Zircon U-Pb age and petrogenesis of tonalite-quartz diorite in the Shibanjing area, central Beishan orogenic belt, and its constraint on subduction of the ancient oceanic basin

The Early Paleozoic Yingyun diorite-quartz diorite is exposed in the Shibanjing area of central Beishan orogenic belt. In this paper, LA-ICP-MS zircon U-Pb dating of the rock mass was conducted, and the ²⁰⁶Pb/²³⁸U average age is 464.4±2.1Ma. According to geochemical data, the rocks belong to the calc-alkaline to high potassium calc-alkaline, quasi-aluminous I type granite. In view of its petrogenesis, the authors hold that the magma source of tonalite-quartz diorite was formed by mixed magma from the crust and magma from the depleted mantle under the conditions of high pressure(>1.5GPa) and low temperature(719～792℃), with the residual minerals of garnet+ruble (without amphibole). Moreover, trace element composition is characterized by enrichment of large ion lithophile ele-ments (LILEs), such as Rb, Ba, Th, U and K, and depletion of high field strength elements(HFSEs), Ta, Nb, P, and Ti, showing the geo-chemical features of the typical island arc granite. It is further proved that the rock mass was formed in an island arc environment on the discriminant map of trace elements tectonic environment. It constitutes "oceanic trench-arc" system with the south side of the Hongli-uhe-Niuquanzi-Xichangjing Paleozoic oceanic basin, indicating that the Hongliuhe-Niuquanzi-Hidangjing Paleozoic basin had sub-ducted northward in the late Middle Ordovician. The closure time of the ocean basin should be later than Middle Ordovician.