy forms 2 - 4% of the rooks in whioh it feeetare,
the importanos of tho relationship of th* pyroxone to tne limeoltis magma from *blot it orjot1,11isild hAs lon; eerc recognised and studied; Kennody (1933) oorrelatod the tyi,e of smiona with p;$rtioular pyroxenes and postulated th-t olivine
- 104 -
basellt. n1°1E:ma is aenociated with diopeidiceaueite or titanqugite whilst the tholeiite -rhyolii,e differentiation earl's ie related to a
lie poor awn:AU.) auglte. Thle ham not 411804
completely acoepted and ninny workers have 4* id the association of diopsidic augite with lime poor anstatite eligitm for pieonite) in olivine baseate. Barth (1931a) after a stuly of basal'4s frees MAM parts of the world cJncladed that pigoonite ie the moot abundlnt pyrexene in volcanic roor and poetulated a trend of cryetallieation in basAltic roof frox diopsidio augitL. to pigeonite or hyperethenic pyroxene "during which the content o gSt03 LL y remain constant or increase elightiy". After conailaring tAs limitations of defilileg eyroxenes on optical proparties, 3arth (1931b) !cwt in Pacific lavas that, wish advancing orjatallioatian, the pyre-Lanes baease richer in lime and poorer in iron; varial from 1.675 to 1.730 and 2V from 0 to +60G. Prom his studies Barth ooacluded that pigeonite 4t ordinary presoure is unstable or will become unstable et a temperature higher than the cryctallisation temperature of gaobron, "where it wilt kes corverted to erthorbmabic'eMmtatite if equilibrium be obtained,"
gaeDeneld (1949a and b), working on liawelian mart teund phenooryste of augite with some aiox'uph.noeryttic rd grouLdmaes pigeonite and a little microphenocrystio
hyperntheneg this association ho correlated with that described by earth. 'Iarlier Slowever, (1944),he found blocke of ma-Arial ejected fr Kilauea in waitth the groundmans pyroxone ia hyperethens and augite, tde latter having 2V me 600 in discukelhg tho absence of pigeonite in the rook he olaimeds
....it appears very proheble teat pigeonite in the ,rotusilmnee of tinny bameltic rooks is metastable at low temperataree and exists only because of the
-105- rapid cooling of the ,avas contninin4; it. Although in some instances pimoonite may form under plutonic crnditions on slow cooling' such conditions generAlly renul in formation of two distinct phacese hyperethen and diopsidic-augite".
In Hawaii MacDonald found diopeide-pigeonite to be the usual association and diopside-hypersthene to be quite rare.
'wards (1938) dercried an :akali basalt suite from the Kerguelam Archipelago ia Which the pyroxenes were "basaltio augite", diopride, titanaugite, aegirine-augite and aegirine; he did not deter stria any pigeonite. The aegirine is dencrioad ae being a late orystallisation phana except in ilmburgite and soda rich rocks where the pyroxene has returnad from low temperature ge.mditions to high temperature condition throwh the aotion of orystal sinking or convection. 7dwardo found calcium poor enstatites and pigeonitee in tholeiitie magma. Tilley and Yoder (1962) have recorded 4 almilnr conclusion in a study of rook from hawail, the tholelitic group giving rise to sub-calcio aucito, hyperethene or pigeomiti or both and the alkali basalt to a high salaam at rite or titanaugite.
Hess (1941) after diecussing nomenclature suggested that the Eroundmase pyroxene of mafie magmas war either augite or pigeonite, or ferry-augite or augite with very rare hyperethene in place of pigeonite. Later with Voldervaart (Poldervaart and Hess 1951) he stated that "in most basalts of oceanic isilnds and in mg4lny other alkalis banalts" Orthopyroxene or pigeonite is generally absent"; a lengthy period of olivi!ie crystallisation is succeeded by. cryritallisatie of pyroxene initially rich in calcium. With succeeding crystallisation of pl4i.loolase and pyroxene, the tie ,Jecomes increasing17 'Talc impoverished' and hence in sat up a crystallieation tread w3 dascrlOad by .̀earth (1931b).
.'Iyengar (1949) discussing pyroxonoe frwl Vi ti Levu, found dionsidic and augitic variation present ever:whore ae phenocryets, though due to their fine-gran else identifica-tion of the groundmos crystAls -.)roved difficult. He recorded one instance of 2V = 39.50 and two instances of 7,V us 48°, which he stated to ae pigeonitic augites he found a trend to less °alai* pyroxenes, tuAnly on the evidence of the honing.
A similar trend in orystallistion corresponding to a decrease in calcium oontent is indioutcd by the figures in table 5. The average 2V of phenocrynts4 microphotooryste and, where measurahle, groundmass cryntals indicates a progressive decrees of 2V through crystallisation, though taken P4P a whole the determinations show that there wl,ls no acute defioiency of calcium up to the completion of pyrexene crystallisation.
*IMO. 1••••••••
7abie 5 s Averaq.e valuorof the optic 4;A:a a ti Dyrox.ense of the ).)hage I rOCkei e,
2)1,029arrEIVAI AkeroTAIonoorr4e firowidause
207 610 58,3 57e 210 53 49 49 261 57 54 453 58 53 56 456 58 58 56 462 56 56 >5 475 61 49 481 58 56 54 484 )7 57 56 508 57 55 52
— )0
As will be ehowE atter tho t llat1o: of a einglo ealc rich 'pyre:Keno i ttr Vatukorlz rockt% i probably controlled by processes of sesimilatlor and mixing. The Vette rooky +pertain both augite and hypernthwoe; sitilar lavas were described by Iyengar Who found aucite with hyperstbene which had a 2V of 55 — 61° olld a consequent Not03 variation from 49 — 57%. Iyeng&r considerA them to be normal two pvroxene rooks; "the crystallisation in these eznes has reaohod the bound&ry of the two pyroxent flolds of Tunboi or has started i the lower field".
There can b€ little doubt that the two pyr ne of the Wtio rocke result from straight orYst,:alistior; differentiation ar dercribed by Tyangur; unlike the 1713.tukoula rocks, differentition in the majority of th2onv evites was not controlled by any factor other than normal fractional cryetAlisation.
4.3 9Uvii, Qiivtne forms phenocrysts and groundrnss grgmulw:
in the rocks of phase° I and It as well as in the menzonites; hence it le present in mere than 50% of the rooks under discuseion, occasionally it forms lees tham 1% of the rook an4 in a few more east.) membery of phar.:e I it exceeds W. Bowever, in spit of the large mount of olivine implie by these state ante, the mineral has be pesudomorphed in a rocks.
The inera1 is =sally pale yellow greet in hind specimen and colo.oriern in thin section; it ie generally cracked but one or two crystals hate a pronounced cleamage parallel to (MO). Euhedral phenocrystio oliviner are rather common but groundmaes olivines are often Fi,:r-Inular. they ara sometimes olustsred with magnetite grains which are also
1.70 - 11Range of olivine
determined. by B
80
+ve
...
1.86 -
1082 -
0
2V
80
70
• Range of olivine letermined by 2V
1.66 ,
1.62
1.78 -
refractive index (B)
1.74-
Figure Io s Composition of the olivine crystals of rocks if
phases I and IT
( After Kennedy 1947 )
Mot. % 20
40 60 80 100 Forsterite Fayalite
frequently included in the mineral, but there is no aesooiation between olivine aad pyroxene. cryoV:as mainly from the phase I rock were doteriainee optically by optic axial angle and refraotivrh index deLrmlnativns. The renulto ,lame tabulated in table 6 and plotted in fig. 16.
Fro the results of thie work, two main oonolusienc can drawn. Firstly the composition-la raw,e of olivines as indicated by the optic angle is 10 390 feyalite; the minerals are nagnesian rich and are fairly constant in com-position as indicated ey their *eft ratios in table 6. owever, refractive index determinations show a nueh sore
restricted range from 9 - 20* fayalite. The disorepanoy revealed by the two sets of determinatione iv due to the fact terit the smaller olivinos on which 2V determinations were made, were not separated oat for refrtotive index work. hence the ranee indicated by 2V is the mom correct one though the nor, fayalitic crystals are uncommon. The later groundmIse crystals are more ferruginous than the earlier phenocrysts.
The deeerminatione also show that there is often a wide variation in the composition of the olivines lc,. a single rook - a variation of 15 - 25 fuyalits being quite common; zoning, however* is peony developed* Barth (1931b) came to similar conolusions after a mineralogicaL study of Pacific lavas and he quotes 11 compositional rang* of 10 - 35% fayalite as being typical.
• ....the any formed orystals are rich in the forsterite molecule, the later crystallisation being ser fayalitic; but so7ling seem not to be very oommonw.
Wither more detailed and localised work wan done by Aurouseeau and 3,ierwin (1928) on recent Hawaiian lavas; from chemical analyses they quoted contents from 12.4 to 16.20 fayalite after making allowances for the presence of other oxides
109'
Tbi,e. 6
172
' a 1 7roertiee O Q]j,tne Cr.7"q
'Ora
e5
2' RANG::: alkeakar=
to
VIASURZOBRANV *MgAo
3.2 from
250 1.690 4.5 280 1.683 1.694 5.7
284 81 85 1.693 3.0
433 79 1.671 1.636 3.0
454 85 89 1.680 4.5 456 83 89(+) 1.674 1.676 4.0
457 87 89(0 1.680 1.681 9.0
458 85 89(0 1.673 1.686 4.9
459 85 89(0 1.679 1.692 4.9 460 87 89 1.688 1.694 4.5 462 87 39(0 9.0
463 83 89 1.671 4.0
464 83 89(+ ) 4.0 473 81 85 1.681 3.3
474 89 89(0 1.674 1.683 11.5
475 89 85 4.0 506 85 89(4) ot- so 1.662) 5.?
508 79 85 2.3
510 85 89(4) 1.672 1.674 4.9 511 85 89(4) 4.9 512 84 1.695 3.2
Daio column fu
ro rrez o ni orstrit to
tmvigiatntg tblee proporad by Kenoody (1947).
g 3.
-110
Waldo' E' kg0 FeO. erevieusly !t4e4 jt.larbge proportion of
the 01ivinee have beeh altered aad pseudomorphedi In the phase I rocks, iddingeite is mpeemodilly develoeed and rarely all. the olivine Le completely pe udowerphed. The iddingeite is usually red—brown in oolour, but a pale yellow variety is noted in eome rocks. This war deporieed by Ross and snannon (1925) who considered it to be errrto*rYptallias and moot probaely a variety o1 chloropal. This alteration is noted in rocks of phase I And II at Ve,tekoula mnd Vatic end though the deeeree of alteration varier), iddinemile has most oowleonly developed on the rims of the olivine crystals. Another *omen altereion produet of olivine ie fibroue earpentine varying in colour fro u a Very pale green to a dark blackish brown; eome magnetite le developed in these instanoes. :earth (1931b) who doeoribed extenuive peeetdoe morphing of olivine by iddingsite from several Pacific areas, Liao pointed out that though olivine le freleently converted to serpentine and calcite, Ayroxene is almost alweye completely fresh; the praeant writer would agree with this with the reservation that calcite is. developed only ae a result of iuteese hydrothermal activity; pyroxenee are also then paeudemorphed by chlorite, calcite and zeolite. Caleite ic primary In sone more acid rocke lett has never bowl rimognif7 d after olivine.
Barth also noted *hat in the Pacific area levee bearing iddineeite were aveocinted with rocks containire biotite and hornhlonde, minerle which are known to be unstable at normal temperatures and pressures.
Olivine and its pseudomorphs are not uniformly distributed throaeh the rocks of phasea I And II; modal
analyses show a variotion from 0 - 23; width may suggest settling of the heqvier minerals it some lavan, though exoosures are intuffielettly good to verify thiv in the field.
4.4 4mehibole Hornblende is the only primary amphibole present
in the rocks of the two volcanoes, though r,ctinolite and uralite hwe been described by Marie Cohen (1962) in her desorlptioa of wall rock alteration at Fmperor. Generally, hornblende is confined to tIle phase Inv rooks tt Vatia though tt;ere are three recorded lnetence” of it f,, t Vatukoula. Fikiba (1953a) deseribed a traenyandesita dyke from the Monlroh Lade OA Loioa 5 levels
"A green rook in 'Attica laro altered, greenish felde2ars fern 4ietinot obetneoryet9. The roOk........ contains lose alkali fel4spar but MQVS laarte and may therefore grade in some parte ilto dacits. A. few peeudomorphs after hornblende with reeoraed edges are present. C1cite an4 L;orid:cy quar416 axe eonfined to vesicles and poscibi to sons peeildomorphe after
augite. A 1.0E48 definite iretance of its ocourrenoe is deeoribed by
site of (in Healy 1960) in a reek collected by Healy at the the Waikatakata sprInget
"Mo olivine tae= been noted,......,The sap10 ill holoorystalline Qorphyrilo rock oontaiaine largo phenooryste of ida6.,00ltime and greenish tlugitts Its coarsely grained groundmaes emeiete essentially of plegioolaeo, but a small amount of chlori-4e and carbonate oocurs in the InterstAcee. Moreover ne6lialila amount of a pleochroi0 green-brown
ferromagneeinn minerl is ttivo pressed. This mineral, probably hornbleuds, ououre iu very minute crystals. lortever, V(7,r rook ronembles wore a lospait than an andePite".
112 -
3m4t1 amount o: zoaondary azaphibels mftlr origia pyrorArla aro niPo known from the mon:mates but nurmally hirlerfis 1* not n constituent of tha rook3 o tiVatUkau.th volcano. In the phao* my rocAs am?hioole ir raetrieted 'to ti t upper Vt,iert12 %nd tba intrncions. tioily it varies from zero to about 30 but tha mzjority of 'oz r lontaim 7 - It somstimse ocours rith olinopyrotene though there it no auPoolatIon botwesr the tr.ro ninorzar; it often resorbr to 2roftce mnenetito std monoclinic pyronone. I' coma rooks ttrovn reeorbi.113! hornblando formt mants to a core of orthltyroxne.
HOrntaendo oryrtals 1„rcl blar with e hrillint lutftre rcri prinmatio in form monctimse booming aolonlar. I, thir eenti.77m they have 1. Tor: voxiable !Irv/armee; torvlAimen the oriAil cryotal ir corpletoly deotroyed and in Only e7,o zectioyI horntlend, oonyloter Usual Inihedrmls i vrrisfl in colour and WYcortion wulording to the follo'llm*: enhomass
4 Zo.J1,) io/44i driaaa Nis yellow brown Deal) br:oun Li 4ralin Yellov brown Pale ',Jrown 'lord Ala lreWnish 6rszn Deep gr3sn brown
Refractive indloaa *era dstlz:Aiaed 04 A W roo1c, sera thr mineral is still iairo Sruins
AL 4 1.06 3.701
1.610 * 8 1.703
1.69 1.C92 lyengar (1949) tonna %but ftnascitoe frou rows
rifts of Fiji contained br.$1,0 trealtis hornblende and in
..)ez
513 'Z.12
supper, of thir ViOW he lists properties ve similar to those giver ;12ove, though he did not measure refraetive ladies,. It i now generally accepted that breve hornblende is the oxidised product of gre,An aaphilmles though the former aas considerably higher refractive indictee that the latter* The amphiboles of Vatia have proportion similar to those Of green hornblende except for the colour. In appearance the u1naral is barkevakitic thouz_.* other eoda minerals are abased/ i,uoroix (19260 considers thit valphiboles of Polynesia are ,laost all barkevekitio and etatea that they are smually resorbed.
Radon aftestta green alfi brown hornblondes talks; eriginall4rIpeetims the rims of the primary amphibole it has produced mmeghtit* and pyrosene. The magnetite °mire as tiny granules which are elumtered in and around the original amphibole, but never astually peeudomorph it. The amount of ore dieeharged boars a relationship to the colour of the original amphibele,brown variotiev produoing such ore and green varieties oomparatively little.
The type of pyroams produoed by resorption bac already been deseribed, amphibole reeorption to pyrosene and magnetite is well documented and was described by ',0,8hingten (1896), Kennedy (1935.4) .0161010,414 that honalles40 eryetellises in deep seated bodies under eemditions of high pressure and is hence Unstable at the treperatures emeountered in normsl lava extrusion; oonsequently the metastable amphibole is converted to stable pyroxene with diedharge of ferruginous oxide**
Awards (1938) deseribed similar phenomena from the make of the Xerguelen Arehipelagol a tug piroxone stage gave place within an alkaline Sepola where there lulp 'L con-► centratin of Volatiles to a hornblende stage, 'Ad with
- 114 s,
subsequent OnC3410 of Alatile compememts, Weeks with a single pyroxeme were formed.
It is notable that resorption of the hornblende h.,s been oarried to a further degree in the slowly cooled Vatubuli Aug rather than in the pyroclastion and lav:Is where there is frequently only little indication of the esconth,ry pyroxene.
4.5 Biotite alotite is the essential ferromagnesian min r.L1
of phase IV rocks And some of the .4raen •ndeoites; it is Very resistant to alteration and small corroded and bleached plates are sometimes found in what are otherwise completely silioified dykes. loulders in the Delaikoul. plug at Vltia beer OGNIO biotite out the amount is very small.
'1-1.0 mica occurs as Well-cleaved prismatic crystal, with _alhedral terminations and only sliehtly ploochreic equidimenmional basal sections which show no cleavage; the abeorptiJn schemes are:
X
light brown yellow
golden yellow colourless to neutral
light straw yellow pals greenish yellow
deep brown
dark brown - almost opaque
very dark brown very dark green brown pale brown sodium brown
red brown drtrk red brown dark red brown very dark green
almost opaque Pleoohroio haloes are not present !vid the crystals
do not contain inoluptIns or form any association with other ferromagnesian miner:10 like hornblende it is most stable under condition of high pressure, but it shows no resorption.
- 115 -
Jarth convidern both biotite and hornblende to be mre minerals 1elani;st the basaltic rook Associltions of the Pacific.
In a trtlelvtio rook from the VatUkoule volcano, a .t,in section shown regrowth of a sing le crystal of biotite, thowh this regrowth is only detected when is paralla to the vibration direction of the lower nicol. In X4 both the crystal and the regrowth are very light brown in colour! in the mess of the crystal is deep red-brown whilst the regrowth is olive green. Zoning in any other form that this hes not boon noted' twin planes have been noted on only few ocomions and these lie at about 550 to the (0001) Cleavages
4.6 I Oxidc* The essential ore mineral of this suite of rocks
is megMettite, the ;h rarely it inv is acoompanied by mall amounts of ilnenite. :10M4 of the rockc were Wished for ore examinatIL,n but they were found to contain only magnititas Magnetite occurs is rounded gr=ules 1,,nd cubes, though the subes tend to be groundnaes constituents whilst the larger crystals are anhedral and granular. It is a primary constituent of all the rooks and v!tries in content from 2 . 10%. Besides Clark; a primary mineral, it is also formed by the alteration of olivine and hornblende crystal*.
Chemical ',4ialysos stow the presenoe of small amounts of titania in the rooks; since ilmenite in very rare, the titania is probably looked in solid solution in the magnetite. In highly zUterid areas the magnetite Itts been replamed by pyrite; other workers hays eonsidered tht in Fiji some migmatite, has developed from pyrite, hod there cin be little doubt that in the present instanoe primary magnetite
. and magnetite liberated by altering olivine - was attacked by sulphurous solutions to form pyrite at the same time that
pyrit muoh smaller quantities of other orseerinerale were depoeited by the minerelining fluids,
in her work on wall rook alteration at the nine, Mari* Cohan descried limonite and leuooxene from 0.-1.tered lodes and country rooks; email amounte of ophene ara aloe recorded from a fee localitiee but the mineral Le not QOMMOU. Since the rocks are eulte deficient in titania (the maximum oontent being 0.9), the preconoe of many titanium bearing ores uay be attributaa to the effecte of mineralieation rather than any prtmary cryellisetion.
Haematite and native, copper are aloes Unewn from mete; toot ere formed by alteration processes %hough acme hematite, notably in the phase IV rooks, is priaary.
4.? Apatite eeetIte is present ae an eecermory mA.erel in All
rockr of the severe,* thoaeh it never for; mere than 2 or 3$ of toe whole. In the b,eio root- It in ecictelar in form end usually vary tiny; in this state :he mineral it colourless or very pals, non—pleoobroi reen. In vote traohyandeniteos of phases ill nd IV llrger prismatic ory44.As up to 1 or 2 met long are alto aolourless; these ltrger cryetele Are Wm.. parativoly rere.
In die Vatia tedeeltte npetite oryntals frequently show a oolouveeee to very dark brown pleochroinn on prismatic sections. Tne ory8tals which exhibit this pleochroic re ueually Nit° with 4ut liountedlo.rd tnoueh aaiouilr and flbrolls Corwi --417e known i% the maunder-Ise, they are not pl000hroio. lyenear described similar apatitd erystele froii otter endesitee aud ashen and find,ie many cuLeol a lineation perellei to the c—axie. This ohereoteristic hAs been noted in only one or teo cryetals trete Yatia.
4.8 1-.14.2921d4.1- ;J-nalir4lt The meolitso are abuzdax2t at lk,tukoulu and rare
at Vt thr ignificance h nlready best diecossed their optical proertioe ure .siven in table 7. They are open associAted with calcite' prticularly in pimmdemervhs of altered f2rromagtesienrt 4uart'g is less cormon and as second9x7 mineral im oonfined the more inten ely altered
areas • Zeolitee ,tria. almost fls.w.',1-0 amygdalo minerals
though primary interstitial zeolite i present it a few rookse A oimilAr oocurranoe of interstitial zeolite has recently been det,tri%,.e.:1 ay Crook (1962) who found up to 251; of zeo is somo tedilliont:.1r7 horizons ia Vii Levy,* Crook defived each major rock rie oztt mineral essemblage *oh he oetzl4erA to be diagetic of that series tld produced in it b7 burial metamorphic offec''- 1 he suggasted that both the aa and 5uva Seriecs 0ould be recognised bjr the following assemblagai
''ohabazite - two of (thomsonits, phillipeite, analcits) calcite - phyllomilicate*
Tho prol:,ent writer hoviever has not found such 4n assemblage to .1te prenent to any considerable extent throuah the Da
Scolecito 512 1.519
nhommonite 1.527 1.530
1,iesolite
natrolite
31axial netive : moderately low 2Y: oblique extinction: length fast. Uaxial positives 2V low est. 20-30': parallel extinction.
1.5014:n46 4. 1.507 Very low birefringence: almost parIllel extinction.
not4 < 1.483 Biaxial positive: parallel. extinct ion.
Table 7 s ic1 P- '..t3o it' s
4e*lite Refractive Index oL 0
Other 2roperties Appearance
analcite
1.491 Very low birefringence almost isotropic: elarvage not well developed and fragments rather angular.
n. 1.487 Aainly isotropic, though some fragments show very low birefringence.
1.500 1.503 Biaxial nek,atives variable extinction: parIllel to oblique.
aditing clusters of crown coloured fibres with velvety appearance. Colourless to White clusters of pearly needless clusters ire small. Fine fibres urranged in small radiating clusters and has satiny white lustre. Aire white very fine fibres; each fibre is independent of the tare and the whole
1111140110f pure white soft hairs. Rhonbohedrat freluently showing interpenetration twins.
Trapesohedra sitting on a formless maze.
Glassy and formless; colourless with perly lustre.
Cha ..va it 1.488
-119—
roko,;y_ and C.,̀,Ilemistrov
5.1 7ho olivine basaltn of ohare I are much more
abundant than rocks of succeeding phases nd their volume over the area mappfgl is estimated at ten cubic miles. They can be subgrouped into three families on he phenooryst minerals/ augite baeldt in much less abulAant than olivinflk augite bas41t and olivine basalt, which contains only groundnuts* vyroxene, in the least abundant of all. Some of the reeks are almost oveanites and mites though generally the olivine and pyroxero eontents are insufficiently high for the rocks to he representativen of these groups.
The olivine basalts have a density ranee from 2.75 to 3.00 thoueh many- rocks are belmsen 2.85 and 2.95. The majority are holooryetalline but differ in theinxture of the groundmass and the proportions of the oonstituent minerals. The basalts contain phenoorysts of au&l.te and
4.1eLspa olivine and usually someAmicrophenocryste; groundm„ canstituents are plagioclar and pyroxene sometimes with olivine and potash feldspar and always with ore mat Asa and apatite.
gnesian olivine phenooryste arc sudral or euhedral and show a great variation it:. sloe from a wilt of 3.3 x 2.1 mm to granules less than 0,5 in diameter* In a few basalts pseudomorphed and partially psoudonerphed olivine eryst:Als rc surrounded by an aureole of grounds**s alteration in which the feldspar and interstitia/ material have been partly converted to very fine brown clay material. This aureole is not more than 1 mm wide and is not assoolated with all olivine cryst1s.
Pyroxene phenocrymts are such more common than those of olivine; they average 2 — 3 mM square, though not
120 uncommonly thly are 4 or 5 mu in either longitudin,),1 or basal eection they range down to miorophanocrysto only slightly larger than groundmass oryotals. In habit they vary from reiz:ular octagonal (001) eac. UonF to stuntad prismatic forms and irregular granules. Away show or form of zoning. 'yroxene crystals frequently contain inclusions of magnetit apatite and less commonly small granular olivines 'aTld some groundmars material* In many rocks it appears that initial olivine and magnetite crystallisation was followed very quickly by gyroxene formation.
21agioclave, the overall composition of which is An 86 — A
n55° ocourn as lathe, usually with irregular
tarminations, and rely an equidimeneional pixtes. The maximum length of '4ile microphenocrysts it; 0.5 mm and MOOS; of them are not lose than 0.3 mmq groundmaos crystals are less than 0.1 mm. Tao minerls are o-ften , all formod though some microphenooryste have indistinct edges; zoning iv not oommon.
in addition to labrulorito, jot and swill amounts of alkali feldspar have been notod in the bwAlts. Prom similar rooks in nearby -areas iyengar (1949) h decried ansiolte which may be primary or se000dary after leucite, tboUgh he favours the latter view. The instability of leucite is well known; 4inchell (1951) considers the alteratior of leucite to analcite to tie oommon in Tertiary and Recent lavas though 1.poroix (1931) convidered that in Pacific k,i.v%s leucite has yialded orthoolavo neoheline. The prezent writer Ms found no indication or leuoite and only tr;r4ces of nepholime, though seolites, includine analcitel are clownon.
The groundmass of the olivine baealts i of variable texture and mineral oomposition. It ranges in courneoees from aphanitio, mioroorystalline or oryptoorystalline to hypohalise ad gl::Isey; when crystiAline the groundmass texture
... le intargranular with sous interstitial Material which is often ,:ltered to iXeen ohloritic mineral*. The commonage* rooke have a groundlut,,ss of *wall crystAle of labradorite (rarely with a little andealne), olivine and augite, though olivine is not avays present; magnetite, usually in finely granular form but sometimes u to 0.5 mn in dimmeterp and Lpatite occur with them. Generally thin latter mineral occurs in , oieular form in the droundmass but not uncammenly it forme elonaitw'A (0.6 mm) prismatic cryntale. Class is sometimes an importnt component; alteration minevas: which 1;44 je p'recat include serpentia, iddingsitel cicite, uralitep ollnozAxite, pyrite and green hornblende.
The olivitle basalt lava pile contains a few floes of traohytie materla in which microliter of feldspar are arranged in parallel ordpr in aansey groundmass; *hese rocks, which ar comparatively rare, contain no forromngneeian 19.11a0ZIO. ually rare are the paIagenitio rocks; a palagonite tuff from a point ono and half miles south-east of Nadarivatti containr anhedml crystal fragments up to 2 mm. aquare of olivine, magnetite and pyroxeno, which together with granules nd cues of magnetite are set in a yellow glass which displays n oolitic texture.
Variations i the modal analynovof some phaso lavas are 1Ilustrted in table 84 the 6roundmaeo is often too fine for micrometric analysis and it it not Reelable in 40y rock to dotermino the proportions of plagioclase and potaoh feldspar. The figures indicate a variation in percentage of ferromngnesian phencoryste fro 48.1 to 7.7%, in total #4441 oontent from 11.1 to 1,O And corresponding groundmaos feldepar fluotuatine from 26.7 to 514%. The figures rs not indioatilo of $ continuous tread through phase I volcanicity
all modal samlysos quoted are volume porceat.
r4
No. MENOCRYSTS
Pyroseem Olivlso petits
Table 8 t XICROPH. Feldspar
mod41 analseee of the rooks of 'hare I.
Gro TJtal 0/10011111AVI
Cass* Olivine ,yroxeno Ore FeldeAr Otherr 462 30.3 10.6 1.8 10.9 7.5 37.7 1.2 100.0 463 25.7 11.4 0.3 24.3 1.0 30.9 6.3 99.9 456 20.2 22.0 0.7 9.8 6.4 38.9 2.0 100.0 460 20.5 4.5 10.4 5.4 51.9 7.3 100.0 4C7 13.6 1.0 2.8 13.5 19.4 5.4 44.3 100.0 506 31.4 7.1 0.9 3.1 12.8 5.1 38.9 0.5 99.8 458 17.5 20.5 3.0 1.5 11.0 3.4 42.0 1.1 100.0 485 11.8 9.8 1.9 17.7 8.8 48.5 1.4 99.9 280 35.6 7.4 0.2 8.2 4.5 31.6 9.5 100.0 284 33.5 5.7 1.2 0.4 7.9 9.6 37.2 4.5 100.0 291 31.0 7.6 1.1 3.8 2.5 54.0 100.0 453 15.7 8.9 0.7 5.3 2.8 16.2 6.2 30.1 14.0 09.9 459 19.0 6.8 29.0 11.1 27.6 6.4 93.9 59 29.0 7.9 0.:I 5.8 9.4 38.0 6.6 99.8 255 25.5 1.6 4.0 2.3 11.9 7.7 41.0 3.0 100.0 461 6.7 1.0 16.0 0.1 76.3 100.1 468 12.1 0.3 2.2 14.8 70.6 100.0 510 15.0 8.4 0.5 4.9 2.0 0.8 4.3 26.7 26.6 100.11 511 512
18.5 32.6
5.8 8.0
1.5 0.2
5.0 0.8 52.5
4.1 15.2 6.8 31.2 11.7 5.8
99.8 99.9
260 22.2 4.0 1.3 1.9 69.6 1.1 100.1 519 37.9 10.2 51.9 100.0 513 29.4 13.9 57.2 99.9 544 25.4 7.8 4.9 61.9 100.0 551 25.6 7.1 64.5 100.2 259 36.1 9.6 54.2 99.9
- 123
and there is in fact the same variation in the minernlogy of adjacent lavas at the top of the pile Ma there 18 at the "ilottoml through the sequenee ere alterations of later rich raid deficient in intretellurio ferremmeteeiane. There is no apparent layering in individual flow*. the*. 01sly of the lavas further away from the 'lint Mey reVeal 4,1demse of crystal sinking. It ie however possible that crystal eilkt:16 prior to extrusion caused the modal variations demonstrated above.
Anroximately 25% of the basalts are vesicular an cortinxted two thirds of these contain zeolite* or
c,Aleite. Vesicles are sometimes a centimetre or more in 31:4seter, thou; a on average they are only half this siell they for up to IQ% of the lavas in which they are found. whoa not infilled vesicles, contain linings of blue and yellow cloy material. No sponge or foam lavas are known.
Plow structure is poorly dew sped and confined to few roOkel the groundmase feldspars rometimes show a rudimentary flow lineation which is best developed in the trachytio The extrusive basalts, both in micro - and maorowAppecimen show little indication of flow and lineations are more pronounced in intrusive bodies,
The order of crystallisation is magnetite and apatite, olivine, pyroxonee feldspar and interstitial matter. Most of the rooks are, mineralogically and texturally, olivine basalts, though ease of them hove potash *entente which are considerably higher than would normally a expected in such basic roOks.
5.2 neTraohybosaltt In addition to the petrology of the traohybasalts
proper, the petrology of the Nasitiu Volcardes ifs aso dieouseed
124 .... in this emotion, since besides ing similar roOke, were fold at the same stage of volcanicity.,
The Ohs-no I1 traehybaerato contain abundant large phenocryste of plagioOlase 4 fewer phonerysts of augito and olivine. The ih neer"y's to fors about 20 - 30 of the rock and ars genera14 snslosed in a dark Mieroorystallino groundmane. They have a specific gravity rftngo of 240 to 2.43. ?es:tut:any they are often fine fined, and porphyritio rooks aro more abundant than among the olivine basalts of phase 1. The majority of rooks examined show sons **area of soolitioation and hydrothermal altsrstiony
Tie rooks contain wart e amounts of &ono rystio plagioclase as shown by the modal analynoe in table 9' In e .o.c of this *Purser make the. banal seett)ne ,41d lat4e are 5 or 6 as long 'and 4 or 5 mm square; these cry; taln are significantly law:or Win any noon in rock* of other Olsson. In the fine-graintd lavnat feldspar in miorophenooryntio emi not more than 0.5 1111 long and often -.T.iliedral$ frequently reek* show a complete variation of pltiloclave also from 5 ma to 0.5 mot. The °Imposition of the phenocrystr ire 1:ttolmita to 13bradorito. the moot cicio crystal determined being Anse.
Pyroxene phenocrysto aria only rarely largo and euhodrall granular oryetale are up to 0.5 am and mere infrequently 1.0 en in diameter. In POMO of the dyke rooks of this phase the pyroxene orystals aro more abundsnt and suhOdral than in the oxtrunivo rooks; aryntale up to 3 or 4 mm long ore sosmon. Th1s inoreane in pyrowle content le not asseelAted with any diminution in nodal phenocryPtic pla ioeles, but with a decrease in the proportion of groundosse conetituents. Olivine, by contrant. is rarely enhedral and -rnules are up 10 0.7 ma in diameter.
The :,Amoundmare in mlwayf, Very fine irked and
125—
the occasional coareer rook, noted from phase are not oOserved in phase II; conversely hyaline and hypolvaline rocks are equally uncommon amongst the traohybaealte and there is little variatin in the intergranUar gronndmase texture. Granules and blebs of ferromagneeian and ore minerals ;1'.e associated with stunt(0 and almost equidimeneleaa lathe of plagioclase and s•=e potash feldspar and potash plagioclase; interstitinl material is not determinate. Frgments of brown pleochroic biotite are recorded in one rock.
The modal analyses of t%ble 9 show a vide varia tion in the proportion of feleic to m-J'ic m4terials and it it not imprkhaUe that some crystal sinkim7 has occurred CrIUSi,:g enrichment of some bands in pyroxene; the basic flows are interbedded with less b-Lsio ones. The modes indicate i range of colour index from 17.7 when glassy, to 27.5 and 47.5 in the crystalline rocks, oomparod with 41 to 66 in the olivine bqsaltel this great difference is not reflectel in hand epeeinen.
‘ of the rocks examined show some degree of alteration. In thin section they have an alteration air ilar to that of the phase I rocks. Fresh pyroxene in first chlortised and ultimately replaced by carbonate; olivine cryntals axe also destroyed thouiJi semetimas the original martAns are outlined by magnetite grains. Zeolite is deposited in fractures in the rroundurs and in pseudomorpho of augite and feldspar, often nefmatite its preoect. The vitreous groundmuss may be brick red to dark slate Frey and there is often a band of calcite up to 1 mm thick surroundt7g each inollIsion. Plates of calcie appear in the groundmns, which. may be veined by striners of carbonate and quartz; some of the quartz tF, etrailed and has a biaxial positive
TWA* 9
011 nD Virgr3
lam Mapper Nagmtite Pei
33.9 52.9 54.9
55.2 46.1
4.7 11.3 13.2 10.1 4.2
3.5 5.7
1.1 7. 5.0 4.5 4.0 3.2 C.4
110 5.5 6.7 4.00 0.4
100.0 19.8 99.9 100.0 100.0 100.1 100.1 100 1
16.6
21.5 14.2 18.8
7.9 13.i r 8,2 29.5
5.5 30. Y.: 2.6 16.9
inoludog
127 figure with a low Axial angle.
One *postmen shows evidence of welding; ntringera of ertisnetite grasslor lying paralll to a linear or 'flow' etruoture suggests that an incomplete welding, or at least a modification of the ori::im.1 structure hae taken plaee, The ;.roundm::,xle, probably oriinally veicular and hyaline or hypohyaline, ba* beon smeared into a linoar arrangomeo.t of Tegnetie, chlorite and glass; a few inocyr are granular.
Srmel This rook !Iry o an ash flow Close and fimisoon 1961) in which the structure is due to compression, heat generation and store in the thlrnules, with subsequeot welding of theses ranules as they disseminated heat throuh 4 ho lava during
emplacement. There are some indioations of devitrifioation though most of t lare has 'leen altered to chlorite. amounts of native copper occur in. these rocks.
The fine-grained bawate of the TimAtiu volcanics are holoorystalline and have a euheirelleu:ohedral porohyritic texture with microphenoorysts of feldr:par, pyroxene and olivine set in i very fine grounds of rimilr minernloa and intergrnnular texture. Groundmase cryst!),1s are subhedral though fAdeparc any sometimes be etihedral; interstitial material of low birefringenoe may Ott potash feldspar, potash plagioclase or zeolite, Fol4opvx microphenocrysts rane in size from 1.4 x 005 UM flOwIt to 0,03 ma x 0.2 sal a few untwinned cryst;Als are 1.0 on in diameter and euhedral. Groundmare feldspArn are not more than 0.1 m long and of the order of 0.01 am wide; they display a patchy and rather weak flow structure. Pyroxene crystals re fewer and larger ttu.al tWoo of feldspar and are clustered with olivine and mAgnetite; they are usually subhedral thou6h the largest (2.5 x 1.5 mm and 1.6 x 0.7 mm) are euhedral. Ground:m.1es granules are 0.05 mm or less in diameter, though a few are 0.3 mu long. Mierophenocryste of olivine are rare and about
O6 mm dianetiq.; the small altered groundeples granules re the nfTime size as the pyroxenc grains. ?henocrysto of pLAgioclase h.Ave a range of An86 An78 but two microphenoorysts are labraderite Ax; the groundmane faidapar le indeterminate. Magnetite and apatite are accessory minerals. The mode of this basalt is estimated at
Phenoerystic olivine 1% ?henocrystio pyroxene 12 Fhenocryntic plagioclase 16 Groundmlee 60 Others including
magnetite and apatite) 9
Chemical analyses of thia rock have not oen made but from the evidence available it seems to differ on. sliehtly from the treohybanalte; it is finer—grained than most lavas and coetaine lane phenooryetio feldspar and more groundmaes olivine than the average traohybesalti
The Masitiu audesite in rather variable but differ from the basalte of phaees I nd II in %hat feldspar and olivine do not occur in phenocryetic form and the grountmans, which is largely l'ehyrict im very light coloured in hand
andesrLes speamen; these ===== contain abun.dant seolitw3. Pyroxe.ne in the essential forromngnesian mineral and forme euhedral phonoryete and granular groundmaox crystals. The rooks are holooryetalline and plagloolape. of which there are a few phenoorysts and microphenooryets, Is the main groandmass component. The range of placioolane in from bytownite to labradorite thowLh groundmase eryntale are too fine for determination*. Magnetite undapatite aro aceeesery minerals and phyllosilioatee are the main alteration products though the rocks contain much calcite. new etructures flro ytt well developed.
-129-
110necryntic feldnears are up to 1.5 mm long or 1 rem square; ?yroxele phenecreste 2 x 1 mm and range to microphenecryetio grzlmalee of 0.3 mm dieeneter; groundmaee orystale are 0.5 or 0.6 mia long. The groundmaen texture is nermally intergranular, but them is interstitial materiel which 1i ueually altred to pale green chlorite. Verde 504 in table Cie typical of thee rodke; the mode Ohne* the high feldsper content And comparatively low proportion of mefic minerAls. The colour indeX is 25.7 and the specific gravity 2.75. The rooks are apparently basic augite andeeitee though if their Z20 content is as high ae that of the trtchy—bltn, they are better termed trachyandeeiter.
10aes traohyte: etrade the rock of the Namitiu eries. Mineraloeioally, texturally and chemically there iv
little difference between them and the green andesites of phase V of the 'inivoce area. They are extremely fine-' reied rick r which contain only feldspar microlitee and plates of calcite in a Vitreous groundaaen. Occaeionelly thl rocke are rather coarser and any contain some pyroxene. Their petrology will be further discussed with tirvt of the phaee V roc.
The olivine aneite basalt and augite basalt which fern the dyke cutting the Nasitin rocks are identical in All respects with the phase I lavae,
It has' been deduced earlier that the basalt dykes re intruded into trachyte s. Hence the reek eelliences indictite differentiatioe of a beet° magma to a trechyte followed by a return to very brixic ceeditioan. Thum olivine )Asalt magma hae produced trachybanalts and tradhyandenites which have been intruded by, or covered by trachyte. A return to very basic c'elditione occurred eith Vie erupti,-el of the Namitiu basalt and eubeequently the Lava pile wee intruded by anether series
of eta sad trash:Nu mats* The 6eo1ogy of Neon of tits 'iaikabukubu area needs
to be studied In grea4er detail thaa has .en i)oesible to date, but the main point le that in tAis 4210A differentiation was not 4 single process producing rooks fromb.rittalts to tradbytes, but was a proce ere0y Individual poskats of magma differentiated in different places at different times to produce a series of rocks in which the oasis to intermediate rock cycle iF repeated to or three times.
5.3 he various types of '.cyandesite Of the Vatutoula
volcano differ widely in texture, colour and mineral composition but chemically tbey are muob alike; they may be eonveniently grouped into a few major reek types for the purposen of petroraphic description.
fhe rook v of phase III are $ily augite trohy Ande:Utes with a few trachytee and are confined to the cone sheets of the basin and certain dykesand sills exposed south of the basin contaet. Tiums is no apparent variation in the petrology of the cone shoots' the traohyandesite is light iseen and eentalum wAny pyroAene phenocryste. 011vi,tte and biotite are not present and in many reepacte the rocks are mineraiogically and texturally similar to the andeeitea of the 1aaitiu 3ories,
in thin seotio nest of the eon, shoots show some alteration; they are generally holocrystalline though one or two are hypohyaline and porphyritio with phonooryste of pyroxene and feldepar in a trachytic groom.. As will be shown later some dyke rocks have a different tolitopo, Pyroxen phenooryots are oUhedral and u to 2 or 3 ma long and 0.5
nn wits, out are few in number and the trachyandosites do
-131- not contain more than 10% of nodal pyroxene. Feldspar phanoorysts are less abmndant and often imoompletely psendonorpheds whereas The cored of most crystals. have been altered to clay minerals, the crystal rims, are gene ly fresh. The rocks contain loss than 51A of phenocrYlege feldspar, all of which is plagioclase,
The roundnass is entirely feldspathic though size because of the fine dialikan4 alteration much is indeterminate. Aproolable astotults Sr sanidino aro identified, but some small twinned plagloolaeo metals are informed. The groundmass also contains pools of fibrous and granular chlorite, and lar!zely secondary calcite is tux important constituent of the
L rook. Magnesite grains are peppered through tie eroundmass but apatite is present in only very small quantities,
Augite tronyandePito dykes are usually less altered but have a different texture. Holocrystalline and porphyritic they cont din oUhodral and subkaidri phenocryste of pyroxons often clustered with peoudomorphed microphenocryf7.ts of olivine and a few pseudomorphe after feldspur. . The ground-mass is intorgranular. Mineralogically the dykes are composed Of plagiotlasO, olivine, pyromsno, potash feldspar, magnetits, apatite, thlovite. **onto, *mall amounts of limonite and aoisite and with very swill quantities of possible feldspatb016.
The 'argent pyromsno phonocrysts are 8 — z 3 11111 though thAy .very e at)out 2.5 z 1.0 nmS the smelleot ono* are 0,6 MO in diameter. The peeudonorphs aftor olivine are from 1.2 z 0.6 - to 0.3 am in diameter and the feldepl4r phenoorysts are consistently 1.0 z 0.4 sm. Yeldspar phencorysts are completely 41tered to clay minors's, though original veins of potash feldspar through the plagioclase Pall remain fresh. Grounds*** granules of pseudonorphed pyroxon* are sot in a w.;srl of feldspar plates and laths, all loos than 0.1 en longs
132
they are ameesited with dranular ind interotitial cAxite and with oabes and elongated eryst%ls of iron Q-xide
5«4 ,,..144aelteri of This sca. riev of tr3ohyandesitwl and trachyten, though
widely distributed does not have a variable petrography. Wle oesential mina ,106y is bietit;e, feldspar and magnetite though in a few rooks of this group, biotite it missing, the rooks may also contain augito, apatite, heamatite and iron stained Galata, seolites and payllorilicate alter_.tion products. Texturully the rocks fall into two groups, those which are bolocrystallie ad those' which are hypohrlliao. The more abundant holoorystalline v3rieties borphyritic rook!, with phonocrysts of feldspar and biotite in an intergrunulr groundrelso which is occasionally oryptoorystallIne. -notate in rarely phonocrystio; it varies in oleo from 1.9 x 0.3 am to fragments about 0.6 new in di3mater and usual 1.1..127 cleavage controlled prism faces with ular terminations. Pyroxene when prevent is swahodral, and thoudn occasionally forming prisms 1.0 x 0.5 mm, the crystala are more eommlnly plat and 0. mm in diametrr. euhedr41 feldspar phoneorynts ars from 3.2 x 1.3 we to the average 2.0 x 0.8 ma and the mmallar 1.0 an square plats,. The grain sloe of the greandmoss is iodinated by the length of the roldspar cryntals, the laths varying from 0.1 to 0.6 ma.
In some rocks the large,-7 biot1:-, crystals Ivwo edges which are sagged and asfg,ciated with narrow zones of granular ore material whit* are indicative of reoorptioni Poldspar phenooryete are often fresh; most of *hem are plagioclase (twinned and loos often zonA), veined with pot%h feldspar. Tha plagioclase Obiaoorysts have an average content et Am,. "elates of sanidine 10, to 0.5 mm square are quite
- 133 numerous in some rocks; the mineral is clear but cracked. ';'rim, with euhedral tenminatiene of apatite are quite common 7:uld up to 0,8 mm long.
The groundziris is essentially feldspathic with potaxh feldspar, p1e4Jociase and granules et pyroxone and apatite ani magnetite. Calcite is prenent in anhedral platey form; it is, in part, primary as are some columnar and radlatLng mattes. Alteraticn products inclizde chlorite a.Id zoleite. Thou& most of the detormin7,te feldspar in p1agioolase4 the hi ,h proportion of potaeh in the rock in suggested bys
1. Phenoorystic nanidine 2. !3tringers of pota'h feldspar in. phenocrystio
plagioclase.
3. xsolution of potaeh feldspar from plagioclase 4. Ground mass pota7,1h feldpar identified by staining.
6 Whilst corfocrsirg to V.Iis ee,oription epeOblin (57%)
also contains toieits sr,'. more serpentire it irre7111%r patches. (575) contains much biotite rith interstitial trmieine and sore plagioclao crystals aro unntled by ranidine; pyroxene is c, rent but apatite is abrnil..:11t and cell developed.. (185) is moro trachytic and shows r excellent flow etructure; there is comparntively little pherecrystio Oarioclese sri even less Ennidine, AUcIte crd biotite. the ;Toundmase plkloctlame hau) the corIpcsition cf ander!.re and oligoclse. One ?Taoism (529) considered to be allied tc these rocks, doee not rentain biotite; Ivroxere ard ranidirt rise creurdmars onnstituentsr and the rock contains *an estimsted lao- of grnurdrams feldspar.
Tho hypotyaline rocks rtre much firer rrnined !-gid often indeterrirt%tc; characterietically they show microlites of feldspar are sore rnued crystals of biotite met in a vitreous groundmtre. Rosks texturally and. oerl,oeitionally similar to these form part of the rean andesite series.
134—
eeecimen (526) it intermediate in texture between the two zein tepee; it iu weok! finer then the holocryetelline tucks but shows an excellent treehytoidel textuee though a few of the fel4spare tend to be platey.
Both the ietresive ane extrusive rock of this phsee contain normative heamatite, thoueh modal beamatite ift rsrely seen. It often etainr eelette, a mineral frequently developed in abundance on biotite dyke marine. Corundum it expreased normatively. Chemielly. the iotite Um:Awl-ends:A:item *re very rich in potaeh, 5 — T beine consieered an average figure; they contaie apereximately half thin amount of sedee Model analyeee of two eiotite andewitee are given in table ID. The modeo indicate a Flour ineex from 12.4 to 8.4; the specific gravity renge ie 2.25 — 2.3$..
eietite andleeitee have been deseribed from many parte of north Viti Levu; the rocks of the present phases Ill and IV were conAdered by Garretty (1936) E be part of the Vabeto voleanice. Augite—bietito and aueite andesites
reeogrused were deeoel .e4 from Sabeto by ekiba (195,a) who deecribed rooks very eimilar to %hone at Vetukeule; he foend that biotite web partly reserbed aee thet poteeh feldeeer was interstitial. Skiba also noted the well developed apatite eryetele. Bartholemew (1959) also investieated these rooks; he concluded that GareetW c earrelation of the eeeeto and Vetukoul derites war Invelid mince he (Bartholomew) considered that at enbeto tee ands:lite is older than the baeelt of the Ba eerier. Hie eeecriptioe of a typical enderite
ie very ?dialler to thee by rkiba: "The biotite octave in ouberdinete amount. It
is etrongle pleoehroic fres pals to deep chestnut brown. er eone eerticre the Mame ere ercloeed
a rie of opaque Iren nre positislon. This peenomenon mey 40 eae Le late *too rak44.10a with the eurroundleg Noma; it is Immo that under the
influence o. high 'temperature, biotito may altar to mngne.tita othr milmsrals. Certain/y the strong pleochrolem suggests th3t it oontains an appreolo.ble a4lount
Jurtnoiomew eattm,At*hi 'ithe ;,:‘tidoultati to aontin ana 30-A pyroxona.
iyengur (1941) als aeocri4ad a specimou from tho Jaaeto series; £ found gioakly ploaohroie wrohene an4 euaordinate otiu 1h a grou4dmaao of ellgoolaae laths'
magnett,:eg ilmanii:e Ana ,kvatite, it4, limonite showing “a radiating and °now-fling struotarai"
tae aiotii;e traohyanaosites of this region therefore, waotner elder or your tiv,a tiAonait re croioicaUy most idea‘loal. Reoently doleat has 'Ocit
tnrown on 23kArthoionewtti interpro44-41oh of the field ralation- 'Iestwe,rt taa wia the adadeolts At ,WJeto and some
workers maInt,Ain'4'14014 ila eo Andealtaa eomprise a thiak northerly, steeply dipping shoot IntruJed into the basalts, whilst Bartholomew oonsiderod it to -4 A norml uncom-rormable
se-luenee of basalt over anitesite. At Valukoalat the 4iotItii tr..ohyandesites rwsoent
the late»t artrusiv* phase; specimens fram t vent agglamerr.te formed by tfAs eruption do not; difiqw fro siva rooks. Ins $c)-tat/h aonte-t at trehytic toguare o listiuguish them from the Vatia andeftlto.
.64114 WNW.-
Td3le 10 ; h-• IV rpcka,
1 lai Totta f1 di and neeoetasie 32.5 77.9
ro 'tens 3.5 0.5 Biotite 5.5 3.2 Ores 4.6 4.9 Alteration products 3.9 13.5
0 I.
1004
134 5.5 V4S irPk(ut 6'114404V',
The green andesites were mapped as & group on their macroscopic features, colour and mineralogy; petrography has shown thwt most of them are trachytio, but considerably sere variable than apparent in hand speeimen. They are einientially pyroxene—feldsparrnmagnetite recite, very fine-grained, often indeterminate and rarely coarse-grained; many are hydro., thermally altered and calcite is a common constituent. The coarser grained traohytes, typified .)3t. epee/men (480) are hypohyaline with euhedral phenoorynte of feldspar and pyroxene set a groundmaas which is in part cryptoorystalline and in prt vitreous* In this opecimen there is a .small amount of grnuLtr olivine with i4dingsito and such seeondry wtterial in the fors of quarts, chlorite, calcite and pyrite* The plagioclase phenocrysts are up to 1.5 x 1.0 an, are fresh and howe excellent zoning and twinning; idiomorpUc, the phenocrysts hIlve t composition of Anw An62 thowh groundmass plagioolaT, e in other seetionn is in the andesine—oligoolase range. "euhedral pyroxenes of similar size to the feldspar* are, in thi rock, .peinadosorpbed by calcite, though a little colourless augite romans fresh. Olivine in also pseudomorphed; a rim of iddingelte encloses plater calcite or a mosaio.of fine quarts crystals or both.
When determinate, the .,;roundmass of these rooks is felispathie; natio minerals are absent though sem ore material is present. The feldspar laths are up to 0.5 - long and usually show oarlsbad but little albite twinning; interstitial arterial tv2,s a. vary low Artfringence and refractive index. The rocky frequently contain considerable amounts of potash feldspar, though thin is , enerally a groundmess constituent. specimen (480) stows intone alteration of the .,roundmass and the primary mintorsAs are indeterminate. The rock is slightly
vosicular and veoieles are 1 nod by chlorite ether* ars intilled 5y quarts and cal e but zeolite is absiont.
(490) oonforms with thin desorivtion apt thst olleime is absent and omen* is proses* se wafted's/ and alhadral miorophenocryete wadi are sm4sligAt4 esareerthsx the send ass eryotals• (492) he. a time indOtermilOalle gronnamaes bat In addition o olivine sod pyroson* oontains a tow 1114.4eveloped orystAle up to 0.1 x 0.2 ma of biotite! they are ,arown pleochroio and ore 00311441 is twin-1404 parullel to the (0001) f4ce. DUhedrni erystals of apatite up to 1 ism long %re not common. Origin allidionorphic. mnd up to I. z 1.0 EN4 plagioclase I* wattled by pot 11 feildsly.4r which nometimee onolorr4 plat** of material of very low birefringence whit& may N. nepheline$ the veining and riteration is oontroiled by cleavage scour in 055) whisk oleo oontainr one orrItAl
x 1,0 as as the °sly l'erramammesiani it i with two swilmodral cryst4lc of magnetite 0.5 mm
soolit** biotite, sociated imeeter.
f:martis in amoomiated with the other soisondary min rale. The ottler type of groom andosite is ao intiotersinate
in thin emotion as in hand opocimon; it consists of lany (1 Mta lens) SIcrelitev of feldspar alisnod in a glase matrix. one such reek from melt* watkobillsbu vill4ge ammtelms. in 4dition to the mierolites. enhedr.11 to subbedval elem. phenceryete up to 00 sm low; of feldspar and welters and n small amount of isotropic analoite as wsll as ealeitee though all oryst3Iline material a subordinate in quantity $0 ass*.
Text ly and anemically those rook* are trachYtosi
ease are quarts ohytes *Mart other* are distinctly undies*. saturated. Non* of then contain phenceryetic eanidine. MOldsL samaysis that follows le one of the fitter meabere of
tole group: Pyroxene 1.0 Feldepar 2.6 Ores 0.5 Alterti
9,8
01,Na8 94.8 The fact tLAt undereaturated and oversuturated
rocks occur to : ether is ,:t.6ain suggoOlOme Of differstiation eing a oyclieal penomerr, and !olklaig pia** in a aerie* of
:11tges rather t one mitinuous process. It has previously been euggeerted that the green andeeltos may he related to the
intruelon; tLie andesits iv a grey-green rock of intermediate grain size with prominent pyroxew, phencerystsi loroncopically audit., feldspar aA #agnetite are the essential minerals an4 the texture is anhodral paphyritio; the groundmass crystals are eut)hedral and a of them vilaller pyroxeues are completely athedral. Plagioclase crystals, which are of partially altered to clay material, have that composition of andesiye and how albit and oarlsbad twinning.
The groundnass 1:2nsiots of plagioclase laths up to 045 as Issis with granular axvite and potash feldspar and some iniaretittal ohlorite. Nagnetite and apatite are the min seoessory Minerals and alteration produote include phy11006 alliostest clays ad oalcite.
Th rook is a traohyte and forme an iutrwAve &vas eft pls. Mineralogically and texturally it is akin to the groan anAlesiter and poseioly also to the phase III trashyandosites.
$.6 The.ronsonivis, Of all tLe rocks exposed at Vatukoula, this group
is Who seat rare; their eounterparte are not known from fatia.
139
They vary in pe ho * Ines, porphyritic a rich in mane no they are cr)nsiderably coarser grained and more strongly porphyritie than moat lavas. Subedral phenooryote and inibsdral to eubhadr:,1 milerephenoorysts are net in an intermediate tixained 4roundmns which is mahatma1 to granular with soA, intereertal material. Pyrexanas r=.ro tb* abundant phenoorysts; the lar act of those are 4.5 Om in diameter though avemge crystals are 1.3 x 1.0 mm. is predominantly a grounduAse oonetituent and microphenooryvts 1.3 x 0.9 mm grade into smallor °metals of 0.1 diameter or less.
The larger py xame oryotale usually contain inclusions of mAemetite and feldspar thoudh the *mallow oryotkle tend to No subhedral and sores microphonaoriets and greundmase :raiment are oomple ely irraeuL:Ar; the reeks oontain only a smell amount of groundmasa pyroxeno. Olivine is seMettmes abundant except in a few it where it is replaced by some chlorite; 11be oo' F, rota are euhedralhouja the few groom as olivines are '4'anular. ere; uolt.uttered togetilsr the olivine oryr,tals ra:ve in size from 2.3 x 1.3 mm to 0.2 — 0 3 mm diameter.
Apiirt frou a very fow eryst -,1t; up to 1 long, biotite is a g,rou..ntkies c4astitaemi occurrin&; as irreoalar plates aad a few lathe the Largest being 0.2 0.3 amt long; other mineral fraexonte aro pleochrole from pure greiOr to green brawt4 have oblique ettLiction and normally exhibit eeeond order red birefringeNne; they ar cleaved, but noes of tha fragments have euhedral form. The minernat which in present in only %y small quantitiss in poseibly primary hornblende.
The edomi g d is feldspar euhedr41; a rudiment:447 flowstructure in
140
POP d but w-lny use ilve outlines and indefini.%s twin please. The plagioclase
range is An 65; a refractive index of 1.547 Indio:taboo that the aroand ans plagioclase is mine An 0. Potash feldspar ie also a groundmass constituent together with small amyante or augite, olivie and biotite. Avtite ie abundant and mtignetit,:$ IP the on4 iron Ohids, nerpertine oaleite, and *lay minerals are the Maim escosdary materials and name heaMotitsilimoni%e staining is associated with a few grains of ore. Chlorite has, in most iastanoss, rsoolted from the alteration of interstitial matter*
Modal comvozitione of two rocks of this group are given lowl
Illenocryntic plagioutue 3.5$ Mit Croundmars feldspar 47.7 Phenocryritio pyrozone 32.2 17.4 Grounds:lass pyroxene 0.9 10.e Riotite 2.6 2.3 ,erpentine after olivine 4.6 11.6 Ores 5.8 7.5 Others 2.7 3.3
100.0 100. Other rodks of the same type ow little variation
from this. In (535) interstitial leotropio Notarial say indioatm %he presence sf susloitel 'mosses phenoeryrts oftsn show a very narrow ri., in optical continuity with the whole, Of groin pleochroio ferroaugite (X • pals green, if • yellow gross and Yi = medim green). The intensity of the colour v-trtes from orynt:a to oryotUl in amount it is greatly subordinate to to norJ. olinopyrouens. 3ma11 putatives of emerald green placairoic pyrozene may be wirinOl 1i the ore
material in pyrite. Pyroxene phenocrygte in rock (554) also have -L rim of ferroaugite and again tht, ore mineral im pyrite. Peldspathoids h:;.ve not been identified from either of theme rockr, the estiwAed modes of which are given belowi
2.12 Total feldspar md mosoetasis 65 67% Pyrogene 12 18 3iotite 5 4 Orer 5 5 Olivine 5 2 Otherr 8 4
k similar rock, which has a texture more charaoteri-stic of the monzonite i decried from the nmperor mine workings. Its mir,er;-1o6y in as that of the rocks described above, but crystals of Ila,4oclatAi, biotite and accesSITY and alteraLion minerals are enclosd in poikiloblastic plates of pota2h feldspar. At LAbeto, :7,kiba (1355a) distinguished these two similar textural forma of monzonite.
"One variety in machos grained monzonitel the other is monsvnite porphyry containing large phenocrysts of
augite and biotite in a relati,fely fine grained matrix." monzonitic porphyry correspondm to the mioromonzinite
of this thesis. Since tho rQokr of the.layered"intrusions are in
some ways Tory eimilar to the monzonitem, it is convenient to discuss here the petrology of the various units previourly desorkaed. (562) is tiken from the au4te rich layer just above the chilled selvedge, the rook is bypohyaline but contains phenocrysts of plagioclase and pyroxene. clms plagioclase phenoerysts, which are up to 245 x 0.8 mm,
142—
almoet oompletely peeudomorphed by oalcitee The plagioclase raneeo is ensi An all bytownite. The rook tam, oontaine rounded eranuleo of pyroxent and one fl%ke of brown 410tite.
From 6 inches above the beee of tho sheet, (563) sheer numerous differencee to (562). It is holoorystellino and the flve—eTained groun3$* is compoled of elegiociaee lathe and eranulen of mafic minerals; it is intergrenuler. The reldelear phenocryate are now more numerous than those of pyrozens and are larger than in (562); tsr ce no micro— ehenocryete. Peldepare ere zon an.1 twinned and the pineloolaee mates) is An80 An; small umounts of poteeh 17eldspar occur in tile greendmaepe Pyroxene im accompanied by biotite, fragments of which re scattered through the cromeftess, host nf the interetitial materil has neen comverted to chlorite.
F,ppoimen (564), whioh is taken fren an eleven feet b:eed in the middle of the eheet is, vary dirt from the foregoing rocks; the texture iv treehytic and phenocrynte of plagioclase, menidiee Nnd mioa are Pet in a groundmees of feldspar lathe which mhow exeellent flow orientation. Tiny granules of eyroxene and biotite p.14, preeent in the groundmaes* Peldeear phenocryeto are aninly potneh feldvenr. (565) is from a bend which oeerlier the previous zones; it differs from thsa in only a few reepecte. Olivine' biotite aad pyroxei are all pfteent and phenooryetie plagieclaee is Ann — ANIL whilst groundmas, plagioclave, associated with some Wasik feldspar is Ann An51* The model analyses of thee* rubs are 4ven in tenle 11.
Ther- e figurer indicr-te the tendency of the derkers howrior minarele to occur in the lower layere, whilst the lighter feleic minerele occur ie the uppermoct zone, though in this inetence the upper zone iv apparently a normal.
.4:44 r the tour loae pi ;LJ34 voce
4111111.0401.111...4.111.101.11....0'
1. 15.2 4.5 Phenoery t e plazioolmee 11.2
eemidine 7.9 pyrorene 3.5 6.5 0.9
Groundmays pyroxene 7.6 Grounds foldrpor 62.8 82.2 3ictite 0.8 0.7 Olivine Ores 0.9 7.6 3.8 Others 8.5 glase 72.0
.212. 25.4
9.4 6.8 47.4 1.6 2.3 6.0 1.2
100.1 100,0 100.0 100.1
micrononzonites The composition of the plagioclase orystle [...1st) reflrct a 1:4ering; ths1owet entv. &Ave plagioclase the comlwsition of bytosnite, but it exadoc to andesine in tile upper layers. &a shown eIrlier, the "savored structures mov result from the 6romity differentiation of 4, lavu of micro-monsonitio affinities, or more pos. ibly froi, mixing of inter-mediate and busic lavas, raso with s,,)me grAvity differentiation.
5.7 Al 4splb1evsle wn4,pites hornblende and hornblende ,Lugite andesites from the
%tia peninsula are roc with 4 consistent texture and neraloa. In thin meet ion they s.re all hypohyaline and
sxhi.)it zany larma phenocryets e t in a grounamaes which is eitaor wholLy or partly vitreous; the Arounaa%ss is often irresolvable. Flow struetare is h,t well developed but some of the phehooryste have a parallel armagement• Feldspar phsnocry*rts are idiomorphic, said prismatia and basal sections re sometimes 4 x 2 mm hut more usually they aro lose than
- 244 2 x 1.5 za and averaoe 1.0 e N7 am; barrel sections 1.5 val aquas.* grade irto leer idloworeohic mioroptenoorys. Pyroxeae weera erevort is elteedrel; a ..aw granules ere mere then Iam in diaster ri aoet are between 0.5 and 0.5 ma. Pyroxono is not e common groundmass constituent and oryatals leaner than 0.3 am diameter are rare. Determination of the ;Ileac of primary emphillole oryetals is often impoesible beoause of cutmeTtent resorption; it usually hal an ecioular or prismatic habit end in never more than 3.5 mm long and usually beteeen 1.0 and 2.5 ma. Anidiaensianal orystals are equally common and between 5.0 and 1.0 in diameter; like pyroxane, amphibole 13 not a groundmase constituent. The zroundmaee in normally composed almost entirely of feldspar with 90M4 ore material and is very finely crystelline or vitreous. Mierophenooryets of feldspar are noted in e few rooks; they are ill-formed and eubhedral and ranee from 0.2 - 0.4 mm in diameter though they ere sometime rather aneular.
letarminete feldnper is ell plazioolate which often shows particle anl eomposition,71. zoning; • the compositionel range in Ans6 Anwt rare determinate crystals indicate the groundmesn feldepar to be An40 An50. Altaretion of plagio-clase to very fine clay materiel hark eroceedel to e further degree in the celoic• °ores of the orystele then in the more sodie
Loet of the rooks contain one pyroxene thou specimens from some of the later rocks "leer hyperethens as well an eweite. The former mlnerel is restricted. to a fee rocks, but the latter, a diopeidic augite, is common. In some cases this mlnerel occurs ir IIrtorzt cubic like form an though the material had eeen broken up intc cleavage fragments during intrusion or extrusion; sic crye4els have mill defined and entire MS110118 though the larger phenocryste often show
145 - 1n1 boundariez. The polarisation ooloura of augito ere
tiv41 oe000d order red; (010) twin pianos are common. Flpsrethene forms small granulok,0 eowatiues subangular in etiapo and up to 0.2 or 0.3 in itianoter; they z*per to e trzlaments of a mach larcer oryst,al. Magnetite with a little
apatite are tlie &Lau aceewsory ainerale anatI eineipal seeoudary se•tter la ealoitat. live vitreous groandiaave i often red 4x. puri,1.* duo to kiak;:tite stAinint.
The analyse's of taro* rooks five* Vatia indicnto-thAt they &re not 'to hIghly alkaline as those at 4atukoula and the avarogo pot Ash sitateat of the aadesites is just over S. ThouGh the groundaass le indotorminates it is considered tho4 it to largely plagloolavo ariJ ,,.1inlog hos shown that only very mafl amoonti or pot.wh feldear aro present. 'Zhoro IP no
1,”rsAtive nopheline, One or too or the rooks, howavtr. aro a little oversaturated. A colourless slooml of weir ooclre interstitially and also in logo eiroulur easose of 0.2 - 0.3 22 diagater iA the rook; ually them) forme re.—?rloaTt Astri4,1tem or tills granules uf quarts, each granule having a olightLy different orientation from it o oeighbours.
eh griauie ie posibivo with A very low axial anele; (sow gramtlee ohowed hlki* axiol dose and
nega'Ave elan but i1.j coa!goolto aal anomalous flaoh fiGurel 9roduoo1 the interforease of two noighbouring ranules.) 1h' minaral lo ntrained quartz; or matmrial
of Amilar aP.014,m0410 iiidshractAtriotios has asn found in the r trscLtlasta of Deialicouie.• Some Qf .41 zi434i tits erretele are bro ,.. in colour.
The roc o he Vatit sories have -.oson divided into two groulm; the lower 7atia grucoins augita at the only forromognecian whilot rooks of the upper aria group contAn hornblende nd au6ite, though ;.,L few of 'Vile oldor rooko of thin group do not contain, any oyroxone and some of the youngor ones
146 oontain two pyrOZOWO without ear tole. The following description of suah a reek is of ec spoil:mem tree the VatuTali island intrusion.
The rook is holocrystallinel waste" and dens* end is estimated to cant In 80% reldsper, 1:40 tsatere is stromaY orphyritic with large euheiral pyreasse errstmls amd subhedral
groundnass feldspar; the rook is froth and the greumdmass is Intel:granular. Feldspar phenoorysts aro sometimes olustored together; they range in sins from 5.7 x 3.6 me down to plates 1.0 ma square end miorophenooryvts rare 0.6 — 0.8 ma in length. Pyroxens phenesrysta rang, fret 2.2 x 0.5 mm to subhodral erystal 0.5 an in diametor; they era colourless and show faint pleoohroism to pink and are emhedral to subhstralf. Pyroxene phenoorysts are frequently surrounded by a halo If magnotite granules; this may suggest resorption of orieinal amphibole to form pyrozone and magnetite. Twinning if the feldspar phenoorysts is unoommon and more usually they show an exoillent oscillatory *ening. The plagioolase range is An93 -.Jaw anorthita to labradorites Partial* zoning is also common. The grountimass material is very finely granular and shows no twinning. NagmeWe, &petit*, phyllosilleates and °slate possibly with some quarts are also present.
The modal analyses presented in table 12 Indies** that the andosite have a eeidlear index from 12.7 to 36.0, amid* most of the specimens lie between 20.4 and 22.2; the spesifie gravity 'arise from 2.18 to 2.74 hat Who majority lie betimes 2.5 and 2.7. The modes show a remarkable consistemey in the content of mafio minerals is the rooks, though the ratio of pyroxene to :amphibole is.; somewhat variable; !wither is there any great wariation in the content of phsnoorystio plagioclase. In this way the rocks 1iffer significantly from those of other groups watch ar, lens consistent in mineralogy, texture -ani
4.00.111.4104(11..,41,,rtwoo..........411.1,1101111111.1110.1111001181111•01110.011011101•1.1..
'4,4a1e 12 10d4 o the ph oZT17.441
nornblende Augita Hyper Ore Alanoor. Ground A/t. Tot ethane ek4rVr Wes OINIMICO11011•1100.441,
175 +208a 206b 219 604 594 592 591
21.8 7.7 3.3
21.8
"19.5 18.5
10.0 10.5 6.7 9.9 0.1
10.5
e* 4.3 0.4
3.4 5.5 7.5 6.2 c, 9 rac9
2.3 2.2
21.2 45.4 38.9 27.6 45.5 30.6 30.3 37.9-
57.3 32.6 39.3 35.4 33.6 .47.2 48.8 49.4
0.9 2.3 1.0 0.6
100.0 100.0 99.8 100.0 100.1 100.0 99.9 100.0
• indicates two specimen- collected from different ;y.'ti of one exposure
• includes estimated 3* quartz 4$4* includes hornblende which has remoraed to pyroxono
and ore
ohomistry. Chemicalll the Vati desites are ci.;nsidorably lose potaosie thri those of the other pirt,men and this is a fundament:a difference between the rocks of Vatukoula and thoos of Vatia. In view of this fact, the tors andesite is cvynkAderad airplioahle to these rocks; those which are over.-satur.3ed could 50 el1oa d4cites, but since the modal per... oant:si,e of quartz is not grelter than 3%, they ,ass termed quartz AAesitcs.
?no foregoing deseription is applioable to all the luveccias, agglrates and intrusions except the DelaikoUla structure which can be represented as a normal andesite plvg, zoned with a rim of hornblende andesite and with a Imre of augite andesite. During tho period of andesite activity there is a well defined tread in terrosagnosian oryst4A1loatiOn• The early Vatia rocks contain only pyroxene; they were followed
by rooks containing hornble *de sometimes with subsidiary pyroxene but in the youngest rocks there is a reversion
s back
to augits andesie. Ns wiil, be domonstrated subseIuenty thip is due to crystallisation conditit'ate.
5 Pifteen rooks have ae ana ed by rapid ill *0
methods. Teel?* of there were from the Vatukoula volcano and three from Vatim; tho results are given in tables 13 :::,std 14. At vatukoul% the samples were selected to cover the complete mnc of rook types. intrUSIve extrusive, but the Vatia Elf:avian were taken frees intrusive bodies. Two older analyses quoted by Poye (1910 and lyeng;Ir (1949) are given for 00.1w parison in table 15.
The analyses of the rooks from the two ral differences and the soot striking of these are cm/alkali ratios and the potash/silica r.tios. Pig.
shows th-t the Vatukoula rooks ara unusually rich in pot-, sh whilst the Vatia rook contain the 1 to 1.51‘ K20 typicul of andeettes. The gruph shows th:At the rate of increane of to t .,t alkalis with silioa is the sqme in both groups. though it awe possible thlt the rote of increase of pot Ash with a little higher at Vatukoula that at Vatia. Ther' is no enrichment of sod-a, and at ekluivAlent wAluos ., , rock s
areas contain similr =punts of soda. Eowever. the di ferenoes between the suites are not
apparent if the rocks aro plotted en a Fe0440e203 I i Rs204: 0 diagram as in fig. 18 where their all lie in a broad belt; this demonstrotee the trend to hiehly alkIline conditions an well as the very slit htly higher Fe0+Fo203 than lig0 content. Pig. 13 of section 5.1 shows the position of the rookn on the Or 1Ab An diagram; this shows the very distinct trend of
Figure 17 : Graph showing the variation of total alkalis and potash
with silica.
14 Total alkalis
Potash
12
10
Alkalis
,/'/Vatukcula
//
6
4
2
/
Vatukoula
Vatic .
1 I i i i 1
49 51 53 55 59 61
Silica %
47
F (Fe203 + Fe0)
Figure 18 : Total iron - magnesia - alkali
diagram of the Vatukoula and
Vatia rocks.
0 Vatukoula rock
• Vatia rock
0508
505 0 (...532 267()
CP9 C) 456
0574 5(W)A £582 597 „
k-J 515 0260 • 604 0280
0 185
0 492
A (Na20 + K20) Ole )
.149
Tibia 13 Analms and Norio of rocks of the Vattikienpi Volcano.
260 259 280 267 456 515 574 504 508 505 492 185 102 47.31 48.08 48.32 48.49 48.78 49.11 49.34 50.59 51.78 52.28 57.47 57.81 A1203 12.52 12.90 11.64 16.06 11.82 15.43 12.83 17.98 19.38 16.18 17.88 18.73 hi203 5.48 5.87 4.92 4.49 4.30 5.04 4.73 4.71 3.11 4.59 2.36 4.74 Fe0 4.40 5.14 5.19 5.63 5.82 4.56 5.42 4.36 3.23 4.70 3.01 0.63 Xg0 9.63 7.72 12.02 6.16 13.26 5.86 8.69 3.69 2.26 3.07 2.08 1.19 Ca0 11,89 12.03 11.03 10.62 9.62 7.78 11.14 8.13 6.51 8.46 2.79 2.87 ha20 2.29 2.55 2.08 2.79 2.44 3.66 2.21 3.83 4.11 3.92 4.26 4.23 1020 3.06 2.82 1.80 2.62 1.75 4.52 2.64 3.81 4.94 4.10 7.62 6.46 H20+ 1.54 1.15 1.0 0.74 0.07 1.38 0.92 0.79 2.50 0.12 0.08 0.52 H20- 0.96 0.76 0.66 0.35 0.50 0.70 0.35 1.06 0.70 1.05 0.56 2.08
7102 0.55 0.66 0.56 0.89 0.60 0.52 0.52 0.66 0.56 0.71 0.57 0.41
P205 0.29 0.35 0.31 0.43 0.26 0.50 0.29 0.41 0.46 0.51 0.29 0.26 Mn0 0.18 0.11 0.20 0.19 0.18 0.13 0.17 0.21 0.17 0.19 0.11 0.07 CO2 0.25 0.41 0.13 0.19 Tr 0.20
100.10 130.22 100.11 )9.71 99.31 99.79 99.88 100.23 99.90 99.88 100.16 100.00 Qts 1,38 Or 18.35 16.68 10.56 15.57 10.01 26.69 15.57 2;7.24 28.91 24.46 45.04 38.36 Ao 6.81 11.53 17.29 19.91 20.44 14.67 18.34 23.58 25.68 25.68 29.87 35.63 An 14.73 15.29 17.24 23.63 16.40 12.23 17.24 20.57 20.02 14.46 6.95 12.23 Ne 6.82 5.40 1.99 8.80 4.83 4.83 3.98 3.41 Cor 0.20
go 17.40 17.86 14.96 9.78 11.40 9.51 14.62 7.08 3.13 9.78 1.39
DI ea 13.70 13.50 11.60 6.70 8.40 6.80 10.50 4.50 2.20 6,20 0.80
fs 1.72 2.51 1.72 2.51 1.98 1.85 2.77 2.11 0.84 3.11 0.53
TT_ an 11'7 fs
1.20 0.26
2.70 0.26
01 fo fa
7.28 1.02
4.06 1.02
12.04 2.24
6.02 2.45
15.40 3.26
5.46 1.63
7.84 2.24
3.22 1.02
2.38 1.43
0.98 0.61
3.08 1.84
3.00
7.89 8.58 7.19 6.50 6.26 7.19 6.73 6.73 4.41 6.73 3.48 1.16
Haan 3.84
Ilaa 1.06 1. 2 1.06 1.67 1.22 0.91 0.91 107 1.06 1.37 1.06 0.76
1.01 0.67 0.67 1.01 0.34 1.34 0.67 1.01 1.34 1.34 0.67 0.67
12° 2.50 1.91 2.04 1.09 0.57 2.68 1.77 1.85 3.20 1.17 1.44 2.60
Cal 0.60 0.90 0.30 0.50 0.50
100.29 100.23 100.07 99.43 99.54 99.76 99.50 100.11 99.93 99.93 100.06 99.83
Table 14 :
582 :402 57.13 A1101 17.93 ta203 4.24
..ie0 1.96 iid0 3.44 Ca0 743 ha.)a 3.52
4
X20 1.10 Y204 0.65 110. 1.02 TiO2 0.52 P205 0.13 Mn0 0.13 CO2 0.37
01 150 40
Analyses -01 VO
597 604 59.2 52.62 17.91 18.36 3.78 3.50 1.87 1.59 2.83 2.62' 6.91 6.99 3.70 4.30 1.10 1.41 ily 0.92 3.19 1.12 0.34 0.43 0.40 0.14 0.09 0.11 0.12
0.29
582 507 604 13.44 15.54 12.60 6.67 6.67 8.34 29.34 31.44 36.15 2).75 28.63 27.80 2.32 1,97 1.63 2.00 1.70 1.40
6.60 5.40 5.20
5.14 0.4,6 4.97 o.4$ ...0 o.76
0.91 0.=:31 ' 0.64 0.34 0.34 0.34 1.68 2.04 0.53 0.80 0.70
k i 01' ' i u.desitesi,
'tz Or Ab AB
wo Di en
fs
" fs
LTA. ilm Ap 1120 .Cal
0.111.0.1,403111011,- • .0114.011•01,..•111•••••••11,1,411111111.0.01...Mio ,• • • • ,11r. •4•11111.^.11111WONIMI.,..,••• •+00111.6.1•011114.1•01.
tomiab.*Orm,e41 111.10
Total 1).98 100.11 100.31 11.67 11.19 100.27
..0400011.110*
• wow rag*
z?lanntion of tmblee 13 and 14 i 260 011v1ne Nita bncalt Lololovu creek, # mile due
wont of Memo,. 259 s olivine audits basnll * mile due west of Nfr-omo
village. 280 t olivine augits basalt Korsboya trig. station. 267 : augite basalt - eLosa location azi (259).
olivine aucAto baoalt dyke - un-amotl tribut-vry of tile ;:latude 2510 ft.
,15 : 4a Pie augite traohyandosite dyke - u„)Jor &A6 -41.a creek, altitude 560 ft.
574 : mioromonzonio It trat4on miler, duo est of 7:kikubukubu village.
504 Na mitiu basalt - Nad.triv:Itu road. * mile above ..ilwoukuOu ridge.
508 t Traohybasdt rimdariv, u rovtd. atitlde W.) feet. 505 Truchybasalt Nadarivtu road, altitude 1000 feet. 492 1 Traohyte WaikubUkubu creek, 300 yarth upstroLum from
v111:10). 1115 i Aotite trrichyte - Kings road, * mile north of
Nalarivatu junction. 582 s "ornblende 3ndesite intrwiion westorn marisin of
Dolaikoula. 597 Ands ito ZroL olre of DolAlkoui iJtrusion. 604 $ Andesite of 7,,tubu1i truoion.
wMpilkilte
Table 15
vua 144 f We4ta 19.11aa.twalt
49.53 47.36 Si0
A1203 20.38 19.38
Fe203 4.65 3.60
Fe0 3.45 6.13
Ne0 3.05 5.31
CaO 1.17 10.54
Wa20 2.20 2,69
K20 2.44 1.74
*20+ 0.47
H2O. 0.99 2.20
TiO2 0.89 0.85
$205 0.44 0.19
Mn0 0.15 0,11
002 0.10
Others 0.30
Total 100.21 100.10
References: lyengar (1949) Foy° (1918)
int:reusing normative orthoclase. There tiwever.
insufficient panto to determine the trend of the Vatia reeks but it e: possfiblo that they may evolve tow,rde the albite corner of the tAanglot mad she %.; only a Very sural increase in
- 152
ertheilese content with evolu found in nom Vfltukoula rocke,
Mere is one other differ between. the Yatia 4,nd Vatukoula. ,14 tbL ounce= P705 content. The Vatukoul% rook.: contain frog 0.29 - 0 54 of P 205 and the average is 0.37, but the reeks of tia contain 0.13, 0.14 and 0.09 respectively. re is, therefore, sews variation in the Eitaunt of Pi0 with 140*
helm this data it is sevement that the reek suites Vatia !void Iralukoula are in general similar and differ from
eh other only in the potash and phosphorous contents. This i furter demonstrated in figs. 19 and 20 in which the norms and molecular percent Aother oxides are plotted 3,gainst silica.
There in t/o ohemicA evidence to suggest whether hest, now buried, olivine balta) were riot or
impoverished in potash. The roseate of X-ray etas fermlner elements
1.4re given in table 16. The yttrium and sire-sal= content of the rooks are low and consistent* The rubidium content, as shown in fig. 21 is, in general, direetly proportional to the amount of potaeh in the rock though a.cents of loos than 40 ppm ruqidlum were apparently indeterminate; there .ore Eye ono or two anomalous pointe. Strontium on the other bend iv v3riable and when plotted againiit clacium, fox no obvious puttern (see fig. 22). Soekolds mnd Allen (1954) doseribing the minor elemont0 of the alkali rachyte rooks of Hawaii
oth r Pacific arse statedi
e si'
"calcium decreases ate 'd function (1/3Si+K) - (CA from the extreme basie end linear ar, in calo.aIkali ign
Y'The ratio. Sr/Oa and r three sarlaz they inoros reached, when th dealing,
each series rtr.; the reason and apart crows is virtmlly ak WOri011or000
behave alike; in all late stage =woke are
of normative minerals against Figure 19 : Plot
35
30
/
/ ....-"" r.....'"
......' r V r
V
55 57 59 51 sic)2 47 49
45
silica in analysed rocks. 40
Olivine
--- Orthoclase
------ Albite
Anorthite
Diopside
V - Vatia
N,
N
V
25
Norms
20
,N .N
N,
I
5 1
'N
Vg0
- • 49 51 53 55 57 59
Fe0
Figure 20 : Graph of other oxides plotted
against silica. _ 20
—15
Other oxides
— 10 \
Silica %
Fiiire 21 : Graph showing relationchip of rubidium (ppm) to potassium (KO) .7
_ 150 7
z - 120 z z _ 90
z 60
•
1- 30
2 3 4 5 6 T 8 0 1
Rubidium (DPI)
Potassium (1(20) %
Figure 22 : Graph showing relationship of strontium (ppm) to calcium (Ca0 %).
- 1500
1200
900
strontium
(PPm) 600
300
2 4 6 8 10 12 14 1
Calcium (Ca0 %)
— 153 — !-uch a p-ttJrn however cannot be doterminea in the
rooke of Vatultoul Vatic. It is possible that the original association between o9.1cium -,nd strontium hae been complieated by the deposition in many lava* of oarbonate gangue bearing some strontium* It 14, known that the norwal strontium content of b'salts is out 500 ppm; in all the rocks) ana11me1* the quantity iv Fre:Lter than this* occasion lly to a famtor of x3.
Laile 16 : e1alkc,,JN%_Iu1152Lt,he v nik0u0 ark v:0771777eks*
4-ray -1,nalysie by A. Zmith !,ccuracy 20% t results in parts per d11i
1112 260 40 267 40 456 40 492 160 504 80 508 90 185 110 259 230 100 505 100 515 100 532 100 574 75 582 40 597 604 40
Yt Zr Nb
830
30 60
1450
40 70
650
70
1620 30 140
1170 30 70
1410 30 110
870 40 40 -
1300 . 40
1000 40
1100 40 . .
1000 - - .
570 110 .
960 - .
760 40 40
750 40
910
40
i.)eciraen localities are listed under table 13. 1001.00/0.00
004 :.
154
5.9 IVT9AntA.V.
5.9.1 GleleraA Ot,atsme,At
The obeervable sequence within the volcanic deposits of Vatukoull.-‘ is a simple rock association of olivLle basalt * tr3chybealt, trewhyoadosite and trachyte with olivine bas-.1t bei the predominant mosbers,. This suite displays cyclical repetition. In may reopeete the alkaline olivini basalt acsociatiorl of Yatakoula eXhibits mnny charactere which are similar to those of basalts of some continental and oceanic petrographic provinces; there is the s?Jas general alkaline olivine awlt - tr4chyte association.
The uain facts to he locounte4 for in petrogenesis are included in the following summarYi 1. The Vatukoula make belon6 to the alkaline rie':1t trachyte
at:moo:dation whilst those at Vatia are andefAtes, evolving (as soon in the Delalkoula intrustna) to more acid types.
2. The following are the main petrographio types representod: givino basalts 31.02 0 48%, m 10-35% Fat ealoic
plagioc14-e Ane6 AZI55, diopeidioaaugite.
40)111,040.1 810 0 50-52%, olivine 0 10-35;4 Fa, plagioolase is bytownize to olie00140, diopeidic*augitee
TrachlrantAstsiApi 402 57%* no olivine, diopoidiagite, biotite*
TrPlAYtiM , potash feldspar, plagioelase Iibradorite to oligoolaoe.
WitoikPI Si02 0 57...59%, no olivine or biotite, diopsidioi‘aui4te and bYperetheney hornblende, plagioclase is labradorits to andesine.
3. Chemically the rocks of Vatia and Vatakeula are similar, except that the reel= at Vatukoula are richer it potash and
— 155 —
phosphorus in proportion to their silica. contents. 4. Although there is e general trend for average plagioclase
to be inoreaeingly nodic along the general eerier olivine baerat trachyte, the compeeition of roue plIjoelase phenocrysts shows anomalous relation to tha chemical oomyosition of the rocks in which they occur. For example, the Vatia andesiter contain highly calcic a4ioal!tset whilst the rock is not enriched in calcium.
5. Orretallisation conditioe have caused the fort .tion of amphibole at Vaillas.
6. The andeeitee are youner thin the ban alts and trchytoos
5.9.2 NE4ure of the,arenlAapa The oompoeition of the on undifferentiated
magma appeurs to approximate to olivine basalt. The forecoing ohapters have illustrated the et.vuotur.11 evidence for, and the diminishing volume of succeedin - increlsingly alkaline ..hears of volcanicity cnd it c an only be concluded that the wtrlient meet abundant rocke are most closely related to the parant magma, To determine the approximate composition of sqch .atertal the most "normAi" olivine basalts of those. .,:lalyslod (280 7141 456) have been (in terms of major oxidos) reealealated to 104C and averaged* the result, which ean be provinionaily regardel the hypetbetioal parent imams is 6iven in table 18, (p.9e160-
The parent sigma of 'tat okoula is similar, bet eheMically and nineralOgioally to the olivine basalt of the Hawaiian volcanic province (tale 18). The main differences between the two parent magn4s lie in the greater magnesia and jotath contents of the Piji magma. It i upeculative whether he high potash oontent of the early Fiji olivine hatalt can be
indicative of the oomposition of an unoontaminn4ted and
-156 undifferentif;ted olivine basalt magma; it is most likely that the processes which produced the derivative rookt from the olivine basalt also played some part in evolving the Fiji parent magma from more usual, lees alkaic material such aa the average llawailan olivioe basalt.
5.9.3 ?victiov4 orystallisaticn and e.stmiIltIla Assuming that the volcanic rocks of koula re
genstioally related to a parent olivine basalt ma6ma, it is now possible to comider whether the traehyhasaitc, trachy—andesiter, nd trachjtes are the renult of fraotionsl orystellination of such a parent ms6mas
Bowen (1928) dismissed the frIctionll crystallisation .of olivine basalt magma to produce traohytic derivntiveet
....the traohyts nag be rearded as a derivative of the basaltic Itapat differentiation having occurred throAgh fractional en" stallisation of quite small bodies. The general tendency towards a trachytio differentiate night then be referred to a similar tendency towAxdil small dimensions of the individual bodies of hssaltio magma."
Though frotional crystallisation hi-Ast bean an important process in Uri, produotion of the derivative rooks it in unlikely that it was the sole prooeve involved. Bowen postulated the fractional orystlIin.ation. of 11cit ti give alkaline de—rivatives under what have 'leen sailed *abnormal" conditions' It is conlAdered that at VatUkoula each conditions were controlled wanly by Crystal sinking and assimilation'
Kennedy (1933) considered thqt there were two hasalt ages, the olivine b!Aralt magma (whioh yields traohytes and -phonolitets) vid the tholeiitio magma (which yields oalc alk9.1ine derivatives); he also oonriders the pyroxono to be identified with the parent tvi471s. type. Yoder and Tilley (1962) also found evidence of two types of magma with their related pyroxen. .n.
-157 Vie work of Barth in 9 respect has been disowned in 4.2, where it has been shown that both basalVirachyte !Ind andesite associations contain similar oyroxeneef thouch the Vatia rooks .loo oontlin orthopyroxene. The Vatukoula rooks belong to Konnedy's Atlantic suite and the Vatia rooks to his 2aeifio suite.
As suggested previously, differentiation by °rye t ,41 oinRin has probably been an important factor in basalt evolution and in support of thim theory the groat varition in intratellurie crystal content of phase I and II. lavas has been described. Some workers on other alkaline basalt a000ciations hrkve suggested th,.tt this process is the prime, and possibly the only cause, of evolution. MacDonald (1949a and b) in dineursi the rocks of both the Island of Hawaii and the Hawaiian Petrogrphio Province as a whole, described examples of crystal sinking in Kilauea and Mauna Kea voleanoen and showed how gravitation of variable proportions of cable plagioclase, olivine and pyroxone could produce truchytoe and trachyandealto lavan as well as the olivine rich pit rite basalts, thouh the picrite baslte of ankarositic type, he concluded, ''cannot be derived by simple enrichment of olivloo Jaw:at magma with crystals formed in another part of the - magma Chamber". As• an alternative he canoidered the augito rich basalts to be renidual from the orystollieotion of olivine basalt, as are the andositee and tradhyten; this theory will be more fully developed subsequently. MacDonald stated that crystallisation differentiation alone could not helve produced the augite rich baealto.
R.W. Le Maitre, recently working on rook o of the mid-Atlantic ridge, has also postulated crystal settling as the main °owe of basalt evolution but his analyseo and con-clusions have not yet 'seen published. Rdwards (1938b),
-151-
reporting o an olivine basalt-tr!lohyte association in the T3rtiary volennio rooks of oentral Victoria, concludeds
"The evolution of the suite has b:.:,en brought about partly by gravity differentiation (that is (1) by oinking of olivine and to lesser extent pyroxehe nd plagioclase crystals, and (2) by risllg uPsf3r4s of anorthoelase and aegirine crystals), and partly by the crystallisation of lime rich pyroxens, which removed the lime from the magma, leaving nn alkali° fluid in which the alumina was formed to Combine with soda and potash's.
In an earlier paper on three such provinces (1935),:..dwards eonsidered that the differentiation was a result of three processes - the control of pyroxene cryst;Allisation from the magma, the growth of cupolai.like extenolons above the main reservoir permitting the accumulation of alknline mAgma concentrated locrilised bodies and tile gaseous transfer of °odic minerals.
There is evidence at Vatukoul that crystal sinking ic a principle cue of evolution though other processes have also been involved i the 7Jreduction of certain basalts. In the °considerations that follow, the luthor has cloulated the composition of the material that must be sWotratted from the parent mageo, to form the derivative lava of iatukoula. ($oe table 17). The norms of e removed material consist of evacic plat diopside, hyperathene and olivine. All of these minerlas might 5o expectei to crystallise in a basaltic magma although the monoclinic pyroxene would be augite rather than diepside. The pro:,ortion of material removed from the parent maca4 is 68% to yield trehybasalt and 81% to yield trachyus. This appears reaNwOle. flowevr it is speculative whether olivine or augite could removed froa7 tne parent basalt in the amount required to produce the large volumes of trachyandevite and trcnyte.
159
Thi figures of table 17 however, show come llomvl.inse, Columns 'a' and 'b' represent the compositi. 4 of th ;! m4-:,,teriLl to be removed from the parimt VIA bafrIlt to produee two augite rioh reeks (259 and 267) of the phase succession, the forrsr 'Ing relatively alumin:4 deficient .Ind the latter alumina rich. Though lb* romervi'd plagioelse in column 'a' is more (inlet* than that rhieh hae crystallised in the rook, it le anolvaous in that it does not conform to the trend to more iodic normtive feldrparl demonstrated in columns, 'o' to 'f' of the erva table. Pic anonvily is further exemplified in column '11, where thouzh the amount of fildsdar removect. is very small, the plgioclase is Any) whilrt in the cryst:_llised rot* it is An52, These anomalies indicate thr3 othr processes were also involved in the differentiatior,
Column 'te of table 17 shows the composition of materiA. to be subtrrAted from Fawalian olivie basalt to produce the Yin parent basalt; the ern shown the removul of similr malaria to that cloulated for the production of anomalous hlt (259) from the Fiji parent, though the plagioolr,se 1.v slightly more sodic nald there is considerably less olivine. Hence there is The possibility that the Fiji rode were derived partly by aravity differentiation from a magma which was not oriflwaly alkali*.
-160-
Table 17 a
(In the ealeul,tionc o potrh used ap7 barim for determination)
81°2 49.2 Al203 9.3 Total F Fe0 7.3 w1éO 23.0
9,8 ri!„‘20 X20 0.0 ,11;4411nt removed % 60
Ab No An
ro Di en
4 [
en fal
[4 c;omfrof,,ltion br,,,T,
61 30 67 67 62 53 4 .11i6;iool0341 '1"'
Tqble shown, oonTeeltion of wateri,-1. to be removed tram parent b . o derive#s
a i olivine (259) b aazalt (267; o a traftlasnli (504) d i raohybemat (508) o a biotite traoLyte (492) f 3 r ieremonznite (574)
Couliloeltion of n4terill to '4e adde3 o
averags basalt to VW. average Hawaiian olivine basalt. Ave' of be.i.ltz (200) and (410, (1,1Tr=iMer
ae the parent olivine basalt. pecimen numbers refer to rooks listed in table 14 Oat, on Howell.= rockF fruL, 1.949b)
011010111010.111•00. rmoo....4.4011010.41.111.04110011...00,SA101.
NO1413
12.1 7.3 4.7 ..).7 11.5 8.9 23.1 0.6 0.6 2.0 1.1 5.1
18.9 3.3 13.9 15.6 20.3 15.8 27.5 12.4 25.6 24.5 24.0 21.0 18.1 10.3
fi 9.1 194 17.2 16.6 13.7 13.0 4.9
2.1. 4.1 5.2 5.4 5.6 3.4 5.3 $04 6.8 2.0 7.4
01 Pril
*.O 32.1 25.5 22.8 20.7 27.2 7.3 7.1 7.5 8.4 7.8 5.7 6.6 7.6
abodefgh 41.4 46.2 48.5 49.2 47.9 51.2 51.2
2. 6.2 1.5 10.1 9.4 /4.6 12.4 7.6 8.7 8.5 8.8 7.9 12.2 8.0 26.0 21.5 11.7 16.2 20.7 8.8 13.3 13.0 14.6 14.7 14.2 11.9 10.5 11.9 1.0 0.7 1.1 1.6 2,2 2.7 2.3 0,0 0.0 0.0 0.0 0.0 0.0 1.9 60 66 73 81 58
161
Table 18 shown the analyses (to 100%) of the major oxides of the Hawaiian lnd Fiji olivine basalt lavas. For
.111110.0,1411.**00.
81.0 2
Ta:1,La 18
a 51,1
$ te,rus
b 51.3
a 51.4
d 51.2
£1203 13.9 13.8 17.1 11. 700 10.8 8.2 8.1 8.0 MO 10.2 8.2 6.6 13.3 Ca° 10.9 12.8 11.2 11.9 giap0 8.6 2.8 3.0 2.3 20 0.6 2.9 2.8 1,9
a : .ver--4e Hacailan olivine basalt b : aajta rich basalt (259) o . . aa:ite rich basalt (267) 4 1 hypothetioal Fiji part (average of 280 ,!k
456)
equivalent 010 0 retie and compared with the 11awaii haselt, the Vatukoul A shows
1. An increase in alumina the extreme ease of (267). 2. A general decrease in irvn. 3. A higher 0a0/24g0 ratio su4,Lgeating either 400retss in
AgO.Fe0 ( t possibly in 'b' and '0) or an increase in Ca0 (as in 041) or both.
4. An increase in the K20 content. This is in general agreement with Am:I:Donald's I indirs though tha adkaramitic lavas of Hawaii ara conAderably more under-satur4ted than those in Fiji. Nio deerease in the Be0 and MgO contents of 'b' and. 00 ma.y be dike to ontraotion, pontibly by sinking, of olivine ogyetals, thou6h thsr is no propor-tionate decrease in th.: ocloilen sontent to &,,Ilote pyroxene
62
in facet there is an env/ that there was any differe nlia sialn4 of °living,
and pyroa ono a if ,:s appeArs ee More uny gravity dif orentiation t nil,the OW toontent of the magma must hsve been kept to a rIcirly constant level by some precwo of 1,4similation or mi ixg or mineral or sl migmtion. Such process*a could account for the olivine 4.214 and rich pyroxen* content of rooks (259) amd (267). is shown previously, the alkali/Silica proportions in (259) can be obtained by rinking plagloolase crystals of An, but in (267) mmy clase that sunk was of the Comosition M. Thug (267) indicates 'that, whilst there might have been some sinking of plagioclase with olivine and pyroxene, there wan also a re-inforoseent of the anorthite content of the residuum.
The procipitatio from the magma of olivine in ex° ,s of its stoichiometric proportion ties the offoot of forming lava %soaring normative, and after onttleient pre capitation, modal quarts; oinking of *elate plagioclase automatically enrichep the residuum in alkalis and eventually, the lava han a composition of the trachyte. This derivation of the lava series is paralleled by an evolution in the forro-magnosian mineralr, such that initial rocks bear olivine and pyroxine, later reek, have only pyroxons and the latest hove biotite and pyroxene, the amounts of pyroxeno dishing with time. The distribution of olivine and pyroxeno have previously been described, but a few comments may b• made on the appearance of biotite. The crystallisation of this mineral app.:4re to ae duo solely to the inerwleing proportion of (140.700)/k20; in the rooks analysed, biotite only mums *hen the potash content is Ude* that of the feml.c oxides. It is possible Writ biotite formed under coaditions of high water vapour prof-Inure and the subsequent resorption of 4 few crystals may
13
in di such an on7;ini heveverp the majori r. of 0 _Lre very fresh and there is no firm evidence to suggest abnormal. eonditions of orystallicmtion causing procipits ion of biotite in preference to other forromagn cianP,
gvidence has o an presented to chow that the roam of Vatukoula probably ow a2,4 not orii4nat by fferentiation alone and that nome prooeses of tion of wall rook 17 a basaltic magutt is neouotmry
to A t for the range of composition obeerved. 'Direct avidity o of oontamination of the nova involved in Iackin. 4-enolith of baoame6t rocks *Lich wiiht indicate materirAl axisimilated by the lavas were not found.
There are neveva processes vh r by a magma tiny be enriched in specific oomponents, rineous transfer wan Wets. oiderod by Awards (1935) to be iwtrumontal in causing flotation of anorthoolase and ao4irino crystals in some Of his lavas ;end he concluded that taken in eenjunition Ida errs's/ slaking, it could produce iiaorsaturated reeks Maellemeld concluded that whilst there may have been sem* enrichment of lavae in iron oxide, and alkalis by gaseous transfer, there wao no definite evidence to suggest that a prowls 1-4v1 ever ocaurredl and indeed he suggested that the Hawaiian rooks could be derived is othe sire. veve he did not dicou ;e vasiculation of the rocks as did 3010n (1928
"If this bubbling of gases is to be the prineipal f otos in 16neous differentiation it must occur in deep
ated bodies. Vesieulation should be a °omen pos. ditiono perhaps the Dorsal condition, ofdeop—eented
see. It nightt of course, be trAnsientt but deep bodies have sent 471se into surrounding rooky at all stages of their careen and these dykes are often at least at their mar a sphanitio to Glassy. have thus orystqlli tepidly that if the were voeieular the iti would be vemicular elect Yin 11001011ar dj ss tr47ke selvedges cre exceeding/7
164 rare and are apv)arently always found in association with flows which indicate for them a shallow depth."
:3owen further discunsed transfer of volatilee by diffusion.. At Yatukoula Vseicies aro atment In the ma3or intrueions !And are only ocoasionally found in the minor ones. Further it is possible that lineations of veeicles in some dykes may not represent ori6inal volatiles but pockets Of air trapped in. the laves duriig intrusion. There is a eeimplete cyet2k of any deposit which may result from volatile compossuteAfor the aeolites awl the cly mineral linings of some amygdaless The continued explosive nature of the volcanism may ba attributed to accumulation of volatiles beneath the crust, but a ooms parAtively small proportion of the precluots of the explosions are voeicular. There is therefore mo suggeetion that gas 0 streaming has played any important role in Cle difforentiation of the basalt or in determining the type of voloninIt.
The Ca° content of faolia;:i can also be inereoxed by limestone ausimilation (Daly 1933). Assimilation of thin type usually produces a desilication and an increase in lime and iron content in the dericallvo rocks which will be lees femic than the original basalt. This is domonetrahtd in (259) and (267) oolumnn 'be ind lot of table 17 = bull is less obvious in OOlumn 'AP since the rook oontaine a larger pro—portion of olivine. The 4oellication is not obviously expresescl in the chomiotry, but in the decreasing amounts of normative undereaturAtecl miner:As.
Calc-rich rocks ars known in the aid and lower Tertiary rockp of Fiji and it is possible that a conduit or chamber of olivine basalt magma ouAing into those eedimente may have Asnimilated suffioient quantities to cause crystalli-sation of the aueito rich olivine bealte.
Two other processes could account for all the
fwvturcs of ;he Vatukonla rocks Tin firet Jo corxt:421n3tien of olivine basalt magma by material whieh may be rich in alluais. A large body of magma contaleinated in this wow could probably pro due all the derivative lavas and would explain the abnormally wide na,nge of plagioelase pheloOmpte in Many rock. Daly and MacDonald have e•ach examined the effects of selective remeltlna of eLluntry rock in the pre*. duction of the alkaline rocks. This reversal of fractional crystallisation could alrA, produce enrichment of a at in alkali oontent. However, there ir no tiro evidence to chow ttv4 such precenses affected the parent roek at Vatukoula.
In summary the data up:oars to Indicate that the magma 0. Itself quite rich in alkali* ... might have
produced the ijvative y fractionli crystvalimation aided by grvity differentiation and possibly by country reek assimilation. The production of be ?iji pit rAla trim IS ao normal alrai poor olivine Iptscat morn my aloe beim taken place by these procePom or possibly by omana contaminep. non or mixing.
?S$ZOenet1. of the Vatia smdesitess The anderites of Vatia pose a different problem
Since there are no definite Lndieationt of the type of mama from which they hay crystallised or been derived. The preblem Of the andesites w fully diecusped by lywar (1949) who given a full rerun of work on the subject. There ie no evidence that the andoeites have 1)een derived from a thole/la* magma; it is unlikely that a major eruption of olivine basalt and its alkaline derivativec sholid ':)e followed almost LMP-diately, in the 844.me area, by eruption of the derivatives of a different ban,it magma; further, such a tholeiltie not exposed. lio%Aarver, the very widespntad and abundant
antiesites are not oloo.yr ano niad by tho
they are alleged to form. The eruption of lard quantities of andeeite bee
been associated with orozeny and crustal deformation by eeee writoro. Other authors have postulated or000nic movements to explain the remelting and mixing of eruetni rocks to produee andesitoc. Anorther theory oonoerning the origin of andeeites we!: put forward by Holmes (1932) who conaidered that they Weeelt from an aysooiation of granite and basalt with sediment. Kennedy did not coneider that granite rd any prime place in the oryetallisation of andeoltes but he thought that bending of the oronitic layer in erogenic zono o will cause an under" lying "tholeliticE shell. to melt and produce anderitee 4nd rhyolites.
such evidence all is tovailalo At Vatia is suggestive of mixing of mooriolo of acid and bosic composition* probably with oadimentary rooks a2 well. liouldaro of granodlorite bare been found in pyrocloatio rocke in many parts of Vatial these amphibole — plagioclaee orthoclase rooks are most common ac xenolith° in the Melaikoula intrueion which io oom000ed of the most eel* lava in Vetia. There are boulders of olivine basalt — apparently oquivolent to the basalt at Vatukoula — whico have been found near this intrusion; these may have been torn fro_ bedrock or in a few inotances trqnsporteii from the olivine barnit oountry to the south* but it is not impossible that esus may represent clots of crystallised olivine Salt trapped in the andesita lav4 and eventually deposited !owns** the pyroolastic rocks. Ooze of the ondesitee in the road sal on the western side of Dslaikoula also contain mall but quite numerous xenoliths of black rock. Macropoopically their ainsralogy is indeterminate* but their edges are nomowhat sorrod144 In thin seotioo toey are olmoot Opaque, crypt°.
— 167
Seretalline and contain small blobs nnd gre lee of ore material; in appearance they cen be likened to fine-grained black elltutonee. There ie 0040 evidence therefore that basalt, granedierita and peltic eedimente have been involved in the production of the andesitese
It is possible that the fusion of the three rook types wee caueed by the ration of hot basalt awe aseimilating granodiorite bodies which hwt been emplaced ie politico sediment; similar emplacemente in caste sediments have been described from the waila Lower Tertiary rooks of Viti Levu. It ie oivo possible that the eaid component wr. mama originating in the granitic lever of the cruet, thouet if this wae eo there Were too periods of assimilation, the fire or the basalt and granodiorite mixed and the second where the oentemineted meow assimilated the mantle Sediments. Since there is little superheat in a basaltic enema the remelting of bodies of emplaced grenodiorite is rather unlikely and the euggertion of eaeze mixing is more feasible. ieeneer (1949) rusgeoted that mane of the Fiji andisitee were due to fusion caused by a granitic magma but the volcaniciel at Vatukoula show* that there wag an independently existing basalt magma* Two more pieces of evident:2 nee •44D proposed, in favour of aesimilation. Holmes (1932) coneldered that the high content of amoo and 81.0 in andeeiten and deeitee is indicative of aenimilation of shales on a oeleiderable scale; he also thought that a cheruoterietio ass of andesites with rhombic pyroxenes is also indicative of assimilation of sediments. Hyperethene is preeset to a small extent in sums of the Vatia rooks. The Sri content of the andesitea i.e 750 . 900 spa; ac shown earlier the content of the besalte ie vy hieh probably due to the effeete of miterelieation, but if the normal avert-lee of about 500 ppm for baealts is acoaptede the high values
168
recorded in the andesitee qre appr °Fted. lyrng%r agreed with the idea of rook mixing, mg be
allied the whole proc,,-n. to a period of mountaiy1 buUdftigt "In the opinion of the writer, during the °regents
periods, the gr4nitio material which also includes *be edimentary material in !evereil instances is remelted
smd mixes, with the melted tholetitic material giving rise to ms.terials of dioritio, granodieritio and andssitic composition."
The Vatia andesites are conniderably li.tor than the early Tertiary °roomy noted from the rocks or tai and it is possible that the acid materiul involved In the mixing was a body of gromitis. magma which had, at leoyth, remained molten since the °regent
The Vatiu rooks display a trend from intermediate to very slit-it y oversaturted Material; it in not known to what extent such evolution. is controlled 4 the proportions of the mixed materials or by other evolutioasry processes, crystal sinking, gaseous trtnsfor etc. As ieMonstruted earliert crystal einkinz and mineral. segregtion by convection currents during intrusion :my have beer responsible for the zonal arrangement of the Delaikoula intrusion and the evolution of tho entire sequenee may be controlled in a similar manner; on to othor hand the xenoliths of odiorite in the Deleikoula ihtrusion :Guy be indicative of a larger proportion of acid, mkwterial in the mix during the later stage of volennioi*y than in the earlier ones. The question cannot be satisfactorily resolved.
Both xenoliths of gran-. and ler widest*. beer hi'eolei this Ice.a not apper to indicate selective remelting.
Xenoliths of granodiorite do not show preferential solution of the eitlio over the femio minerals and there are also considerable physical difference between the amphiboles of the two rooks. In the acid rook the amphiboles are Invariably
quite large and though Oohed they ar* better developed fL,,,.a
in the andesitem; amphibole in the andosite!7, however, usw,aly has olee,r fresh unstched fum.
The conditions of horrblondo crystsllimtion hove been doscribed. There can be little doubt that amphibole foraod in a maepa which was cooling am a result •of heat lot ay arsimilation of country rock, and that when the pressure WAR intermittently released and the hornblende moderates eruptedl they ra-ored to pyroxims and t/to. At the beginnitc of the eruption the basolte underwent oomparatiVely little 'dicing and produced the lower Yatta rocks; later, with
mere mixing the inerwming t7,,perature and preentre *Mired orystalltsq,tion of hornblende, and eventually, when pressure was released and volcanism was dying, there was a return to pyroxens orystollisation. Unlike the biotite of the Vatukoula rooics, hornblende is an indicator of erY stall/soften oonditions and not nacessArily of magma cos It has be,A. rioted that the umphisole at V.;,tie is simllur to they« of the alkaline afinociations of oceanic islands.
finally, it in rsleva,it to re—examine 04 Pp, °entente of the lavaA in the alldeltes it in ahem* 0.150 and in the basalt about 0.40%, The andasttes aloe contain siihtly less TtO and ka20 than the banalta. The diminution of these elements is probably duo to the dilution of the basalt by gramodiorita and modiaant, thouzb since the 220, sentent of Piji gronodiorites is shout 0.1V and %Termini;
a Arpothstioal andesite famed of squal quantities of basalt
and granodicrita, there emy be a suggestion that the tea is enriched in 405 and that any liaostone added to she INE4119. at Vtukoula mf4, live Allen phosphatic. Alternatively awing low phosphatic seditaent, tho mix uAy oe cowposed of about one poxt buivAt, one iyIrt politic seditlant t,tyo parts gr4no—diorite.
ocoale ko of other RA40 Part 6.
- 170 - The Vati. rooks -cfo believed therefore to reprerent
asolmilation of quantities of granodiorite nd politic sediments by a banalt ma6ma whLoi, out probably he *voted with that material which proiuceI the Vatukoulr, rooks.
Geologists havqA ,;roupod the volcanic rooks of the Paoifio noean ara into two dictinet petrophic provinoost the basalt-andosite-daoite-rhyolite ancociatio of the Paeifto mrtreln, olio the alkaline olivine baaalt amsoarits traohyte phonolite association of the islo.ntis lying with the ?acifio barin. The boundary between these two provinces, commonly referred to an the andeuite line, marks a zone of petrological and strueturA trvaleition betwee t:1 the two provinces. The rooko of the 7awaiian Te1f41.de form a. classloal example of the olivine basalt is. trachyte mosoain.tion with olivihe bsalt predooinating and representing the -Arent raitiogr4 of the Hawaiian lavw,, These rock' gro.de into mafia olivine riot oceanitec or augite rich ankaramitev and into ltws abusIdant flown of mugearite (oligoclase anderite and aadosine andewite) and tmailYtei Zrtaller amounts of "nephe/ino basalto,
haralt" rind "nepheline basanito" Ptre found in some tvf the icl
Average obemieal annlysee of than principal rock typer compor to both Hawaii and Fiji are .4ven it table 19.
It is interestin6 to note thaL the parunt ma:4mm show mo,.ny
simileritieF: in both provinces but Lere in a wider variety
of difforentiates in taii. Thal 311mli-line index of the H%walitn roc lce is in the vicinity of 54 (gacL)onaid 1949b) plIcite; the province ao a whole in the alkali-cola* series, The alkali lino index for the iiji rookp ie aoout 55.
Table 19
a b 0 d a f lAon 46.35: 49.76 51.35 51.58 62.77 59.35 Al 0 ' 2 3 13.18 11.87 16.34 18.24 17.20 19.23 Fe203 2.35 4.66 4.64 4.77 3.14 4.85 Fe0 9,08 6.02 6,10 4.42 1.30 0.64 40 9.72 12.79 3.73 3.75 0.69 1.21 Ca0 X0.34 10.46 6.61 8.25 1.43 2.97 Na.0 2.42 2.27 5.01 3.38 6.74 4.33 Y 0 0.58 1.81 1.94 3.87 4.11 6.51 TO 2.77 0.55 2.74 0.66 0.53 0.41 T; 2- 5 . ' 0.34 0.28 1.00 0.41 0.21 0.26
Yri0 0.14 0.19 0420 0.21 0.19 0.07
a : Averge Hawaiian basaltC..ki.aaDolvad 19494 : Averaige VatukoulA olivine bacalt
Oligoolne ain4esi 4 (Hawaii) aver* of 11 alyeem 4 : Traohybasalt. analysis 504
Hawaiinxi trac4ta aversgs of tour an1yees f Aotite traoilyte (185) VetukolAig.
raknoftosaawianot Vicssamel.0.
analyseo show thlt where th later differentiates of the Hawaiian basalt becomo doiinnt1y the Fiji oner became more potansio. ran of the Hawaiian rock contain nepheline in the norm If it is absent in the mods.
3oth as oho eycle of differentiation with sequences of 4asoltic eruption l/owed by trAchytee and a return to basic conditions; tha volume of thl tme toe in always coacidevoly loos thsn thqt of the proceedIng basalt eruption. AacDonald danoriows flows of tra4hyte. it at
172
Vatuicoala t to i u 11j an in k exceptions to this rule, 4 in few Aotite t lows and sons eollopsed *ewe Ill rocks. UseZona L,d r cord up to 15$( of nodal quarts insonet Ichytes which mLy oleo tsar hornblende or seveml boles, come of them so is, have boon recorded fromftaali o they are abeont from the Vatukoula rlit;e*
The Marquesas Islands lio in longitude latitude 10°S; they consist of niue islands arranged in three rouos* oroiz (1928, 1931 and 1933), Chubb (1930) and
3arth (1' 31) have each descried some aspect of the Teology Chubb stateds
*rho predominant rock types throughout the group are labradorite basalts onth phonoorysto of augite ;Ind oltVies, and trachylaasalte, the latter including the andeOlne basalts and andesine andositoe of Lacroix* These rocks do not oont711 any isportsat_porcentag e of virtual free silica, or of virtual nophslines.,........ in ,..111 soda predominates slightly over potash***.*****
?roz other alrellm of the giono he described later andeeine andeeitees sIbitaplelisseleso trachytes and finally sanidine traohyt***
*So far as evideneo is available it would appear that eruption of the different types always praeeeded in the order of decreasing 'l e i of tires
Re ale° desori.)ed final product* which he considered to '0444; ho not*d that sow trathytes contA.ned 6% moC,1 salon, that a bananitoid contained $ nodal nepheline and that the Ua.?ou phonolite eontained SO modal nophelinos This latter rock vma graphically domoribled by Lacroix (1933)1
periphory of ths Leland ifs composed of basaltie volcanic 'oaks de fiOient in silica and tsphritoids, Chemically %separable with the Tahltitee from Tahiti and labradoric basalts*
*From this barn* of black rook rive phonolitis *rests consisting of five sharp peaks. Thee* ltd
73— heatre* at wh-dh be ns a narrow v
'Inyon doesending into the sea in a wee direction. A very large number of sample. have 4een. colleeted in this rogion* all son, ist of ovapaot leek green, fah or- yellow phonolitos resembling flints lith the naked eye one oan hardly distitiguish anything but crystals of sanidine* ono or two millimstres in sine* and omallor crystal* Of nee line with a vitreous or Very bright oily lustre. Under the mierosoope o= be seen trs4os of hornblende in p -lees of resorption =d 7 to 12% of as rive or
rine mmits of an extremely warted structure* nsedlee or palms scattered or distributed in. parallel band. or pockets. Another tyo *doh iv homogenous or appsars in the form of igneous br000las* is poor in nepheline* but contains a collsiderable number of t peeeh*dress of leucite* spigenised with **ghee. and
Th sums zeolite, assooiatod with ohri atianito sometimes borders small cavities."
Ssrlier (1931) ACTOIX oonolmled that the phonolite sem =Oh later than the basnits; he naiad the lath of erosion and dedusod recent volcanism* the traohytio eruptions posoibly ieine eilian in type eines the phonolite foru alguillos. very similar to the spine of N3.M.
Howevor* Lacroix and Chubb differed in their conclusions in one important respect. 3oth recognibed the basalt-trachyte 3ssociation, but whereas Chubb elon w ldarmii the phonolite to ese a final stage in the basalt evolution, Laoroix thought that Ua-Pou was the result of a dietinot eruption not connected with the toasalt activity and that it was comparable try the rooks of and Moorea, He pointed out that the depth of water separating Ua-Pou from the other islands Is 20-00 metres deep. Analyses of rooks from the Marquesas are given in table 204, 'ram the analyses quoted in tPie table and Others published by Lacroix there can be little doubt that the basalt on Ua-Pou in related to that on other island, of the group. The Ua-4?ou phonolite (column 'o* in table 20) show*, an extreme enrichment in soda A3Qd lens pronounced increase
SiO2 Al203 Pe f03
1W) ONO
y 4,
1102 P205 P205 knO
a. 1
40,
8.4 4.)2, "33
1. 1.10 1.9 2.4 ).3 0.2 0.19
b.
48.36 18.00 4.33 5.40 3.60 7.80 3.76 2.76 0.96 0.81 2.80 0.67 0.26
c.
61.94 19.12 2.03 1.25 0.19 1.38 5.99 6.16 0.81 0.59 0.59 049 0.20
d.
46.52 16.00 5.56 4.20 6.04 10.40 2.45 2.45 1.39 1.12 2.78 0.50 0.23
e. 54.74 22.01 2.72 1.26 0.11 1.62 9.76 5.53 0.91 0.14 0.41 0.11 0.19
* WIwcit laoradorique. Nuku Hiv Reference Lacroix (1925)
b andeeite .;ndtosinique, 11$
biotite trtohyte, N N
basalt, Usr.P013, a t phonoll$0# N N
in polar ; in other respeets it diffium only a little from the trachyte and can probably be considered a late Ot10 derivative or the basalt.
Barth (1931) c;yoside rd that leusite ens not normally present in Pacific lavas but Lacroix (1931) found both nepheline and leucite in the UsmPou phonolite. though he noted tams tendency of leucite to be converted to orthoclase assi nephsline. Soma of the features of the mineralogy of the MarquesnsI deserit.led by Barth. have previously been discussed.
The late stage rooks of the Marouteee eontain large
175-
amounts of potash (6f) by co-1..priso-zi with the Fiji traebytee which contain about 58% S102 and K20. However the flarquesae rocks contain more soda than potash rmd in the latest deri... vatives the total alkali content ie comparable to that of 7ome of the lava of the Society Irlands and is much greater than at Vatukoula (*arqueeas 15.53%4 Vatukoula 11.88%). These high alkaline oontents are indicated by modal and normative leucite and nepheline in the Uarquenan and normative azd very rarely modal nepheline
at.
Vatukoula. There h not been any work done on trio peerogeneele
of the Marquesai lava, but Chubb recognised an interesting pattern of volcanicity. He noted period of quiet effusive activity followed by violent explosions and concluded by minor intrusion.
"It is not suutented that oath of the three 2haseil took place simultaneously in all parts of the group or that the third phase was not initiated before Vie close of the second or even the first phase. The second and third phases represent the expiring struggles of the voloano prior to extinctions*
The theory of gaseous transfer has already been disowned and rejected* but Chubbin implication of differentiation associated wii exploeiVe activity in different places at different time,
euggent that in this area at least, the volatile colindonento have played some part in mamma evolution.
One rock, described by Barth from HiVa Oa is apparently anomalous to the alkaline banalt-trAthyte suitel Barth called it a rhyolite* but AseDenald (1)49) mammas that it may be a quartz trachyte. Both have allied it to similar rocks in Samoa. The nbrms and mode of this reek, which are tabulated below, are oomparable to those of Vatukoula tm.eki-te (185), except that the rhyolite contains more normative quartz; otherwise the rooks differ only in the proportion of potash feldspar to plagioclase. In spite of Barth's nomenclature
e
176 -
Nom G ;3 14% guar , 12 Q 24.4 % Orthoclase 25 Or 25.6 Plagioclase 40 Ab 35.6 aiotite 2 An 2.5 Corundum 3 C 5.6 Ore 3 Hy 1.0 Apatite 0.3 11 1.4 41rcon 1.1 Km 1.5
Tn 1.8 Ap 0.3 th
(1931
the rhyolite m.t.n probly be considered as an alkalis*
Work on the geology of the 3ooletir Islands 4$$W re S.) by Williams (1933). Lacroix (1928), Mareball 15) and Iddings and Morley (1915 and 1918) has theien that in this alkaline suite the early differentiates cont An much more soda than potash, the101 potash beeomes dominant in those lavas which contain about 12%. total alkalis. Again, olivine basalt 3.e considered to he the parent lava type; williams,eonsidering trip rocks of the inland of rahlti. stateet
"All who have studied the lwas of these two cones age aF;reeleth;_t the dominant types are hus-Altio. or we 0=441/ '3asanitos and tiasanitoide with subordinate ssikereadtee and oc,;,.anites. Though the volume of these beeLe levee has not been determined. it seems safe to 'predict that they constitute no lose than 95';', of the 'Ones. Accompanying these rooke -,nd without doubt differentiates from them, °°,zse the tahititest trA3hytes and phonolitee, which appear, from this prAiminary survey, to be moot plentiful on the minor cone of la "
lutonio reeks' Amite, nepheline sienite, aspheline monsonite, eesexite and peridotitecare recorded from Tahiti
- I
emit lohlialos* whioh bear hawse t e diti n to foldspathol4e haws not been recorded fro any othe AK:ktine suite; leucite in apparently not present* h ftimilar soca Lion oecure on tae neicAbouring island i400rca wi e parent Isse.It ovolving to product Isltrm-basic And tra-:t pee; the limootice of Bonito was described by Iddin, e, Mo ley (191$).
Willie." Stately that biotite is entremoly amenget the traohytea 4nd p4onolites** Re doe, however
record ► fee wmaLte wiaoks oontain biotite' or phlogopito* and to whitish he tentatIoly amoribod the name trlohyL.meAlt. AU the rooks of Ti 4i contain either frot or virtual nopheline; Tahiti is included in the nepholino free none of Lacroix though* in isontramt, the other islands of the group belong to the mined or nephelino free zones* Analyses of the Tahitian Mks axe given in table 21.
Dismuosinz the origin of t4 Man lava". Willi 'totems
adt nom rem the itch osoon
, it ill push matorial
6 frost beneath 4 orysLallime ormsts•110-4.4,4,4,1,0*
"To what **tont the formation of high primmer. lime riot silicates ma$ have boon setivo beeeoth the folded mountain ranges of the oireum.Pacific and bow mph a procosa night load to 4 migration of aitalis into such robion. of oszpar,'4ivo sta4ility 40 the 'waft. floor is a natter for ur• spooulation.
"It is however dittfieplA 1, understand mon Ina() yet are *Aka ataains 4110i from CO3 aadooltic 4-Lsaill OF shi WNW* of differentiation... If the !Amory magma mere aft alkali basalt* She diffiou ty of explaining the promos of still more baste imt yet diatinctty mlwaltips derivatives woutd bo uoh reduced**
*A' far se oan be judg tha,e that the alkaline roar, of 7nhiti absorption of ealeareoun material* floor be littoral gull osamasiegg difficult to ocnoolve the nelhet Chore °madinflows* the baso.ltio magma ris
a s average of 4 ankarianites b s labrAorite undeni%e of
a
s ehoehonite
average of 2 phonolites of Tahiti (Lacroix biotite nepheline eyenite, Tahiti (Lacroix oqolitio traoAyte, Moorea (Iddinge
Morley (Lacroix
of Tahiti %hiti
(Lacroix 190) (Lacroix 190)
190) 1910) and 1918) 1928)
178
fable 21 a
a1. ser of ro s of Tahiti •and Room'. b is
'74°2 43.25 47.62 55.47 52.25 57.61 56.42 A1 0 2 3 8.33 17.31 19.00 18.70 16.47 17.52 f+e203 4.90 6.18 3.22 2.55 2.32 3.52 P.O 7.58 5.33 2.22 3.69 4.73 5.07
9.02 3.51 1.68 1,78 0.86 0.72 040 12.36 8.86 3.71 3.95 2.61 3.38 a20 1.36 3.31 7.80 5.10 5.06 4.58
K20 0.55 1.72 4.87 6.62 5.97 4.81 H20+ 0.94 1.43 1.39
20- 0.97 0.35 1.00 TiO2 2.27 3.40 1.34 2.29 0.51 1.04 P 0 2 5 0.25 0.46 0.45 0.20 0.42 0.31 Nn 0.13 0.12 0.24 2.75 1.39 D.
aaa-aaaakaa
In the oiroum-acific are the alkalino rooks are disseminAtod in great tilickaesees of oaao alkaline andenite and,volumetrioally, make of the basalk-trachyte are almost iTtnignific,.xnt. from the lavn,s of Central Victoria, Australia, ,d ,Ards (19381p) min desori4ed an Older and, a Newer Volo4nic Seriati: the Older eerier in Oliocene to eidsidocene in age and the Youn6er :•erien is Pliocona- Reoent. The rook types deeeriated. by Edwards are shown in table 22.
- 179 -
atollroto.
Table 22 1 C, non of rook teot 2rj lkalini ass©e 4zt Ohs. Reference: Edwards
Victoria Older Newer
Solvebergita +? soda rhyolitq
(1935)
Otago Ka , , '1°)
Traohyte +? + + Phonolite 4? + Trachyphonolit..k + + Trachyandesita + +? O1.ivire-trac4yte Traehybaealt Oligoolase eaw4it Andeeine basalt 1 + Labradorite basalt Liaburgite 3 Olivine nepholinite lasenite
+1
Crimemite aftemilP11101111•000.6
This table, which includeo information on the Otago and Kerguelan provinces to ")er discussed later, indictes that VI, only rock types common to Olo four suites ,a‘e labradorite-basalt aaJ limburgite, though trachyealt and tm,chyte may be present in all the regions. Analyses of lava s frqD tfe tustrali:m provinca are riven in table 23.
The Australian labradorite basalt contains 8f Ug0 and the Vatukoule materi'a 10-11A there is Ilea lees Ca0 in Australia and the maximum content in the limburgites in 11% and in the Fiji basalt awlat 131. Sdwarde quotes hiWaier percen ages of alumina in hi rooks than are known at Vatukoula.
Again the potach content of the rooks it high and increases markedly with evolution; an in the rocki, of the Marquesas, the nod-. content is always slightly grater than
-160-
1- 3 20
Table 23 : of 1.1kaline rooke r v. Reference 24wards (1938b)
a a
4.86 11.15 51.55 57.77 66.46
14.35 16.06 16.61 18.77 17,05 11 2.03 4.21 2.82 5.61 1.68 2.54 FeO 7.02 3.48 5.59 4.59 1.48 g0 3.25 5.29 2.62 0.76 0.1 Ca0 5.45 8.05 4.84 2.64 0.67 Na20 2.80 3.16 3.94 5.38 6.13 Y 0 1.23 2.07 3.45 5.21 4.30 CO 2 0.04 0.79 TiO 2 1.62 1.99 1.81 0.55 0,20 2-20
5 0.38 0.46 1.11 0.39 0.18 0.27 0.27 0.20
a verge of 16 labradorite : Ii 12 andeeine basalts
d 3 oligeolase traohybawats (ftw,oedonitee) 4 traehytes (Trentham dietric.t)
e Ot, 3 solvebergites.
v4;;;:i ,..;;1=tytt or potash. Ards considered that mukt4 of the potash is in volid solutiw in plagioclasal inoresingly co plaijoclaes becoreo more .o‘lic.; he illustrated CAB point with the following !tears* whio% represent partial analyses on hand picked samples a? phenooryste of folds?4re of the earlier period of Mower Ilasalt eruptions. Whoa plotted on the OrisAbtAn diagram, these faldsps.m reprenent 4 trend not diseimilA04 to the Fiji material except that in the late stage rooks the Or ooaponent is not no strong.
1 2 3 4 7 8 Bap 5.39 5.78 6.49 6.02 6,03 5.29 5.21 5.03 4,81 410 6.53 6.10 4.29 3.63 1.21 0,30 0.43 0.96
40iocu1er Poreent,%gell
Ab An
39 :70
11
39 49 12
36
J
25 51 24
22 51 27
7 46 47
5 44
3 43 54
6 41 33
1.13 33114 tO the concluml,,m thats "Precipitation of olivine, Lime-pyroxeno ;And
pleat:J*1am, bas Net Wee he polo ri.otor ln the derivation of the tesehrtitl types. There ham !usen A reL400317 imay elystailisltion of anorthoolatm and aeigiViMee ansethol,:,:mo ham Voided ti float and ;4eenennIate in the upper regionm of the MMONS0 but 11,,w iu pari boon eitr urried down int* mews ,)Amie layer* ef Magna, such afA Limbareitep sima. agents as sometion ourrents, or ham boon oramOli up 47 the more basics JM LA sons waif."
kIkaline rooks in New 4ealaad cosUr on the Otago peninsula on the eAmtern °east of ths south Island, They woos deseriOed by VArshill (1906), eseMpeeei by ldvarde (1935) to the Vietevia r.nd Kerguelen multem, sad smipmeites from 'Vie ampoclation nave 40en dellori'led by Tieason and Turner (1940). Subseluentl, (1)61) therm are reforenoes to a manuscript by 3enston on tam petrology of the Otago penineulew Marshall deecribed maalts, an4Oeibee, trAonytes and phonolites, many of theg nephsliae Wiring; he coAmidere4 that t;,„,ere win a griAuAl merime from trachytos to trachyteid photolitee and mudgeeted th:4 tn traohydolorites und .!Jesanitem form connecting linkz between tIst tmoio rook.q and the more alicmaine reeks. Thou4h moor. of t4p, lavam con'.;Aitl more moda than potanh, motie late stage derivativem outn more potaAa than isodal tvle are ilIumtrato4 in tablo 24.
4 7,-,1,02 5E.64 41203 14.44 Po,z03 6.72 Yoe 4,5C CaO 3.60 g$0 1.6t La20 4.7 K20 7.09 fl 20 4,a0 P 0 2 5 0.11
b &paw :tf.26 5*72 3.52 3.10 1.05 csce 4.17 2.64
a 1 .;411-4to a plinedini (Lkiriftpial 1900 b : :r,whytoid phouolits: ° ( 0 * )
;Urn:114U described the altornstion of haste and oPleolins 4oa4itlorT and corviuglo4 tiv,t "the cr4er At 0,taint dotes not seem to rollow an/ of thou t44-1; aro ov,ger;o4 0 thoories of uzgaAtio difforontiatlon.° Re not t:0 that whilst the :Anemic: cf. tho 4ae4itelp traehytelt . 4n41 Wmati phonalite were 17ritah uneerrodody *ono wineralo (no4the1ine And so4alite
Puli01.41440 in tLe tt,AE!.1titos wort. corrode4 1:41,1 that th* wroxeas at e4 a ()pro ell atoeirLaep Ie c4nel:Idod thAt this
111 oryrtvalising orAltiono Waa dW4 to mmot mixtne betare eise4i4.>n and he c:sidead thmt the eononant luvus were tr3o0ihonolito an4 dolorito, ha oklw.l. guirtt theme two nagmaot. §day t1-4eiatt4;Aroo 4i") e4191;x4tadbyeafferortition from
a sem= par-mt# In the AZ441 Indies trclnl aro of limited
dietrOution of Terlilary Ace; tnoy follow04 4 pr.0$0&0
And 11mootos$10 volo:lnieity of rranitop zabbro and cmlo-aititialt
-
rock. In Java, 3rouwer (1928) hat' desetibed the formation of late Tertiary trt3.0 .frm pyrominendenitis naLma 1.1esoeiati
with A%lcording to Iyengar, Norker
attributes the lute development of leucite rooks after crac-alkaline voloanicity to the encroachment of an Atlantic type mgma *non ths frontie7. of the Pacific rozion." Alknline roak are alto recorded frmn the Phillippine Islands and japan.
There are fever instances of alkaline barult tmchyte suiLns way frtw, the eacifie areal though alkaline rock: have ear. dev,oribod frov everca plaoes. Asslyses of sorts rock frc trig Thizlean ProviLes are given in table 25.
Thane annlyre- a6;ain show u remarkable similarity to those of the 17ucific rocks. Columns 'a and 'e' compare
oottish olivine dolez.its and a Fijian trachybasait; there 4x41 few importnt difrrences. Th FLJ. rock has 2 less alumina and oonniderably mare potash that the Scottish lava, and the 1i n cf Fa201 and reo differ, thoueh the totals are almost equivalent. The rocks of he Thulean Province are aneociated with oalo-allulline lavas. Thai relationship between the alkaline and calc-alkaiine reeks of Arran mu) discussed by Ling (1955) who considered that- the calc-elkaline rocks may have been XorLled by hybridisation of basaitio or habbroic rooks by later acid macma, or by progreeive crystallisation of a bawaltic Mar.
An olivine w4.suit-traohyto association was described from the Kerguelen Archipelago by Awards (1938)f the Archipela6o is situated off the Any coast in the south of the Indian Ocean. Aeon he rock types whica he descrijed are phonolites, solvsbergi4a, anorthoole troh,yte, hornblende eanidine traohyto, anorthoolaso tirnblende, trac4te, rhyolitos, traohybacalts, leuoit,e and nopheline tephrites, nepheline ;Ind aaalcite barvImitan, crinanite, various types of basalt,
184
Si 02 A1203 7203 Te0 1040 Ca0 Ka20 Y20 Li 0+
2 TiO2
1110 °theta
a
6.23 3.24. 8.03 4.30 1.19 0 99 0.37 1.81 0.17 0017 0.06
49.24 46.42 55.76 50.5 15.84 14.01 16."5 17.%1 6.09 5.03 3.10 4.71 7.18 9.02 6.02 4.36 3.02 3.82 1.0a 3.69 5.26 8.10 3.23 8.13 5.21 3.$2 6.28 3 89 2.10 2.00 3.87 3.81 1.61 7.22 0.95 0.7' 1.08 0.80 1.06 1.84 a.4 1.78 0.66 1.47 ned 0.22 0.41 0.29 asd 0.40 0.21 0.30 0.03
or nyri io olivine dole :rite Sky o.
✓ rite, Skys
O ite -ill, shy*
4 1Ihmeemite, ull.
e trnahrbesalt, Wttukoula.
rid oeeanitec. non series tro:'i wisullised it an pertly coseurring later in a 'eapola chamber in the
establiebed a cliffs 1,:1-onoIitee, but he
her and chamber he considered
that differentiation, hy ery Sal sinking, of olivine basalt
185 produced undesine 4ae*it f.Ind later oligoolalls basalt; Iimburgites and ocoaniteo were alro formed in the same°. ehambor. The evolution fro tp oligeolexto basalt to trachyphonolits took place by fractional orystallisf4tin in the cupola througt the intermediate stager of traehybaralt and hornblende traohyte• Be considerad that, in by fractional Crystallisation, the traohybqsalte gave rir;e to leuoite aid nepholins tephritts, basanites 4Tad subsequently limburgitesi the hornblende trachytol are viouallred as producing sanidine and?morthooluesatmohytee, nolvsbergite and sanidine rbyoiite and the trf,ohypiwnelite* IrS give sanidino phonolitee nephelina phenelite, tres4
is towards quartzosa tmohytio rooks (ranidine r4yolite) and higLly undereaturnted forms bearing feldepathoids, Coda is again dominrimt over potash, but in the extreme derivatives, traohyten and phonoliteel the pot,m1h content in approxiMetlar equal to, or o:411eads that of sodas in the Fiji reeks the talk assumes dominAnce at a much earlier stage of ovolutiona Zdwards strAtedi
"If it iv assumed that the a:0mila* rocks have diftarantiated tram a parental basaltier am, it is seeosoary to postulate that the name Wese thr044P,A the crust al, a series of mall, stogy dewed. Oupola like bodies, rising at irimmular Interval( in advance off but connected pith the- main moomm reservoir. *
asopettold (1949) 'Lo suggested a similar fora of the. imam Chamber and interconnected poOkets of laVa have been 'suggested at Vatukoulal the formation of such cupolas is described ay Holmes (1931). Simorde also considered the possibility of limestone assimilation playing any- * part in evolutiont
The possibility of the productiwf of trachy4o. and 1,14 phonolitio lavas through the assimilation of onloareous sediments hy basaltic -Ha ha been disowned Or all who hmirm mailmen a ut the lavas
and the weight of opinion is everehe
opposed to ito The low lions of do Ie Rue mai
of 1awsort have shown the island to be almost exclusively igneous in ohareot,Jr. The only sediments knowr to emir are beats of lignite ieteroolated with the sheets of basalt and b1ooh of Qobigerin4 ooze it the volcanic debris.*
Lcroix (1924) recorde xenoliths of granite, nyonite ,And ihline eyenite from Kerguelen Ivland.
The rook e and islands of the old Atlantic ride an largely alkaline, though sons Thyolites Ind other clo-lltaline rooks are known. Currently, 'me work on this province is bein6 done oy R.. 41 Maitre. Daly (1925 and 1927) described the geology of Asoensien and $1. Helene Islands. From Aseension he noted rooko of the olivine basalt..trchyte aesociation with quartz treohytes or rhyolitea (eve table 25) and xenoliths et alkaline hornblende biotite granite, hornblende eyenite with quarts, suite hornblende eyenite, diorite and z.allbro. Duty eoneidered that fractional crystallisation was important in the generation 'of *alio magmas from basalt', hut he cam thought that gaseous transfer wee controlled by the movement of water gee derived from rooks surroundin8 the vents. Pe correleSed the rooks of Asoonoion with those of Tutuila (W. NNW whioh are also alkaline in character. in (1927) he defier/bed basalts, tmohytes and phonolitet from Y.t. relena but here there are neither rhyoliter nor xenoliths.
Table 27 by Le Maitre, indicates the representativen of the ika1ine suite on the various islands; he hae recognised that :toot contain either olivine or amphibole and in some rocks from Tristav de Cunha he hes identified :Tehedra leucite which io represented in the norm as nepheline. Le ],laitro fialls that generally traohyearEat occurs in greater volumes than either the proceeding olivine 4fterIAs or the suooeedin traohytes and phonolitee$ this is unusual since in many ,r.reas the inittAl olivine bacatst., produces the
-0-4www.linwou,4914.....irpl.,...09.111.1106.11111NIe.101w
T'a4I.e
1 7
26 APAIYFIL.91
-
9 ;It v1„.214,, Or C •
102 47.51 52.12 57972 67.0 71.88 A1203 16,21 16.68 17.64 15.43 12.85
116203 2.20 4.45 4.47 3.25 3.60 140 2.78 1.2'; 0.05 410 7.15 342 1.01 0.16 0.18 CiO 10.02 7.32 4.36 1.0G 0.60 N*20 2.87 • 4.63 5.50 6,12 5.32 gl 0.64 2.06 3.90 5.32 4.78 N 0+ 2 B 2
0.19 0.30
0.09 0.40 1.65 0.17
0.56 0.18 t 102 2.79 • 2.01 0.37 0.10 0.25 k,205 0.55 0.52 0.57 0.04 0.35
0.17 0.37 0.03 0.29 a
3
Olivine title at. Athydo:crit-%.
Truellyuz.441eits, T.1.1yte,
t. lialona t3t. Us Iona ft. Helms St. LeIena
RofIrenoes DUy '4925)
Quartz trvto (rhyQiits) ;!elona
188
mere ablindrInt lawls. Le Maitre also describes brown besqltio hornblende ::.4cs known in the :Pacifie.
Several facts emerge from_ the foregoing comparison of alkaline suites. The vrent rvt= in each case is an olivine basalt containing 2 — 3% tet“A alkalis and with modal plagioclame, divine, pyrexeno (titc..Lnaueite or uagite depending oq the TiO, content of the maosa) 7:;.nd or material; the parent
4
rock 9roduces late tr:_Lohytes 1,1d cc;metimes phenolites and in most inat,Inces there is development of intermediate types, tradaydoleritos, trachyndesitso and tr714hybasalto. Basalts enriched in olivine are away pt eut ankaramitie lavas are limited in their distributin, In all the suites from wnich arikarftmites have been described (with the exception of the mid Atlantic ridge), there are known supplies of limestone (Hawaii, ?ehiti, Fiji), whereas. elsewhere (isespepas, Victoria, New Zealand and Kerguelen) thowt limestone nor be sysilsbiet there is no definite evidence t4 s.uggest assimilatial4 GrwitY differentiation has been desoried fro most arel3 but indioa—tlons of gaseous trunsfsr balm not been noted cv:oept by Awards
Mareht4.11 in Australasia. flow much of the evidence which they cite is due to true gaseous transfer and holt:much in due to normal accumulation of lase dense minerals by the sinking of the heavier ones is unknown and irresolvable. It as, be near floe truth to suggest that gravity differentiation is a :principal c=use of evolution of the :tlkaline rocks though there might be wwy oliint effects of 6. 1seous transfer in some places. Assimilation of limestone in the early stages of volcanicity will produce ankaramitio rook; no indieations of limestone aseimilation are known JJ1 the latest derivatives*
Thera are however other conditions goTarnine orystallisation of these rooks; ].airs of similar Chemistry may in one area contain no feldspatholde and in another area. oontain
-139- abundunt nepheline or leucite. It ha? been noted that both alkali and nilico oontontoinoreooe it evolution though the rate of inor000e of 14h silica z alkali ratio must be leoe in the foldspatholdal rooke thon in the non feldepatheidal rock. Tolo questioo of eaturation iv aooin dependoot on the omount of olivioe which has oryntollised oni sunk Toot the conditions wOich produce feldepathoidal rock?? may be thole' foyourio4 the oontioned orystallisatiou of olivine.
It has been ehown that with evolution, the alkali oontent of the maoma increaser) and in all 0$001 the potash inomies is most marked. Ultimataly it becomes predominant, but the stage at wrioh this happen o difforo from suite to suite; many times rocks oontainiog about 58% Si02 and 12% alkali have equal amounts of Na20 used K20, or with K20 oliohtly reoter. The difforing pr000rtiono of alkali minerals ay be related to the amount of gravity 4ifforentiation and, in some rooks, of limestone assimilation; the potash rich traohytes may result from sinking of sodic plagioclave in quiet and till ConditOone of crystallisation in otioh s000 of the lens dine ainersio were sunk.
The association of alkaline with cole-aIkaline rocka in arson other thon the intro,o0aoific ie suogestive of production of (talc-alkaline roc*' by mIxino of baoalt And gronite, soorottr O0Nro-,004,0,9-, particularly oc many ouch suit contain xenolith of granite, gabOro ood diorite; Xenolith. are absent only in regions wbere grauitic laysr ir supposed to be absont. hore hot basalt and granite magwo, mix without assimilation, compooito lavan are formed and where they combine they form calc-alkalino andeeiteo ond derivativoo.
6.2 pousiQz Alkaline basalts are apparently confined to the
Sato T rti4 okr ' of yi it. tea the earl/ and 1.--,raz arc mnin27 onle-aakaline ander/tee, though interbedded baenite Ore known. taiba (in press) F7-1; litisation of sums wainimala rocks, but oth&wiec no enclose
inity har been recorded. Nov that the u:tkaline nature of VatUkoula axt ociation has been defrt r straatcd, ether are -1L
are of late Tertiary balvate in Y ji c be reviewed there it no
seri available for that roots. of 7weuni Islandp tTabouvv.lu (Vanua Le) anti Tiotuna Islad. 11 of whit are hasn.Ltic aad in the write4. opinion, 11110/ sumer. in age. Tavemni Islaml lien a few miles to the of Vanua Levu and riper to a heig„ht of 4,000 feet; the ere -t of the island form
orth-north-exiti
dote
a ado, 26 miles lon> tied strikes general south-south-west. Many craters lie on a summit fissure but since the flanks of the ridge represeflt the invl n/opesonly the rocke of the latest eruptions are exposed - theme are olivine augi4 basalts. .ash and turf deposits occur in the north-east and south and Rickard 4e171%,„ ems) has recently found pebbles of hornble do andesite though the rook ht.a! n °en examined Xligtmit. The Nabouwalu bapalts surround an old vela no in Bua Provinee i.n sclIth west Vf:Lnua LOVU/ they -e feldspathic and comparable mineraleRiolly and texturally with the Wctukoula truchybasalts# Rotuma lies 300 miles to the north of VL.nua l'evu ,;,nd or' the andesite line as LS delineated by MacDonald (1949101 though only about 3 miles long and 1 mile vide it conoists of several small orators of olivine basalt (oomparAle with the phase I rocks at V%tukoula) risiag to about 600 feet above sea level. In all these maw though the baaalto are simil;Ir to those at PA ukoula, the later derivxtiver are absent.
In Fiji then i t t f e ti ct ff0r stggeetlyg that caled.alkalins aotivity is o lowed by alkaline volcanicity
Tel entarelie.tiou 'as irot musde ey lonnedy Ana tazeh 14) by Iyengar who applied it to Viti Levu. Hower, the Vatia rocks show um almost immediate reture to cello-alkaline conditions; a compari, on of aid-Tertiary and Vette andesitea reveals few difterenoee in either ooweoeitien or textlere, exoept that the Vatia rooks usually contain larger phenocryste of faideear and smaller phenooryets of amphieole then earlier endesites. On this evidonee, it seems inadnissaule to postulate a different petrogenseie for the surly °Ale-alkaline rooks than for the later ones and it is protmble that most of this pre-Pliocene volcanic rocks of Fiji meat from coltamination of aoid and basic muterialv. There is soe evident,o to justify this euggestion, iia(2101, oomm.) believe* that there is an absence of trek - granite in Fiji and all the plutonic rock reoorded by his. are gmnediorite; Fouti ne.s alo coneiderad
many plutons axe hybrid roc czused by mixing of materialm. It is noteworthy that basalts are almost as rare at= granite in this thick lava pile.
The acid and basic co.-exieting magma:: involved in the mixing mor have formed dnrinz prooessee of down..buckling of the °rust correlate. with the eyntectonie stage of early 2ertiary orogeneeie. Two weemle would readily six in different pro-portions; hence would be formed the suite of rocks baealt-andesite-rhyolite with the posnible oomplete absence of wits or two members.
The irrecular and podkety nature of the awe Ohomber has been considered and it has been implied that the oOraliber migrated to a high level in the cruet before eruption oommeneed. It has been eht)wn that collapse into. the Chamber at Vatukoula penetrates to at least ono or t-Yo miles below the present surface; further, although they mm, be present at greater depths, limestone and politic sediment* are prevent at high
192
ive1 in the eswAtlid* /row ti ti evidenue o: ale and boulders in a&lomar,4.tesit apporeht thAlt undsrl:i the basalts “,s aro exposed i the older south and west of he island, nawe4y that there 044tnente (limestone, elltaCines) i.i andestUes on4401$00) in4ruhed by griAmitic rock.
Gold has 4eon found at Vatukouli, ut that ts to a-v, mineralisation is known to have
4enoliths elail,4r rocks aariee in tie are inter Cbiotite
not at Vatia; occurred in
4.raa where has untisr6one extreme differentiation to produce high.ly alkaline rocks but not where aseintlatten occurred on an extensive scale. A structure at liskirOkip dtKaribed by Rodda (in press), is possibly eiuilar to that deiceribA from Vatia, and i3artholomew (1959) has described a. t4iUr assemee of rocks fro •avusavI Bay, Vanua Levu, thouh on fossil evideilee the rooks there. have ':1c4ein tentati-vely
dated az TertliarY 4, (Upper 141Qcauo). Rartholomow descri'aed olivine free and olivine poor banaltn, the eruptiGh of which
followA by the emplacement of hornblende ande4te pluv associated with some andesitic laroolstics. '1,ta stlee utaeratisin8 fluid carried preoious and base metals and in places econotuic daposits of co1a were formed On faults formed durig andesite intrusion. Be considered that the young hornblende anderites represe.nted the late stage differentiates of banaItic magma. This severce shows ramarkable simila—rities to the Vatia volcanicity apArt from the P;;ct that it carries gold ntmermlisation.
Volcanoes like Vatukoul may' also bear old. In Taveuni, Naboupalu and Rotuma late mantic differentiates are not exposed but thip may be due to the erosion level and it is not impossible thrl.t trachytio derivatives,' may be intruded, and mineralised, at depth. Reform*e has already been made to the :3abeto rocks where biotite animate* and monzonites, similar
to those seen at VatUkoulat are intruded into the Be. basalt; batve metpl mineralisstion is known fror this area. It is quite probable *that sore voleanoes lie to the ev.',st and south
of Vatukoula and for *mumble ronnonn it is vital that tbe”s structures should be identified and 0r4ifUlly ped. I Ire possible tkl,t all the baenit was eruyte from the Vatnkoula volcano, but to the writer it seems more proh ie that there wer2 several. such centres.
It is beyond the soope of this thesis to dieouse the detailed frasture pattern in the :aperor wine* though it is raggented that generally the more importanI4 faults aru collasse planes along whioh some eliding and perhaps br000ltion took plc,und which often dip towards the centre of collapse; study of some of the minor structures leads to the suggestion that the sflatmakess rteepen quickly towards the basir centre and die out away from it. It le pointed out tilt few minor intrusions, which are formed alone collapse planer, whel;par steeply or shallow dipping, and which lis to the south of the basin contact, can be traced for any appreciable dist2xme; honge tne lflutmakee may nut 'LA, very extensive laterally. Amilarly,dykos ars not very 1,?orcistent and though sometimes mineralised, they are liable to die out very quickly along the strike and also to steepen with depth..
The pattern of outcropping ere bodies ean be correlated with the pattern of secondary mineral deposits; zeolites are related to basalt eruption and dyke emplacement but it has'40,Tm shown that mineral deposits have a zoned •aireola of quartz and quartz and calcite. Thin in inevitable since the temperatures of mineralising solutions must destroy the previously formed zoolitne and deposit higher temperature minerals in their place. Mau the width of the quarts and quartz and calcite cones are controlled by the thermal gradient
.194-
induoed in the rooks by the solutions; wherever quarts occurs an emygdale or replacement mineral at VatUkoula, it denotes
the effect of mineral bearing solutions. Calcite is loss critical ,:tins is ubiquitous where the temperature ws sufficiently hi 6b to destroy seolites; in some instances it spq se an original amygdaloidal. Admiral.
There %re no apparent controls of deposition other than the collapse structures and the preference of the solutions for basalt host rock; home* oonditionn are similar all around the basin andl thoughl mineralisatien is known from a few areas, its absence in ot4evo is due to the erosion level. when it is considered that the ma ..aprob%bLy originated in a high level source, the rate or 000ling of lavas and fluids rising to the surface must have been fact and if the fact that the phase I reservoir was probably not mach higher thin 700 feet above sea level is also considered, it beclomes possible that the bulk o the mineralising solutions: cooled before they reached the present erosion level :,:kr1 hence that exploration to depth will show further deposits.
6.3 Strotary and ConolTIVIne
Calc-alkaline volwmiem in early ,Ind mid...Tertiary times was followed by a thick accumulatio;; of sediments and, in Pliocene times, by eruption of olivine b;salt in several parts of Fiji. A large volcano formed at Vatukoula tad as differentiation by tractional crystallisation and gravity differentiaAon of the parent olivine basalt magma and assimilation of foals rooks prodnoed traohybasalts, trachy-andesites, trachytes %.nd monsonitic rooks as well as more >A810 types; much of the aotivity was exploeite. The relation-ship uetwoon the rock types is further expretTmed in their strucLura associati,ins and the derivative rock °. which were
195
erupted in well defined Amos, are %Jelly situlited within the caldera of the voleame of the p reseeding phase. The': extrusion of large quantities of lava is related to at leasttwo periods of caldera Euidenos; this 0411%pm, took plane: on inward dipping ring frt.toturen, some of which have nu. beequently been intrude1 to A:ry, cone oheste and dyke p, road 'exults In a basin etructuro. :cost of the rooks in the basin are deformed and breociated and zonen of intensity of oollapee are reco6nised in the surrounding phase II lavas; some of tame to the east of tho basin may represent an e3riter volcanic feature rather than an effect of subsidence*
A north.eouth striking basalt dyke swarm indicates the fissure zone on which the phase I volcano was built and the root sons of the phase II volcano han been mapped in the Wainivoo areal this has subsequently been re-intruded by later dykes. Multiple dykes are also thick and common in the faiknbukuhu area where they strike co,:xintently north west and where the geology ie complicated by the products of spatter conee,whicil developed in a corner of the phase I caldera during ph Yoe II or III times; these rooks, the Naeittu yoloanics, are intruded by thiek traohyte domes. A third set of dykes include', many biotite trachyteo and trachyandeeites, sees of whin L in tho Wainivooe area are zoned* The cone sheets intruded into the phase III rocks'', have formed on oollupoe freatures and from a study of the fracture pattern it is concluded that what are apparently distinct nets of vertical and horizontal frnaturee are related, the fractures flattening out from the centre of collapse.
The pattern of zeolite distribution in the phane I rooks . and probably also in the phase II rooks is eentrolled by the higher temperatures developed on the 47 o smarms, though such a pattern is locally and some tines regionally, distorted
- 196
by eubsequent minex iiration. At Vatia peninsula a hornblende andesit vo aano
grew on a platform of phage I and II rooks; the early produato were mite andeeite pyroclastios and later, after a prolonged period of hornblende and site eyroclestic deposition. many
11 hornblende andeeite bodies were intruded in the region of the calderas; the latest intruzion of all in sonedl probably by gravity or convectian currents/ and beam xenolith e of granodiorite, diorite and seeiments* There is no evidence of collapse at Vatia and the volcano apparently formed as a result of the pressuroe which, caused the elongation of tie Vatukoula basin in the fined stages of collapse. Minor intrasione. and seolites are absent from Vatit:4.
Chemioally the Vatukoula rocky, are highly alkaline, be, teldepathoide are very rare and even in highly peteesic reeks, moat of the potash is locked in solid solution in plagieolaset the range of whicL is quite large in west rocks; in the basic la's* the main ferromagneelan mineral is olivine, but in th interiediate rocks; pyroxene becomes predominant and biotite is the important natio mineral in the traehytes. Hornblende- is developed only in the alkali poor andeeltee of Vatia. Petrological and chemical determinations are oonsistent with the idea of a eamplete evolutionary series. The petro-genesis of the rooks has 'Jaen disowned in detail and the swixaetion made that they are derived frat olivine basalt magma by gravity differentiation and limestone assimilation and that the Vetia rocks formed by mixing of a similar as
th with acid aukIwal;mad subsequent aesimilation of politic esdiments.
Comprieen with similar rooks of other Provinces has shown Almost identical features and the main difference lies in the ,steer of bnealt evolution a% whioh potash becomes dominant over soda. Colo-alkaline rooks of pre-basalt e,ee are
191 -
comparable to thc, Vatia rooks. The controls of mineralisation it Vutukoula hlve
been diseuseed •And the suggestion m-de that though ildividual gold-bearing bodies may not be extensive, it is probable that below the present erosion level they oover a larger area, may be sore steeply dip, ing and are possibly richer.
6.4 AoNnowledgemenkp I am indebted to the Chief Geologist ...
Dr. L A. Phillips and the staff of the Geological nurvey Department, Fiji' for sash esoistanoe during the periods of field ms.ppingi my thanks are !Llmo duo to hoe Technical Amid/tests of the Survey who A000mpanied me into the field. I as also grateful for snoh help and hoopitzaity at the hands of the e=-Chief Geologist of the Omperor Goldmine and his wife (Mr. and Mrs.R.M. Cohen) and for assistance from hie staff and for much helpful discussion with all the3o people. My thanks are 41po extended to Drs. W. loboand G.P.L. walker for such adviee and for orittoally reviewing the maammeript• My thanks also go to Mrs. Skiba for typiiv out the thesis. The provinion of a ISIS grant and of air fare is gratefully acknowledged.
198.*
3,uson, W.N. and Turner, F.J. 3latchford, A.
Jowen, N.L.
3rady, H.B.
1899 The islands 1,7141 Cork]. Reefs of Fiji* Hav* Niue,. Comp. 7,001. Moo., v.13
1936 The dynamics of the formation of Cone Sheets, Ring Dykes ,aid Cauldron Subsidences.
Proc, Roy. !",-oo. :din., v.INT, p.128 1928 Olivine from the nawslAian Islands
and pure forsterite* km. Min., v.13, ao.11, pp0,559-564
1924 Tertihry and post Tertiary Histlry of Mull.
gem* Geo. z"larv. Scotland 1956 Hot Springs of AA.
Report to t Fiji Dept. of Iin; unpub. 1930 Fps an anemousito basalt.
3. wash* Acad. Sot., v.20, p.60 Crystallisation of pyroxsnori from basalts. kin., v.16, no.5 aneraloloAl ?otrography of .'acifie lavas. Jour. oi., v.21, p.377 Geoloy of the Lautoka rea, north-west Viti Levu.
Gool,t1; Yiji Dull* no.2 1959b Geology of Savusavu Nest,
evu* Geol.Surv. Fiji lull. No.5
1960 Gooloa of the Nandi area, western Viti
Gael, 'e.urv. Fiji 3011. No.7 1940 gugearites in the Dunedin District*
Trans. Roy. noes N.Z., v.70, Pt.3, p•188 1953 Geology of the Tavua Goldfield.
Proc. Auot. IAA*, usa.168-169, p.13 1928 Zvolution of the Igneous Rocks.
rinceton Univ. Press 188R Notes on the so-called eoapstones
of Fiji* v.liv, pp.) 10
6.5 References
&g,r,scis, A.
Anderson, Lai.
Aurrousseate, M. and Mervin, H.E.
E.B.et al
Ca. and Tarvey, S.R.M. rta, T.F.W.
3artholsess, a. w.
1931a
Ara. 1931b
M. 1959a
Dana, J.D.
;aokle, 1099
R.A.
1927 Geology of Helena. roc. Am. &aac. Arts, Sci.. v.62
1933 igneous rocks and the Depth of tits
raw Hill, Now York 1849 Report of the U.S. p3,or w d.
(Wikes). v.10 (Geology). pp.337-352
1951 The Fiji Islands, a goo phi.ctal handbook.
Govt. Printing Office. Petrological mates the, PSji zoo.
Proc. P,ms Acad. Arta. Sol., v.34, 21).581- 595
?uva on some rock; from
99
Brouwer,
kitoh of the Coology of Vi.ti Um, ;Ivo. Roy. floc. Can., *set 30 18, p.63-83
1928 „'rAuction of try; and phono from ;yroxen dositic magma assoeiated with limes io e.
J. Geol., v.36, 2.545
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To Ard heinn Area of the Centr.1 Ignoous Complon of Arran.
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1949 ?rincipl s of ::itructur-1 Geolocf.
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Winchell, A.N.
woolnough, W.0
Geol. ;301*.
•,,.~ `,,,
;;,_
7, V.'41*
_ /01117:
• r
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4 w,
it< • •
-401111w, • - - ' .=0...„,t_ •
41:4to 9.
4.1444 11«
Pinto i?.
'$t Instel
aCt *Pita
Pla!; 15.
Plate 2$.
Plate 17.
plate 18.
,,La • 11.
PIM* 10.
Plate 21..
21ae 22.
{fait 2♦
new 24.
eLelateg Plate 1 s A general view over the Vatemilla basin fOest the
olivine basalt ridge to the south of .Nadler.The treeecapped hill in the middle dints noe maths the southern limit of the phase III rooke. The basin stretchea. amity to the right. Tha hills is
n c the far dicta o form the Vatia peninsula. f l=ate The pile of tuffs, aeelomerates and breccia*
forming the loweet rodke of the 3a- Series; they are exposed in. the 3a valley about six Miles south of Vatukoula.
Late 3 Pyroolastio rocks of the Vatia ;3eriee *hoeing tha predominance of fine tenet:ewer material; a quarter of a mile north of Dr.attarib trig,
Plate 4 t Ununually fine oontaot brecciasabout three feet below tlw actual basin contact; they are exposed 200 yards west cif the Wainivoce bride'
Plate 5 s le amain Gout:act. Amminio i oitting on breedss let the bedded And. faulted material forming upper part of the seotiee le normal phase III
mudstone. Plate S $ A dyke of olivine basalt cu'eties through trathil
of the Hasitia Series. The dyke is two feet wide* A contact of the dyke illustrated in the previous plata. The seAle is six inches long. The Aseatele$ of the dyke appears as a leached on ami the calcite veins cot the trachyte host rook,.
Plate 8 : Hornblende andesite pyroolastice of the Yalta !,erien. The tuffeceouo matrix shoes a fa ail but r,leular grooving. clot of hornblende granoillorite embedded to
hornblende endeeite in the 1elaikouLa intrusion* The pencil is two inches long.
elate 10 s livine rich baqalt. ?hen= cryste of olivine with narrow rim of iddingsite embedded in typieal pharo I groundneve: P.P.L. x20 : (456).
Plate 11 4 Alleits rich olivine basalt of Naha: a I : P._ z20 : (259).
elate .12 I Trachybaoalt of phase II contains ehenooryets qf plaeioclase and augitet P.P.L. x20 : (504).
Plate 7 I
Plate 9
altered traOkytmmat (with an $1101nsio ) hae, )(gen subjeeted to son* smeldings
'*L x20 : (533). Plats 14 *malt of the til.sitiu &miss ohowine high feldspar
oontent and tr,chytic tortures the main !env magnesian minerid in !,ugiteis P.P.,. x150 * (573)
Plate 15 i notits traobyte showIng crystal of biotite in a trachytio Groundmase of pot:iAl feldspar and plagloolanot crossed nioo/s 120 (185).
Plate 16 1 cremonzonite with crystals of plagioclase' olitte, olivine and biotito Imbedded in plute of potash feldei44.Lr showing some ex-olution: P*P*1,* x40 3 (242)*
Plate 17 1 As plate 16 crossed nicols *40. Plate 18 Masortiod hornblende oryotril with a core of
otithopyroxonos in hornblendo andosites crossed niools x60 $ (580).
Plate 19 s Vornblende andssite of CatubIlli intrusion thcwinz typical andesite tutust with phenocrystic plagiocle:se, potash foldep,,r w:Al some resorbed hornblende: ?Jai. X20 04)
Plate 20 : At plate 19* crossed siools x20 Plato 21 t Vitreous andesite from the lam:Pt of four on
in a "layered" intrusions phenocryste of fresh and altered plagioclaro with toms o,uf:it in vitreous groundmass: ?.P.L. x20 : (562).
Plate 22 z second zone fro il the same intruoion* eho rock is essentially iiJr to plate 21 though the ground A "3 to n..tw finely crystalline; Pavia. *20 : (563).
P1 ?hird sore of this intrusion: crystals of biatito and sanidine are a>,t in a tru.chytio grounemsmet crossed ntools x20 * (564).
Plate 24 ?ourth sone of this intrusions this basic rock has phencocrysto of plagiooluse, auate ,Lnd uatoW06 olivine in a basaltic groundmass: crossed niceois x20 s (565).
