拉萨地体东南部林芝杂岩形成与变质演化的锆石U-Pb年代学限定

Origin and metamorphic evolution of the Nyingchi Complex,eastern Lhasa terrane,southern Tibet:Constraint from the zircon U-Pb geochronology

本文对拉萨地体东南部林芝地区分布的变质岩进行了岩相学和锆石年代学研究。结果表明,林芝杂岩中的变质沉积岩主要由片麻岩和片岩组成,它们经历了中压角闪岩相变质作用。变质岩中的锆石多由继承的碎屑岩浆核和新生的变质边组成。继承锆石核给出了新太古代至晚古生代的年龄范围,其主要年龄峰值在～1560Ma、～1190Ma、～620Ma和～340Ma,而锆石变质边给出了53Ma和27Ma的变质年龄。这一结果表明,林芝杂岩中的变质沉积岩很可能形成在古生代,其物质源区具有Grenville和Pan-Africa期造山作用的构造热事件记录。这一研究和已有的成果进一步证明,拉萨地体起源于Gondwana大陆北缘,在新生代印度与欧亚大陆的碰撞/俯冲过程中,拉萨地体作为俯冲带的上盘经历了多期变质作用改造。本研究为拉萨地体起源与多期构造演化提供了重要信息。

A petrological and geochronological study was produced on the Nyingchi Complex rock from the southeastern part of the Lhasa terrane,Tibet.The results show that the meta-sedimentary rocks of the Nyingchi Complex consist mainly of gneiss and schist,and metamorphosed under the medium-pressure amphibolite-facies.The zircons from these rocks contain an inherited detrital core and an overgrowth metamorphic rim.The detrital zircon core yielded variable ages ranging from Neo-Archean to Late Paleozoic,with main populations at ca.1560Ma,ca.1190Ma,ca.620Ma and ca.340Ma,respectively.The zircon rims of two samples yielded metamorphic ages of 53Ma and 27Ma,respectively.These indicate that the sedimentary rocks of the Nyingchi Complex deposited during Paleozoic time,and their material provenance recorded mainly the tectono-thermal events related to the Grenville and Pan-Africa orogenies,and that the Lhasa terrane derived from the north margin of the Gondwana continent.The Nyingchi Complex,representing the hinging wall of the subduction zone,experienced the Eocene and Oligocene metamorphism during the collision/subduction between the India and Eurasia continents.