西藏扎雪—门巴韧性剪切带变形时代及机制研究:来自同构造花岗岩体的证据

Age and Genetic Mechanism of Deformation of the Zhaxue-Menba Shear Zone in Tibet: Evidence from the Synorogenic Granites

从同构造花岗岩体宏、微观变形特征入手,拟定出扎雪-门巴韧性剪切带内同构造花岗岩体的存在,并揭示出剪切带在空间上为一条近南北向逆冲推覆兼具左行走滑的斜冲型剪切带。同构造花岗岩体岩浆锆石LA-ICP-MSU-Pb年龄为(86.57±0.57)Ma,表明剪切带变形变质发生在晚白垩世,属冈底斯板块早、晚白垩世构造事件正反转后挤压体制下的初期产物。新特提斯闭合及洋壳俯冲所致的挤压构造环境导致晚白垩世冈底斯带普遍遭受区域变质作用,剪切带正是变形变质作用由南向北传递至该区减弱的表现,因此该变形年龄可能作为新特提斯闭合、洋壳俯冲作用延续到晚白垩世的年代学证据。

The occurrence of the synorogenic granites in the Zhaxue-Menba shear zone was determined according to the character of macro- and micro-deformation in the synorogenic granites. The shear zone is a diagonal shear zone with N-S thrusting and sinistral strike slipping features. The LA-ICP-MS zircon U-Pb age of the synorogenic gran- ites, which represents its crystallization age, is （86.57 ± 0.57） Ma, which indicates that the deformation and met- amorphism in the shear zone occurred in Late Cretaceous, and pertain to the initial products by the positive struc- tural inversion under the compressional system in Gangdese in Early and Late Cretaceous tectonic events. The com- pressional tectonic environment induced by the closure of the Neo-Tethys and underthrust of oceanic crust resulted in regional metamorphism in Gangdese during Late Cretaceous. Then the shear zone formed when metamorphism gradually weakened and passed from south to north, therefore, it is evident to suggest that the closure of the Neo- Tethys and underthrust of oceanic crust may last until late Cretaceous.