Fiorentini, Erna: Oxygen isotope thermometry based on a refined increment method and its application...

GEOLOGICAL SURVEY DEPARTMENT

PROFESSIONAL PAPER NO. 5

THE CRYSTALLINE CRUST OFSRI LANKA

Part ISummary of Research of the

German-Sri Lankan Consortium

GEOLOGICAL SURVEY DEPARTMENT48, Sri Jinaratra Road - COLOMBO 2

MIMSTRY OF INDUSTRIES, SCIENCE AND TECHNOLOGYREPU"'16 OT SRI LANKA

Provided for non-commercial research and education use. Not for reproduction, distribution or commercial use.

Author's personal copy

All rights reserved, including fianslation into other languages. Material in this document may be

copied without rcstraint for library, educational or personal research service. However, republication of

any portion requires the written permission of the authors as well as appropriate acknowledgement of

this publication.

Deutsche Forschungsgemeinschaft (DFG)Priority Research Project "Composition, Structure and Evolution of the

Lower Continental Crust"

German-Sri Lankan Research Consortium

IGCP Project 304Lower Crustal Processes

THE CRYSTALLINE CRUST OF SRILANKA

Excursion Guide for Geological Field Trip in Sri Lanka,September 1991

This publication may be cited as: - Part IKröner, A., ed., 1991. The crystallini crust of Sri Lanka, Part I, Summary of Research of the Summary Of ReSearch of the German-Sri Lankan ConSOrtiumGerman-Sri Lankan Consortium. Geol. Surv. Dept. Sri Lanka, Prof. Paper 5, 271pp.

papers in this publication may be cited as: compiled and edited by A. KrönerAuthor, A.A., 1991. Title of paper. In: Kröner, A., ed., The crystalline crust of Sri Lanka, Part I,Summary of Research of the German-Sri Lankan Consortium. Geol. Surv. Dept. Sri Lanka, Prof. MainZ, JUne 1991Paper 5, pp. xx-yy.

PROFESSIONAL PAPER NO. 5

GEOLOGICAL SURVEY DEPARTMENT48, Sd Jinaratna Road - COLOMBO 2

MINISTRY OF INDUSTRIES, SCIENCE AND TECHNOLOGYREPUBLIC OF SRI LANKA

t99t

@ Geological Survey Department of Sri l^anka, 1991

Printed in Germany with funds of the German Ministry of Economic Cooperation (BMZ)



v

PREFACE

This Guide is intended to serve as an Introduction for a 4-day geological excursionin Sri Lanka, organized as part of an international Workshop on "Composition andevolution of high-grade gneiss terrains". It summarizes most of the research under-taken by a joint German-Sri Lankan Consortium and funded by the DeutscheForschungsgemeinschaft (DFG) as a contribution to the Special Research project"composition, structure, and evolution of the lower continental crust".

The idea for such a project originally took shape in discussions with my friend andcolleague P.G. (Jeny) cooray who, after retirement from his position abroad,returned to his home country and undertook considerable efforts in order to initiateinternational collaboration on the basement rocks of Sri Lanka. I visited Sri Lanka forthe first time in 1985 and undertook a reconnaissance field trip with Jerry and p.W.(Das) Vitanage, the two internationally recognized experts on Sri Lankan geologywho, in their early professional careers, had mapped portions of the basement for thäGeological Survey Department. on this trip we. came to Kurunegala and, in the nowfamous Udadigana Quarry, we saw dark patches of apparently irregular distribution ina light-grey gneiss. I immediately recognized this to be similar to the in-situ chamock-itization phenomenon previously only known from Archaean gneisses in the well-known quarry at Kabbaldurga in southern India. This, together with the excellentourcrops in countless qua:ries and the obviously complicated structural and meta-morphic evolution of the Sri Lankan basement, convinced me and several colleaguesin Germany to submit a proposal to the DFG for an interdisciplinary research ptoj"ct,to study, together with our Sri Lankan colleagues, the evolution of what we thought tobe a segment of typical Archaean deep continental crust.

The Project began in 1987, and it soon turned out that we were not looking at apiece of Archaean crust but rather at a segment that had a long Proterozoic and evenearly Palaeozoic history of deposition, deformation and metamorphism. It was our aimto unravel this protracted evolution, but we must now concede that, although havingmade some contributions to an understanding of deep crustal processes, as summar-ized in this contribution, we are far from understanding how the various domains,which we are now able to identify, came together and where they originated.

My first and foremost duty, on behalf of the German members of the Consortium,is to thank our friend and colleague Das Vitanage for his ccntinued interest and con-siderable suppolt during numerous field sessions in Sri Lzurka, and his wife Rana forall these fantastic oriental dinners which we had in the "Blue star" in Kandy. JerryCooray deserves our gratitude for many interesting scientific discussions and for hisorganisational skills to make the Workshop in Kandy a reality. We also thank ourcolleagues in the University of Peradeniya and in the Geological Survey Departmentfor cooperation and assistance during field work.

Last, not least, we thank the DFG and the German Ministry of EconomicCooperation (BMZ) for funding our research efforts and this Workshop. We alsoappreciate financial assistance from the Ministry of Economic Cooperati,on and theMax-Planck-Institut ftir Chemie towards the printing costs c,f this contribution.

Mainz, 2I June I99l A. Kröner, Editor



CONTENTS

THE GERMAN - SRI LANKAN CONSORTIUM ON LOWER CRUSTALSTLIDIES IN SRI LANKA: INTRODUCTION AND HIGHLIGTTTS(A.W. Hofmann)............. ....................I

LITHOTECTONIC SUBDIVISION OF THE PRECAMBRIANBASEMENT IN SRI LANKA(4. Kröner, P.G. Cooray and P.W. Vitanage)...... .........................5

SRI LANKA: STRUCTURAL, MAGMATIC AND METAMORPHICDEVELOPMENT OFA GONDWANA FRAGMENT(G. Voll and R. Kleinschrodt) ..........22

STRUCTLIRAL GEOLOGY OF THE SRI LANKAN BASEMENT .A PRELIMINARY REVIEW(L. Kriegsman)............... .................52

STRUCTURAL EVOLUNON OF HIGH-GRADE TERRAINS IN SRI LANKA WITHSPECIAL REFERENCE TO THE AREAS AROTIND DODANGASLANDA AND KANDY(K.V. W. Kehelpannala)................... ...................69

GRAVIMETRIC INVESTIGATIONS ALONG TECTONIC BOL]NDARIESBETWEEN T}IE HIGHLAND/SOUTHWESTERN COMPLEX AND TT{EVIJAYAN COMPLEX IN SRI LANKA(G. Büchel). ................89

CFIEMICAL COMPOSITION OF TIIE SRI LANKAN PRECAMBRIANBASEMENT(J.R. Pohl and R. Emmermann) .......94

GEOCFIEMISTRY OF TI{E MAFIC INTRUSION IN T}IE SYNFORMS OFTFIE KAT{DY AREA(H.-G. Stosch)................. :i................... ..................125

COMPOSITIONAL CHARACTERISTICS OF THE VIJAYAN COMPLEX(C.C. Milisenda)............. ..............135

GEOCHEMISTRY OF CHARNOCKITE FORMATION AT KURUNEGALA.SRI LANKA(C.C. Milisenda, J.R. Pohl and A.W. Hofmann)................... .......................... l4l

METAMORPHIC ZONATION AND P-T HISTORY OF THE HIGHLAND COMPLEX INSRI LANKA(V. Schenk, P. Raase and R. Schumacher). .........150

CHARACTERIZATION OF HIGH.GRADE METAMORPHISM IN THE SOUTI{ERNPART OF SRI LANKA(M. Raith, S. Faulhaber, R. Hoffbauer and B. Spiering)................. ...................164



vlu

SAPPHIRINE-BEARING GRANULITES FROM CENTRAL SRI LANKA .

OUTCROP DESCRIPTION AND MINERAL CHEMISTRY(L. Kriegsman)............... ...............179

METAMORPHISM AND NATI.JRE OF THE GRANULITE-FACIES CRUST INSOUTHWESTERN SRI LANKA: CHARACTERZANON B Y PELITIC/PSAMMO-PELITIC ROCKS AND ASSOCIATED GRANTILITES(W.K.B.N. Prame)......... ................188

PETROLOGY OF THE KATARAGAMA COMPLEX, SRI LANKA: EVIDENCE FORHIGH P,T GRANL]LITE-FACIES METAMORPHISM AND SUBSEQUENT ISOBARICCOOLING(W.K.B.N. Prame)......,.. ................200

OXYGEN AND CARBON ISOTOPE RATIOS IN HIGH-GRADE ROCKSFROM SRI LANKA AS A MONITOR OF FLUID.ROCK INTERACTION(S. Hoernes, E. Fiorentini and R. Hoffbauer)..... .......................225

GEOCHRONOLOGY OF THE SRI LANKAN BASEMENT(S. Hölzl, H. Köhler, A. Kröner, P. Jaeckel and T.C. Liew)............. .................237

ISOTOPIC CHARACTERIZATION OF TI{E HIGH.GRADE B ASEMENTROCKS IN SRI LANKA(T.C. Liew, C.C. Milisenda and A.W. Hofmann)...... ................258

EXSOLUTION AND ORDERING STRUCTURES IN FELDSPARSFROM HIGH.GRADE METAMORPHIC ROCKS IN SRI LANKA(C. Evangelakakis, H. Kroll and G. Voll)........ .........................267



224

Raith, M., Raase, P., Ackermand, D and Lal,.R.K., 1983. regional geothermobaro-metry in the granulite facies terrane of South India: Tansac. Royal Soc. Edinb.Earth. Sc., 73, 221-244.

Ramiengar, A., Ramakrishnan, M and Viswanath, M.N., 1978. Charnockite-gneisscomplex relationship in southern Karnataka. J. Geol. Soc. India, 1,9,411-419.

Schumacher, R., Schenk, V., Raase., P and Vitanage., P.W., 1990. Granulite faciesmetamorphism of metabasic and intermediate rocks in the Highland Series of SriLanka In Ashworth, J.R and Brown, M (eds), Allan and Unwin, London, High-gnde metamorphism and crustal anatexis.

Sen, S.K and Battacharya, A., 1984. An orthopyroxene-garnet thermometer and itsapplication to the Madras charnockites. Contrib. Mineral. Petrol. 88, 64-71.

Shiraishi,K., Hiroi, Y and Onuki,H, 1984. Orthopyroxene-bearing rocks from the

Tenmondai and Naga-Iwa rocks in the Prince Olav Coast, East Antarctica Firstappearance of orthopyroxene in progressive metamorphic sequence. Mem.National Institute for Polar Research, Special Issue 33, 126-144.

Stormer, J.C and Whitney, J.A., 197'7. Two-feldspar geothermometry in granulitefacies metamorphic rocks. Sapphirine granulites from Brazil. Contrib. Mineral.Petrol 65,123-133.

Vitanage, P.W., 1959. Geology of the country around Polonnaruwa. Geol, Surv. SriLanka, Prof. Paper 1, 75 p.

Vitanage, P.W., 1985. Tectonics and mineralisation in Sri Lanka. Bull. Geol. Soc.

Finland, 57 , l5'7 -168.Volfinger, M., Robert, J.L., Vielzeuf, D and Neiva, M.R., 1985. Structural control of

the chlorine content of OH-bearing silicates (micas and amphiboles). Geochim,Cosmochim. Acta, 49, 37 -48.

Voll, G and Kleinschrodt, R., 1991. Sri Lanka: Structural, magmatic and metamorphicdevelopment of a Gondwana fragment. This volume, 22-51.

Wickramasinghe, O.C., 1969. Geochemistry and geochronology of charnockites and

associated rocks of Ceylon Unpubl. Ph.D thesis, Univ. [.eeds, U.K.Winkler, H.F.G., 1979. Petrogenesis of metamorphic rocks, 5th ed. Springer-Verlag,

New York.Wones, D.R., 1989. SignificancO of the assemblage titanite+magnetite+quartz in

granitic rocks. Am. Mineral., 7 4, 7 44-7 49.

Yoshida, M., Hiroi, Y and Vitanage, P.W., 1987. Tectonics and metamorphism ofPrecambrian rocks of central to south-western Sri Lanka. In: Abstr.-vol.. Pre-

cambrian Events in the Gondwana Fragments, IGCP 236 meeting, Sri Lanka, 34.

225

OXYGEN AND CARBON ISOTOPE RATIOS IN HIGH.GRADE ROCKS FROM SRI LANKA AS A MONITOR OF

FLUID.ROCK INTERACTION

S. Hoernes, E. Fiorentini and R. Hoffbauer,Mineralogisch-Perologisches Institut, Univenität Bonn, Poppelsdorfer Schloß,

5300 Bonn, Germany.

INTRODUCTION

Stable isotope compositions of minerals and rocks can be used to model fluid-rockexchange processes. The sensitivity of the method depends on the degree of isotopicdisequilibrium between any invading fluid and the reacting rock-system and on therelative amount of the interacting isotopic species. For example, the amount of meteoricwater in hydrothermal systems is most effectively modelled by hydrogen isotopes, be-cause of the predominance of hydrogen in water relative to any rock as demonstratedconvincingly by Taylor (1977).

Stable isotope data have been used to address the problem of granulite formation(e.g. Valley and O'Neil, 1984; Vry et al., 1988; Jiang et al., 1988; Wilson and Baksi,1983, 1984; Fiorentini et al., 1990; Jackson et al., 1988; Stähle et a1.,1987; Baker andFallick, 1988). Models of granulite formation can be grouped roughly into two cate-gories as reviewed by Touret (1989): (i) fluid-absent metamorphism, and (ii) carbonicmetamorphism. In the frst case isotopic compositions acquired prior to gpanulite-faciesmetamorphism should remain unchanged In the case of carbonic metamorphism,changes in the isotopic composition of those rocks can be expected, which originallywere in equilibrium with fluids different from the infiltrating carbonic fluid.

Considerable variations in the granulite-facies lithologies of rocks in the Highland/Southwestern Complexl of Sri Lanka with repeated changes of sedimentary and ig-neous precursors at different scales, represent an ideal region to test whether influx ofdeep-seated CO2 can be regarded as a possible mechanism for regionally extensivegranulite-facies metamorphism. In order to address this problem several hundred stableisotope analyses were carried out on whole-rock and mineral samples, collected fromthe central and southern pan of Sri Lanka (Fig. 1). This paper summarizes the resultsof stable isotope analytical work on whole-rock samples and on graphite. A large partof our analytical results, including O-isotope data on silicate minerals, is published inFiorentini et al. (1990). The complet€ dataset is presented in Fiorentini (1990).

I The nomenclature of major lithotectonic units follows Kröner et al. (this volume).



226

WANN I

1300

98

! 99/oE,'

128 /

172tr

otzt iToMPLEX tztp Potonnaruwa

d11

VIJAYAN

1100

trao

121D

82

r2o

TOMPLEX

Fig. l. Map of central and southern Sri hnka showing location of samples used for stable isotope

and chemical analysis Inset map delineates major lithotectonic units. Symbols are as follows:ü Whole-rock oxygen isotope analysis (WR)I WR and O-isotope analyses of minerals (min)

O Wn and major element chemistry (chem)

V WR, chem and mineral microprobe analysis (EMS)

V WR, min, chem and EMS

O Wn, min, chem, EMS and modal analysis

227

SOME THEORETICAL CONSIDERATIONS

Before we discuss the isotopic composition of the high-grade rocks of Sri Lankawe introduce briefly the possible isotopic effects of infiltration of mantle-released CO2into lower crustal rocks.

The oxygen isotopic composition of mantle-CO2 can be estimated based on the av-erage isotopic composition of carbonatites (Deines and Gold, 1973) and the calculatedCO2-calcite fractionation at high temperatures (Bottinga, 1969). The resulting AlSOvalue of mantle CO2 is close to +12 permil on the SMOW-scale. This estimate is inagreement with direct analyses of CO2 inclusions in fresh MOR basalts (Pineau andJavoy,1983).

The effect of CO2 of this isotopic composition on the l8O/l60 ratio of metasedi-mentary and metamorphosed igneous rocks is demonsffated in Fig. 2.ltcan be shown

lnteraction of lower crustal rockswith mantle derived CO,

6tto o/oo

20sedimentary rocks

qtz (i)1 . .: .:rr: i.:. . rr.l

plg (f)

10 I4,7!',t..

f:::i-=-rI ::: IL;;-Jffi ffi qtzttl

plg (i)

magmatic rocks

o

Fig. 2. Due to CO2-min. fractionation at granulite-facies temperatures, infiltrating mantle-CO2 willlower primary sedimentary ElSO values, while mafic rocks remain essentially unchanged.

that metasedimentary rocks with originally high Ol89 values will be shifted to lowervalues, the extent of isotopic shift being only dependent on the CO2lrock ratio. An ex-



228

ample is given below: To lower a marble from a primary öl8O value of +25 Voo to +10Voo,aCO2,lrock ratio of 1.8 is necessary. This can be calculated based on the abovementioned composition of the infiltrating fluid, the CO2-calcite fractionation of +5 Vooat 800 oC, and assuming open systenr conditions (Iaylor, 1977). This ratio is related tooxygen proportions in rock and fluid and represents a time-integrated value. A fluid/rock ratio of one, calculated according to this model, but assuming a closed system,means that the exchanged rock contains equal amounts of oxygen derived from infil-trating fluid and from the solid precursor.

Metamorphosed igneous rocks of mafic chemical composition and typical ö18O val-ues around +6 Voo, on the other hand, will practically remain unchanged, if invaded bymantle-derived CO2 Gig. 2).

The carbon isotopic composition of mantle CO2 can be estimated from available di-amond, carbonatite, kimberlite, and fluid-inclusion analyses (Hoefs, 1987) which, de-spite some remarkable scatter, have a pronounced maximum between -5 and -8 permilon the PDB-scale. Direct analysis of magmatic CO2, sampled from active volcanicareas, yielded even heavier öl3C values between -3 and -4 permil (Gerlach and raylor,1990), where these values are thought to represent primary values, because degassingwould lead to El3 depletion of the remaining carbon. In contrast to the oxygen iso-topes, only very low COzlrock ratios (<0.1) are sufficient to lower and homogenizecom-pletely the carbon isotopic composition of primarily light organic carbon (Vry etal., 1988), since normal graphite-contents in granulite-facies rocks are frequently in therange of 0.2 weight vo and less. Any substantial pervasive influx of mantle Co2, aspostulated by the CO2-streaming model (Newton, 1985), should thus buffer the Al3Cvalues of graphite to the equilibrium value of the CO2-graphite fractionation atgranulite-facies temperatures (-800 oC). If the invading carbonic fluid has a 6l3C valueof -7 Voo, all graphite crystals which equilibrated with that fluid should have 613Cvalues of about -14 Voo (Bottinga, 1969).If the invading fluid was more enriched in13C, the resulting equilibrium graphite would also be isotopically heavier.

In contrast, if the infiltration was not pervasive in the sense of a continuouslystreaming gas, but rather batch-like and channelized, where each batch of COz was de-posited qui:kly as graphite, 6l3C graphite values could be expected, ranging from theequilibrium-value to that of the infiltrating fluid. It is important ro note, that El3Cvalues in the "fypical mantle"-range do not a priori mean that the CO2 was derived froma magmatic source. Different models involving fluid mixing (Rumble et al., 1986) orsolution-redeposition of pre-existing graphite (Hoernes and Raith, 1988) can alsoexplain öl3C values of graphite in that range.

RESULTS OF STABLE ISOTOPE ANALYSES

The oxygen isotope variations within the main rock types of Sri Lanka are illus-trated in Fig 3. Several points are of special interest:

1. Amphibolite-facies rocks and their granulite facies equivalents like gneisses andcharnockites or amphibolites and mafic granulites do not differ isotopically.

729

2. Marbles have oxygen and carbon isotopic compositions which are typical forsediments. Calcite in calcsilicate rocks and mobilized calcite in veins are iso-topically lighter. The so called "carbonatite" from Eppawala (Jayawardena,1976) is characterized by high O- and C-isotopic values: calcite and dolomite6l8Q = +15.3; 6l3Q= -1.8; apatite 6l8e = 11.9 permil.

3. The oxygen isotopic composition of metapelites conesponds to values reportedfrom Archaean metasediments (Longstaffe and Schwarc z, 197 7 ).

4. Mafic granulites and enderbites are enriched in l8O relative to unaltered mantlematerial (5.7 permil; Kyser, 1986 and references therein).

5. charnockites and gneisses have compositions typical for both, I-type and S-typeorigin. A few samples within the goup of charnockites and gneisses show ex-tremely light 0186 values.

6. The various rock-types from different geological and/or geotectonic units do notdiffer in terms of their oxygen isotopes.

Fig. l.6l8O variation of the main lithologies in Sri Lanka.

The variation of ö13C values of graphite from silicate and carbonate rocks is shownin Fig. 4. Disseminated graphite in silicate rocks and massive graphite from minedveins cluster around -7 to -8 Voo on the PDB-scale. The only difference is the largerspread of the data, and particularly some light values within the group of disseminatedgraphite. Graphite in marbles is isotopically buffered by the carbonate-carbon.

One example of small-scale variation of stable isotopes in a chemically inhomoge-neous rock sequence is presented in Fig. 5. It is apparent that there is some degree ofisotopic homogenization within calcsilicate-dominated parts of the profile, whereassteep isotopic gradients exist in immediately adjacent rock parts.

Oxygen isotopic relations between closed pair gneiss/chamockite and chamockite/bleached charnockite situations are prcsented in Fig. 6. There is no obvious isotopicdifference between the respective rock types.

il

.il

2

6

10

14

sI2

6

10

14

3I€

N 10 6 202 6 10 N

maficanphholites(N=l;d{ gr4tites (N=301

mv=8.3t1.07.. I mv=78t1.4'l*

N 10 6 ?92 6 10 N

hH-bio-qrpiss€s I enderbitesamphibotites { {N=E)

mv=&0! 0.8 "/*

2

6

10

l4

so3

4

I12

16

3:o

I

.--J----q

qt-tio-qrEbses _

(N=30 - I

tEtüites(lE6;el

nY= 10.0!2.0ol€

L

-----r-------J

chrnockites(N=421

mv=9.5r15'/-



230

Fig. 5. Whole-rock öl8O variation inplanation see text.

Fig. a. ö13C variation of graphite.

Distance Icm ]

sample BSL 88-58 near Hakgala. SE of Kandy. For ex-

o\oo\

E6

g raph itein carbonaie rocks(Hoffbauer, pers. comm.)

q raph itein silicate rocks

Q secondary (0igana)

202

202

-6

dUF

0

-6o

\o

;@

0

pn-L:r?.ilpF r? (s

b io I

tli11llti111

{l I l

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231

ARRESTED CHARNOCKITE FORMATION

KURUNEGALA (HLS), Angangata quarry

11

10

9

I7

:40 cm

11

10

9

I7

Fig. 6. Closed-pair analyses of in-situ chamockites and bleached charnockites.

DISCUSSION AND CONCLUSIONS

From theoretical considerations given above, it is clear that penrasive influx of largequantities of mantle-released CO2 should:

(i) 1sws1 1867160 ratios of metasedimentary rocks,(ü) homogenize isotopic differences in chemically different rocks, at least on a

small scale, and(üi) buffer C-isotopic values of graphite to the equilibrium fractionation value

CO2-graphite at granulite-grade conditions.None of these conditions are fulfilled in the metamorphic rocks of Sri Lanka. All

rock-types acquired their O-isotopic signatures essentially in upper crustal levels, mostprobably close to the earth's surface, as documented by the following observations.

E18o tz.l

rt'

GNEISS

---1

GNEISS

I-

IHARNOTKITE

BLEACHED CHARNOCKITES (HLS)

CHARNOTKITE

D igana

Hettipola

Horana

Jayanthipura

BLEATHED THARN.



232

1. Marbles, in most cases, have primary sedimentary isotopic compositions. A groupof carbonates, however, is markedly depleted in l8O and, to a lesser extent, also inl3C. In principle, three mechanisms can be responsible for this depletion: (i) Chan-nelized infiltration of an external fluid, e.g. mantle-COz, (ii) Rayleigh fractionationduring decarbonation, and (iii) isotopic exchange with silicates of the immediate neigh-bourhood (Rumble, 1982). The light, nearly carbonatite-like A18O values and normalsedimentary öl3C values of calcite in the small-scale profile shown above indicate thatthe last mentioned mechanism might have been most effective. Rayleigh fractionationmight also be important, especially in the case of calc-silicate-dominated felses whichcontain only limited amounts of calcite. This calcite is also depleted in l8O and in l3C.

The isotopic data of the Eppawala carbonates do not support a magmatic origin forthese rocks as suggested by Pohl and Emmermann (this volume) based on trace ele-ment data. lsotopic exchange with silicate country-rocks could theoretically producepositive shifts in l8O, but not simultaneously in l3C. Therefore, we assume a sedi-mentary origin of these carbonates.

2. Metapelites and quartzites. If the chemical composition of these clastic meta-

sediments represented by the pelite index (Hoernes and van Reenen, in press) is plottedagainst the öl8O values, a negative correlation becomes apparent (Fig. 7). This essen-

tially means that the metapelites cannot be derived from marine sources with high con-tents of autigenic clay minerals. In contrast, the fact that the most Al-rich metapeliteshave the lowest öl8O values indicates that these rocks have high proportions of detritalillite, typical for an environment of rapid sedimentation. It is apparent from Fig. 7 thatmetapelites from different lithotectonic units (e.g. Wanni Complex in the Colomboarea, Highland/Southwestern Complex) do not differ isotopically.

20 40 60 80

pf = Si02 I 4 + 2' A1203 + K20 -( FeO+tu203+ Mg0)

Fig.7. Pelite-index versus 8180. Open symbols represent metapelites mainly from Colombo area,

collected by B. hame. If these metapelites were originally marire sediments. they should plot in the

band between illite-smectite in shales and quartzites. The low öl EO values are interpreted as a primary(not granulite-facies acquired) featue of the sediments.

233

3. Charnockites and gneisses. The rare, extremely ligtrt l807160 ratios indicate ex-change with meteoric water in high-crustal ievels prior to granulite facies metamor-phism. Since the 8180 values of the granulite-facies minerals are also lowered, it can beconcluded that lowering was not caused by a late-stage, retrograde process.

4. Mafic rocks. The isotopic shift to higher ö180 valuqs cannot be due to granulite-related interaction. The Li-Siot-8180 correlation, as defined by Mengel and Hoefs(1991), presented in Fig. 8, indicates a modification of primary manrle values by spili-fization in an oceanic environment (field A). Only fwo samples plot in the field of intra-plate basalts where the SiO2, Li, and l80 increase indicate crusrai conraminarion (fieldB).

Si02 (wf"Äl

Fig. t. lilöl80 versus SiO2 diagram, with the discrimination fields of spilitized basalrs and inrra-plate basalts according to Mengel and Hoefs (1991). Filled circles: samples from northem part of theHSWC. Open circles: samples from southern part of the HSWC.

Nearly all gpanulite-facies parageneses in Sri Lanka still contain a cerrain amounr ofwater-bearing phases, mainly biotite and/or amphibole. This means that there musthave been a H2O-activity >0 throughout the entire period of metamorphism. Based onavailable diffusion data, Baker (1990) calculated that, in the presence of a fluid-film of10 A thickness, isotopic homogenization would be achieved over 5m in 1 Ma. Sinceisotopic inhomogeneities are preserved on a much smaller scale, it can be concludedthat an interconnected water-bearing fluid phase was present only for shon periods oftime, most probably during decarbonation/dehydration reacrions. Similarly. the onlyperiodic presence of fluids has been deduced from stable isotope data in amphibolite-facies rocks from the Damara orogen in Namibia (Hoernes and Hoffer, l9g5).

From the preservation of light öl3C values of disseminated graphite in silicate rocksand the fact that marbles contain graphite with carbonate-controlled compositions, it isclear that the rocks at the time of granulite-facies metamorphism were not flooded

100

50

:820ö-o-

10

5B50lr2545046Q

20

10

E18o tz.l

.utnis.ni, JTlraolinites ! |Ä_ -i'li

littite diaqeneiicQ minerals -kaoliniies

I I in ..nd.ione, It_

ttliie /smecliiein shales

-l-

I

shales/slates fiTo l-o -I oaJo Rt\ oo

sandstones! ao""i o



234

pervasively by large volumes of mantle-derived CO2. The only form of fluid infil-tration which is consistent with the stable isotope data might be a periodic, channeled,influx of limited barches of a carbonic fluid, which did not give rise to the formation ofan inter-connected fluid-film in large rock volumes. The source of such fluids could bemantle-related melts, underplated to the lower crust, but also repeatedly mobilized CO2from the graphite-bearing metasediments themselves (Srikantappa et al., 1985; Raith etal., 1989).

If progressive "arrested charnockite"-formation was triggered by a mechanismsimilar to this batch-infiltration model, it also becomes plausible why this phenomenonis not accompanied by changes in the oxygen isotope composition Gig 6). The amountof infiltrating fluids is too small to have an imprint on the oxygen isotopic compositionof the rock, but it is sufficient, to reduce the H2O-activity such that dehydration reac-

tions could elapse.Similarly, rehydration along discrete pathways leading to bleached charnockites can

occur without imprint on the oxygen isotopic composition (Fig 6), if the infiltratingfluids were not derived from a source with extreme isotopic composition (e.g. mete-oric water). Regionally extensive rehydration of granulite-facies rocks in the Limpopobelt of South Africa is also not accompanied by changes in the oxygen isotopic com-position of the rehydrated rocks, which can contain refogmde formed anthophyllite up

to 30 volume percent (Hoernes and van Reenen, submitted).

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