Abstract:

The Xianshuihe (XSH) fault zone is an important lithospheric strikeslip fault in the eastern margin of the Tibet Plateau. A migmatite zone, about 120 km long and 35 km wide, developed along the fault. Kinematic and chronological studies reveal twostaged anatexis and metamorphism of the migmatite zone and indicate two periods of deformation of the XSH fault. The first stage migmatization occurred in 3227 Ma, suggesting that the Xianshuihe fault zone underwent intensive deformation during the Oligocene. The second stage migmatization occurred in 2520 Ma. Zircon UPb dating of two rock samples taken from the leucosome of the secondstage migmatite yields respectively the ages of ca. 25.06 Ma and ca. 20.9 Ma. Field investigation and microstructural study show a synshearing anatexis of the secondstage migmatite, with intensively developed horizontal lineations. Five zircon UPb dating of the granite intrusion, which closely related to the secondstage migmatite zone, yield crystalline ages of 2019 Ma, representing the ending of the migmatization. The new finding of the twostaged migmatization constrains the Cenozoic transformation from compressional regime to sinistral strikeslip regime of the XSH fault. And it confirms that the initiation of the Cenozoic strikeslip along the Xianshuihe fault zone occurred in 2725 Ma which is significant for understanding the processes of Cenozoic tectonic deformation in the east Tibet Plateau.

Key words: Xianshuihe fault zone, migmatization, Oligocene, zircon U - Pb dating