The San Pedro and San Pablo volcanoes of northern Chile and their hot avalanche deposits

P. W. FRANCIS et al. - - The San Pedro and San Pablo volcanoes of northern Chile

The San Pedro and San Pablo volcanoes of northern Chile and their hot avalanche deposits

By P. W. FRANCIS, M. J. I~OOBOL~ G. P. L. WALKEIt~ P. R. COBBOLD, M. COWAIID *)

With 16 figures and 8 tables

Zusammenfassung

Die beiden 6000m hohen Vulkane San Pedro und San Pablo erheben sich etwa 2000 m fiber der breiten, von Ignimbri t gebildeten Hochebene Nordehiles. San Pablo ist 1/ingst erloschen und tief erodiert; dagegen weist San Pedro mehrere historisehe Eruptionen und dazu eine noeh aktive Fumarole auf. Bei beiden hat sich die vulka- nische T~itigkeit im Laufe der Zeit nach Westen verlagert. Die Vulkane sind aus Pyroxen- und Hornblende-Andesit-Laven gebildet; ihre iiuBeren Flanken bestehen aus breiten vulkanoklastischen Ablagerungen, die heiBe Erdrutseh-Ablagerungen einschlie- Ben. Chemisehe Analysen von 80 Gesteinsproben ergeben, dab die Produkte der Vul- kane nur geringe ehemische Sehwankungen zeigen und dab die SiO2-Werte zwischen 56~o und 66~o liegen; deutlich h/Sher als die SiO~-Werte der quart~iren west- pazifisehen vulkanischen Serien. Ihrer M~ichtigkeit naeh (100--200 m) miissen die Horn- blende-Andesit-Laven eine hohe Viskosifiit gehabt haben. Wo solehe Laven hoeh auf San Pedro ausflossen, verloren gr6gere Teile der Lava ihr Gleiehgewicht und rutsch- ten ab. Die so gebildeten Erdrutsch-Ablagerungen sind ein wichtiger Teil des ganzen Vulkans geworden. Die meisten Erdrutsehe entstanden, als die Lava noch in heigem Znstand war, aber schon so hohe Viskositat hatte, dab sic mit spr6dem Brueh ab- brach und niederstiirzte. Die gr613eren Brocken dawm zeigen radiale, s~iulige Klfifte, die senkreeht zur Oberfl~iche laufen. Dies zeigt, dab sic naeh dem Abrutschcn als einzelne groBe Broeken gekiihlt sind. Es wurde bewiesen, dab der Transportmecbanis- nms dieser ,,heiBen Erdrutsehe" wahrscheinlich sehr ~ihnlieh den nicht-vulkanisehen Erdrutschen ist. Bimsstein-pyroklastische Lavadecken sind auch vorhanden, eine befin- det sieh an der Flanke von San Pedro, w~ihrend andere die Ignimbrit-Hochebene bilden. Die letztgenannte erh~ilt ebenfalls viele pyroklast~sche Lavadecken yon ge- ringerer M:,tehtigkeit als jene von San Pedro. Es ist sehr wahrscheinlich, dab auch diese Ignimbrite ihren Ursprung an den andesitischen Vulkanen fanden. Manche von den Ignimbriten sind I/hyolithe, viel saurer als die andesitischen Stratovulkane. Die Trennung der rhyolitisehen und andesitiscben Laven wurde auf der Erdoberfliiche durch Transport vollzogen: die Andesite wurden nahe am Vulkan angeh~iuft, w~ihrend die meisten der Rhyolite wegen ihres v611ig anderen Eruptionsvorganges weiter ent- fernt vom Vulkanzentrum abgelagert sind.

Abstract

The twin 6000 m volcanoes San Pedro and San Pablo rise 2000 m above the extensive ignimbrite plateau of northern Chile. San Pablo is extinct and quite deeply eroded, San Pedro has an active fumarole and a record of several historic volcanic eruptions. Activity on both has on the whole migrated westwards with time. The volcanoes

*) Author's addresses: Dr. P. W. FRANCIS, Department of Earth Sciences, The Open University, Bletehley, England; Dr. M. J. ROOBOL, Department of Geology, University of the West Indies, Mona, Kingston 7, Jamaica; Dr. G. P. L. WALKER, and P. R. COB- BOLD, Department of Geology, Imperial College, London SW 7 2 BP, England; Dr. M. P. COWARD, Department of Earth Sciences, University of Leeds, Leeds, England.

357

Aufs~itze

consist of pyroxene and hornblende andesite lavas with broad aprons of volcaniclastie rocks, including hot avalanche deposits, on their flanks. 80 new chemical analyses show that the composition range of the rocks composing the volcanoes is small, with silica varying between 56 and 66Yo, significantly higher than that of Quaternary volcanic suites of the western Pacific. The hornblende andesite lavas are very thick (commonly 100 to 200 m) indicating a high viscosity. Where such flows were erupted high on San Pedro, large portions of them collapsed when they became mechanically unstable and the resulting avalanche deposits form an important part of the volcano. Most avalanches took place when the source-lava was still hot, though so viscous that it broke by brittle fracture when it collapsed. The larger blocks have a prismatic jointing normal to their outer surface, showing that they cooled down in place after collapse. Evidence is presented that the mechanism of transport of these hot avalanches is not likely to be different from that o~ non-volcanic rock avalanches. Pumiceous pyroclastie flows also occur, one on the flanks of San Pedro, while others comprise the ignimbrite plateau. The latter is made up of many pyroclastic flows different from the San Pedro example only in size, and there are good grounds for believing that the ignimbrites likewise originated by eruptions on the andesitic volcanoes. Many of the ignimbrites are rhyolites, much more acidic than the andesite stratovolcanoes and a process of surface transport separation has operated whereby the andesites are concentrated on and near their source, while most of the rhyolites, because of their different style of eruption, have spread widely from it.

R6sum6

Les deux volcans San Pedro et San Pablo, hauts de 6000 m, du Chili Septentrional, s'616vent fi 2000 m au-dessus d'un vaste plateau d'ignimbrites. San Pablo est 6teint et profond6ment 6rod6, tandis que San Pedro montre une activit6 fumerollienne apr6s plusieurs 6rnptions survenues au cours de la p6riode historique. L'activit6 volcanique chez les deux volcans s'est d6plac6e dans le temps vers l'ouest. Les volcans sent compos6s de laves d'and6site ~t pyrox6ne et fi hornblende, avec, sur leurs flanes, des roehes volcanoclastiques comprenant des d6p6ts dfis ~t des 6boulements chauds. 80 nouvelles analyses ehimiques montrent que ]es limites de composition sent 6troites, avec de 56 /t 66~o de silice, soft une composition nettement plus 61ev6e que celle des roehes voleaniques quaternaires du Paeifique Occidental. Etant donn6 leur 6paisseur (100 fi 200 m), les laves and6sitiques fi hornblende doivent avoir eu une viscosit6 tr6s grande. Dans les endroits 61ev6s de San Pedro off de telles coul6es se sent 6panch6es, une grande pattie d'entre elles se sent effondr6es. Les produits de ces 6boulements ferment une pattie importante du volean. La plupart des 6boulements se sent produits quand la lave 6taft encore chaude, mais si visqueuse qu'elle s'est fraetur6e avec une eassure nette pendant l 'effondrement. Les blocs les plus grands sent cass6s le long de lignes perpendiculaires fi lcur surface ext6rieure, ce qui montre qu'ils se sent refroidis sur place apr6s leur effondrement. Cela montre avee 6vidence que le m6canisme de transport de ces <<6boulements chauds>> n'est vraisemblablement pas diff6rent de celui des 6boulements non-volcaniques. I1 y a aussi des coul6es de laves pyroclastiques ponceuses, dent une sur les flancs du San Pedro, tandis que d'autres fomlent le plateau ignimbritique. Le plateau est aussi tempos6 de nombreuses coul6es pyroclastiques d'6paisseur moindre que eelles du San Pedro. I1 est tr6s vraisemblable que les ignimbfites aussi proviennent des volcans and6sitiques. Beaucoup d'ignimbrites sent rhyolitiques, encore plus aeides que les and6sites des stratovoleans. Un processus de s6paration au tours du transport a concentr6 les and6sites pros du volean, tandis que la plupart des rhyolites, ~ cause d'un style d'6rnption diff6rent, se sent r6pandues fi plus grande distance.

358

P. W. FRANCIS eL al. - - The San Pedro and San Pablo volcanoes of northern Chile

Resumen

Los volcanes gemelos de San Pedro y San Pablo se devan 2000 m. sobre la extensiva altiplanicie ignimbrltica deI norte de Chile. El San Pablo yaee extinto y fuertemente erosionado, rnientras que el San Pedro dernuestra actividad furnar61ica y ha registrado erupciones en tiempos hist6ricos. En ambos volcanes la actividad se ha desplazado prpgresivamente hacia el occidente, Los volcanes se hallan consfituidos per lavas de andesita pirox6nica y hornbl6ndica. Tienen amplias falds de rocas volcanocl~sticas, con dep6sitos de ripe "hot avalanche". Treinta nuevos anal(sis demuestran que las rocas no varian mucho en cuanto a composie~6n qulmiea. E1 poreentaje de silice es de 56Yo a 66Yo, cant(dad que supera la de de las rocas volcAnicas cuaternarias del mergen pacifico occidental. Las laves de andesita hornblendica alcanzan grandes espesores (de entre 100 y 200 in.), lo que indicaria una alta viscus(dad. Cuando una lava de estas brotara en la superficie alta del San Pedro, gran parte de ella devendrla inestable, deslizAndoso y originanado dep6sitos que ahora constituyen una importante fracci6n del edificio volcAnieo. En su mayor(A1 los dep6sitos provinieron per disagregaci6n mecfinica de la lava paterna, la que fractur6 forrnando bloques rnientras se hallaba aun c~dida. La presencia de un sistema de un sisterna de fracturas perpendicular a la superficie externa de los bloques mas grandes indicaria que estos alcanzaron ternpera- turas ambientes en el propio site donde ahora se hallan. E1 mecanismo de transporte de estos deslizamientos vulcan(cos no serla tony differente al de los deslizamientos formados per transporte flnicarnente terrestre. Flujos piroel~sticos con piedra p6mez se hallan, uno sobre las faldas del San Pedro, otros eonstituyendo la altiplanicie ignimbritica. Estos 61tirnos se distnguen del primero finicamente per su mayor volflmen. Uno qne otros probablernente originaron per erupci6n en los prop(us volcanes. En su mayoria las ignimbritas son rioliticas, es dec(r, rnas acidas que los estratovolcanes andes((ices. Per lo tan(o, ha operado un proceso de separacj6n en transporte super- ficial, cual proceso ha concentrado las andesitas cerca de su origien, mientras que las riolitas, t en su propio estilo eruptivo, se han dispersado mAs, alcanzando grandes distancias.

t~l)aT~me eo~epmaHne

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1. Introduction

S a n Pedro and San Pablo are two large adjoining volcanoes in the Antofagasta province of northern Chile, near the frontier with Bolivia (fig. 1). They form part of the high Andean Cordillera, the upper parts of which are composed entirely of Quaternary volcanoes built on a Cainozoic basement. The Pacific/Andean plate margin is widely quoted as the best developed example of a subduction zone at a plate margin, yet surprisingly little is known in detail of the volcanic processes operating there. The area studied is adjacent to the deepest part of the Peru- Chile oceanic trench, which closely parallels the coast line (HAYES 1966), and it overlies the most seismically active part of the Andean Benioff zone. HAYES has noted that at lat i tude 25 ~ S there is a difference in surface elevation of 14.7 krn over a lateral distance of 870 kin. Seismic studies also show that the continental crust reaches nearly its greatest known thickness of some 65--70 krn in the Cen- tral Andes (JAMES 1971). This has some implications for the geochemistry of the volcanic rock suite. The pair of volcanoes described in this paper are among the highest summits in the Andes, being both above 6000 in, and they rise some 2000 in above an extensive level surface of ignimbrite which slopes gently westwards to the Rio Loa. They form a ridge trending east-west on which the main peaks, 5 km apart, are separated by a col at a height of 5250 rn (fig. 2), which is occupied by a large crater, called Central Crater in this account.

San Pedro has a fumarole near its summit, and there are reports of eruptions in 1877, 1891, 1901, 1911 and 1960 (CASERTANO, 1962). Prolonged fumarolic activity has extensively altered the rocks in the summit region and the deposits of sulphur there were mined up to the 1930s. The summit is accessible from the north-west by mule or on foot, along tracks made by the sulphur miners. San Pablo is on the whole older than San Pedro. I t is much more dissected by erosion and is regarded as extinct.

San Pedro and San Pablo like many of the other high volcanoes in the region show clear evidence of glaciation, although today they lack glaciers. Terminal moraines of small valley glaciers are found as low as 4300 in (cf. HOLLINGWORTH ~X: GUEST, 1967), together with striated and polished boulders and rock pave- rnents. San Pablo has not been active since late-glacial times and Central Crater on its western side has been eroded by ice and contains erratic blocks. San Pedro on the other hand shows evidence of considerable post-glacial activity and one example was found of lavas overlying a glacial moraine containing blocks. The volcanoes are surrounded by broad aprons of their elastic products, including conspicuous hot avalanche (nude ardente) deposits, extending up to 20 km from their source, while extremely thick hornblende andesite and thinner pyroxene andesite lava flows extend up to 10 km from their source.

360


30' 15' ' C}H'IL~E ", BOLIVIA ' , "-',,

/ ' . .~ E / ~ ' . , ,'-. k PERu ,) =

' I O udcha ~ r~ ~ Z

.~176 < / ~ 1 ~ . ! , -.-. ~,

!I "

~ i i ~176 6-, \ ((q~IP l a b ~ "~':<:-:'~':q~<4":'::3 ,~ u__ / . , ~ X ' - ~ X . / ; t5 c~o,,~ .:.::--:--:-~:.:.::--..:-,:. ,

o~o. 5997 V':~::::: �9

~:.;::.:~ ~176 .. ~ . ".,

Railway J Basement ~ Graben ~ " Road ..--'" Salars :iiiiiiii i~: Fumarole v

Ignimbrite ~o:;~176 ~ Frontier-'"" outcrop SCALE

0 5 10 15 20 AItEtudes in meters km Contour lines at lO,OO0ft

{c 30Om) intervals

68~ 21~

22~

Fig. 1. Index map of the volcanic chain lying between 21 ~ and 22 ~ S showing the location of the two volcanoes described in the paper. The inset map shows the location of this area (B) in relation to those described by KATSUI & GONZALEZ, 1968 (A) and

GUEST, HOLLINGWORTII et al. (C).

361

Aufs~itze

The field work was carried out by the authors in 1969, as part of a broader research project in northern Chile. This paper describes the structure and petrology of the two volcanoes which were selected from the 80-odd volcanoes mapped partly because they are relatively accessible and partly because San Pedro is the only one in the area which has a record of historic activity. This is the first systematic account of any volcanoes in the 400 km stretch of the Andes lying between latitudes 18 ~ 25' and 22 ~ 05' S. At the northern end of this stretch, KATSUI & GON-

o 5 110 ~ Pyroxene andesite lava _,,,,~i ~:)[- ...... ~ Hornblende andesitelava

['~:,• ~L','{,':',','L Debris f low :%:TL!f:':: Pumice flow H

- \" . : ? ~ ::."...: Ignimbrite plateau -..\ ":.:~. ~ Crater ~: M~'(~.J i:~. tlr Fumarole

:"-... Analysed sample number " \ e%

__ 6 - - - - - ~ Z _

~ =col z~/ .. . . . . . . ,,~SAN PABLO~%,

_ - ,,_~ ~:----

~/S~'~ .... ' ~ ~'~'r ;--'=--Summit Gp.] older lavas ~%, :[,C,. ?_iL_ Older lavas j s" PedrO

, Summit Gp. ] ~ , Middle Gp, i' S.Pablo

A ~ ~ LowerGp, J . . . . . . . . . . . . . z . . . . . . . ~- Dip

Fig. 2. Geologieal sketch map of San Pedro and San Pablo volcanoes, based in part on aerial photographs. The nmnbers give the location of analysed samples.

ZALEZ (1968) have described volcanoes in the Nevados de Payachata area just south of the Peruvian border (fig. 1, inset, A), while at the southern end ignimbrites and related volcanic and tectonic features of the country arround Toconce and Toeonao have been described by DINGMAN (1965), HOLLINGWORTH I~UTLAND (1968), GUEST (1969) and GUEST & SANCHEZ (1969), (fig. 1, inset C).

PETERSEN (1958) has described the large scale setting of the Andean volcanism in Peru, Chile, Bolivia and Argentina, and summarised its relationship to the structure and uplift of the Andes.

RUTLAND (1971) has attempted to relate volcanism in this general area with its plate tectonic setting. HAUSEN (1988) has given petrographic descriptions and three analyses of rocks from San Pedro and its vicinity. ZEIL (1960 and 1964), ZEIL .~K PICHLER (1967), PICHLER & ZEIL (1969), EL-HINNAWI et al. (1969) and SIEGERS et al. (1969) have given accounts of the petrology and trace element chemistry of rocks collected from many localities distributed over northern Chile, the latter including one analysis of a San Pedro andesite.

362


2. Structure of the volcanoes

S a n P e d r o V o l c a n o

The San Pedro volcano itself is a composite of two cones (figs. 2, 8 and 4). The higher, eastern one is capped by a dissected flatqying pile of pyroxene andesite and hornblende andesite lavas (The Summit Group) which form the top of a dominantly pyroxene andesite volcano. The lower, western one consists mainly of hornblende andesite resting on or banked up against the other cone. The prod- uets of this younger cone are spread out widely over the western and northern

B

D - / - . , . ~._ " - - - ~ . .

Fig. 8. General composite view from the north-west of San Pedro volcano illustrating the principal structural units: A - - main, eastern summit of San Pedro capped with flat-lying lavas; B - - younger, western cone of San Pedro with fumarole at top; C - - thick andesite lava (i9i on Fig. 2); D - - extensive apron of hot avalanche

deposits; E - - prominent collapse scar above D.

slopes of San Pedro and include some very large volume andesite flows and elastie deposits derived from them. The rounded top of this cone is made of hornblende andesite and it has a summit crater at 6060 m, and another crater on its north-east side at 6040 m inside whieh the aetive fumarole is located. These andesites occupy and overspill an earlier crater 1.2 km in diameter on the north- west side of the older eastern cone.

The superstructure of the eastern cone is best exposed on its eliffed southern face where relatively thin lava flows are exposed, non-vesicular andesitie rock al- ternating with prominent red and black scoria horzons to form a pile which dips gently westwards. Six steeply-inclined discordances - - they may be unconformi- ties - - are visible between groups of flows. The oldest group occupies the ex- treme south-east of the summit area, and later activity has extended the cone both upwards and to the north-west.

Some of the extensive debris flows which are found on the lower western slopes of San Pedro probably belong to the older cone. At least one of these

363

Aufs/itze

Fig. 4 a. General view of San Pedro volcano from the north-west (from Polapi railway station, 15 km distant from the summit) showing A - - main, eastern summit capped with flat-lying lavas; B - - younger, western cone with fumarole at top; C - - thick andesite lava (191 on Fig. 2); D - - extensive apron of hot avalanche deposits; E - pro-

minent collapse scar above D.

Fig. 4 b. Close-up of 4 a taken from altitude of c. 5,000 m on rubbly surface of hot avalanche deposit. A, B, D and E as in (a).

flows, conspicuous on account of the dark-coloured pyroxene andesi te boulders in it, extends 200 km f rom the volcano as far as the gorge of the Rio Loa, Similar deposits are also exposed on and b e t w e e n ignimbri tes in the Rio San Pedro south of the volcano.

364


The north-western cone is morphologically a very youthful feature. Much of its activity is post-glacial and it is likely that most of the historic eruptions of San Pedro have taken place there. It eonsists very largely of thick hornblende andesite flows which radiate to the north, west and south-west from a central vent. Most of the flows are 100 m or more thick, though none exceeds 10 km in length.

Flank eruptions are responsible for two young pyroxene andesite flows which occur between the hornblende andesite flows on the western slopes, for the extensive flow from the scoria cone of La Poruna and for the massive hornblende andesite lava flow on the south-western flanks (fig. 5) which skirts the Rio San Pedro gorge and has sent a scree of blocks down the side of it at two points. This last is the largest flow known on the two volcanoes; it has a flow front up to

Fig. 5. Part of Iower reaches of large lava on S.W. flanks of San Pedro (61 on Fig. 2). Flow here is about 60 metres high, and rests on flat desert surface. Blocky texture, typical of lavas in this area, is well developed. Age of flow is unknown, but is certainly

more than 100 years.

200 m or more high, covers an area of 19 km ~, and has a volume of the order of 8 km 3. While large for a lava flow, it is substantially smaller than the 24 km a Chao lava described by Guest & SANCHEZ (1969) 80 km south-east of San Pedro.

The flows on the north-western eone can be broadly separated into two age groups by a particularly prominent white pumice horizon (table 1) which includes an air-fail deposit and a pumice flow and represents what is for San Pedro an usually large explosive episode. The older flows include very large ones which reach the foot of the mountain north of La Poruna (specimens 166, 184 and 187) and have pockets of pumice between the flow-ridges on their upper surfaee. The largest of the younger flows is that which skirts the Rio San Pedro gorge.

The pyroxene andesite flow of La Poruna is also young, though its age relative to the youngest flows on the north-west cone is mxeertain. According to HAUSEN (1988) it dates from the late 19th Century, though this seems unlikely to the present authors. The lava flow (specimen 64) covers 12 km ~ and has a volume of the order of 0.8 km a. It is of aa type, contrasting with the block lava of the thick andesites, and has well defined lava channels in it. The scoria cone of La Poruna is

365

Aufs/itze

Table 1. Stratigraphic succession established on San Pedro volcano.

N o r t h w e s t a n d W e s t e r n S a n P e d r o Hornblende andesite lava and hot avalanche N (172) Hornblende andesite lava and hot avalanche M (169) Hornblende andesite lava (191) and hot avalanche L Pyroxene andesite lava (186) Hornblende andesite lavas (182, 184) Hornblende andesite lava (166) Hornblende andesite lava and hot avalanche K (174)

Pumice flow H and marker air fall horizon

Boulder Clay Hornblende andesite lava (167) Hot avalanche J of hornblende andesite fragments

(168) Hot avalanche G of pyroxene andesite debris (170) Avalanche E Avalanche D

80 m thick Rio San Pedro Ignimbrite

80m thick ignimbrite

S o u t h e r n S a n P e d r o

Hornblende andesite lava (61) Small debris flows F

Marker air fall horizon

Debris flow C Debris flow B Debris flow A Pyroxene andesite lavas

m thick ignimbrite m thick ignimbrite

Hot avalanche of hornblende andesite debris

Rio San Pedro Ignimbrite

DebriS flow

180m high, is extensively reddened by steam action, and contains scattered spindle-shaped bombs up to 2.5 m long.

S a n P a b l o V o l c a n o

This volcano is sufficiently dissected by erosion to reveal that it is structurally quite complex. The oldest parts consist of a group (The Lower Group) of three buried piles of pyroxene and hornblende andesite lavas with much scoria, each pile having its own radially dipping structures. These are blanketed by two younger and structurally distinct groups of scoriaceous lavas, here called the Middle and Summit groups.

Of the three early piles, the north-western one is the largest and probably the youngest and is cut though by the Central Crater (figs. 2 and 6). The core consists of thin red pyroxene andesite flows (e. g. specimen 281) whieh are overlain by thick hornblende andesite flows now well exposed on the southern flanks of San Pablo. The second pile, in the eastern part of San Pablo, eonsists of hornblende andesite flows and the lower part contains sulphur and iron and copper sulphides. These flows are now exposed as 'flat-irons' and are overlain by red pyroxene andesite scoria. The andesite lavas of the third pile also occur as 'flat- irons' on the southern flanks of San Pablo. In the west these dip under the lavas of the north-western pile, which is therefore younger.

The Middle Group eonsists mainly of pyroxene andesite with a few hornblende andesite flows, separated by much red scoria. The total thickness varies from 50 m to 800 m. Where it overlies the southern pile, the difference in dip indicates that considerable erosion occurred before the Middle Group was superpose& The

366


Group is best exposed in the eastern wall of the Central Crater where it consists of lavas and red scoria in roughly equal proportions. At the western extremity of San Pablo a fault trending 25 ~ with a downthrow of about 800 m to the south- east isolates an outerop of this group.

The Summit Group of San Pablo consists of reddish pyroxene andesite flows, totalling about 200 m thiek, which are amongst the youngest on the volcano. These flows, whieh occur only above 5700 m, dip gently outwards and suggest a source vent somewhere near the present top of the mountain.

Fig. 6. San Pablo volcano from northwest, 2 kin from Polapi railway station. The volcano has smoother outlines and is more dissected by erosion than S. Pedro. A - - main smnmit of San Pablo, capped by lavas of Summit Group. B - - flat-topped ridge in north wall of Central crater, interior only visible from above. C - - flanks of

eastern summit of San Pedro volcano. D - - relatively old pyroclastie deposits.

The Central Crater is elliptical and measures 8.5 km by 2.5 km and is open to the west. It is developed largely in the Lower and Middle groups which are well exposed in its walls. The crater is partly infilled with scoriaceous lava and pyro- elastic material from a low scoria cone in the middle. This later cone has been rounded by glaciation and a scatter of erratic andesite blocks derived from San Pa- blo are strewn over it and the rest of the crater floor. The final eruption from this vent in the crater must therefore pre-date the latest glaciation. The sample collected from it (spee. 280) has the lowest silica content (56 %) of all the rocks analysed from the two volcanoes.

The distinctive black vesicular lavas and pyroelastics from the Central Crater interdigitate with red scoriaceous lavas and pyroclastie deposits which can be correlated with the Summit Groups of both San Pablo and the older cone of San Pedro. The Middle Group of lavas of San Pablo are truncated in the Central Crater walls but lavas of the Summit Group of this same volcano drape the southern walls of the Crater. These relations indicate that the lavas and pyroelastics of the Central Crater, the red Summit Groups of San Pablo and the older cone of San Pedro are all of broadly similar age and that the Central Crater

367

Aufs~itze

formed after the Middle Group of San Pablo but before the Summit Groups ceased to be extruded. The following sequence was observed within the Central Crater:

Erratic blocks, indicating glaciation

2 4 m Black vesicular lava and bombs from vent in Central Crater

4 m Brown air-fall pumice from vent in Central Crater

1--2 m Red scoriaceous summit group of older San Pedro cone

2 m Black vesicular lavas and bombs from vent in Central Crater

> 4 m Lavas of Summit Group, San Pablo, draping the southern walls of Central Crater.

Fig. 7. Part of lower reaches of hot avalanche apron, San Pedro volcano. Note boulder strewn surface of deposit, and tendency for boulders to line up in low ridges, Margin of flow shows up clearly as contrast between light and dark tones in lower foreground,

left. Large volume extrusive dacite dome of Chanca right middle distance.

3. Debris flows and pyroclastic flows

Fragmental deposits constitute a large proportion, possibly half, of the total volume of the younger andesite cone of San Pedro. The first type consists of monolithologic rock avalanche deposits resulting from the gravitational collapse of hot but highly viscous andesite lava which formed mechanically unstable masses high on the steep slopes of the volcano. The fragments are largely non-vesicular and can be matched with parent lava bodies which are marred by collapse scars. The second type includes various avalanche deposits similar to the first except that they lack clear evidence of having been hot when they were formed. Some are polylithologic and these are unlikely to be due to the collapse of hot lava bodies. The third type consists of mudflows which are always polylithologic. The fourth type is represented by a non-welded scoria flow and a non-

368

P. W. FRANCIS et ah - - The San Pedro and San Pablo volcanoes of northern Chile

Fig. 8. Prismatic jointed block on surface of hot avalanche deposit on north west slope of San Pedro. Blocks characteristically resting on surface of deposit rather than buried within it. Note distinctive polygonal fracturing, particularly on left side. The block

has partially disintegrated since it was transported int(~ its present position.

Fig. 9. Fragment of small prismatic jointed bh)ek from hot avalanche deposit on north- west slope of San Pedro, showing closer spacing m c the prisms immediately below the

surface than farther in.

24 Geologlschc B.undschau, Bd. 6S 3~9

Aufsiitze

welded pumice flow, both of which are supposed to have resulted from explosions in a central vent and which are truly pyroelastie in the sense that both the fragmentation of the magma and the transportation of the fragments were the direct consequence of explosive aetivity.

Hot avalanche deposits:

The main evidence that some of the rock avalanches were hot when emplaeed is the presence, both within them and scattered over their surface, of large blocks which have a well developed prismatic jointing normal to their margin (figs. 7, 8 and 9). Many of these blocks, which mostly exceed 50 em in size, have since fall- en apart into heaps of polygonal-shaped fragments by separating along this sys- tem of prismatic joints. The rock avalanche deposits in general may be distin- guished from mudflows in that they show a "jigsaw structure", where a eon- tained block has broken almost in situ so that the fracture is infilled with fine- grained matrix although the fragments have not moved apart.

Five hot avalanche deposits have been mapped on the western flanks of San Pedro (J, K, L, M and N on fig. 2) and a sixth is exposed between ignimbrites in the Rio San Pedro gorge. A block from each of the first five or its parent lava flow has been chemically analysed (specimens 168, 174, 191, 169 and 172 respec- tively, table 2) and all are leucoeratie hornblende andesites. The deposits range from large-volume accumulations which locally bury lava flows over 100 m thick (deposits M and N) to small deposits which are little more than a surface scatter of blocks (deposit L). The large deposits have well defined flow fronts and a surface pattern of channel and lev6e structures running downslope. Individual deposits can be subdivided into flow units as the flow fronts and ridges of one unit truncate and bury those of older units. The four youngest deposits have been matched with their parent lava flows which lie immediately above the highest points of the deposits and are truncated by collapse sears.

The youngest hot avalanche deposit (N) covers 22 km e on the northern slopes of San Pedro and is derived from the youngest andesite flows on the northwestern summit (figs. 4, 7 and 8). It is divisible into at least 9 clearly defined flow units (fig. 10) which on the higher parts of the volcano have channels with lev6es on either side. Prismatic jointed blocks up to 5 m in size are conspicuous everywhere. Four flow units of the parent lava are seen immediately upslope draped on 80 ~ slopes and terminate in collapse sears (fig. 4).

The next youngest deposit (M) has a flow front less than 1 m high at its distal end but traced upslope the surface becomes level with and partly overlaps of lava flow 166 which, near its lower end, is 140 m thick. Tile avalanche deposit could therefore be of comparable thickness there. Prismatic jointed blocks up to 2 m across are common, and well-defined surface ridges are developed which curve around the front of the same lava flow. One flow unit belonging to this deposit is particularly conspicuous and has been outlined separately on the map (M'). Whereas most flow units have flow fronts 1 to 4 m high, this particular unit has a flow front 25 m high.

Among the oldest and largest avalanche deposits, D (fig. 2) has a flow front l0 m or more high with good linear ridges on the surface parallel with the direction of flow and clearly seen on the aerial photographs. Another, E, of nearly the same age has a flow front 20 m high and is well exposed in railway cuttings,

370


where, for several kilometres, the rai lway follows the flow front. Neither deposit possesses prismatic jointed blocks.

Other avalanche deposits and mudflows: On the southern flanks of San Pedro is a group of three polylithologic debris

flows of uncertain origin. The two oldest flows overlie the ignimbrites on both

~ Young scree apron

[~ ~ ~ ~ ~ ~ ,o, | Fumarole :o: . . . . . ~, Parent lava lobe

~ ~ ~ ~ (~/ Hot avalanche ~ P '4 ~ ~ ~ ~ Dashes show subunit.

A z a z ~ A ~.~ . surface ridge trends Per cent prismatic jointed blocks and size range Older deposits

1 N

ont

Fig. 10. Detailed sketch map of the north flanks of San Pedro volcano, showing successive flow units in the youngest hot avalanche deposit G, fonning a braided

pattern. The short lines represent flow-ridges on the deposits.

sides of the Rio San Pedro gorge and the older contains a high proport ion of pyroxene andesite fragments and is dark-eoloured in coilsequence. I t has a thickness around the edge of only a few metres. The youngest flow is a massive, lobed deposit which has a flow front 120 m high and extends only 8 km from the cone. Well preserved small mudflow lobes locally overlie it.

The scoria and pumice flows: The only example of a pumice flow is H (spec. 175) which spreads widely

around the northern and western quadrants of San Pedro and contains pink pumice blocks up to 1 m in size. Part of the area of this flow is indicated on the map fig-

~4 * 371

Aufs~ttze

ure 2, although it includes some redeposited and air-fall pumice. North of La Po- ruua the flow front is about 1 m high, and the deposit is probably not very thick. In a small roadside pi t near La Poruna the deposit is 2.5 m thick and is very coarse. The pink pumice blocks which are abundant in it are distinctly rounded and reach 30 cm in diameter. A sample of the deposit (spec. 58) from this pit has been sieved and the cumulative curve and derived statistical parameters are typical of pyroclastic flows.

The scoria flow G is largely buried beneath younger andesite lavas and the described avalanche deposits and has been seen at only two localities, several ki- lometeres apart. I t consists of black pyroxene andesite (specimen 170) and is a monolithologic and unsorted deposit. I t contains bombs up to 2 m in diameter which have a highly vesicular core and a ropey surface which indicates that the magma was less viscous than that from which the pumice flow formed. The flow front has been smoothed by secondary processes and the deposit near its distal end is only a few metres thick.

T h e i g n i m b r i t e p l a t e a u

The extensive ignimbrite plateau forms a gently inclined pedimentl ike feature west of the line of andesite cones along the axis of the Andes in Northern Chile. I t is one of the most striking morphological features of the Andes, and contains an enormous volume of volcanic material, probably comparable with the andesite in the cones themselves.

The term ' ignimbri te ' is appl ied in this account to the pumiceous pyroclastic flows constituting this plateau whether they are welded or not. In fact most in the San Pedro area are almost totally non--welded. The plateau normally consists in any one place of several superposed ignimbrites but little is known of the volume of individual sheets in the area. GUEST (1969) has estimated a volume of at least 100 km z for one in the Toconao area further south, but some in the San Pe- dro area are probably much smaller than this.

In the present area, ignimbrites are best exposed in three gorges, those of the Rio Loa, and its tributaries the Rio La Turbera and the Rio San Pedro. The section in the Rio Loa at Conchi Bridge (18 km south-west of San Pedro station) was described by BRtrGCEN (1950). Here a part ial ly welded ignimbrite about 15 m thick rests on sediments which unconformably overlie basement rocks, and is in turn overlain by lacustrine deposits of the Loa formation including a thin freshwater limestone.

Individual ignimbrite cooling units are coloured either white or pink. The greatest thickness observed was 80 m in the Rio Loa, but the thickness is very variable and some iguimbrites occur only as lenses infilling depressions. West of the Rio Loa they wedge out against the gravel fans extending down from the sills of basement rocks to the west. In gullies west of the Rio Loa individual ignimbrites - - as many as four can be seen in one section - - interdigitate with the gravels and can be traced westwards until they have a thickness of only a few centimetres.

The two ignimbrites exposed along much of the Rio Loa can be traced into the gorge of the Rio San Pedro to above San Pedro railway station. The upper one, the Rio San Pedro Ignimbrite, has a maximum observed thickness of 6 m and is welded with flattened fiamme near the base. I t is conspicuously rich in biotite

372

P. W. FRANOS et al. - - The San Pedro and San Pablo volcanoes of northern Chile

flakes, which have a parallel orientation. Only 10 m of the lower one is exposed and it is non-welded and contains abundant pumice and lithic (andesite) fragments. A hot avalanche deposit about 2 m thick is locally seen between the two ignimbrites upstream from San Pedro station. It contains prismatic-jointed blocks over i m in diameter, and is similar to the younger avalanche deposits which occur on the younger cone of San Pedro, described in the previous section. It is important because it indicates the existence of San Pedro (or San Pablo) as a high cone at the t ime when the ignimbrites were emplaced. An incomplete analysis of one of the blocks (not quoted in the table of analyses) shows that it is an andesite with 66 % SiO2 lying near the acid end of the San Pedro suite.

The source of the Rio San Pedro ignimbrite is not known but it is possible that it came from an ancestral San Pedro or a volcano lying farther east. It is signifi- cant that San Pedro is known to have produced at least one pumice flow, H, which is in reality an ignimbrite, albeit a small one. Other similar examples, some welded and some not, were found by the authors high up on the flanks of the cones of Palpana, Chela and Puntilla 85 to 40 km farther north (fig. 1) showing that it is not rare for such flows to originate on the stratovolcanoes.

4. Pyroclastie fall deposits

Numerous beds of white, brown and black air-fall pumice occur throughout the area, generally covered by wind-deposited sand or dust. The most prominent is the youngest one which may have been produced by the same eruption as the pumice flow H, possibly during the opening stages of the eruption.

Although the data are very incomplete, isopach and isograde maps have been compiled for this pumice bed (fig. 11). The deposit is a typical plinian one in that it is virtually unstratified and is coarse grained to a considerable distance

P%lapi / 5krn r / ~ /

/ / 70" 83 /

" _d / /'~ 4.5

/ 20 / x 33 / / '~

14// / / / / /

i I 2.6 I i I I 1

i I I / I

I I I i i l I 1 S.Pedro t6 33406~, A S.P~blo/ / /

'-- 7.0 .

x . . &PEDRO ~ / RLY STrt ~ ~ ~ 2+3~ ~

Polapi

4;7 4:3

NN 5;5 \ / /-

/

/ " / / - " \ \ A z u ~ ( 52 / /

I 6;5 ~ - I

I , / I I

[' I SPz~dr ~ A 4;9~ 7.9.10.11 S.Pablo

x 4"I\ \ 3.7

\ \k X \ I

x

Fig. 11. The wrongest pumice fall deposit of San Pedro. Isopach map (left), thickness given in em;'isograde map (right), giving the average maximum diameter of the five largest pumice clasts seen at each exposure. There is some uncertainty regarding the

correlations on the south side of the volcano.

373

Aufsatze

from the vent. Thus the 10 em isograde of maximum pumice size encloses an area of the order of 1000 km 2, and a sample collected from the edge of Salar de Asco- tan, 27 km north-east of San Pedro, has a median diameter of 5 mm similar to that of the famous AD 79 deposit of Somma Vesuvius (WALKER & CaOASDALE, 1971; LINER et al., 1972) at the same distanee from the source. The sample is also well-sorted, and gives a o~b value of 1.2, virtually identical with the AD 79 example. The 1 m isopaeh encloses an area of about 500 km 2, about the same as the AD 79 pumice, and the deposit therefore probably has a comparable volume, of the order of 2 km 8. Several other pumiee beds thicken towards San Pedro and seem to have been erupted from it. Such large volume deposits appear to be rather unusual in the area, however, and on the whole it seems that pyroclastic fall deposits have a smaller total volume than lava flows, unlike many other areas around the Pacific where the opposite is true.

5. Petrochemistry

Chemical analyses of 80 rock samples are given, together with norms and modes, in table 2. Most of the lavas are porphyritic so up to 2 kg of each sample was crushed. The elements Si, Ti, AI, Fe, Mn, Mg, Ca, K and P were determined by X-ray fluorescence, Na20 and K~O by flame photometer, FeO by the rapid volumetric method of Shapiro and Brannock, and water (including other vola- tiles) by heating and ignition. For the calculation of the norms the analyses were recalculated to 100~ on a water-free basis, with the ratio Fe20~: FeO adjusted to 0.8. Modal analyses were by point counter and are given as volume percent- ages. The roeks are ealc-alkaline, with a Peacock index of 58, and the frequency distribution is weakly bimodal, with a major peak at 63--64 ~ SlOe and a minor one at 57--58 ~ though the latter may result from a sampling bias. The rocks have been divided into two groups, namely pyroxene andesites and hornblende andesites.

This division was reeognised in the field and has since been found to corres- pond closely with mineralogical and chemical differences.

The first ten of the analysed rocks in table 2 are pyroxene andesites. They form relatively thin (usually less than 10 m thiek) and dark-eoloured lava flows asso- elated with seoria which is commonly red. They generally have less than 60 ~ siliea, less than 10 ~ normative quartz, and normative plagioelase more ealeie than Ana0. They are sparsely porphyritic and are eharaeterised by the presence of both ortho- and elinopyroxene and varying amounts of olivine, and the absence of hornblende and biotite. The most olivine-rich are specimens 186 and 280. The latter contains 8 ~ of ill-formed, often skeletal, olivine crystals up to 5 mm in diameter, together with smaller augite and plagioelase crystals in a groundmass of brown glass containing plagioelase, elino- and ortho-pyroxene and opaques. The plagioelase phenocrysts in this group reach 8 mm in size, are mostly labradorite (the composition range, based on refractive index determinations, is An 86 to 60), invariably strongly zoned, and commonly show partial fusion textures inside the rim. Speci- men 190 has tridymite infilling amygdales. The prominent flow of La Poruna (spec. 64) is the most aecessible and also the youngest example of this group.

All but one of the remainder are hornblende andesites (a few are perhaps bet- ter called daeites, though laeking modal quartz). They form thick (usually 100 to

374

P. W. FRANCIS et al. - - The San

SAN P E D R O / S A N P A B L O

Pedro and San Pablo volcanoes of nor thern Chile

T A B L E 2. ANALYSES, N O R M S A N D M O D E S - Pt. 1

280 186 285 176 190 283 64 287 289 170 SiO2 56.03 56.20 57.30 57.60 57.80 57.80 58.10 58.50 58.80 59.30 TiO~ 0.91 0.85 0.84 0.76 0.86 0.95 0.76 0.86 0.79 0.86 AlzO a 16.80 16.50 17.10 15.90 18.20 16.90 16.50 17.30 17.20 18.00 F%O a ~ 1.20 1.65 2.54 2.04 2.77 1.75 2.33 2.26 1.67 1.94 FeO 5.87 5.05 4.05 4.42 3.76 4.56 4.01 3.95 4.17 3.61 M n O 0.12 0.11 0.10 0.11 0.10 0.09 0.10 0.09 0.10 0.08 M g O 5.65 5.75 5.16 5.65 3.56 3.91 4.95 3.58 3.66 2.71 CaO 6.94 6.98 6.83 6.33 6.56 6.48 6.38 6.23 6.20 6.10 Na20 3.56 3.57 3.93 3.64 3.95 3.72 3.72 3.76 3.88 4.38 K20 1.65 1.63 1.84 1.86 1.72 2.03 3.27 2.15 1.66 2.35 P205 0.20 0.19 0.21 0.18 0.22 0.25 0.12 0.24 0.21 0.23 H 2 0 + 0.74 0.78 0.44 0.96 0.14 0.72 0.49 0.51 0.75 0.48 H 2 0 - 0.32 0.33 0.11 0.21 0.14 0.30 0.12 0.26 0.18 0.12

100.25 99.60 100.45 99.65 99.77 99.46 100.75 99.69 99.27 100.93 Fe20 a : FeO 0.24 0.33 0.63 0.46 0.74 0.38 0.58 0.57 0.40 0.54

C. I. P. W. N O R M S (calculated to 100~o water free) Q Z 5.12 5.26 4.70 6.87 7.42 8.33 3.37 8.95 10.15 7.45 OR 9.81 9.75 18.88 11,17 10.22 12.18 18.68 12.83 9.99 13.95 P L A G 55.44 55.13 58.84 53.07 60.69 55.66 50.32 56.36 58.45 59.72 DI 6.74 7.60 7.35 7.24 3.63 6.01 9.67 4.57 4.11 5.24 HY 18.33 17.90 16.01 17.62 13.74 13.31 14.17 13.04 13.33 9.65 M T 2.39 2.26 2.17 2.17 2.16 2.13 2.09 2.07 1.97 1.84 IL 1.73 1.63 1.60 1.46 1.65 1.82 1.44 1.65 1.52 1.63 AP .44 .44 .46 .39 .48 .55 .26 .52 .46 .52 COR 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Total 100.00 99.98 100.00 100.00 100.01 99.99 100.00 An Percent 43.9 43.0 40.1 39.7 43.1 41.2 36.2

M O D E S Phenocrysts 26.2 6.8 42.1 27.6 Plag. 7.0 2.6 9.2 36.1 21.8 Q t z . - - . . . .

Oliv, 8.1 1.2 tr - - 0.7 Cpy. 5.2 1.3 7.3 1.9 2.1 Opy, 4.6 1.7 5.0 1.9 2.9 Hb. - - . . . . Bio . . . . . Opaques 2.3 tr 0.1 2.3 0.1

100.00 99.99 100.00

41.5 41.4 36.4

2.9 16.4 0.3 13.6

- - tr 0.6 0.4 2.0 1.4

- - t r

- - t r

150 m thick) and l igh t -co loured lava flows. T h e i r silica c o n t e n t is more t h a n 60 ~ t he re is more t h a n 10 ~ of n o r m a t i v e quar tz , a n d the n o r m a t i v e plagioclase is less calcic t h a n An40. T h e y are h igh ly po r phy r i t i c a n d con ta in phenoe rys t s of h o r n b l e n d e a n d b io t i te in add i t ion to va r i ab le a m o u n t s of cl ino- a n d or thopyroxene .

T h e h o r n b l e n d e is genera l ly a dark b r o w n var ie ty ( lamprobol i te ) excep t in spe- c imen 175 w h e r e a g reen var ie ty is p resen t . H o r n b l e n d e forms wel l s h a p e d pr isms, c o m m o n l y 1 to 2 m m long, occas ional ly r o u n d e d a n d w i t h an o p a q u e oxi- d ised rim. Biot i te is genera l ly sparse a n d is da rk b r o w n a n d more s t rongly oxi-

375

Aufs~itze

T A B L E 2 - pt. 2

487 281 490 56 187 184 169 182 61 284

SiO 2 60.20 61.00 61.60 61.90 62.30 62.50 62.80 62.90 63.00 63.10 TiOz 0.66 0.68 0.65 0.75 0.72 0.63 0.64 0.66 0.77 0.65 A120 3 17.50 18.10 17.30 16.60 16.20 15.90 16.20 16.20 15.00 16.40 Fe203 1.59 2.87 2.01 2.25 1.62 1.29 1.92 1.66 1.80 2.06 FeO 3.55 2.44 3.06 2.95 3.11 3.31 2.50 2.82 2.88 2.46 M n O 0.09 0.10 0.08 0.08 0.07 0.07 0.07 0.07 0.07 0.07 M g O 2.15 1.84 2.20 2.70 2.95 3.53 2.29 2.70 2.41 2.19 CaO 5.10 5.19 4.75 5.36 4.67 4.57 4.52 4.46 4.16 4.47 Na20 4.27 4.31 2.69 3.18 4.11 4.19 4.13 4.23 5.01 4.22 K20 2.33 2.21 2.59 2.92 2.93 2.81 2.87 2.93 2.44 2.93 P20 5 0.21 0.24 0.21 0.17 0.19 0.17 0.24 0.22 0.25 0.17 H~O + 1.15 0.19 0.55 0.37 0.77 1.26 1.14 1.04 1.06 0,23 H 2 0 - 0.63 0.58 0.62 0.18 0.30 0.53 0.32 0.34 0.31 0.74

FezOz : FeO 99.73 99.75 98.31 99.37 99.95 100.76 99.64 100.23 99.16 99.69

0.45 1.17 0.54 0.76 0.52 0.39 0.77 0.59 0.63 0.84

C. I. P. W. NORMS (Recalculated to 100~o water free) QZ 11.23 12.30 21.65 15.88 12.53 11.93 14.60 13.30 12.79 13.97 OR 14.13 13.18 15.84 17.44 17.49 16.79 17.26 17.49 14.72 17.55 P L A G 59.23 60.58 46.59 49.83 52.39 52.27 53.03 52.89 54.79 53.61 DI 1.95 .68 0.00 2.64 4.05 4.42 3.21 3.53 6.58 3.35 HY 9.94 9.68 10.59 10.68 10.13 11.47 8.66 9.55 7.47 8.38 NIT 1.75 1.74 1.61 1.73 1.58 1.55 1.48 1.49 1.58 1.49 IL 1.29 1.31 1.27 1.44 1.39 1.22 1.23 1.27 1.50 1.25 AP .48 .52 .48 .37 .44 .37 .52 .48 .59 .37 COR 0.00 0.00 1.95 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Total 100.00 99.99 99.99 100.01 100.00 100.01 100.00 100.01 100.02 100.02

An Percent 36.2 37.8 48.0 44.0 31.7 30.3 31.7 30.4 20.0 31.2

M O D E S Phenocrysts 27.5 50.9 54.2 43.6 28.6 39.0 37.1 36.4 39.6 Plag. 19.8 39.1 39.0 30.4 21.9 27.7 25.1 28.7 26.8 Qtz. - - tr . . . . . . Oliv. - - - - tr 0.5 . . . . Cpy. 0.64 2.4 1.1 5.0 2.9 3.1 2.6 1.2 0.6 Opy. - - 4.8 3.1 5.8 2.2 1.3 7.5 5.5 2.2 Hb. 5.0 0.5 5.1 - - 0.5 3.8 - - tr 7.9 Bio. - - 0.5 tr - - 0.5 1.8 0.2 tr tr Opaques 2.1 3.4 0.8 2.5 0.1 1.3 1.7 1.0 2.1

dised than the hornblende. I t is par t icular ly abundan t in the ignimbri te spec imen 56, which is also no tewor thy because it is the only rock which contains moda l quartz. Or thopyroxene is invar iably present as wel l - formed, faint ly pleochroic pr ismat ic crystals, general ly smaller than those of hornblende , and is commonly r immed by hornb lende or el inopyroxene. T h e plagioclase phenocrysts are ve ry similar in overall appearance to those in the basalt ic andesites, but are dist inctly less calcic be ing most ly andesine (the plagioelase for the most part covers the range An 60 to 40 though it is sometimes more calcic, as in 281). Specimens 184 and 187 are anomalous in containing a small amount of olivine. The remaining sample, 462 is rhyolite.

376

P. W . FIIANCIS et al. - - T h e San Pedro and San Pablo volcanoes of n o r t h e r n C h i l e

T A B L E 2 - pt. 3

177 174 54 172 500 175 191 166 168 462

SiO2 63.20 63.20 63.30 63.40 63.50 63.80 63.80 63.90 66.00 78.26 TiOz 0.64 0.66 0.51 0.63 0.65 0.56 0.64 0.65 0.46 0.13 A12Oa 15.80 15.80 16.70 16.40 16.10 15.40 15.90 15.90 16.00 12.29 F%O3 1.73 1.88 1.72 2.11 1.47 1.61 1.65 1.88 1.36 0.53 FeO 2.67 2.57 1.46 2.32 2.84 2 .19 . 2.60 2.50 1.68 0.38 M n O 0.07 0.07 0.05 0.07 0.07 0.06 0.06 0.07 0.05 0.06 M g O 2.39 2.58 1.62 2.35 2.18 2.02 2.56 2.37 1.06 0.19 CaO 4.20 4.18 3.83 4.47 4.48 3.16 4.15 4.15 3.29 1.00 Na=O 4.25 3.90 4.43 4.18 4.18 4.11 4.21 4.01 4.45 4.13 KzO 3.08 2.94 2.78 2.91 2.95 3.21 3.09 3.02 3.17 2.99 P=O5 0.21 0.20 0.22 0.18 0.22 0.21 0.17 0.22 0.16 0.03 H20 + 1.03 1.24 3.40 0.44 nil 1.81 0.57 0.55 0.99 - - H=O- 0.33 0.43 0.33 0.14 0.06 0.63 0.14 0.16 0.46 - -

99.40 99.65 100.35 99.60 98.70 99.22 99.54 99.38 99.13 99.99 Fe=Oa : FeO 0.65 0.73 1.18 0.91 0.52 0.75 0.64 0.75 0.81 1.39

C. I. P. W. NORMS (Recalculated to 100% water free) Q Z 13.92 16.01 16.64 14.46 14.85 16.99 14.59 16.18 19.26 40.02 OR 18.56 17.73 16.90 17.38 17.67 19.56 18.44 18.09 19.15 17.67 P L A G 51.86 50.91 56.55 53.24 52.57 50.43 51.52 51.02 53.15 39.73 DI 3.91 2.23 .19 3.05 3.72 2.18 3.45 2.35 .90 0.00 H3, 8.57 9.91 7.18 8.80 7.99 7.96 8.98 9.17 5.28 1.45 MT 1.48 1.49 1.06 1.46 1.45 1.29 1.42 1.45 1.01 .29 IL 1.23 1.27 .99 1.22 1.25 1.10 1.23 1.25 .89 .25 AP .46 .44 .50 .41 .48 .48 .37 .48 .35 .07 COR 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 .52

Total 100.00 An Percent 28.1

Phenocrysts Plagioclase Qtz. Oliv. Cpy. Opy. Hb. Bio. Opaque

100.00 100.01 100.02 99.99 100.00 100.01 100.00 99.99 100.00

32.5 30.5 31.6 30.5 27.7 29.0 31.4 26.4 11.4

M O D E S 33.3 21.9 39.2 15.0 28.8 33.0 40.0 23.5 16.1 26.8 10.4 16.9 22.5 33.8

. . . . 1.6 - - 0.7 3.6 - - 1.7 1.5 2.7 5.0 4.0 2.2 3.9 1.7 4.7 5.9 4.6 - - 2.5 1.9 7.5 tr 0.5 1.1 0.1 1.4 - - 1.9 - - 1 .2 1.0 1 .4

Ahnost all of the analysed rocks are porphyri t ic . The content of phenoerysts ranges f rom 8 ~ (specimen 287) to 55 ~ (specimen 56) and averages roughly 30 ~ No a t t empt was made to analyse the groundmass separately. Some small xenoliths occur in some of the lavas. T h e y are of volcanic rocks. Most of the hornb lende andesi te lavas are more or less non-vesicular , bu t the pyroxene andesites are sometimes h ighly vesicular, for example nos. 287 and 64.

One of the analysed samples (sp. 56) is a densely we lded ignimbri te . I t is conspicuously rich in crystals of biot i te and plagioclase, wh ich together const i tute about 50 0/0 of the total volume. Orthopyroxene, hornb lende and quar tz a r e also present in subordinate amounts in a glassy matrix. Since there is reason to doubt

377

280 - - 186 - - 285 - - 176 - - 190 - - 283 - - 6 4 - -

289 - - 287 - - 1 7 0 - - 487 - - 281 - - 490 - -

5 6 - - 187 - - 184 - - 169 - - 182 - - 6 1 - -

284 - - 177 --- 174 - - 5 4 - -

172 - - 500 - -

175 - - 191 - - 166 - - 168 - - 462 - -

Aufs~itze

Table 8. Specimen localities.

Pyroxene andesite lava, small vent, Central Crater, San Pablo Pyroxene andesite lava, W. San Pedro, Lower flanks Pyroxene andesite lava, Older cone, E. San Pedro Pyroxene andesite bomb. Near roadside, W. San Pedro Pyroxene andesite lava, Corry, high flanks S. San Pedro Pyroxene andesite lava, Middle Group, San Pablo, N. side central crater Pyroxene andesite lava, La Pornna flow Pyroxene andesite semi-breadcrnst bomb, roadside, W. San Pedro Pyroxene andesite vesicular lava, Col between San Pedro and San Pablo Pyroxene andesite bomb. Scoria flow, W. San Pedro Hornblende andesite. Breadcrust bomb, E. Cone San Pablo Hornblende andesite lava, cliffs in E face Central crater (collected from scree) Hornblende andesite lava, Lower Group, N. E. San Pablo Hornblende andesite upper Ignimbrite, Rio San Pedro Gorge Pyroxene andesite lava, W. flanks San Pedro Hornblende andesite lava, W. flanks San Pedro Hornblende andesite "hot" block from hot avalanche deposit. W. San Pedro Hornblende andesite lavas, W. flanks, San Pedro Pyroxene andesite lava, massive flow S. W. San Pedro Hornblende andesite lava, summit group, E. cone, San Pedro Hornblende andesite lava W. flanks, San Pedro Hornblende andesite "hot" block from avalanche deposits, W. San Pedro Hornblende andesite Pumice, lower ignimbrite, Rio San Pedro gorge Hornblende andesite "hot" block from youngest avalanche, W. San Pedro Hornblende andesite breadcrust bomb. Adjacent to massive lava, S.W. San Pedro Hornblende andesite pumice. Pumice flow W. San Pedro Hornblende andesite lava, thick flow high flanks W. San Pedro Hornblende andesite lava, coul6e flow, roadside, W. San Pedro Hornblende andesite hot block from avalanche deposit, W. San Pedro Rhyolitic pumice from lowest ignimbrite in Polapi gorge, 20 kin. N.W. San Pedro

whether a whole rock analysis of an ignimbrite is representative of the source magma, because of crystal concentration processes operating during eruption and transportation of the material 1) (HAY, 1959; LIPMAN 1967; WALCER 1972), the other analyses relating to ignimbrites (specimens 54, 462) have been made on large pumice clasts separated from the ignimbrite. A pumice sample (175) from the pumice flow on San Pedro was also analysed. Of these three pumice samples, 54 and 175 are hornblende andesite containing abundant plagioclase and hornblende phenocrysts, while the third (462) is a rhyolite containing mostly biotite phenocrysts. The last is the only rock which is chemically distinguishable from the lavas of San Pedro.

Great care was taken to collect the freshest material possible but the rocks show variable degrees of oxidation with the ratio Fe~O3: FeO varying from 0.15 to 1.4 and averaging 0.6, and in many of the andesites the biotite and hornblende are partially to completely oxidised. This average is lower than that of other published Andean data and in particular is much lower than the average

1) In this connection it is worth noting that the analysed wholerock welded ignimbrite (spec. 56) has a composition incompatible with the phenocryst assemblage present: the silica content is 61.9, yet there is a great abundance of biotite and even a little modal quartz. This, coupled with the very high content of crystals, suggests that there has been a substantial loss of siliceous glass.

378


F F Fe203 + FeO ~ M MgO

A Na20 + K20 �9 Lava fl?w

A M

Fig. 12. (above) FMA plot of the new analyses of San Pedro and San Pablo. (below) Comparative FMA diagrams of rocks from northern Chile from A - - KATSUI & GON- ZALEZ (1968); B - GUEST (1969); and C - - ZEIL & PICHLER (1969).

F

/' /

j ~ • ~ o /~ ~

// ~• / . z ~ • * ~

,,/xx x �9

Fe 0 + Fe203 MgO N a20 + K20 Lava flow Ignimbrite

\ \

\\\ \ \

\\

'\,M of 14.0 given by sixteen analyses of ignimbrites (GuEsT 1969) which presumably reflects a systematic difference in cooling conditions between lavas and ignimbrites.

The major element data is plot ted on FMA, CaO - - Na~ - - K~O, alkali-silica and potash-silica diagrams (figs. 12--15). Plots for other Andean analyses also given show a remarkable similarity in major element chemistry for volcanic rocks in N. Chile between the latitudes of 18 ~ 25' S and 22 ~ 05' S.

There is a complete gradation both mineralogieally and chemically from the pyroxene andesites to the hornblende andesites. Some hornblende andesites which contain 60 to 68 ~ silica have only a sparse scattering of part ly resorbed hornblende and there is no sharp break between the chemical and normative differences listed for the groups. There does not appear to be any systematic change of composition with time, but the pyroxene andesites predominate amongst the earliest products of each volcano and the hornblende andesites

379

Aufs~ttze

amongst the latest. The single analysis of the La Poruna flow (spec. 64) is, however distinctly richer in potash than the other lavas of similar silica content.

The detailed discussion of the volcanic chain between latitudes 21 and 22 ~ south is at present being prepared. KATSUI (1972) has discussed the large scale significance of the petrochemistry of the volcanic rocks of the Central Andes in the light of the available analyses as part of a study of Cenozoic volcanic provinces in the Andes and Antarctica, so the discussion here is therefore limited, and will be confined to a few general points.

Firstly, the overall range in composition of the analysed rocks of San Pedro - - San Pablo is rather small. All but one of the 80 analyses fall in the range

CaO . ~ Lava flow

Na20/ . ~ \K20 Fig. 18. CaO/Na~O/K~O plot of the new analyses.

56--66 ~ SIO2, with a main peak in the frequency curve at around 68 to 64 ~ SiO2. This distribution is typical of 115 other analyses made by the authors and is in agreement with the findings of ZEro & PICnLEa (1967) who analysed 24 specimens collected over a wide area of northern Chile. ZEIL & PICnLER, though they preferentially collected "olivine-bearing dark rocks of basaltic appearance", found no rocks with less than 520/o SiO2 and this, in conjunction with the present evidence shows that, contrary to earlier views (e. g. PETEaSEN, 1958) basalts are rare or absent from the Central Andes. Secondly, the Andean volcanic suite appears to be significantly more siliceous than the volcanic suites of the western Pacific area (see, for instance, the average andesite of NOGKOLDS, 1954, and TAYLOR ~x: WHITE, 1965, 1966, and the compilations of DICKINSON, 1968). This is not merely a difference in terminology (though some of the San Pedro rocks would be called dacites by other authors), but does seem to reflect a basic difference between the most abundant 'andesitic' rocks of the western and eastern Pacific. Typical andesites from the Western USA and Mexico are more similar to those of the Andes than the West Pacific andesites, but the andesites of both the West Pacific and of the USA are associated with basalts to varying degrees which may be a func- tion of the variation in thickness of the continental crust in different areas around the circum-Pacific belt. Thirdly, ZEIL & PICHLER showed in their analytical work

380

Na2o +

K2o


�9 Lava flow

x I g n i m b r i t e

o o l

• j ,

o

0 50

x

x x

�9 .7. : ' . ~ x •

Q o o

60 70 50 60 70 SiO 2 SiO:z

Fig. 14. Plot of total alkalies against silica for the new analyses of San Pedro and San Pablo (left) and other rocks from northern Chile (right) by KxrsvI & GONZALEZ (1968),

GUEST (1969), and ZEIL & PICHL~R (1969).

i : �9 Lava .flow

x Ignimbrite

x 34 .Xl~x

K~O ...

2~ �9 i �9 �9 %

• Xx xX

�9 x x X X Xx~ >.:x :

i e l �9 t o �9

�9 �9 A % , -~-3

- - : ' 7 . - ; " -2

|

50 60 Si02 70 50 dO SiO:~ 70

x

x x x �9 x x

x x x Xe •

x x

Fig. 15. Plot of potash against silica for the new analyses of San Pedro and San Pablo (left) and other rocks from northern Chile (right) by KATSUI & GONZALEZ (1968), GUEST (1969), and ZEro & PICHLER (1969). The dashed lines are for other circum- Pacific areas (after DICKINSON, 1968); 1 - - Izu Islands and Peninsula; 2 - - Central

North Honshu; 8 - - Central Mexico; 4 - - Ryukyu Islands and Kyushu.

that the andesites from the Central Andes are notably rich in potassium, and sug- gested that there might be a broad zonation of potash content along the length of the Andes. KaTSUI (1972) has compiled the available data and eommented that the volcanic rocks of the Southern Andes are eharacterised by the occurrence of abundant high-alumina basalts ass�9 with calc-alkaline rocks, while rhyolites are rare or absent, whereas in Patagonia, east of the main Cordillera alkali-olivine basalts predominate. The extension of the Andean range in the South Sandwich islands is charaeterised by tholeitic basalts notably poor in KeO.

The average K20 content of the 80 new analysed rocks is '2.8 ~ strikingly higher than that of the andesites from many other areas. DmKINSON (1968) and

381

Aufs~itze

others have shown the importance of the K~O content of andesites in interpreting their origin, and it is well established that andesites from different regions show very different K20:SiO2 trends. On DIGKINSON'S plot, the An- dean andesites occupy a distinctly different field, having higher potassium contents than other eireum Pacific groups (fig. 15). High potassium contents in igneous rocks have often been used to infer an origin by the melting of silica continental crust, to the KzO content has been related to the crustal thickness and to the distance above the Benioff zone. Zeil and Piehler concluded from this and other evidence that the andesites of the Central Andes were in fact derived by such crustal melting. KATSm (1972) has discussed the problems of the genesis of the high-potassium high silica Andean andesites more extensively.

6. Discussion

San Pablo, San Pedro and La Poruna form a 25 km long ridge aligned eastwest across the trend of the Andes. Other notable alignments within the same general area are the 25 km long Aucanquileha ridge, trending west north-west, and the 85 km long line of volcanoes Carasilla, Polapi, Cebollar, Ascotan and Palpana (fig. 1). Only Aucanquilcha and San Pedro have had any postglacial volcanic activity and in both cases this occurred at the western end of the ridge. The San Pablo--San Pedro- -La Poruna ridge is the best known structurally, and shows evidence of a westwards migration of activity on each cone as well as from one cone to another. In contrast, plutonic foci in the Central Andes have been migrat- ing towards the east since early Mesozoic times (RuIz FULLER et al. 1960; GILLETTI & DAY 1968; FaRRAR et al. 1970) and the same appears to apply to the Meso- zoic and Tertiary volcanics (RuTLaND 1971). This has lent support to current ideas on the evolution of the South American Plate margin and it is supposed that the present andesite volcanoes are located above the youngest plutonic foci (HAMILTON 1970). However, what evidence there is in the present area suggests a samller-scale migration in the reverse direction, the significance of which is uncertain.

Very thick andesite flows are conspicuous on the volcanoes, and the thickness (commonly 200 m or more) indicates that the lava had a high viscosity. A large part of the clastic rocks which form an extensive apron around the lower slopes of San Pedro are coarse monolithologie breccias made of nonvesicular hornblende andesite. These breecias are interpreted as the product of the gravitational collapse of large masses of lava built up on steep slopes high on the volcano. In many cases this lava was hot, at the time of its collapse being in the condition either of a solid rock or a highly viscous melt, though behaving as a brittle rather than a plastic material. I t is also evident that many of the avalanches originated from lava bodies some distance down-slope from the eruptive vent, the collapse scars left by these avalanches being in some cases clearly preserved, and this is cogent evidence that explosions are not likely to have partieipated in causing the collapses. Well authenticated small-scale examples of such collapses are known on Ngauruhoe (GREGG, 1956) and were seen by some of the present authors in 1970 on Santiaguito, Guatemala.

The authors have used the term 'hot avalanche' advisedly because they regard the phenomenon as secondary to the effusion of lava and not directly related to

382


primary happenings in the vent area. The deposits described here are nu6es ardentes pel6ean d'avalanche in the classification of LAc~OiX (1980) and MAcGRE- COR (1952) or nu6es ardentes in the strict sense in that proposed by ArtAUAKI (1957). ARAMAKI & YAMASAKI (1968) subsequently revised this classification, and proposed another based on the volumes and densities of the pyroclastie flows, suggesting that the most dense and least pumiceous flows were also those of smallest volume. The present authors have found that while in the Pedro area there may be a crude correlation between volume of flow and density (and hence vesicularity) of material it is difficult to apply divisions based on volumes; the dense pyroclastic flows listed by ARAMAKI & YAMASAKI all have volumes less than 0.8 km, whereas some on San Pedro are probably much larger than this.

The hot avalanche deposits are remarkably similar to those of conventional 'cold" rock avalanches such as are found in non-volcanic areas, for instance those described or collated by CRANDELL & FAHNESTOCK (1965), KENT (1966), SHREVE (1966, 1968) and WATSON & WalCHT (1969). These similarities include the ratio of the vertical fall V to the horizontal distance travelled H (this ratio being called the "coefficient of friction" by SHREV~, 1968). A plot of V against H (fig. 16) for described examples shows that cold avalanches occupy a well defined field with V/H ranging from 0.8 to less than 0.1, and the hot avalanche deposits of San Pe- dro plot within the same field, giving values lying between 0.2 and 0.15.

Many of the described nudes ardentes of other volcanoes have also been plot- ted on this figure. They include hot avalanche deposits which, from the available descriptions, appear to be similar to those of San Pedro, for instance the 1930 nude ardente of Merapi (BEMMELEN 1949) and probably also Chaos Jumbles and the 1916 nu6e ardente of Lassen Peak (WILLIAMS 1928, 1982; FINCH, 1985; HEATH, 1960). Several others, attributed to directed explosion, such as M. Pelbe (LAcaolx, 1904, 1980); could be of similar origin. These likewise plot in the same field as the cold rock avalanches. While there is evidence from San Pedro of purely mechanical instability being the cause of collapse, it is of course possible in other instances that explosions triggered the avalanche, and this must be so in the 1888 Bandai-san (SEKIYA & KIKUCHI, 1890) and 1956 Bezymianny (GORSHKOV, 1959) examples.

Figure 16 also includes nu6es ardentes rich in pumiceous material which are manifestly not due to the gravity collapse of lava bodies, examples being Mayon, 1968 (MooRE & MELSON, 1969), Hibok Hibok 1950--1 (MACDONALD & ALCA~AZ, 1956), Komagatake, 1929 (MuRAl, 1960) the 1788 Agatsuma flow on Asama (ARA- MAKI, 1956, 1957) and the pumice flow H on San Pedro. These plot in the same field and their flow could be by the same mechanism.

The mechanism of transport of hot avalanches, such as those on San Pedro, and of nu6es ardentes in general may not therefore be any different from that of 'cold" rock avalanches. It has been concluded by SHREVE and others that a cold rock avalanche or landslide starts as a simple rock fall high on the steep slopes of a mountain and then travels by "overriding and trapping a cushion of compressed air upon which it traverses the gentle slopes below with little friction" (SHREVE, 1968, p, 87). There is no reason to doubt that the same mechanism applies to the hot avalanches, and the fact that they are hot may be of little significance, although McTACCAaT (1960) showed that the heating of trapped air could increase to some extent the distance travelled. For the Saidmarreh land-

383

Aufsiitze

slide, however, WATSON & WmGrIT (1969) concluded that sliding took place on pulverised marl and gypsum bedrock.

I t is noteworthy that, perhaps contrary to expectation, the hot avalanche and pumiceous nu6e ardente deposits on the whole give a larger V/H value than the large cold avalanches. The authors suspect that the horizontal distance travelled for a given vertical drop is related more to the size of the debris flow than to its temperature: the greater the volume of the flow the farther it is likely to travel. Measurements on the volume of the debris flows are included on fig. 11 where they are available, though the data are insufficient to test fully this relationship.

V o Nude ardente deposits k~ �9 San Pedro avalanche deposits . . . . . . . . .

J ~ / . . ' / * / o / / /o.~ ." o / ,8

/ / "% / ~ o / 15

x O 0 0

~b i~ is H ~s km

Fig. 16. Plot of vertical drop V against horizontal distance travelled, H, for cold rock avalanches in non-volcanic areas; hot avalanches and pumiceous nu6e ardente deposits; and two cold avalanches of San Pedro. Lines of specified V/H ratio are given. The figures, below some of the numbers give the volume of the deposit in units of 108 m 3.

Large-volume ignimbrites are highly pumiceous deposits which have a remarkably wide extent. I t is usually not possible to determine the vertical fall, V, because caldera collapse consequent on the ignimbrite eruption has also removed the top of the volcano, but many ignimbrites must plot well below and to the right of the field of debris flows recorded on fig. 16. Ignimbrites are thought to be transported in a fluidised state as what can justifiably be called "pyroclastie flows". The transportation and maintenance of the fluidised state of the flows may have been aided by the evolution of juvenile gas. The long distance of travel is no doubt facilitated by the relatively low density of the pumice fragments, but it is possible that the large volume of erupted material is also important. Mud- flows likewise often give very low V/H values, flow being facilitated in this case by water.

One of the main problems of the area is the surce of the ignimbrites which form an extensive inclined plateau west (and probably also east) of the andesitic volcanoes. In many other volcanic areas, ignimbrite eruptions are related to the formation of calderas, but calderas are conspicuously lacking in northern Chile. The present work throws light on the problem in three ways. Firstly, the pumice flow which occurs on the western slopes of San Pedro is known to have originated from that volcano, and there is no reason to believe that the style of eruption responsible for it was in any way different from that responsible for the ignimbrites, the only difference being that the pumice flow is less extensive and smaller

384


in volume than most of the ignimbrites. Secondly, the hot avalanche deposit and debris flow deposits between ignimbrites in the Rio San Pedro gorge indicate that the period of eruption of the ignimbrites overlaps that of the andesite cones. The hot avalanche deposit indicates the existence of a steep cone before the top- most ignimbrite was emplaced. Moreover the ignimbrites contain andesite clasts which must have come from andesite volcanoes. Thirdly, the chemical composition of the pumice in some of the ignimbrites is similar to that of the andesite volcanoes proper, suggesting a common source.

The ignimbrite plateau of northern Chile covers more than 150,000 km e and undoubtedly has a volume of at least several tens of thousands of km 8 (ZEIL, 1964). In any one place it is generally seen to consist of several separate ignimbrites and the plateau as a whole is probably composed of a great many separate ignimbrites. An analogy can be drawn here to the structure of a typical plateau basalt pile made up of many separate lava flows. Available data on the volumes of individual ignimhrites are scarce but two (GuEsT, 1969) are an order of magni- tude larger than the largest known lava flow in the area. (GUEST • SANCHEZ, 1969). However, there is likely to be a wide range in size and the smallest could be comparable in size with the pumice flow on San Pedro.

Various authors have drawn attention to the striking contrast in the Central Andes between the andesitic stratovolcanoes (the 'Andesite Formation ' of some) and the ignimbrite plateau (the "Rhyolite Formation"), this contrast being chemical, morphological and apparent ly also a stratigraphieal one. I legarding the chemical contrast, the two groups overlap in composition and plot on the same variation curves, and this might imply a common source, but they appear to constitute separate populations. Thus the published chemical analyses of rocks from the stratovolcanoes of northern Chile (CASERTANO, 1962; KATSUI &GONZALEZ, 1968; ZEIL & PICHLEI/, 1967; SIEGERS, PICHLEB ~: ZEIL, 1969; GUEST & SANCItEZ, 1969) mostly lie in the range 55--64 ~ SiO,, and the 30 new San Pedro/San Pa- blo analyses are mostly in the range 53- -66 ~ SiO2. In contrast, the 50-odd available analyses of ignimbrite in northern Chile and Southern Pern (JENKS & - GOLDICH, 1956; KATSUI ~x:GoNZALEZ, 1968; ZEIL &PICHLER, 1967, GUEST, 1969, this paper and unpublished analyses from the country north of S. Pedro) mostly lie between 60 and 76 ~ SiO,, with a peak in the frequency curve between 70 and 75 ~ SiO,,. The difference must be a real one and is not due to a collecting bias.

Regarding the stratigraphic contrast, the statement has often been made that the ignimbrites on the whole precede the stratovolcanoes. Thus for example, PICH- LER & ZEIL (1969, p. 167) state that "The young andesitie stratovoleanoes pierce vast sheets of rhyolite to rhyodaeite ignimbrites", and KATSUI & GONZALEZ (1968, p. 6), state that "Two groups of volcanoes overlie the Rhyolite Formation". A contrary opinion is, however, stated by GUEST (1969) who suggests that the nmin period of ignimbrite eruptions was both preceded and followed by the eruption of andesite lavas, There is no doubt that morphologically the stratovolcanoes appear to be built upon the ignimbrite plateau. Available K/Ar dates for the ignimbrites range from Upper Miocene to Lower Pleistocene (CLAm; et al. 1967; HOL- LINGWOltTH, 1964; DINGMAN, 1965; RUTLAND et al. 1965); but dates are still lacking for the stratovoleanoes. Some volcanoes are morphologically youthful but little is known of the rate of erosion in this part of Chile, and at present the rate is

385

Aufs~itze

certainly exceptionally low. Some of the morphologically youthful volcanoes could well be relatively ancient.

The reason for the contrast stems from differences in the prodominant style of eruption of the andesite-rhyolite magmas. The former tend mostly to produce lava flows which do not travel far, and their gravitational collapse derivatives. In contrast the more acid magmas tend to form pyroclastic flows which travel a long distance from the source. We have here an example of a process which might perhaps be termed "transport separation", whereby volcanic products of different compositions are transported to different distances from their source and ac- cumulate in different areas as a result of their differing styles of eruption.

Evidence presented above suggests that in the San Pedro area at least, the eruption of lavas and ignimbrites proceeded concurrently and this suggests the need for caution in interpreting the relative ages of the volcanoes and plateau ignimbrites, and the need for more age data especially on the volcanoes themselves.

Pyroclastic fall deposits, though they do occur, and locally can have a large volume comparable with that of an andesite lava, are less abundant relative to lava flows than in many other areas around the Pacific. This may be taken to imply a generally low gas content of the eruptive magmas.

Acknowledgements The authors acknowledge the financial support of the Royal Society and of the

Imperial College Exploration Board towards the cost of the field work in northern Chile; of the Natural Environment Research Council towards the cost of the follow- up work; the help of Drs. B. Gunn and G. BoRImY at the Geology Department, Imperial College, and Dr. G. HoanuNC of the University of Leeds for providing XIRF analytical facilities. They are greatly indebted to the Instituto de Investigaeiones Geo- logicas de Chile for assistance and the loan of aerial photographs, and to Drs. G. Caono and C. CHAVES of the Antofagasta office for their help and hospitality, also to Dr. J. MrEns for assistance in the field.

References ARAMAKI, S.: The 1788 activity of Asama volcano. Part 1. - - Jap. J. Geol. Geog., 27,

189--229, 1956. - - : Part II. - - ibid., 28, 11--33, 1957. AnAMAXI & YAMASAKI, M. : Pyroclastic flows in Japan. - - Bull. Volcanol., 26, 89--99, 1968. VAN BEMMELEN, IR. W.: Geology of Indonesia. Vol. VIa General Geology of Indonesia

and adjacent archipelagos. - - 782 pp., Batavia (Govt. Print. Off.) 1949. IRaUGCEN, J.: Fundamentos de la geologia de Chile. - - Inst. Geograf. Militar., 874 pp.,

Santiago, Chile, 1950. CASEaTANO, L.: Catalogue of Active Volcanoes of the World, including solfatara fields.

- - Part 15, The Chilean Continent, 55 pp., 1962. CLAaK, A.U., MAYER, H.E.S. , MOnTIMER, C., SILLITOE, IR. H., Coors, R.U., & SNEL-

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The San Pedro and San Pablo volcanoes of northern Chile and their hot avalanche deposits

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