鲜水河断裂带渐新世至早中新世两期变形相关混合岩的锆石U-Pb年代学及其意义

Zircon U-Pb study of two-staged Oligo-Miocene migmatization along the Xianshuihe fault zone,East Tibet Plateau

鲜水河断裂带是青藏高原东缘重要的走滑断裂带。沿该断裂带发育长约120km、宽约3~5km的混合岩。运动学和年代学资料表明其与鲜水河断裂带渐新世至早中新世的两期变形有关。第一期变形发生在32~27 Ma,混合岩显微构造显示强烈压扁机制,反映了沿鲜水河断裂带的挤压变形机制。第二期混合岩中两个淡色体的样品获得了25.06 Ma和20.9 Ma的锆石U-Pb年龄,限定混合岩化的时间约为25~20 Ma。这期混合岩以发育左行走滑的矿物生长线理为特征,反映了走滑的应力体制。与这期混合岩密切相关的5块花岗岩样品均获得了20~19 Ma的锆石U-Pb年龄,代表了混合岩化作用的结束。两期混合岩的发现限定了鲜水河断裂带新生代由挤压体制到左行走滑体制的转变,也即鲜水河断裂带新生代走滑起始时间27~25 Ma。该发现对于认识青藏高原东缘新生代的构造变形过程具有重要意义。

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 3-5km 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 32-27 Ma,suggesting that the Xianshuihe fault zone underwent intensive deformation during the Oligocene.The second stage migmatization occurred in 25-20 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.20.9Ma.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 20-19 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 27-25 Ma which is significant for understanding the processes of Cenozoic tectonic deformation in the east Tibet Plateau.