Sm-Nd and Rb-Sr Ages for Archaean Rocks in Western Karnataka, South India

Abstract A Sm-Nd whole-rock isochron age of 3020 ± 230 Ma for basal rnetavolcanics of the Kudremukh-West Coast greenstone belt and a Rb-Sr whole rock age of 3280 ± 230 Ma for the underlying granitoid gneisses are presented. These ages indicate that the filling of an ensialic basin with volcanic and sedimentary rocks began at around 3000Ma and confirm earlier studies which concluded that the dominant gneisses of Karnataka represent a major period of primary crust formation in the South Indian Craton at about 3300 Ma. Both the early granitoid gneisses and the younger metavolcanics were derived from unfractionated mantle sources which had not previously been depleted or enriched in LIL elements.