The Oldest Supracrustals of the Dharwar Craton, India

The metamorphosed supracrustal rocks of Holenarasipur, Nuggihalli, Krishnarajpet, Hadnur and Ghattihosahalli belts appear to represent the oldest (3.5 b.y.) greenstones of the Dharwar Craton. Detailed geological mapping of these belts and surrounding gneisses shows no evidence of a sialic basement for these predominently ultramafic belts. The sediments of these belts are mostly chemogenic-volcanogenic-volcaniclastic and fine-grained argillites. Fuchsite-quartzite, which is found at the top of the succession is the only siliceous rock unit. Its δO ¹⁸ value at+ 12.5, suggests that it is a metachert and not of detrital origin. REE pattern of the sediments of HNSB (Holenarasipur Schist Belt) is distinct from the sediments derived from acid plutons. The ultramafic schists of these belts, interbedded with the sediments, are generally komatiitic in composition and although metamorphosed under amphibolite facies conditions, locally exhibit relict pillow-structure and spinifex-texture.

The development of these greenstone belts was followed by at least three episodes of intrusion by sodic plutons. Erosion of these bodies and of the greenstone belts provided the sediment for the younger schist belts. The first evidence of detrital sedimentation derived from acid plutonic rocks is preserved in the form of current-bedded arkoses and quartzites in the younger greenstone belts. An interfingering sequence of the various successive greenstone belts and acid plutons, precursors of the Peninsular gneisses, can be recognised on the basis of available data.