北秦岭松树沟榴闪岩变质演化:石榴石矿物学和锆石年代学制约  

Metamorphic Evolution of Garnet Amphibolite at Songshugou in the North Qinling Orogen:Evidence from Garnet Geochemistry and Zircon Geochronology

作　　者：林少伟 张强强 高晓英[1,2] 郑永飞[1,2] Lin Shaowei;Zhang Qiangqiang;Gao Xiaoying;Zheng Yongfei(CAS Key Laboratory of Crust-Mantle Materials and Environments,School of Earth and Space Sciences,University of Science and Technology of China,Hefei 230026,China;Center of Excellence for Comparative Planetology,Chinese Academy of Sciences(CAS),Hefei 230026,China)

机构地区：[1]中国科学院壳幔物质与环境重点实验室,中国科学技术大学地球和空间科学学院,安徽合肥230026 [2]中国科学院比较行星学卓越创新中心,安徽合肥230026

出　　处：《地球科学》2024年第8期2714-2735,共22页Earth Science

基　　金：国家自然科学基金委优秀青年项目(Nos.41722203,41572042)。

摘　　要：北秦岭造山带早古生代的构造格局和变质演化过程长期以来存在较大争议,是地学界比较关注的问题之一.尽管前人对松树沟变质岩进行了大量的同位素年代学研究工作,但是峰期榴辉岩相变质时间的限定仍然存在较大争议.基于此,以松树沟榴闪岩为研究对象,进行系统的石榴石矿物学和锆石年代学研究.根据岩相学显微结构、矿物包裹体和矿物地球化学成分研究,在榴闪岩中识别出进变质(522~611℃/1.4~3.0 kbar)、峰期(742~749℃/>3.0 GPa)以及退变质(726~764℃/7.3~8.3 kbar)3期变质矿物组合.锆石年代学研究识别出3组年龄:G1为残留锆石核部,具有典型岩浆振荡环带,富集HREE,明显的Eu负异常,具有746±16 Ma到720±7 Ma的锆石206Pb/238U年龄,表明榴闪岩的原岩结晶时代为新元古代时期;G2锆石无环带或保留较弱的切割原岩锆石的振荡环带,具有陡峭的HREE配分型式,明显变化的Eu异常,属于变质重结晶锆石,具有从492±15 Ma到547±17 Ma较大变化范围的206Pb/238U值.G3锆石主要靶上位于锆石边部以位于石榴石边部与其平衡共存的原位锆石为主,具有云雾状或冷杉叶状CL特征,206Pb/238U年龄范围为491~480 Ma(加权平均年龄为488±10 Ma),富集MREE,亏损HREE,低Th/U比值(0.01~0.04),微弱的Eu异常,表明其形成于榴辉岩相变质阶段,代表峰期变质年龄.此外,我们在榴闪岩中代表石榴石峰期的边部发现包裹大量呈定向的棒状出溶体Rt+Ap+Qz,可能表明该岩石曾经历过超高压变质作用.松树沟榴闪岩原岩是来源于Rodinia超大陆在新元古代裂解出来的北秦岭微陆块,于寒武纪开始深俯冲并在488 Ma达到峰期榴辉岩相,而后发生折返并经历角闪岩相退变质作用.The metamorphic evolution and tectonic framework of the North Qinling orogen in the Early Paleozoic has long been controversial and is one of the widely concerned geological problems.Although much isotopic work has been done,the timing of peak eclogite-facies metamorphism is still debatable.Therefore,an intergrated study of garnet geochemistry and zircon geochronology was carried out for garnet amphibolite from Songshugou Complex in the North Qinling orogen.Based on the characristics of the petrographic microstructures,mineral inclusions,and mineral compositions,three metamorphic stages are recognized for mineral assemblages in the garnet amphibolite:prograde stage(522-611 o C/1.4-3.0 kbar),peak stage(742-749 o C/>3.0 GPa)and retrograde stage(726-764 o C/7.3-8.3 kbar).Geochronology study reveals that zircon grains in thin sections and mounts record three episodes of growth or reworking,respectively:(1)relict magmatic cores(G1),showing typical oscillatory zoning with middle Neoprotrozoic U-Pb ages of 746±16 Ma to 720±7 Ma and steep HREE patterns with obvious negative Eu anomalies,indicating the protolith of garnet amphibolite was produced in the Neoproterozoic;(2)metamorphically recrystallized domains(G2),exhibiting faintish oscillatory zoning or unzoning with discordant U-Pb ages ranging from 547±17 Ma to 492±15 Ma and steep HREE patterns with positive to negative Eu anomalies;(3)metamorphically grown domains(G3),which occur not only as overgrown rim in mounts but also as mineral inclusion within garnet rims in thin sections.They gave consistent 206Pb/238U age of 491 Ma to 480 Ma(average age is 488±10 Ma),flat HREE patterns with low Th/U ratios of 0.01 to 0.04,high MREE contents and no obvious Eu anomalies,indicating the timing of peak eclogite-facies metamorphism.In addition,the rod-like exsolution composed of rutile+apatite+quartzis identified in the garnet rim that formed at the peak metamorphic stage from the garnet amphibolite,indicating that it might have ever underwent ultra-high pressure metamorphism

关 键 词：榴闪岩 多期次变质 石榴石 锆石 北秦岭造山带 俯冲带 地球化学 

分 类 号：P581[天文地球—岩石学]