Geochemistry of Granite-Greenstone Terrain of South India

A critical evaluation of the available major and trace element data on rocks of the granite-greenstone belts or South India leads to the following inferences: Barrovian and Abukuma facies series of metamorphism are recognizable in the terrain. Rocks of peridotitic and basaltic komatiite affinities occur in the pre-Dharwar and early Dharwar ultramafic-mafic sequence. The bulk of the greenstones are island arc tholeiites with a strong calc-alkaline affinity. The presence of detrital pyrites in the oldest conglomerates, and the occurrence of red beds in the topmost Dharwar sequence, indicate that an oxygenic atmosphere evolved through the Dharwar times. The pre-Dharwar granitoids were probably tonalitic, and through time, alkali fractionation gave rise to the Champion gneisses, Peninsular gneisses and Closepet granites. The available data are inadequate to elucidate the fractionation pattern of the granitic crust of Peninsular India.

Pyrolite and eclogite mantle are invoked for tectonomagmatic events during the pre-Dharwar and Dharwar times, respectively. Convection, and eclogite sinkers, are proposed as alternative geodynamic models.

From the geochemical evolution, it is contended that the granite-greenstone belts of Peninsular India represent a more evolved crustal condition as compared to the Archaean granite greenstone belts of South Africa and Western Australia. Obviously, the granite-greenstone belts of all the shield areas, did not evolve simultaneously.