The Early Precambrian Crust with Reference to the Indian Shield: An Essay

The existence of several greenstone generations and of continuous transitions between gneiss-greenstone terrains and granulite terrains in the Indian Shield allow a unique insight into the structure and evolution of the Archaean crust. The long standing controversy on granite-greenstone relations reflects contrasting basic assumptions, methods and criteria. Many of the problems reflect the uncertain significance of Rb-Sr ages, which often represent thermal resetting events. Correlations between volcanic units in separate greenstone belts and between shallow and deep crustal zones should be soon enabled by U-Pb and Sm-Nd isotopes. The concept of greenstone belts as primary basins is not applicable to the oldest supracrustals whose remnant enclaves and xenoliths are distributed on all scales throughout the Peninsular Gneiss. The oldest directly identifiable rocks in south India consist of ultramafic-mafic-anorthosite-pelite-chert assemblages of the Sargur Group intruded by ca 3.3 Gyr gneiss. While sialic nuclei may have been present, a pre-existence of regional sialic crust is inconsistent with the petrological, geochemical and isotopic characteristics of the dominant tonalite-trondhjemite components of the Peninsular Gneiss. The gneisses are extensively imprinted by a ca 3.0 Gyr thermal event which reset Rb-Sr systematics, but left least-reset isotopic 'windows' such as around Hassan. The age of the discrete depositories in which the Dharwar Supergroup volcano-sedimentary successions accumulated remains to be clarified. These rocks were possibly deposited in rifted zones floored by Peninsular Gneiss blocks, older greenstones and simatic gaps. The formation of charnockites and the generation of K-rich adamellites about 2.7-2.6 Gyr ago may have been genetically linked and appears to have been essentially imprinted on the older gneiss-greenstone system. Repeated thermal rises at infracrustal levels during the Proterozoic resulted in isotopic resetting of Rb-Sr systems in the Eastern Ghats granulite-gneiss belt. The temporally unique nature of the Archaean Era and major unresolved questions are pointed out.