Geochemical, Sr-Nd and zircon U-Pb-Hf isotopic studies of Late Carboniferous magmatism in the West Junggar, Xinjiang: Implications for ridge subduction

Abstract

Three rock groups: the dioritic rocks, charnockite, and alkali-feldspar granite, are identified based on our petrological and geochemical studies on the voluminous granitic batholiths in the West Junggar. The dioritic rocks are calc-alkaline, Mg enriched, and show high Sr/Y ratios, with weak negative Eu and pronounced negative Nb-Ta anomalies. Their depleted Sr-Nd and zircon Hf isotopic compositions are compatible with those of adakites derived from partial melting of subducted oceanic crust and interacted with mantle materials. Charnockites show transitional geochemical characteristics from calc-alkaline to alkaline, with weak negative Eu but pronounced negative Nb-Ta anomalies. Their Sr-Nd and zircon Hf isotopic compositions also indicate a depleted feature, suggesting a hot juvenile lower crust origin. Alkali-feldspar granite are alkali and Fe enriched, and have distinct negative Eu and Nb-Ta anomalies, low Sr/Y ratios, and depleted Sr-Nd and zircon Hf isotopic compositions. These characteristics are comparable with those of rocks derived from juvenile lower crust. Despite of different petrology, geochemistry, and possibly different origins, zircon U-Pb analyes indicate that these three rock groups were coevally emplaced at - 305 Ma. A ridge subduction model is proposed to account for the geochemical characteristics of these granitoids and coeval mafic rocks. As the ‘slab window’ opened, the upwelling asthenosphere provided enhanced heat flux, and triggered the magnificent magmatisms: partial melting of the subducting slab formed the dioritic rocks; partial melting of the hot juvenile lower crust produced charnockite and alkali-feldspar granite, and partial melting of the mantle wedge generated mafic rocks. This study suggests that subduction was still under processing in the late Carboniferous, and supports that the final collisional termination of the Central Asian Orogenic Belt took place in North Xinjiang not earlier than 305 Ma, possibly in the Permian.