柴北缘锡铁山韧性剪切带的基本特征及其形成时代

在柴北缘锡铁山地区元古宇达肯大坂群与奥陶系滩涧山群之间发育一条呈NW向展布的基底韧性剪切带。本文对该韧性剪切带中的宏观构造(鞘褶皱、不对称褶皱、塑性流变组构、S-C组构、旋转碎斑系、构造透镜体、粘滞型石香肠构造)和糜棱岩微观构造(云母鱼构造、压力影构造、动态重结晶、雪球状旋转构造、波状消光和云母扭折)特征作了详细的研究。对锡铁山基底韧性剪切带中花岗质糜棱岩进行白云母Ar-Ar同位素测年,获得坪年龄为398±4 Ma,等时线年龄为399±4 Ma,二者基本一致,代表了锡铁山基底韧性剪切带的形成时间。综合上述特征判断,锡铁山韧性剪切带是一条由元古宇达肯大坂群向SW斜向逆冲所形成的右行逆冲韧性剪切带,该韧性剪切带与早古生代柴达木地块向NE斜向俯冲碰撞造山作用有关,并可能导致了超高压变质体折返到浅部地壳层次。同样,该韧性剪切带的存在,也对锡铁山铅锌矿起到了十分重要的后期构造控制作用。

Characteristics and Muscovite 40Ar/39Ar Age of Ductile Shear Zone in the Xitieshan Area,North Qaidam

A north-west trending ductile shear zone was recognized in between the Proterozoic Dakendaban Group and the Ordovician lower Tanjianshan Group in the Xitieshan area, North Qaidam. This paper illustrates various macroscopic structures including sheath fold, asymmetrical fold, plastic flow fabric, S-C fabric, rotating mortar system, tectonic lens, and viscous boudinage structure, as well as the microscopic structures including mica fish structure, pressure shadow structure, dynamic recrystallization, snowball shape tectonic rotation, wavy extinction and kinking mica. These structural data indicate that the ductile shear zone in the Xitieshan area is dextral transpressional ductile shear zone with southwest ward thrust of the Proterozoic Dakendaban Group. Ar-Ar dating of muscovite from the granitic mylonite of the Xitieshan base ductile shear zone yielded Ar-Ar plateau age of 398 Ma and inverse isochron age of 399 Ma, respectively, showing that the ductile shear zone formed in Late Silurian. Combined structural and dating results, we suggest that this ductile shear zone is related to the northeastward oblique subduction of the Qaidam block during Early Paleozoic, which might have led to the exhumation of the ultrahigh pressure metamorphic rocks to the shallow crust. The ductile shear zone might also have played a very important rule in controlling the mineralization of the Xitieshan lead-zinc deposits.