Geochemistry of Archaean amphibolites from Karnataka State,Peninsular India

Ortho- and para-amphibolites occur throughout the Dharwar schists, gneisses and ultramafics in Karnataka State, India. From the chemical data alone, these ortho- and para-amphibolites cannot be very clearly differentiated. While the ortho-amphibolites follow the Karroo trend, the para-amphibolites appear to have been evolved from the metamorphism of low-Si and -K graywackes. The amphibolitic xenoliths occurring in Peninsular gneisses follow both the igneous and graywacke field, indicating that probably both para and ortho types constitute these enclaves. Their trace element concentrations, like those of Ni, Co and Cr, are not related to the Mg and Fe contents of the amphibolites. Probably these amphibolites have been derived from basic rocks, which were predominant over the Archaean crust, either directly or after having gone through a sedimentary cycle. The higher concentration of Ni, Co, Cr and other elements, such as Mg, in these amphibolites, as compared with younger oceanic basalts, thus indicates the higher concentration of these elements in Archaean basalts and the proto-mantle.