拉萨地体东部早侏罗纪变质和深熔作用  

Early Jurassic metamorphism and anatectic melting of the eastern Lhasa terrane

作　　者：董昕[1] 张泽明[1] 田作林[1] 李冰[2] DONG Xin;ZHANG Zeming;TIAN Zuolin;LI Bing(Institute of Geology,Chinese Academy of Geological Sciences,Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources,Beijing,100037;Geological Corporation of China Gold Group,Beijing,10001)

机构地区：[1]中国地质科学院地质研究所,自然资源部深地动力学重点实验室,北京100037 [2]中国黄金集团地质有限公司,北京100011

出　　处：《地质学报》2019年第10期2446-2462,共17页Acta Geologica Sinica

基　　金：中国地质局地质调查项目(编号DD20190057);国家自然科学基金项目(41872070);国家留学基金项目(201809110024)联合资助成果

摘　　要：位于青藏高原南部的拉萨地体不仅记录了中生代的新特提斯洋俯冲及随后新生代的印度-欧亚板块陆陆碰撞造山作用,而且还记录了晚古生代-早中生代南、北拉萨地体的拼合作用。本文对拉萨地体东部东久地区的片岩和脉体进行了岩石学和锆石U-Pb年代学研究,表明片岩经历了峰期高角闪岩相的变质作用和部分熔融,中压角闪岩相退变质过程以及晚期的降温、降压过程。片岩记录了峰期矿物组合蓝晶石+石榴石+黑云母+斜长石+钛铁矿+石英,退变质矿物组合石榴石+夕线石+堇青石+黑云母+斜长石+钛铁矿+石英,晚期退变质矿物组合堇青石+黑云母+白云母+绿泥石+斜长石+钛铁矿+石英。相平衡模拟研究表明,片岩的峰期变质作用温度、压力条件约为720℃、0.9GPa;退变质条件约为670℃、0.59GPa以及480℃、0.12GPa。全岩地球化学研究表明,含石榴石长英质脉体具有显著的Eu元素正异常(δEu=3.57),为斜长石堆晶的产物。锆石U-Pb年代学表明,片岩和脉体在早侏罗纪的181Ma和195Ma发生了变质和部分熔融作用。本文结合已发表研究结果表明,东久地区的高级变质岩可划分出不同的构造岩片,在早侏罗纪先后经历了相似温、压条件的变质作用,为南、北拉萨地体碰撞造山作用的产物。The Lhasa terrane located on the southern Tibetan Plateau not only records the Mesozoic subduction of the Neo-Tethyan oceanic lithosphere and subsequent Cenozoic continental collision between India and Asian plates,but also the Late Paleozoic-Early Mesozoic amalgamation of the southern-northern Lhasa subterranes.This study carried out petrology and zircon U-Pb chronology analysis of schist and veins in the Dongjiu area of the eastern Lhasa terrane.The results show that the schist experienced peak high amphibolite-facies metamorphism and partial melting,then retrograde medium-pressure amphibolite-facies metamorphism and later retrograde metamorphism of cooling and decompression.Schist records peak mineral assemblage,including Ky+Grt+Bt+Pl+Ilm+Qz,while the retrograde mineral assemblages are Grt+Sil+Crd+Bt+Pl+Ilm+Qz and Crd+Bt+Ms+Chl+Pl+Ilm+Qz,respectively.Phase equilibrium simulation shows that the peak-metamorphic conditions are 0.9GPa and 720℃,and retrograde conditions are 0.59GPa and 670℃,and then 0.12GPa and 480℃.The garnet-bearing felsic vein has significant Eu positive anomaly(δEu=3.57),suggesting that it was the products of plagioclase cumulation.Zircon U-Pb dating suggests tht the Dongjiu schist and vein underwent metamorphism and partial melting at the Early Jurassic of 181and 195Ma respectively.With the published results,we suggest that the Dongjiu highgrade metamorphic rocks contain two tectonic blocks and expereniced the similar metamorphism at early Jurassic period.They were the products of collision of the southern-northern Lhasa subterranes.

关 键 词：早侏罗纪 变质和深熔作用 拉萨地体 碰撞造山作用 

分 类 号：P61[天文地球—矿床学]