羌塘中部高压变质带的退变质作用及其构造侵位

Retrograde metamorphism and tectonic emplacement of high pressure metamorphic belt in central Qiangtang Tibet

羌塘中部的高压变质带主要由榴辉岩、石榴石白云母片岩和蓝片岩等组成,它们在遭受高压变质作用之后折返,构造侵位于晚古生代展金组地层中,二者以韧性变形带为接触边界。本文以高压变质带中的榴辉岩和韧性变形带为研究对象,讨论了高压变质带折返过程中的退变质作用特征及折返时代。研究表明,榴辉岩在高峰期变质作用之后的折返过程中经历了由榴辉岩相→蓝片岩相→绿帘角闪岩相的退变质作用演化过程;在高压变质带构造侵位过程形成的韧性变形带中,白云母石英片岩的白云母^(40)Ar-^(39)Ar坪年龄为219±2Ma。高压变质带在219Ma左右构造侵位于展金组地层中,并于214Ma之前最终抬升出露地表。

The high pressure metamorphic belt, located in central Qiangtang Tibet, is mainly composed of eclogite, garnet-phengite schist and blueschist. They underwent the exhumation after high pressure metamorphism and tectonically emplaced into the Late Paleozoic epimetamorphic rocks. The high pressure metamorphic belt contacts with epitmetamorphic rocks by a ductile deformed zone. In this paper, based on studies of both the eclogite and the ductile deformed zone, exhumation age and retrograde metamorphic characteristics are discussed. It is proved that the eclogite underwent retrograde metamorphic evolution after peak metamorphisin, from eclogite facies, blueschist facies to epidote-amphibolite facies. (40)Ar-(39) Ar dating of muscovite from muscovite-quartz schist, in the ductile deformed zone, obtains the plateau age of 219 +/- 2Ma, which represents the emplacing age of the high pressure metamorpbic belt into the strata of Zhanjin Formation. Moreover, they were uplifted toward surface before 214Ma ago.