西昆仑库地韧性剪切带的40Ar/39Ar年龄

西昆仑库地以南有一套变质变形较强的岩系,前人依照区域对比关系将其划为前寒武的古老基底。对西昆仑早期构造演化的论述均基于该观点,但没有提供确凿的同位素年代学证据。笔者通过野外观察、室内研究,确认库地以南的变质变形岩系是大型韧性推覆剪切作用的产物。通过对新生变质矿物角闪石和黑云母单矿物的⁴⁰Ar/³⁹Ar年龄分析,确定剪切变质年龄为426-451Ma,说明库地的变质变形岩系是形成于早古生代晚期的一条大型韧性剪切带,这对于解释西昆仑的早期构造演化具有重要意义。

40Ar/39Ar DATING OF DUCTILE SHEAR ZONE IN KUDA, WEST KUNLUN, XINJIANG

The study region located in Kuda, west Kunlun, Xinjiang, which is an area with low research level and very limited geological information. There is a set of metamorphic rocks with low amphibolite facies metamorphic grade from 153km to 174km along Xin Zang highway (from Yecheng, Xinjiang, to Shiquanhe, Tibet), which was thought as Precambrian basement by other authors, but no one provided isotopic age. This paper provides valuable ⁴⁰Ar/³⁹Ar data for the study of geological evolution of west Kunlun. The authors realize that the metamorphic rocks are products of ductile shear activation on the basis of field observation and lab analysis. Metamorphic minerals hornblende and biotite from ductile shear zone were dated by ⁴⁰Ar/³⁹Ar method in this paper, plateau and isotope age were obtained. The plateau ages of 452 Ma and 428 Ma have been obtained in hornblende and biotite which were determined at five temperature interval from 900℃ to 1300℃ and from 900℃ to 1400℃ respectively, representing the cooling time of the main metamorphic event. The isotope ages of hornblende and biotite are 451 Ma and 426Ma, respectively. It is concluded that the plateau age coincides with isochron age. The initial argon ratio of hornblende and biotite is 292.3 and 290.5 respectively, which means no excess radiogenic Ar existing in hornblende and biotite and no argon loss. It is interesting that the metamorphic age of ductile shear zone is approximately to the age of the "No. 128" pluton which was dated by ⁴⁰Ar/³⁹Ar method, implying they are the same products of the tectonic movement. The age of ductile shear zone in Kuda, west Kunlun is belong to early Paleozoic, which is not the Precambrian basement. It is possible that the duration of ductile shearing is at least 25Ma.