藏北纳木错西缘前寒武纪辉长岩变质变形年代学研究

Geochronological studies on the deformation and metamorphism of the Precambrian gabbro from the western area of Nam Co, northern Tibet

前寒武纪辉长岩出露于纳木错西缘逆冲推覆构造带中。辉长岩经历了早期角闪岩相变质和韧性推覆、晚期绿片岩相变质和脆性推覆。早期变形的斜长角闪质糜棱岩中变质矿物角闪石、斜长石和石榴石的 Rb-Sr等时线年龄为173±10Ma;角闪石^(40)Ar-^(39)Ar坪年龄和等时线年龄均显示174Ma的变质和变形年龄。受晚期构造-热事件的影响,斜长石的^(40)Ar-^(39)Ar坪年龄和等时线年龄分别为109Ma和110 Ma。综合上述结果,可以确定前寒武纪辉长岩早期角闪岩相变质作用的年龄是173～174 Ma,可能与羌塘地块和拉萨地块初始碰撞引起的深层次的韧性剪切变形有关;晚期绿片岩相变质作用年龄是109～110 Ma,指示了碰撞造山晚期的冲断构造变形时代。两期变质变形年龄的确定对研究羌塘地块与拉萨地块碰撞过程具有重要的意义。

The Precambrian metagabbro, recently discovered in the thrust fault zone about 25km west to Nam Co, experienced two tectono-metamorphic events. The early metamorphism is amphibolite-facies with ductile shear and the later one is greenschist-facies with thrust slip fault. The Rb-Sr isochron age obtained from the metamorphic minerals ( hornblende, plagioclase and garnet) in the amphibolite-mylonite is 173 +/- 10Ma. The hornblende separated from the amphibolite-mylonite gives Ar-40-Ar-39 plateau and isochron ages of 174Ma, respectively. As the late tectono-metamorphic event, Ar-40-Ar-39 plateau and isochron ages of plagioclase are 109Ma and 110Ma, respectively. Consequently, we conclude that the early metamorphic age of the Precambrian gabbro is about 173 similar to 174 Ma coherent with the deep ductile shear during the collision between the Lhasa and the Qiangtang blocks, and the later is about 109 similar to 110 Ma, perhaps perfinent to the thrust movement under the greenschist-facies in the late orogeny. The ages of two deformation and metamorphism can be used to study the process of collision between the Lhasa and the Qiangtang blocks.