Geochemical evidence for a paired arc-back-arc association in the Neoarchean Gadwal greenstone belt, eastern Dharwar craton, India

摘要

Neoarchean Gadwal greenstone belt is situated in the eastern Dharwar craton, southern India. A wellpreserved volcanic sequence which includes a boninite adakite suite and normal tholeiitic to calc-alkaline basalt andesite dacite/rhyolite suite occurs in this belt. The focus of this study is the basaltic rocks from Gadwal greenstone belt. Based on their high field strength element and rare earth element (REE) data the basalts have been broadly grouped into two types: type I basalts are characterized by relatively high Nb/Th (5-9.2) and display slightly depleted to flat chondrite normalized REE patterns, whereas type II basalts display light-REE enriched patterns and Nb/Th ratio <4. Both the types display uniform Nb/Y (-0.12) over a narrow range of Zr/Y (<3) ratios, and collectively exhibit negative Nb, Ti and minor Zr anomalies on a primitive mantle normalized trace element variation diagram. Alteration, metamorphism and contamination by assimilation of Archean upper continental crust in the study area can be ruled out as the cause of these anomalies. The incompatible trace element characteristics are consistent with a subduction related intraoceanic-arc setting for these volcanic rocks. The geochemical variations in Gadwal basalts cannot be explained by a two-stage mantle melting model as recently proposed for certain basalts from elsewhere (e.g. Pickle Crow Assemblage, Canada). The geochemical behaviour is attributed to the lateral variation and batch melting of a primitive mantle source involving subduction zone components. The Gadwal data together with recently published data on basaltic rocks from Archean and Phanerozoic subduction regimes have been used to suggest a Neoarchean paired arc back-arc association in the eastern Dharwar craton.