Timing of deformation along the Tan-Lu Fault Zone in eastern China: Constraints from zircon U-Pb geochronology of the Malongshan Shear Zone

The Tan-Lu Fault Zone is a major fault system in eastern China, and its timing of activity has been the focus of much research. We studied a ductile segment of the Tan-Lu Fault Zone, the Malongshan Shear Zone, which is located in the Feidong Complex (the southern part of the Zhangbaling Uplift). A dark biotite-adamellite mylonite has been recognized in the Malongshan Shear Zone, and the mylonitic foliation strikes NNE, consistent with the regional orientation of the Tan-Lu Fault Zone. The occurrence of abundant syn-tectonic leucocratic veins within the biotite-adamellite mylonite indicates that ductile shearing was accompanied by hydrothermal activity. To constrain the timing of deformation along the Malongshan Shear Zone, we dated the biotite-adamellite mylonite and the leucocratic veins using zircon U-Pb geochronology. The weighted mean ages of zircon grains from the biotite-adamellite mylonite (159.0 +/- 5.5 Ma) and the captured Group 2 (156.3 +/- 3.7 Ma and 173.2 +/- 4.2 Ma) of leucocratic veins are interpreted as the ages of protoliths of the biotite-adamellite mylonite during the Jurassic. Additional weighted mean age of the hydrothermal zircon grains (130.9 +/- 3.6 Ma) represents the timing of crystallization of the veins. Therefore, the results indicate that the Malongshan Shear Zone (and hence the southern segment of the Tan-Lu Fault Zone) was active during the Early Cretaceous (similar to 131 Ma). Based on the present results and existing data, we consider that sinistral shearing of the Tan-Lu Fault Zone during the Early Cretaceous was controlled by oblique subduction of the Pacific Plate at a high angle.