The primitive crust: Evidence from the Indian shield

Abstract

Abstract Four distinct geological settings appear to have existed in the Indian Peninsula during the Precambrian Era. These are represented by (1) Early and Middle Archaean 1 ‘true greenstone belts’ of Holenarasipur, Nuggihalli, Kolar and Sargur, (2) Late Archaean—Early Proterozoic ‘greenstone-like geosynclinal piles’ of Shimoga-Chitradurga, (3) high-grade derivatives of greenstones and granulites of Tamil Nadu-Kerala and (4) Proterozoic platforms of Cuddapah-Vindhyan basins. The ‘greenstone belts’ of Holenarasipur type are elongated, sometimes symmetrical, belts of rhythmic volcanism and fine-grained detrital and chemical sedimentation. The base of these greenstone belts is made up of basic and ultrabasic flows of komatiitic affinities and these are intruded by tonalites and trondhjemites. The main Dharwar greenstone-like geosynclinal piles start with orthoquartzite-carbonate facies showing ripple marks and current-bedding and these develop into thick, graded-bedded eugeosynclinal piles and do not show the characteristic rhythmic volcanism, sedimentation and symmetry of ‘true greenstone belts’. Their volcanism is of a calc-alkaline type. Recent data on these geological settings from the South Indian Shield suggest a thin simatic protocrust with a highly undifferentiated protomantle existing during the Early and Middle Archaean. In the cracks developed due to degassing of the protomantle, ‘true greenstone belts‘ were laid down and the products of partial melting of the basic crust at comparatively shallow depth, i.e. tonalite-trondhjemites were emplaced. This process formed the continental nuclei of mixed, but predominantly basic, composition. Between these continental nuclei, geosynclines were formed in which Dharwar-type greenstone piles were laid down. Partial melting of the lower crust of the upper mantle, at a deeper level than the depth of genesis of trondhjemite, gave rise to potassic granites and resulted in more than two (?) episodes of widespread potash reactivation marking the completion of cratonisation, following which emplacement of dike swarms took place.