Characteristics of Early Paleozoic Magmatism in Fengxian Area, West Qinling: Evidence from Petrogeochemistry, Zircon U-Pb Age and Hf Isotope

In order to study the tectonic environment and dynamic background of the magmatic activity in the West Qinling orogenic belt in Early Paleozoic, it chooses the granite and diorite in Longwanggou rock mass and granodiorite in Tangzang rock mass, which near the Shang-Dan fault zone, for study. Geochemistry and LA-ICP-MS zircon U-Pb chronology were tested for all type of rocks and Hf isotope analysis for granite and diorite in Longwanggou rock mass. The dating results show that the age of Longwanggou granite is 433±4 Ma and 439±5 Ma, the diorite emplacement time is later than granite with the U-Pb age of 453±3 Ma. ΣREEs of Longwanggou diorite and Tangzang granodiorite are (107-181)×10^-6 and (209-321)×10^-6, respectively. Rare earth element distribution curve characteristics are similar, both showing clearly to the right-inclined pattern, strong light and heavy rare earth element fractionation strongly ((La/Yb) _N at 7-13 and 31-57, respectively), and no obvious negative Eu anomaly (δEu are 0.88-1.00 and 0.79-0.87, respectively). Relative to the Longwanggou diorite, Tangzang granodiorites have high ΣREE, light and heavy rare earth element fractionation is more intense, less negative Eu anomaly. The emplacement times of Longwanggou diorite and Tangzang rock mass are consistent, both of originate and granodiorites are from the lower crust, mixed with mantle-based magma to a certain extent during the rising process. The Tangzang granodiorite experienced relatively sufficient magma evolution. Hf isotopic compositions have distinct differences between the two Longwanggou granite samples. Although the ε_Hf(t) values are all positive, they concentrated on the values of 0-3 and 7-10 respectively, with corresponding two-stage model ages (t_DM2) 1.1-1.4 Ga and 0.8-1.0 Ga. Combined with geochemical characteristics, the granite formed in the active continental margin tectonic environment (island arc) and derived from the partial melting of Qinling Group. At the same time, much of basic rocks from oceanic crust were brought into the melt during the plate subduction process, the magma have experienced a relatively strong crystallization differentiation and no obvious mingling process during the emplacement occurred. Longwanggou rock mass is a polyphase complex, experienced the tectonic regime transformation from subduction to collision and compression, and recorded the tectonic evolution information of Qinling ocean from subduction to closure. The subduction and orogenesis of Qinling area in the Paleozoic lasted for a long time and the accompanying magmatic activity also continued from Early Paleozoic to Middle Paleozoic.