Zircon U-Pb Geochronology of the Shangma Fault, Dabie: Constraints on the Transformation from Ductile to Brittle Deformation, and Its Significance to the Tectonic Evolution of Dabie Orogenic Belt

According to the particular investigations on the geometry and kinematics of the Shangma fault, three different crust-levels of extension movements have been differentiated in sequence of development from the deep to the shallow: ① ductile shearing with top to the NWW in low dip angle; ② brittle or brittle-ductile transition normal fault with top to the NWW in low dip angle; ③ brittle normal fault with top to the NWW in high dip angle. Two samples were chosen for zircon U-Pb age dating to test the constraints on the activity age of the Shangma fault. A bedding intrusive granitoid pegmatite vein which was parallel to the foliation of the ductile shear zone of the country rock which exhibits a boudinage deformation, showing syn-tetonic emplacing at the end of ductile deformation period and deformation in the brittle-ductile transition domain. The zircon U-Ph age dating of the granitoid pegmatite vein gives a result of 125.9±4.2 Ma, expressing the extension age in the brittle-ductile transition domain of the Shangma fault. Another sample, which was collected from a granite pluton cutting the foliation of the extension ductile shear zone, gives a zircon U-Ph age of 118.8±4. 1 Ma, constraining the end of the ductile shearing. Then the transformation time from ductile to brittle deformation can he constrained at 126-119 Ma.Compared with the former researches, the formation of the Luotian dome, which is located to the east of the Shangma fault can be constrained at 150-126 Ma. This study offers a new time constraint to the evolution of the Dabie orogenic belt.