折多山‒贡嘎山杂岩体晚渐新世退变质过程:来自长英质熔体的记录  

Late Oligocene Retrograde Metamorphism of the Zheduoshan-Gonggashan Massif:Records from Felsic Melts

作　　者：热孜亚·麦麦提 田云涛 姚欣博 刘志超 刘一珉 张帅 潘黎黎 张贵洪 张增杰 REZIYA Maimaiti;TIAN Yuntao;YAO Xinbo;LIU Zhichao;LIU Yimin;ZHANG Shuai;PAN Lili;ZHANG Guihong;ZHANG Zengjie(Guangdong Provincial Key Laboratory of Geodynamics and Geohazards,School of Earth Sciences and Engineering,Sun Yat-sen University,Zhuhai 519082,Guangdong,China;Southern Marine Science and Engineering Guangdong Laboratory(Zhuhai),Zhuhai 519082,Guangdong,China)

机构地区：[1]中山大学地球科学与工程学院,广东省地球动力作用与地质灾害重点实验室,广东珠海519082 [2]南方海洋科学与工程广东省实验室(珠海),广东珠海519082

出　　处：《大地构造与成矿学》2024年第3期566-579,共14页Geotectonica et Metallogenia

基　　金：国家自然科学基金项目(42172229、41772211);南方海洋科学与工程广东省实验室(珠海)自主科研项目(SML2021SP315)联合资助。

摘　　要：青藏高原东南缘鲜水河断裂带中段折多山‒贡嘎山杂岩体东侧,出露新生代混合岩化片麻岩和侵入其中的淡色花岗岩,是该地区构造变形和地壳加厚过程的重要岩石记录。本文在详细的野外调查基础上,对两个长英质熔体样品开展了系统的岩相学、锆石U-Pb年代学和微量元素成分分析。获得的锆石U-Pb年龄分布在195~23 Ma之间,其中最年轻的一组年龄来自锆石增生边,集中在25~23 Ma之间,代表了熔体最后的结晶时代。锆石幔部的年龄范围为195~28 Ma,其中新生代年龄数据(54~28 Ma)在谐和图上呈串珠状分布,记录了该地区新生代以来的变质历史。锆石年龄与微量元素成分相关图显示,35~33 Ma以来重稀土及Y元素含量随锆石年龄减小而逐渐升高,指示变质基底发生石榴石分解,反映该区域自渐新世开始转变为减压退变质作用。另外,锆石结晶温度自35~33 Ma以来呈降低趋势,反映该减压过程中同时伴随着温度的降低。对比哀牢山‒红河断裂带高级变质作用及相关岩石的锆石U-Pb年代学研究结果,发现两个地区变质历史相似,表明川滇地块南、北两侧的大型边界断裂带在始新世‒晚渐新世均经历了显著的地壳加厚。本研究认为青藏高原东南缘始新世‒晚渐新世块体边界薄弱带的构造活化,吸收了印度‒欧亚陆陆碰撞所导致的陆内构造缩短和斜向挤出。Cenozoic migmatite and felsic melts developed on the eastern edge of the Zheduoshan-Gonggashan Complex Massif along the middle segment of the Xianshuihe fault zone are essential records of tectonic deformation and crustal thickening in the SE Tibetan plateau.Based on detailed field observations,petrographic,zircon U-Pb dating,and trace element geochemical analyses were carried out on two felsic melt samples.The results showed that zircons from the samples have typical anatectic and magmatic signatures,displaying core-mantle-rim textures.U-Pb dating of the zircon rims yielded a cluster of young ages of 25–23 Ma,with a weighted mean of 24.4±0.2 Ma,providing quantitative constraints on the final crystallization of the felsic melts.The mantle sectors of the zircons have dispersed ages of 195−23 Ma.Plots of the early Cenozoic zircon ages against their trace element contents(e.g.,the total heavy rare earth element content,Yb/Gd,Y content)show an evident increase of heavy rare earth element content at 35−33 Ma,indicating garnet breakdown in the source of the melts during retrograde melt production and crystallization.The Ti-in-zircon thermometer shows a decreasing trend at 35–33 Ma,suggesting the retrograde is likely to be associated with a decrease in both pressure and temperature.Comparing our results with zircon U-Pb results of high-grade metamorphic and associated rocks from the Ailaoshan-Red River fault zone,where peak metamorphism occurred at or before 35–33 Ma,it is found that the two regions probably experienced a similar metamorphic history during the Eocene-late Oligocene,indicating that the north and south margins of the Chuan-Dian block experienced significant crustal thickening during the early Cenozoic.This study highlights the importance of pre-existing crustal weak zones along block boundaries in accommodating the intracontinental crustal shortening and oblique extrusion caused by the India-Eurasia continental collision during the Eocene-Early Miocene.

关 键 词：青藏高原东南缘 鲜水河断裂 折多山‒贡嘎山杂岩体 混合岩 石榴石分解 

分 类 号：P542[天文地球—构造地质学] P597[天文地球—地质学]