柴北缘锡铁山地体花岗质片麻岩退变质作用和^(40)Ar/^(39)Ar年代学研究及其地质意义  

Retrograde Metamorphism and ^(40)Ar/^(39)Ar Geochronology of Granitic Gneiss from Xitieshan Terrane,Northern Qaidam

作　　者：胡荣国 白秀娟[3] 刘希军[1,2] 杨启军 冯佐海[1,2] 蔡永丰 赵义来[1,2] 邱华宁 HU Rongguo;BAI Xiujuan;LIU Xijun;YANG Qijun;FENG Zuohai;CAI Yongfeng;ZHAO Yiai;QIU Huaning(Guangxi Key Laboratory of Exploration for Hidden Metallic Ore Deposits,College of Earth Sciences,Guilin University of Technology,Guilin 541004,Guangxi,China;Collaborative Innovation Center for Exploration of Nonferrous Metal Deposits and Efficient Utilization of Resources,Guilin University of Technology,Guilin 541004,Guangxi,China;MOE Key Laboratory of Tectonics and Petroleum Resources,Faculty of Earth Resources,China University of Geosciences(Wuhan),Wuhan 430074,Hubei,China)

机构地区：[1]桂林理工大学地球科学学院,广西隐伏金属矿产勘查重点实验室,广西桂林541004 [2]桂林理工大学有色金属矿产勘查与资源高效利用协同创新中心,广西桂林541004 [3]中国地质大学(武汉)资源学院,构造与油气资源教育部重点实验室,湖北武汉430074

出　　处：《大地构造与成矿学》2023年第6期1330-1344,共15页Geotectonica et Metallogenia

基　　金：国家自然科学基金项目(42073049);广西自然科学基金项目(2020GXNSFAA297049)联合资助。

摘　　要：本文对柴达木盆地北缘锡铁山地区含榴辉岩透镜体花岗质片麻岩进行了系统的矿物学、相平衡模拟以及黑云母和石英^(40)Ar/^(39)Ar年代学研究,旨在查明花岗质片麻岩在加里东超高压变质之后,折返过程中发生角闪岩-绿片岩相退变质作用的变质条件和变质年代。矿物学和相平衡模拟显示,发育变形石英细脉的黑云母花岗质片麻岩高角闪岩相变质矿物组合(M1)为黑云母+钾长石+斜长石+石英+金红石+矽线石+钛铁矿,对应变质条件为T>620℃,P>0.16GPa;低角闪岩相-绿片岩相变质矿物组合(M2)为黑云母+白云母+斜长石+微斜长石+石英+榍石±绿泥石,变质条件为T=390~420℃和P=0.10~0.19 GPa。花岗质片麻岩中黑云母激光阶段加热^(40)Ar/^(39)Ar定年获得了一个上升的阶梯状表观年龄图谱,在中高温阶段则形成了一个平坦的表观年龄坪,坪年龄为353.9±1.8 Ma,对应的等时线年龄为356.7±5.6 Ma。变形石英脉样品真空击碎提取流体包裹体^(40)Ar/^(39)Ar定年,则形成了下降的阶梯状表观年龄图谱,10~23阶段的数据点给出了357.9±1.6Ma的坪年龄,对应的等时线年龄为356.1±3.9Ma,记录了锡铁山地体晚泥盆世一期重要的构造热事件和流体活动发生的时间。黑云母和变形石英脉记录一致的~357Ma晚古生代年龄,代表了锡铁山地区超高压变质岩及其围岩在经历了加里东期深俯冲以及晚泥盆世造山后岩浆热事件叠加后,折返到浅部地壳发生角闪岩相至绿片岩相退变质作用的时代。Detailed mineralogy,phase equilibrium modelling and ^(40)Ar/^(39)Ar geochronology studies have been conducted on the eclogite-bearing granitic gneiss from the Xitieshan terrane,the North Qaidam high-ultrahigh-pressure(HP-UHP)metamorphic belt,in order to evaluate the timing and conditions of the amphibolite to greenschist-facies retrograde metamorphism after the Early Palaeozoic HP-UHP metamorphism.Petrographic observations and phase equilibria modelling with pseudosection of the granitic gneiss show that the high amphibolite-facies mineral assemblages(M1)are represented by biotite,K-feldspar,plagioclase,quartz,sillimanite,rutile,and ilmenite,formed at P-T conditions of T>620℃and P>0.16 GPa.Low amphibolite to greenschist-facies mineral assemblages(M2),which are represented by the biotite,muscovite,plagioclase,microcline,quartz,titanite,and chlorite,formed at P-T conditions of 390–420℃and 0.10–0.19 GPa.Stepwise heating analysis of biotite from the granitic genies yielded low initial ages,and then gradually rising apparent ages in the rest of the experiment until a final plateau age of 353.9±1.8 Ma is reached.The dating points contributing to the age plateau yield a well-defined isochron with a normal isochron age of 356.7±5.6 Ma and an initial 40Ar/36Ar ratio of 249±125.The deformed quartz sample dated by ^(40)Ar/^(39)Ar in vacuo crushing method yields monotonic declining release pattern:anomalously old apparent ages are obtained at the first steps and relative flat age plateau over the final several steps with plateau age of 357.9±1.6 Ma.The data of the late steps construct a well-defined isochron in the plots of 36Ar/40Ar vs.39Ar/40Ar with an inverse isochron age of 356.1±3.9 Ma and initial 40Ar/36Ar ratio of 299±8.Microthermometry analyses and ^(40)Ar/^(39)Ar in vacuo crushing dating results suggest that the secondary inclusions should be released in the early steps due to their distribution characteristics along cracks,which should be originated from the excess 40Ar rich post-hydrothermal fluid an

关 键 词：锡铁山 花岗质片麻岩 变形石英脉 ^(40)Ar/^(39)Ar定年 真空击碎 

分 类 号：P597[天文地球—地球化学]