西昆仑布伦阔勒岩群中泥质高压麻粒岩的岩石学、年代学及其地质意义

西昆仑的深变质岩类主要发育于布伦阔勒岩群之中,其中的高压麻粒岩是西昆仑造山带中目前已知的变质程度最高的岩石。本文以其中的泥质高压麻粒岩为研究对象,结合岩相学、相平衡模拟以及锆石年代学分析等方法进行研究。结果显示其峰期变质矿物组合蓝晶石+石榴石+钾长石,是典型的泥质高压麻粒岩岩石组合。根据相平衡模拟估算,高压麻粒岩相峰期变质的温压条件高于850℃及1.4GPa,退变质的温压条件约为650℃和0.6GPa。SHRIMP U-Pb锆石定年结果显示泥质高压麻粒岩记录了两期变质,第一期暗色变质锆石年龄为ca.185Ma,代表岩石从高压麻粒岩相峰期变质退变至近固相线阶段的年龄;第二期亮色变质增生边年龄为ca.166Ma,代表后期退变质年龄;而高压麻粒岩相峰期变质时代应在200~185Ma之间。高压麻粒岩的变质条件、顺时针的P-T轨迹及锆石年代学的结果指示了晚三叠世-早侏罗世的碰撞造山事件(ca.200~166Ma)。结合区域地质资料,推断在西昆仑山内存在一条中生代的中-高压变质带,这条变质带代表了古特提斯洋关闭塔里木与羌塘地块碰撞拼合的位置。

Petrology and geochronology on high-pressure pelitic granulite from Bulunkuole complex in West Kunlun and its tectonic implication

High-grade metamorphic rocks in the West Kunlun orogenic belt are mainly developed in the Bulunkuole Group,among which some high-pressure granulites are recognized and represent the highest-grade metamorphic rocks known in the West Kunlun orogenic belt.In this paper,a comprehensive study of the high-pressure pelitic granulite is carried out by combining petrography,phase equilibrium modelling and zircon geochronology analysis.The results show that the rock experienced high-pressure granulite facies metamorphism,and the peak metamorphic mineral assemblage includes garnet+kyanite+K-feldspar.Based on the phase equilibrium modelling,the P-T conditions of the peak metamorphism of the high-pressure pelitic granulite are no less than 850℃at1.4 GPa,and the P-T conditions of the retrometamorphic stage are about 650℃at 0.6 GPa.SHRIMP U-Pb zircon dating results show that the high-pressure pelitic granulite recorded two stages of metamorphism.The first stage age for the dark grey metamorphic zircon is ca.185 Ma,which represents the age of the rock retrograding to the solidus line.And the second stage age for the light grey metamorphic rim is ca.166 Ma,which represents the age of later metamorphisms.The peak metamorphic age of high-pressure granulite facies metamorphism should be between 200 Ma and 185 Ma.The metamorphic conditions of high-pressure granulite,the clockwise P-T trajectory and the results of zircon dating constrain an important orogenic process during the Late Triassic-Early Jurassic(ca.200~166 Ma).Combined with the regional geological data,it is suggested that there exists a Mesozoic medium-high pressure metamorphic belt in the West Kunlun orogen,which represents the location of the final collision of the Tarim and Qiangtang block.