The configuration of the Indian Shield — Precambrian tectono-thermal events and constraints on the thermal history of Gondwana

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conditions of granulite metamorphism and postpeak evolutionary trend of SGT show resemblance with that of Madagascar (Nicollet, 1990; Paquette et a/., 1994 and references therein) and parts of the Eastern Antarctic Rayner complex and the Lutzow-Holm Bay area (Yoshida, 1995; Harley and Fitzsimons, 1995; Motoyoshi, 1990 and references therein). REFERENCES Anil Kumar et al. 1990. International conference on geochronology, cosmochronology and isotope geology, Canberra, Australia. Abstract Volume 7. Bartlett, J. M. etal. 1995. JournalGeologicalSociety India Memoir 34, 391-397. Bhaskar Rao, Y. J. et al. (In press). Contributions Mineralogy Petrolog ye Buhl, D. 1987. Unpubl. Ph. D. thesis, University of Munster, Germany. Chacko, T. et al. 1987. Journal Geology 95, 343358. Choudary, A. K. et al. 1992. Geological Magazine 129, 257-264. Dasgupta, S. et al. 1995. Journal Petrology 36, 435461. Drury, S. A. 1984. Abstracts 28th International Geocongress 1, 420-42 1. Dunai, T. J. and Touret. J. L. R. 1993. Earth Planetary Science Letters 119, 271-281. Frost, B. R. and Chacko, T. 1989. Journal Geology 97, 435-450. Grew E. S. 1982. Journal Geological Society India 23, 469-505. Grew, E. S. 1984. Journal Geological Society India 25, 116-119. Hansen, E. C. et al. 1984. Archaean Geochemistry ~~162-181. Hansen, E. C. et al. 1985. EOS 66, 419-420. Hansen, E. C. et al. 1995. Journal Geology 103, pp. 629-65 1. Harley, S. L. and Fitzsimons, I. C. W. 1995. Geological Society India Memoir 34, 73- 100. Harris, N. B. W. et al. 1994. Journal Geology 102, 135-l 50. Janardhan, A. S. 1989. Abstracts. In: Structure and dynamics of the Indian lithosphere pp90-91. NGRI, Hyderabad. Janardhan, A. S. et al. 1994. Journal Geological Society India 44, 27-40. Jayananda, M. et al. 1995a. Contributions Mineralogy Petrology 119, 314-329. Jayananda, M. et al. 1995b. Journal Geological Society India Memoir 34, 373-390. Mahabaleshwar, B. et al. 1995. Journal Geological Society India 45, 33-49. Mohan, A. and Windley, B. F. 1993. Journal Metamorphic Geology 1 I, 867-878. Motoyoshi, Y. 1990. Interim report of Japan-Sri Lanka joint research ~~132-139. Muthuswamy, T. N. 1949. Proceedings Indian Academy Science 30(6), 295-301.

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Nicollet, C. 1990. Granulites and crustal evolution pp291-310. Paquette, J. L. et al. 1994. Journal Geology 102, 523-538. Peucat. J. J. et al. 1989. Journal Geology 97, 537550. Peucat, J. J. et al. 1993. Journal Metamorphic Geology 11, 879-888. Radhakrishna, B. P. and Naqvi, S. M. 1986. Journal Geology 67, 145-l 66. Raith, M. etal. 1990. Granulites and crustalevolution pp339-366. Rameshwar Rao et al. 1990. Journal Geological Society India 35, 55-69. Rameshwar Rao et al. 1991. Journal Petrology 32, 539-554. Santosh, M. et al. 1992. Bulletin Indian Geological Association 25, l-25. Sengupta, P. et al. 1990. Journal Petrology 31, 97 1. 996. Shankara, M. A. and Janardhan, A. S. 1995. Indian Mineralogist 29, 60-73. Sivasubramanian, P. et al. 1991. Journal Geological Society India 38, 532-537. Sivasubramanian, P. et al. 1992. Journal Geological Society India 40, 287-290. Srikantappa, C. et al. 1985. Journal Geological Society India 26, 849-872. Wiebe, R. A. and Janardhan, A. S. 1993. Journal Geological Society India Memoir 25, 1 13-l 17. Yoshida, M. 1995. Journal Geological Society India 34, 25-45.

The configuration of the Indian Shield - Precambrian tectono-thermal events and constraints on the thermal history of Gondwana T. M. MAHADEVAN Sree Bagh, Ammankoil Road, Cochin-682 035, India The Indian shield largely owes its configuration to northeast trending faults along the east coast and the north-northwest faults along the west coast. The faulting has been sequential, the east coast faulting having been initiated in the Jurassic and the west coast faulting in the Cretaceous (Krishnan, 1953). The structural history of the west coast is better understood than that of the east coast. The transition from continental to intermediate/oceanic crust is more abrupt and the west coast rift was the source region for the large Deccan continental basalt volcanism. The continental boundaries of the east coast extend well into the Bay of Bengal and the coast is covered over large segments by post-Jurassic sediments. The Rajmahal-Sylhet volcanism may

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have been ushered in by the East coast faulting, though this is not well established. The purpose of this note is to present evidences arising from recent research suggestive of a possible Precambrian ancestry for the two coastal faults and its implications on the thermal evolution of these regions. West coast evolution Geological and geophysical data identify the Cambay rift as an active rift characterized by a distinctive rift cushion above the Moho of high density material and thinned lithosphere in the thickness range of 18-33 km (Kaila et al., 1981). The rift widens southwards into the Gulf of Khambat. Further south it is masked by the sedimentary sequence of the Bombay High and younger sediments on the continental shelf. Several faults with opposing dips have been identified along the continental shelf and the presence of seismic reflectors dipping towards the Arabian sea (Hinz, 1981) suggest that the west coast and the geomorphologically high Deccan Plateau form the eastern collar of the rift system. The plateau is possibly held high due to magmatic underplating. The rift magmatism culminated in the Cambay (Khambat) region with the emplacement of alkaline complexes and carbonatites. The more prominent of these include the occurences at Ambadonagar, Sirivasan-Nakal, PanwadKawant, Mer-Mundwara and sarnu-Dandeli (Krishnamurthy, 1988). Recent research have emphasised widespread Meso-Neoproterozoic events of extensional tectonics and alkaline magmatism in parts of western Rajasthan and Kerala. Modelling the Precambrian evolution of Rajasthan, Sinha-Roy et a/. (1995) suggest alternating extensional and compressional regimes, culminating in the post700 Ma period in the retro-arc foreland rifting in the Malani block. The block is characterized by extensive acid to alkaline plutons and granite bodies. The Kishengarh syenites and the Newania carbonatites of Rajasthan belong to an earlier Mesoproterozoic event (I 400-I 900 Ma) (Srivastava, 1989). In Kerala, a widespread suite of alkaline granites and syenites has been identified by dating as having being emplaced during the PanAfrican extensional event (Santosh et al., 1989). These include the granites of Ambalavayal in north Kerala, associated with molybdenite mineralization, the alkali granites of Munar in central Kerala and the gem, REE and rare-metal

pegmatites in the Trivandrum area (Santosh et al,, 1994; Soman et al., 1996). Taking a spatial view, it is possible that the Rajasthan-Kerala belt of Pan-African and MesoNeoproterozoic extensional events were a Precambrian precursor of the major rifting in the late Mesozoic. The recent correlation by Santosh et al. (1994) of the molybdenite mineralization of south India across the Gondwana assemblies strengthens such a possibility. The region between Rajasthan and Kerala, however, is under cover of thick Deccan volcanics and, therefore, cannot provide a continuous record of Precambrian extensional tectonics and attendant magmatism. East coast evolution There are several lines of evidence for recognizing the Meso-Neoproterozoic tectonothermal events in the Eastern Ghat mobile belt (EGMB). Prominent among these is the Pakkanadu-Samalpatti-DharmavaramSevathur belt of carbonatites associated with alkaline rocks, which have given mineral ages of -770Ma. Along with the Kullampatti granites and pegmatites of -530Ma (whole rock isochron ages) this linear belt extends northeastsouthwest between Salem and Vellore. Gopalakrishnan (I 994) suggests that these alkaline rocks were emplaced during an extensional tectonic phase along a suture zone of earlier microcontinental collisions. Several syenite plutons occur in the EGMB, in the Prakasam Province (Ratnakar and Leelanandam, 1989) and in the region northeast of Vijayawada up to the junction of this belt with the Singhbhum craton. These include the Rairakhol-Lankarakhol alkaline province (north of the Mahanadi graben) and the Khariar, Koraput, Kunavaram, Elchuru, Purimetla and Uppalappadu plutons. According to Leelanandam (I 9941, the emplacement, alkaline magmatism is facilitated when the regional horizontal stress system is tensional or neutral but not compressional. Leelanandam suggests that deep abyssal faults provide a channel for repeated ascent of mantle-derived material over a long period of time. The whole rock isochron ages of these alkaline complexes range from 1350 to 1480 Ma. Thus, in the EGMB, there is evidence for deep faulting, extensional tectonics and magmatism during the Meso-Neoproterozoic which could be the precursors of the Mesozoic rifting events. The fact that the actual continental boundary is away from the EGMB

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well in the Bay of Bengal renders these suggestions somewhat speculative. This calls for a systematic study of the geological setting of the alkaline rocks and the incidence of distensional tectonics in the other members of the Gondwana configuration that may then throw more light on the possibilities discussed here.

Society India Memoir 15, 145-176. Santosh, M. et al. 1989. Lithos 24, 6579. Santosh, M. et al. 1994. Journal Geological Society India 43, 585-590.

Sinha-Roy, S. et al. 1995. Geological Society India Memoir 31, 63-69. Soman, K. et al. 1996. Journal Geological Society India 27, 41 1-418.

Srivastava,

R. K. 1989.

Geological

Society

India

Memoir 15, 3-24.

Bearing on thermal tectonics

history

and extensional

The Precambrian ancestry of rifting on the western and eastern coasts, rather speculative at present, if established by more detailed evaluation/studies, has great implication in constraining the thermal evolution of the Gondwana. The role of mantle plumes in bringing about the rifting along the present west coast and the break up of the Gondwana has been emphasised by many workers. Extensional tectonics and decompression are also considered to be an important mechanism in the generation of asthenospheric upwelling. Precambrian ancestry would imply that crustmantle interactions acting over a long period and weakening the lithosphere is possibly a prerequisite before large scale continental break up takes place, especially when the lithosphere had already attained large thickness of tectospheric scales by the end of the Archaean in most continental regions of that time. Perhaps more than one episode of dilatory tectonics and attendant magmatism is needed to break up continental blocks of such large thickness. As is true with several other aspects of continental evolution, it is necessary to view the Gondwana configuration as a whole in building up models of thermal evoltuion and continental break up. Possibly the Precambrian history may have an important part in determining where a thickened lithosphere will break up. REFERENCES Gopalakrishnan,

K. 1994. Geological Survey India, June 1994 (Abstract) pp6-9. Hinz, 1981. Geologisch Jahrbuch 22, 3-28. Kaila, K. L. Krishna, V. G. and Mall, D. M. 1981. Tectonophysics

76, 99-l 30.

Krishnan, M. S. 1953. Memoir

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India 81, 80-88.

Krishnamurthy,

P. 1988. Atomic Minerals Divison 1, 8 1 -1 15. Leelanandam, C. 1994. Geological Survey India, Abstracts pp28-29. Ratnakar, J. and Leenandam, C. 1989. Geological Special Publication

Significance of Wollastonite- and Scapolite-bearing assemblages from the Kerala Khondalite belt, southern India M. SATISH-KUMAR Department of Geosciences, Osaka City University, Osaka 558, Japan The role of fluids in granulite petrogenesis has been a topic of intense debate during recent years. The discussions mainly concentrate on two end-member models of granulite formation viz., carbonic metamorphism (e.g. Newton et a/., 1980) and vapour-free metmorphism (Lamb and Valley, 1984). Evidence in support of both models has been put forth from petrologic, fluid inclusion and stable isotope studies, reflecting diverse processes in the Earth’s deep crust. The presence of wollastonite-bearing assemblages from the granulites of the Adirondack Mountains, N. America, has been cited as primary evidence against the model of carbonic metamorphism. On the other hand, the paucity of wollastonite-bearing rocks in the southern Indian granulite terrain has been held in support of a CO,-dominated granulite regime in this terrain. This article summarises the recent discovery of wollastonite-bearing assemblages from the Kerala Khondalite belt of southern India. The Kerala Khondalite belt forms a vast metasedimentary sequence metamorphosed to upper amphibolite and granulite facies grade. The occurrence of numerous “incipient charnockites” (local transformation of garnetbiotite gneiss to vein- and patch-type of orthopyroxene-bearing anhydrous granulites) throughout this terrain has been a theme of detailed study in the recent years, yielding convincing evidence for the role of externally derived carbonic fluids in inducing dehydration reactions (e.g. Santosh et al., 1991). Recent findings of wollastonite-bearing assemblages