GRANITE FORMING EVENTS AND THEIR ROLE IN CRUST FORMATION OF THE INDIAN SHIELD

Abstract

Continental crustal evolution in the Indian shield appears to have started around the middle Archaean and completed by the early Proterozoic. In this time span of around 700 - 800 Ma, the primordial gneissic crust has evolved, recycled and stabilized. The earliest phase of the granitoid activity, from the middle-to late Archaean consists predominantly of a tonalite-trondhjemite-granodiorite (TTG) suite, while the second phase of the Archaean - Proterozoic transition (APT) period consists predominantly of granodiorite-adamellite suite. Minor granite forming events have occurred in the middle-to late Proterozoic. The compositional changes in the granites from the Archaean to the Proterozoic appear to have been mainly controlled by the tectonics and the source compositions at each stage and contribution from the mantle appears to be negligible. Increasing of some large ion lithophile (LIL) elements and decreasing of compatible trace elements in the APT granites compared to the earlier gneissic phase and the overall compositions of the latter (the gneisses) suggest that the middle- to late Archaean polyphase gneisses are the partial melts of the mafic rocks from greenstones and the various phases of the tonalite-trondhjemite-granodiorite (TTG), viz., the hornblende-biotite- and feldspathic gneisses (younging in the same order) are the successive fractionate of similar or same source. The tectonic setting at the time of formation of the TTG gneisses is more of a compressive regime and crustal thickening appears to have played a role in the initiation of partial melting of the source. The appearance of extensive K-rich batholithic granites throughout the shield in a limited time span of 200-300 Ma during the early Proterozoic, increase of LIL elements and high Th, Rb and Sr variations in these batholithic granites suggest that these granites are anatectic in nature and have formed due to the release of compressional forces and initiation of an extensional regime in the shield area. Though sporadic important alkalic granite bodies have formed in the middle to late Proterozoic, they are mostly restricted to the orogenic belts like the Aravalli, the Delhi, the Satpura and the Eastern Ghats. Many of these post-tectonic granites (mostly A-type) give evidence for crust-mantle interaction in their formation and their emplacement in a stable continental crust setting.