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山东半岛高压麻粒岩中锆石的U-Pb定年及其地质意义 被引量:16

In-situ U-Pb dating of zircons from high-pressure granulites in Shandong Peninsula,Eastern China and its geological significance.
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摘要 在山东半岛早前寒武纪变质基底中,高压麻粒岩常常呈透镜体或不规则脉状体广泛分布于TTG片麻岩之中。锆石中矿物包体激光拉曼测试、阴极发光图像分析及原位U-Pb定年结果表明,山东半岛高压麻粒岩中锆石成因十分复杂,可划分为3种类型:第一类锆石显示明显或弱的岩浆结晶环带,部分锆石保存磷灰石(Ap)等矿物包体,U-Pb定年结果显示岩浆锆石记录了2 890-2 915 Ma和2 510-2 763 Ma两组207Pb/206Pb年龄,表明研究区变质基底在中太古代末期和新太古代至少存在两期岩浆事件;第二类为变质锆石,保存了典型的高压麻粒岩相包体矿物组合石榴石(Grt)+单斜辉石(Cpx)+斜长石(Pl)+石英(Qtz)+金红石(Rt)+磷灰石(Ap),记录的207Pb/206Pb年龄集中在1 850-1 900 Ma,应代表高压麻粒岩峰期高压麻粒岩相变质时代;第三类也为变质锆石,保存了峰后中低压麻粒岩相包体矿物组合斜方辉石(Opx)+斜长石(Pl)+石英(Qtz)+磷灰石(Ap)±角闪石(Amp),记录的207Pb/206Pb年龄集中1 820-1 840 Ma,应代表高压麻粒岩峰后中低压麻粒岩相退变质时代。该项成果不仅查明了山东半岛早前寒武纪变质基底麻粒岩中变质锆石普遍保存了高压麻粒岩相包体矿物组合,而且准确限定了麻粒岩的原岩形成时代、峰期高压麻粒岩相变质时代和峰后中低压麻粒岩相退变质时代的年代学格架,对深入探讨华北克拉通东南缘高压麻粒岩的成因机制及其形成的动力学背景具有重要的科学意义。 High-pressure(HP)granulites as enclaves and irregular vein-like blocks are widely distributed in TTG gneisses of the Early Precambrian metamorphic basement of Shandong Peninsula,in the southeast of the North China Craton.Zircon grains separated from HP granulites are subdivided into three main types based on cathodoluminescence(CL),Laser Raman spectroscopy and in-situ U-Pb dating;they are inherited(magmatic)zircon,HP metamorphic zircon and retrogressive zircon.The obviously or weakly zoned,dark luminescent inherited(magmatic)zircon with mineral inclusions of apatite(Ap) records two group 207Pb/206Pb ages of 2890-2915 Ma and 2510-2763 Ma,indicating two magmatic events occurring at Mesoarchean and Neoarchean epoches.In contrast,the homogeneous,HP metamorphic zircon with index HP mineral inclusions of garnet(Grt)+clinopyroxene(Cpx)+plagioclase(Pl)+quartz(Qtz)+rutile(Rt)+apatite(Ap) records younger 207Pb/206Pb ages ranging from 1850 Ma to 1900 Ma,which represent peak HP granulite-facies metamorphic times.The retrogressive zircon with mineral inclusions of orthopyroxene(Opx)+plagioclase(Pl)+quartz(Qtz)+apatite(Ap)±amphibole(Amp),however,records the youngest 207Pb/206Pb ages of 1820-1840 Ma,which represents late medium-and low-granulites-facies retrograde metamorphic times.This result indicates that Mesoarchean and Neoarchean igneous protoliths of mafic HP granulites,together with their TTG country rocks and meta-sedimentary rocks are subducted to coeval Paleoproterozoic HP metamorphism(1850-1900 Ma),followed by late medium-and low-granulites-facies retrogression(1820-1840 Ma).This new result is of very important significance for deeply discussing the genetic mechanism and geodynamic model of the HP granulites distributed in the southeast part of the North China Craton.
作者 刘平华 刘福来 王舫 刘建辉 Liu Pinghua,Liu Fulai,Wang Fang,Liu Jianhui (Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China )
出处 《地学前缘》 EI CAS CSCD 北大核心 2011年第2期33-54,共22页 Earth Science Frontiers
基金 国家杰出青年科学基金项目(40725007) 中国地质调查局地质大调查项目(1212010811065)本文的锆石矿物包体激光拉曼测试在国土资源部大陆动力学重点实验室完成,并得到了阎玲老师的指导 锆石SIMSU-Pb定年测试在中国科学院地质与地球物理研究所离子探针室和北京离子探针中心完成 数据处理过程中得到了李献华、李秋立研究员,董春艳、杜利林、宋彪和赵子然老师的帮助 野外地质调查得到了薛怀民研究员和王金光高级工程师的指导和帮助,在此一并致谢.
关键词 高压麻粒岩 锆石 U-PB定年 变质时代 山东半岛 high-pressure granulites zircon U-Pb dating metamorphic ages Shandong Peninsula
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