A new correlation of sapphirine granulites in the indo-antarctic metamorphic terrain: Late proterozoic dates from the eastern ghats province of India

Precambrian Research, 33 ( 1 9 8 6 ) 1 2 3 - - 1 3 7

123

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

A N E W C O R R E L A T I O N O F S A P P H I R I N E G R A N U L I T E S IN T H E INDO--ANTARCTIC METAMORPHIC TERRAIN: LATE PROTEROZOIC DATES FROM THE EASTERN GHATS PROVINCE OF INDIA

EDWARD S. GREW

Department of Geological Sciences, University of Maine at Orono, Orono, ME 04469 (U.S.A.) W.I. MANTON

Department of Geology, The University of Texas at Dallas, P.O. Box 830688. Richardson, TX 75083-0688 (U.S.A.)

Grew, E.S. and Manton, W.I., 1986. A new correlation of sapphirine granulites in the Indo--Antarctic metamorphic terrain: late Proterozoic dates from the Eastern Ghats Province of India. Precambrian Res., 33: 123--137. Uranium--lead and thorium--lead ages were obtained from perrierite and zircons in two samples from the Eastern Ghats Province of South India, a sapphirine-bearing granulite from Anakapalle and a charnockite from Visakhapatnam. The isotopic data are concordant or close to concordant at 1 Ga, which is interpreted to be the age of granulite-facies metamorphism and charnockite plutonism in the vicinity of Anakapalle and Visakhapatnam, and provides the first report of a late Proterozoic granulite-facies event in the Eastern Ghats Province. In reassemblies of Gondwanaland based on DuToit's original proposal, the Eastern Ghats Province is juxtaposed against a high-grade terrain in East Antarctica affected by a granulite-facies event and charnockitic plutonic activity 800--1100 Ma ago. The ages reported here suggest that a portion of the Eastern Ghats Province represents an extension of the late Proterozoic terrain from Antarctica into India. The radiometric results on the Anakapalle rocks are the first evidence in South India for sapphirine formation during the late Proterozoic; other South Indian sapphirine localities appear to be Archaean in age. A review of sapphirine localities in the Indo-Antarctic metamorphic terrain indicates that sapphirine developed under a range of pressure-temperature conditions during the Archaean and Proterozoic. Evidence for a systematic change in peak metamorphic conditions with time is obscured by these regional variations. Characterization of the entire pressure--temperature path, in addition to estimates for the peak conditions, is needed to distinguish the Archaean and Proterozoic metamorphic regimes.

INTRODUCTION In D u T o i t ' s ( 1 9 3 7 ) studies have confirmed Bay of Bengal coast of A n t a r c t i c a b e t w e e n 45 °

0301-9268/86/$03.50

reassembly of Gondwanaland, which subsequent (e.g., C r a w f o r d , 1 9 7 4 ; B a r r o n e t al., 1 9 7 8 ) , t h e S o u t h I n d i a is j u x t a p o s e d a g a i n s t t h e c o a s t o f E a s t a n d 7 0 ° E ( F i g . 1). T h e s e c o a s t a l a r e a s ar e l a r g e l y

~:. 1986 Elsevier Science Publishers B.V.

124 INDEX

~~Ar~Afric°

DIOMETRIC AGES billions of yeors) 25U-Pb II U-Pb (leod loss ) 2 6 Rb'Sr 3 5 Sm-Nd Sedimenfory Cover

2 =1 NonchQrnockiticRocks ] Chm'nockitK;~ Gronulitefa¢~ll Rocks Sopphirine

'ROVINCE

Fig. 1. Selected geochronologic data from the coast of East Antarctica between 40 and 65°E, Sri Lanka, and portions of South India. The division into sedimentary cover, non-charnockitic rocks (includes amphibolite and greenschist facies metamorphics), and charnockitic and granulite-facies rocks, are taken from Dahanayake (1982) for Sri Lanka, sources given in Grew (1982b) for India, and from Hiroi et al. (1983) for Antarctica west of Molodezhnaya Station. Configuration of continents and mapping of Napier and Rayner Complexes are from Barton et al. (1978) and Grew and Manton (1979a). Sapphirine localities are from summaries by Grew (1982b) and Grew (1984a) except for locality west of Molodezhnaya Station, reported by Yanai et al. (1984). Sources of radiometric data for Napier Complex, most of which are summarized in box, are Grew and Manton (1979a), James and Black (1981), DePaolo et al. (1982). Grew et a]. (1982), Black and James (1983), Sheraton and Black (1983), Black et al. (1983a,h), Sobotovich et al. (1983), Black et al. ( 1984 ) and McCulloch and Black (1984); for Rayaer Complex and other late Proterozoic rocks in Antarctica, Maegoya et al. (1968), Grew (1978), Grew and Manton (1979b, 1981 and unpublished data for Fold Island), Tingey (1982), Shirahata ( 1 9 8 3 ) a n d Sheraton and Black (1983); for Sri Lanka, Crawford and Oliver (1969) and Wickremasinghe (1970); for India, Vinogradov et al. (1964), Crawford (1969a,b), Venkatasubramanian and Narayanaswamy (1974), Rao (1976), Perraju et al.. (1979), Sarkar et al. (1981), Halden et al. (1982), Buhi et al. (1983), Grew and Manton (1984), and this paper. A U--Pb (lead loss) age refers to the lower intercept with concordia of the chord defined by isotopic data. The 2.6 Ga date in parentheses near Bhubaneswar is based on lead isotopes only (Vinogradov et al., 1964). Radiometric data related to the early Paleozoic event at about 0.6 Ga have not been shown.

125 underlain by granulite-facies rocks and charnockite (summarized by Fermor, 1936; Subramaniam, 1967; Ravich and Kamenev, 1975; Grew, 1982a) and can be interpreted as fragments of a single high-grade metamorphic terrain in Gondwanaland, an Indo--Antarctic metamorphic terrain. Picham u t h u (1966) was among the first to recognize the overall petrologic similarities between the South Indian and East Antarctic terrains and Fedorov et al. (1977) noted that structural trends in these terrains are parallel in DuToit's (1937) Gondwanaland reassembly. On the other hand, geochronologic data indicate that a complex series of metamorphic and tectonic events affected both areas, and experience suggests that petrologic or structural aspects of the rocks are not an infallible guide to their geologic history. Consequently, comparison of these terrains on such features alone is not entirely satisfactory. A more rigorous approach is to compare the geologic histories as deduced from geochronologic data and attempt to match provinces having similar ages. In particular, a late Proterozoic granulite-facies event (800--1100 Ma in age) has affected large segments of this section of Antarctica, whilst no granulite-facies or charnockitic complexes of this age have yet been recognized in the portions of South India believed to be contiguous with Antarctica (Tingey, 1982). We present here the first reported evidence for a late Proterozoic granulite-facies event in the Eastern Ghats Province of South India. These results suggest that the late Proterozoic granulite-facies belt which extends from 40 ° to nearly 80°E parallel to the coast in East Antarctica (Grew, 1982a; Tingey, 1982) continues into South India as part of the Eastern Ghats Province. Moreover, our geochronologic data indicate that sapphirine (Fig. 1) developed in India during the late Proterozoic. Sapphirine (approximately (Mg, Fe)TAl,~Si:~O40) forms in rocks rich in MgO and AI:O~, metamorphosed under relatively high temperatures and pressures and low water activities, generally in the granulite-facies (summarized by Deer et al., 1978). Sapphirine is found at a number of localities in both the Indian and the Antarctic granulite-facies terrains. However, to date no firm evidence for sapphirine formation in India after the Archaean had been reported {Grew, 1982b). In contrast, sapphirine occurs in Proterozoic rocks in Antarctica (Fig. 1) at Mawson Station (Segnit, 1957) and west of Molodezhnaya Station (Yanai et al., 1984), where the age of metamorphism is probably 1000 Ma (Hiroi et al., 1983). AGE PROVINCES IN ANTARCTICA AND INDIA Soviet geologists (e.g., Kamenev, 1972; Ravich and Kamenev, 1975) proposed that the granulite-facies rocks of Enderby Land be divided into the Napier Complex and Rayner Complex, both of Archaean age. The division was originally based on petrologic and structural criteria, but whole rock Pb--Pb ages near 4000 Ma (Sobotovich et al., 1974) later provided supportive evidence for an early Archaean age of the Napier Complex.

126 The Rayner Complex was interpreted to be reworked Napier Complex. Recent isotopic and geochemical studies in Enderby and Mac. Robertson Lands have by and large confirmed the Soviet geologists' proposed division and have extended it to cover the sector of East Antarctica shown in Fig. 1. In the Napier Complex, the older of the two age provinces, radiometric data indicate events at 3500--3800 Ma, 2900--3100 Ma, and 2400--2500 Ma. According to our preferred interpretation (Grew and Manton, 1979a: DePaolo et al., 1982; Grew et al., 1982; Sandiford and Wilson, 1983L the first two events were periods of crust formation, including sedimentation, volcanic activity and plutonism, while the third event was the peak of granulite-facies metamorphism accompanied by intense deformation and minor plutonism. An alternative interpretation of the metamorphic history is that peak conditions in the granulite-facies and most intense deformation were attained 2900--3100 Ma ago and the 2400---2500 Ma event is a metamorphism in the upper amphibolite to granulite facies {e.g., James and Black, 1981; Black et al., 1983a,b; McCulloch and Black, 1984). A late Proterozoic event was recorded by lead loss in zircon and monazite at three localities in the westernmost Napier Complex (Fig. 1) {Grew et al.. 1982; Black et al., 1983a, 1984}. The second province in Antarctica was affected by granulite-facms metamorphism and charnockitic plutonism in the late Proterozoic (800--1100 Ma ago, summarized by Grew, 1982a; Tingey, 1982). This province includes the Rayner Complex, which is largely reworked Napier Complex (Sheraton et al., 1980; Sheraton and Black, 1983) and charnockitic rocks intruded at the time of the late Proterozoic metamorphism, e.g., Molodezhnaya (Grew, 1978; Grew and Manton, 1981). Reworked Napier Complex rocks extend at least as far west as Molodezhnaya and as far east as Fold Island (Fig. 1). Zircons from charnockitic gneiss on Fold Island lie on a chord intersecting concordia at 748 and 3060 Ma respectively, suggesting that the precursor to this gneiss was emplaced during the 2900--3100 Ma event recognized in the Napier Complex proper (Grew and Manton, unpublished data}. In Mac. Robertson Land much of the late Proterozoic Province does not consist of remetamorphosed Archaean igneous rocks, but instead includes a large proportion of gneisses derived directly or indirectly from sedimentary rocks, possibly Proterozoic in age (Sheraton and Black, 1983}. These gneisses are intruded by late Proterozoic (900--1100 Ma) charnockite at Mawson Station {Fig. 1) (Sheraton, 1982}. A portion of the granulite facies terrain of South India appears to have had a similar geologic history to the Napier Complex. A minimum age ()f 2800--3100 Ma is indicated for sedimentation, plutonism and, possibly, a high-grade metamorphic event, while emplacement of charnockite and the main upper amphibolite to granulite-facies metamorphism occurred in the late Archaean at 2500 -+- 100 Ma ago in india (reviewed by Balasundaram and Balasubrahmanyan, 1973 and Sarkar, 1980; also recent data by Buhl et al., 1983). This late Archaean event has been documented in

127 Madras, Kabbaldurga Quarry, and in the Nilgiris (south of Kabbaldurga, Fig. 1) (Vinogradov et al., 1964; Crawford, 1969a,b; Venkatasubramanian and Narayanaswamy, 1974; Buhl et al., 1983; Grew and Manton, 1984). The significance of this late Archaean event in India (see Shackleton, 1976) led us to suggest that the Napier Complex may be a continuation of the Indian terrain into Antarctica (Grew and Manton, 1979a}. No discernible pattern emerges from the data on the Proterozoic of South India except that a large number of model Rb--Sr ages obtained on single rock samples and several U--Th--Pb ages suggest a major metamorphic event, the 'Eastern Ghats Orogeny', at about 1600 Ma ago, with plutonic activity continuing for 300 Ma afterwards (Holmes, 1955; Aswathanarayana, 1956, 1964, 1968). DESCRIPTION OF SAMPLES AND RESULTS In the course of a study of sapphirine-bearing rocks of South India (Grew, 1982b), the first author had the o p p o r t u n i t y to collect sapphirine granulite in a quarry 5 km north of Anakapatle (Nanda and Natarajan, 1977) and porphyritic charnockite at a quarry 8 km west of Visakhapatnam and 21 km east of the sapphirine locality. The Anakapalle sapphirine granulites contain zircon and perrierite, a rare earth titanosilicate suitable for age determination by U--Pb techniques (Grew and Manton, 1979a,b), while the Visakhapatnam charnockite contains abundant zircon. The perrierite is from a feldspathic rock (sample 3080T), typical of the migmatitic variety of sapphirine granulites from Anakapalle (Nanda and Natarajan, 1977; Grew, 1982b). Portions of the rock consist of mesoperthitic feldspar up to several centimetres across and subordinate quartz. In other portions, abundant garnet, o r t h o p y r o x e n e and biotite are found in plagioclase that is in part antiperthitic and grades into mesoperthite. Sapphirine, sillimanite and possibly cordierite occur in traces in and around garnet, while zircon, monazite, rutile and perrierite are found throughout the rock. Orthopyroxene forms grains up to 2 mm, locally embayed by biotite. Sapphirine in thin tablets mostly 0.1--0.3 mm across occurs in swarms in plagioclase near garnet, as inclusions in the margins of garnet, or with biotite in seams cutting garnet. Sfllimanite in prisms up to 0.05 by 0.4 mm typically accompanies sapphirine in plagioclase and c o m m o n l y forms overgrowths around it. Perrierite in pleochroic grains up to 0.1 mm long occurs as inclusions in garnet and in association with sillimanite, sapphirine, biotite and rutile. Perrierite also forms nearly isotropic (metamict) grains several millimetres across. Textures in sample 3080T indicate that sapphirine crystallized during garnet growth and coevally with perrierite. Millimetre sized grains of perrierite were picked for analysis from a coarse crush of the rock. Fragments of feldspar were also selected and were repeatedly crushed, sieved and picked until a clean 100--200 mesh fraction was obtained. The rock was then pulverised to < 80 mesh and zircon sep-

128 arated by standard techniques. Only fragments of clear, pink crystals appeared m the separate, indicating that the zircon is present in crystals of the order of a millimetre. The porphyritic charnockite (sample 3074) from which the analysed zircons were separated consists of major plagioclase, quartz, K-feldspar and hypersthene. Feldspar phenocrysts with crude preferred orientation 0.5--6 mm across are mostly plagioclase, in part antiperthitic; finely perthitic K-feldspar is subordinate. Hypersthene is largely fresh. Minor hornblende and rare clinopyroxene occur in grains 0.1--0.5 mm surrounding hypersthene, whilst a few flakes of biotite are found in K-feldspar. Zircon grains are anhedral to subhedral and lack cores. Perrierite, apatite and opaque minerals are also present. Bent plagioclase twin lamellae imply that the rock underwent mild cataclasis, but cataclastic textures in quartz have been largely annealed out. Contact relations of the sampled charnockite with country rock are not exposed. We interpret this rock to be an intrusive plutonic charnockite that was emplaced at roughly the tim¢~ when the c o u n t r y rock, of which the Anakapalle sapphirine granulite is a part, underwent granulite facies metamorphism. The charnockite is similar to the charnockites of East Antarctica described by Ravich and Kamenev (1975) and Sheraton (1982). An example of these charnockites, the Mawson charnockite, is interpreted to have formed by partial melting under granulit( ~ facies conditions {Sheraton, 1982}, a possible origin for the Visakhapatnam charnockite. The Pb in the feldspar from sample 3080T {Table I) is highly radiogenic, indicating that the rock existed with high U/Pb and Th/Pb ratios for a long time prior to granulite facies metamorphism. Presumably, highly radiogenic Pb expelled from pre-existing radioactive minerals was incorporated in the feldspar during this event, but without wholesale redistribution of Pb isotopes. If the feldspar's lead ratios are used to make the c o m m o n Pb correction in the perrierite data, the a m o u n t of ~)~Pb to be subtracted exceeds the a m o u n t analysed. The c o m m o n Pb retained by the, perrierite appears to have a model age of about 2 Ga, for the perrierite data corrected with a 2 Ga c o m m o n lead yield U--Pb ages concordant at 989 Ma (Fig. 2) and a Th--Pb age of 993 Ma. The zircon in sample 3080T contains a relatively large amount of common lead. Correcting the zircon data for a 2 Ga c o m m o n lead fails to produce Pb/U ratios on concordia; instead, these lie close to a 0--1 Ga chord. consistent with Pb loss subsequent to closure at about 1 Ga (Fig. 2). The U--Pb data for the three zircon fractions separated from sample 3074 lie close to a chord intersecting concordia at 979 Ma and 500 Ma (Fig. 2). Our interpretation of the zircon data is original crystallization of the zircons 979 Ma ago and loss of lead during an event 500 Ma ago. This early Palaeozoic event has reset biotite Rb--Sr and K--Ar ages to 480--510 Ma in charnockite from Visakhapatnam (Aswathanarayana, 1964) and has affected large areas elsewhere in the lndo--Antarctic meta-

235.1 257.8 280.4

----

-2.34 --

Th %

42.2 46.2 50.1

4.26 1103 59.5

Pb ppm

2,,~pb/2o',pb

0.20718 0.17699 0.12316

0.00021 0.00034 0.00018

0.07363 0.07263 0.07339

Sample 3074

0.00771 0.00756 0.00352

Sample 3 0 8 0 T

2o.pb/2O~pb

Note: (1) F o r sample 3080T c o m m o n Pb age a s s u m e d 2 Ga. (2) F o r sample 3074 c o m m o n Pb age a s s u m e d 1 Ga.

80--100 mesh 100--200 mesh <200 mesh

-209.4 227.4

K-feldspar Perrierite Zircon

Zircon:

U ppm

Mineral

U, Th, P b c o n c e n t r a t i o n s a n d P b i s o t o p e r a t i o s o f m i n e r a l s d a t e d

TABLEI

0.32457 0.29903 0.28193

2.0018 31.284 0.71950

2o~pb/2,,~p b

1.445 1.459 1.491

1.648 1.577

:,,'pb*/:"U

0.1484 0.1507 0.1527

0.1658 0.1555

:,,~pb./'.,~U

~D

130 0-17i

, 9 8 9 Mo

,

~

9 7 9 Me

to 5 0 0 Ma 0 1 4 i ., 14

15

6

17

2O~pb/23~U Fig. 2. U--Pb data displayed on concordia diagram. Dot in hexagon--zircons I'rom Vi sakhapatnam charnockite, sample 3074. Cross in hexagon--zircons from Anakapalh, granulite, sample 3080T. Hexagons represent analytical uncertainty. The dashed lira, connecting the squares represents the trajectory of the perrierite from the Anakapalle granulite corrected for common leads from 1.0 Ga (left of concordia), 2.0 Ga (con cordant) to 2.5 Ga (right of coneordia).

morphic terrain (Grasty and Leelanandam, 1965; Crawford, 1969a,b; Grew. 1982a). Because the perrierite ages are close to the age of the upper intersecnon with concordia of the chord for the charnockite zircons, we believe the, perrierite ages are meaningful and date the time of perrierite crystallization during the granulite facies metamorphism. It is not clear why the zircon in 3 0 8 0 T is discordant and the perrierite is not. Among the samples of perrierite and zircon we analysed from the Napier Complex {Grew and Manton, 1979a), perrierite was m o r e discordant than zircons from the same locality {No. 2006), but the least discordant sample overall was a perrierite from a n o t h e r locality (No. 2098). Thus we have no reason to believe that either perrierite or zircon is intrinsically more liable to lead loss than the other. By analogy with monazite, which was discussed by Black et al. {1984), lead loss in a given mineral may largely depend cm access to fluids after crystallization (in this case, during the early Palaeozoic event}. DISCUSSION OF THE RADIOMETRIC DATA

We conclude that the perrierite and zircon data reflect the ages of granulite facies metamorphism and of crystallization of the associated charnockite intrusion in the Anakapalle and Visakhapatnam areas. Although there are no o t h e r reports of m e t a m o r p h i c ages near 1000 Ma in the East-

131 ern Ghats Province, U--Pb and Rb--Sr (total rock and mineral) ages of 800--1100 Ma have been reported for pegmatites in Orissam, inland from Bhubaneswar (Vinogradov et al., 1964; Halden et al., 1982), aplite near Visakhapatnam (Crawford, 1969b), and for granitic rocks in the Srikakulam district, Andhra Pradesh {between Visakhapatnam and Bhubaneswar, Fig. 1) {Perraju et al., 1979). The 854 Ma pegmatite from Angul, Orissa {nearly 100 km northwest of Bhubaneswar, Fig. 1) dates late deformational phases in this area (Halden et al., 1982). These scattered data suggest that a late Proterozoic event has affected parts of the Eastern Ghats Province beyond the vicinity of Visakhapatnam. Rocks older than 1000 Ma are definitely present in the Eastern Ghats Province, but the data are conflicting and a pattern does not emerge. We do not regard the U--Th--Pb age of 1585 Ma on allanite from Anakapalle (Aswathanarayana, 1956), the U--Pb age of 1570 Ma on detrital monazite from near Bhubaneswar, Orissa (Holmes, 1955), or the model Rb--Sr ages of 1500 Ma or less reported by Aswathanarayana (1964) and Perraju et al. (1979) to be firm evidence for such older rocks. For example, model ages of 1000--1354 Ma can be calculated with an assumed initial s"Sr/S6Sr ratio of 0.705 from the three samples of granitic rock defining an isochron of 816 Ma (initial 87Sr/~Sr ratio = 0.7353) (Perraju et al., 1979). On the other hand, good evidence for rocks older than 1000 Ma in the Eastern Ghats Province is found in the model Rb--Sr ages of 2100--3090 Ma on khondalite, charnockite and granite from the Visakhapatnam district, Andhra Pradesh and Puri district, Orissa near Bhubaneswar (Crawford, 1969b; Perraju et al., 1979). The 2°~Pb--~-°6Pb age of 1900 Ma on alianite from a crosscutting vein at Kasipatnam, 55 km north of Anakapalle (Vmogradov et al., 1964; Rao, 1976) and the Rb--Sr isochron age of 1400 Ma on post-metamorphic anorthosite near Chilka Lake in Orissa (Sarkar et al., 1981) imply that the granulite facies metamorphism in these areas (Fig. 1) is older than 1900 and 1400 Ma, respectively (see also Sarkar et al., 1981}. By extension, the belt of sapphirine localities inland from Anakapalle and passing through Kasipatnam probably lies in a terrain metamorphosed prior to 1900 Ma ago, long before the Anakapalle sapphirine formed. The highly radiogenic lead in the Anakapalle feldspar is characteristic of a rock significantly older than 1000 Ma. The Anakapalle sapphirine granulite may be an Archaean or early Proterozoic rock (age ~> 2000 Ma) reworked during the late Proterozoic metamorphism. The Anakapalle--Visakhapatnam area would thus be analogous to the Rayner Complex at Molodezhnaya, where older metamorphic rocks (possibly 2000 Ma) are cut by late Proterozoic (1000 Ma) charnockites (Grew, 1978). In summary, our data provide further evidence that the Eastern Ghats Province in northeastern Andhra Pradesh and southern Orissa has had a complex geologic history in the Precambrian. Extending the suggestions made by Halden et al. {1982) and Perraju et al. (1979) for late Proterozoic plutonic activity in this region, we propose that the late Proterozoic ac-

132 tivity included granulite-facies metamorphism and charnockitic plutonic activity 1000 Ma ago. The portions of the Eastern Ghats Province affected by this 1000 Ma event can be interpreted as an extension into India of the late Proterozoic terrain of East Antarctica. The distribution of known exposures of Proterozoic and Archaean rocks in India and Antarctica is consistent with the Gondwanaland reassembly originally proposed by DuToit {1937) and now generally accepted {Fig. 1). Moreover, a segment of the late Proterozoic terrain apparently also extends into Sri Lanka as the Vijayan Complex (Grew and Manton, 1979a), which is dated at 1150 Ma and intruded by granite dated at 968 Ma (Crawford and Oliver, 1969). Indeed, the boundaries between the Archaean and Proterozoic terrains could be used to more precisely constrain the relative positions of continental fragments in reassemblies of Gondwanaland. CONCLUSION The Gondwanaland reassembly illustrated in Fig. 1 implies that the granulite facies terrains of Antarctica between 40 ° and 70°E and of the Bay of Bengal coast of South India are fragments of what was formerly a single Precambrian granulite-facies terrain. This Indo--Antarctic metamorphic terrain is remarkable for the widespread development of sapphirine, in partmular sapphirine--quartz, sapphirine--garnet and the associated paragenesis sillimanite--orthopyroxene (summarized in Sheraton et al., 1980; Grew, 1982b; Grew, 1984a). Fortunately, petrologic information on both the East Antarctic and Indian continents is sufficient to draw some preliminary conclusions regarding variations of metamorphic conditions both in space and time for this Indo--Antarctic metamorphic terrain. Regional variations in metamorphic pressures and temperatures are characteristic of both the Archaean and Proterozoic terrains. In the Archaean Napier complex, peak metamorphic temperatures for sapphirine formation reached 900°C and pressures ranged from 5 kbar in the Napier Mountains of the central part of the complex increasing southwestwards to 7 kbar in the Tula Mountains and to 9 kbar in the Casey Bay region of the westernmost part of the Napier complex {Ellis, 1980; Grew, 1980; Harley, 1983; Sandiford and Wilson, 1983). In the Archaean rocks southwest of Madras, metamorphic temperatures for sapphirine were probably near 750--850°C, whilst pressures ranged from 6 kbar (Palni Hills) to 8 kbar (Sittampundi Complex) (e.g., Grew, 1982b; Lal et al., 1984). An example of sapphirine formation in the Proterozoic is Anakapalle: 800--850°C and 7--8 kbar {Grew, 1982b). For the area west of Molodezhnaya Station where Yanai et al. {1984) reported sapphirine, Hiroi et al. (1983) estimated temperatures of a b o u t 750°C and pressures in the range of 6.5--7.5 kbar. For Proterozoic rocks at Molodezhnaya, where no sapphirine occurs, pressures were somewhat lower (5.5 kbar) at comparable temperatures (700°C)

133 (Grew, 1981), whilst elsewhere in the Rayner Complex pressures reached 10 kbar (Ellis, 1983). In summary, the spatial variations have obscured any systematic changes in metamorphic conditions with time. Moreover, geothermal gradients during the late Proterozoic event were locally as high, namely 29--46°C km -~ at Molodezhnaya, as the highest gradients estimated for the late Archaean event, 30--44°C km-' in the Tula Mountains of the Napier Complex (Grew, 1981). Thus, metamorphism in the Rayner Complex as now exposed at Molodezhnaya could have resulted from a thermal regime similar to that for the Napier Complex, but at a shallower level in the Earth's crust. We are tempted to suggest that a thermal regime similar to that for the late Archaean event was reimposed during the late Proterozoic event in the Indo--Antarctic metamorphic terrain. However, another aspect of metamorphic history needs to be considered, namely changes in pressure and temperatures following peak metamorphic conditions. These changes are indicative of the overall tectonic evolution of a terrain (England and Richardson, 1977). Ellis {1983) reported that peak conditions in the Napier Complex were followed by a nearly isobaric decrease in temperature (see also Harley, 198;~; Sandiford and Wilson, 1983), whilst those in the Rayner Complex were followed by a nearly isothermal decrease in pressure. Textures in some sapphirine bearing rocks from the Archaean southwest of Madras also suggest an isothermal decrease in pressure (Ackermand et al., 1982; Grew, 1984b; Lal et al., 1984), whilst detailed chemical studies of charnockitic assemblages with garnet and pyroxene, but not sapphirine, indicate near isobaric decrease of temperature for the same general region (Raith et al., 1983). Although it is possible that the metamorphic regimes differed in the northern and southern parts of the area (this region has been much affected by tectonic fragmentation and differential uplift, e.g., Raith et al., 1983; Drury et al., 1984), we cannot exclude the possibility that the different conclusions reached regarding the metamorphic evolution may be related to the different approaches used to reach these conclusions. In any case, a full characterization of the pressure--temperature paths followed by the metamorphic complexes, in addition to estimates of the peak conditions, is necessary to determine whether the Proterozoic regime was fundamentally different from the Archaean regime that preceded it. ACKNOWLEDGEMENTS We thank J.K. Nanda, A.V.R. Shastri, D.N. Kanungo, and D.S.N. Murthy for guidance and assistance in the field work, and P.C. Grew for a thoughtful review of earlier drafts of this manuscript. E.S.G.'s field work was supported by a grant under the lndo--American Fellowship Program. Financial support from the U.S. National Science Foundation Grants DPP80-19527

13,1 t o t h e U n i v e r s i t y o f C a l i f o r n i a , L o s A n g e l e s , a n d D P P 8 4 - 1 4 0 1 4 to t h e University of Maine, Orono, and from the Alexander von Humboldt-Stiftung ( B o n n ) are g r a t e f u l l y a c k n o w l e d g e d .

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