Petrogenesis and Tectonic Implications of the Carboniferous Andesites from Bayingou in the North Tianshan Belt, Xinjiang

The Late Paleozoic volcanic rocks in the North Tianshan complex belt are the products of closure of the North Tianshan Ocean, which are important for understanding the Paleozoic evolution history of the Paleo-Asian Ocean. In this study, we present new petrological, mineralogical, geochronological and geochemical data on the Carboniferous Bayingou andesites in the North Tianshan belt, and discuss their petrogenesis and tectonic implications of the andesites. The results show that: ① the andesites belong to the calc-alkaline series that formed in a convergent continental margin setting related to the plate subduction in this study, whereas these andesites have low MgO (＜3%) contents that are different from typical high-Mg andesites (＞6%), whereas the andesites have high Mg^# values (＞45), indicating mantle materials participating in the formation of the andesites. ② In-situ LA-ICP-MS zircon U-Pb dating on the andesites of the Bayingou Formation yielded an weighted average age of (324.1±1.1) Ma (MSWD=1.5), representing the emplacement age of the andesites. The Carboniferous andesites were a mixing product of the felsic and mantle-derived melts, and the felsic melts were derived from partial melting of the mafic lower crust in the North Tianshan orogenic belt. Integrated with the previous studies and geological data, this study holds that the Bayingou Carboniferous andesites were generated in a subduction zone or arc-related environment, which further indicates that the North Tianshan Ocean was still not closed in the late Early Carboniferous.