Geochronology of Late Mesozoic syn-tectonic granite veins in the Shangdan tectonic belt of the Qinling Orogen and its tectonic implications
Intense intracontinental orogeny occurred in the Qinling Orogen during Late Mesozoic,and the ages of syn-tectonic granite veins founded in the Shangdan tectonic belt can effectively define the time of intracontinental tectonic deformation.In this paper,zircon U-Pb dating and Hf isotope analysis were conducted on the granite veins.The microstructural observations suggest that the granite veins are characterized by mylonitization,preferred orientation of minerals and medium to high-temperature solid-state deformations,such as the dynamic recrystallization of quartz by grain boundary migration and subgrain rotation,and myrmekite growth textures at the edge of feldspar.CL images reveal that the zircons are marked by spongeous texture with weak cathodoluminescence.Meanwhile,the zircons have lower Th/U ratios in the range of 0.03 to 0.07 and are enriched in heavy rare-elements.According to the discriminant diagrams of(Sm/La)N-La and Ce/Ce*-(Sm/La)N,it can be concluded that the zircons of syn-tectonic granite veins were formed in the transition stage from magmatic to hydrothermal process,belonging to the hydrothermal fluid metasomatic alteration type zircon.Zircon U-Pb dating yields an age of 130～124 Ma for the hydrothermal zircons,representing an important tectonic thermal event.Furthermore,the zircons of granite veins have concentrated negative εHf(t)values of-9.47～-0.99 and Hf two-stage model ages of 1 752～1 218 Ma,probably resulting from the partial melting of the Qinling Group and the Kuanping Group.Combined with previous works,this study reveals that the Yanshanian intracontinental orogeny in Qinling Orogen is characterized by two episodic tectonic deformations,and the age of the syn-tectonic granite veins in Shagoujie ductile shear zone corresponds to the second episode of the intracontinental tectonic deformation.
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