Abstract

Widespread metasomatic addition of U has apparently occurred in Archean tonalitic–trondhjemitic (gray) gneisses in the Western Dharwar craton of southern India. Gneisses of 3200–3000 Ma age near the Holenarasipur schist belt contain average concentrations of about 10 ppm Th and 3.5 ppm U. The U is considerably more abundant than average U (1.5 ppm) in Archean gray gneisses of the Superior Province of the Canadian Shield, and consequently the Th/U ratios are low in the Indian gneisses. Comparatively massive trondhjemitic plutons associated with a 3000 Ma event in the Dharwar craton are very depleted in both Th and U, and we propose that the metasomatism of the gneisses occurred while the plutons were being emplaced and were expelling fluids from the magmas. There is no evidence for metasomatism 2500 Ma ago, the age of extensive granulite metamorphism in southern India.

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