地学前缘 ›› 2019, Vol. 26 ›› Issue (2): 233-248.DOI: 10.13745/j.esf.sf.2018.7.3

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中祁连地块北缘退变榴辉岩的发现及其地质意义

宋志杰, 张宏远, 侯迪, 柳长峰, 刘文灿, 吴晨   

  1. 中国地质大学(北京) 地球科学与资源学院, 北京 100083
  • 收稿日期:2018-02-28 修回日期:2018-07-22 出版日期:2019-03-30 发布日期:2019-03-30
  • 作者简介:宋志杰(1982—)，男，博士研究生，构造地质学专业。E-mail：laomo913@qq.com
  • 基金资助:
    中国地质调查局地质调查项目(1212011121188，12120114014401)

Discovery and the geological significance of retrograded eclogites from the northern margin of the central Qilian block

SONG Zhijie, ZHANG Hongyuan, HOU Di, LIU Changfeng, LIU Wencan, WU Chen   

  1. School of Earth Sciences and Resources, China University of Geosciences(Beijing), Beijing 100083, China
     
  • Received:2018-02-28 Revised:2018-07-22 Online:2019-03-30 Published:2019-03-30
  • Supported by:
     

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摘要： 1∶5万区域地质调查首次在中祁连地块北缘发现的退变榴辉岩，呈构造岩块分布于大羊陇一带的变质基底中。岩相学和矿物学研究显示，石榴石的矿物包体和化学成分具有进变质环带的特征，属于C类榴辉岩。石榴石核部成分以及残留于核部的黑云母、斜长石等矿物包体代表了进变质阶段(M1)矿物组合，计算得到其温压条件为568～580 ℃和0.80～0.82 GPa。大致估算得峰期榴辉岩相阶段(M2)温压条件为(669±5) ℃和(2.1±0.2) GPa。石榴石“白眼圈”结构指示了等温减压退变质作用，利用局部的平衡矿物获得高角闪岩相退变质阶段(M3)的温压条件为681～705 ℃和0.68～0.71 GPa。进一步的退变质作用发生在低角闪岩相条件下，以基质中出现粗粒的角闪石和斜长石为特征，估算得到这一阶段(M4)温压分别为500～545 ℃和0.38～0.43 GPa。上述变质过程形成一个顺时针的p-T演化轨迹，暗示板片经历过快速俯冲和折返。榴辉岩的锆石CL图像显示锆石大部分发光度低，为无分带、弱分带或海绵状分带，边部发育宽约5 μm的强阴极发光带，主体表现为变质增生锆石的特征。LA-ICP-MS锆石U-Pb定年获得峰期榴辉岩相变质的上限年龄为(485±22) Ma。根据岩石地球化学特征和构造环境判别，大羊陇榴辉岩的原岩为MORB，推测属于北祁连洋壳的组成部分。结合中祁连地块北缘广泛发育弧岩浆岩，确定了晚寒武世—中奥陶世北祁连洋壳存在向南的俯冲作用，其俯冲极性为南北双向俯冲。

 

关键词: 退变榴辉岩, 高压变质, p-T轨迹, 中祁连地块, 北祁连造山带

Abstract: In the 1∶50000 regional geological survey, we discovered retrograded eclogites, occurring as tectonic lenses in the metamorphic basement of the Dayanglong area in the western part of the northern margin of the central Qilian block. Petrographic and mineralogic examinations revealed that garnets in eclogites preserved prograde metamorphic zonation, exhibiting annulus characteristics in both their chemical composition and mineral inclusions, and that Dayanglong retrograded eclogites belong to C-type eclogites. The prograde metamorphic assemblages (M1) are mineral inclusions (such as biotite, amphibole and plagioclase) preserved in the garnet core. The p-T metamorphic conditions were estimated to be 568580 ℃ and 0.800.82 GPa, with peak metamorphism (M2) taking place at 669±5 ℃ and 2.1±0.2 GPa, consistent with high-pressure (HP) metamorphism. The M3 stage is characterized by the amphibole+plagioclase+biotite corona around garnet porphyroblasts, indicating an isothermal decompression process with estimated p-T metamorphic conditions of 681705 ℃ and 0.680.71 GPa. The p-T conditions for the subsequent retrogression under amphibolite-facies environment were estimated by coarse-grained matrix amphiboles to be 500545 ℃ and 0.380.43 GPa in the M4 stage. The estimated metamorphic conditions revealed a clockwise p-T path, suggesting that the subducted slab has undergone a process of rapid subduction and exhumation. Most of the cathodoluminescence (CL) images of zircons from retrograded eclogites showed low luminous intensity with no zoning, weak zoning or spongy zoning with 5 μm wide strong CL belt at edges, indicating that most zircons are metamorphic or inherited zircons. Results from LA-ICP-MS zircon U-Pb dating showed that the 485±22 Ma zircon age may represent the top of the HP metamorphic time. According to geochemical analysis and tectonic discrimination, the protolith of retrograded eclogites is MORB, which likely came from the North Qilian oceanic crust. The above results, combined with arc magmatic rocks in the northern margin of the central Qilian block, confirm that the North Qilian oceanic plate had experienced south-north bi-dive subduction during the Late CambrianMiddle Ordovician.

Key words: retrograded eclogite, high-pressure metamorphism, p-T path, central Qilian block, North Qilian orogenic belt

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