SOURCES OF CONTINENTAL MAGMATISM ADJACENT TO THE LATE ARCHEAN KOLAR SUTURE ZONE, SOUTH-INDIA - DISTINCT ISOTOPIC AND ELEMENTAL SIGNATURES OF 2 LATE ARCHEAN MAGMATIC SERIES

Conspicuous Nd, Sr and Pb isotopic differences exist between the Arche an gneiss terranes adjoining the suture at the Kolar Schist Belt, sout h India. These gneisses, which are the deformed equivalents of plutoni c and volcanic rocks, have known or inferred igneous ages of 2630 to 2 530 Ma. Initial isotopic ratios of Nd, Sr and Pb suggest that metaplut onic gneisses west of the Kolar Schist Belt were emplaced into, and va riably contaminated by, an evolved continental crust that formed prior to 3200 Ma. Felsic metaigneous gneisses that occur as slivers on the western margin of the schist belt have an isotopic character similar t o that of the metaplutonic rocks on the same side of the Kolar Schist Belt. On the east side of the Kolar Schist Belt the isotopic evidence suggests that the 2530 Ma granitic gneisses were not derived from or c ontaminated by an older continental crust. Their source probably evolv ed with a Nd isotopic composition similar to that of typical Archean m antle, but became light rare earth element enriched after 2900 to 2700 Ma. The inferred tectonic setting for the west side of the Kolar Schi st Belt is an Andean continental magmatic are. For the east side of th e Kolar Schist Belt, a possible Phanerozoic analog is an evolved islan d are, such as Japan.