柴北缘超高压变质带都兰地区的构造演化历史)))来自片麻岩独居石原位电子探针定年的证据  

The tectonic evolution of the Dulan terrane,North Qaidam UHP metamorphic belt:evidence from in-situ EMP monazite dating of paragneiss

作　　者：张聪[1,2] 张立飞[2] 张贵宾[2] 陈梅[3] 黄杰[3] 

机构地区：[1]中国地质科学院地质研究所,大陆构造与动力学国家重点实验室,北京100037 [2]造山带与地壳演化教育部重点实验室,北京大学地球与空间科学学院,北京100871 [3]中国地质大学地球科学与资源学院,北京100083

出　　处：《岩石矿物学杂志》2013年第6期1037-1056,共20页Acta Petrologica et Mineralogica

基　　金：国家自然科学基金资助项目(41202034;41310104002;41172196);中国博士后特别基金资助项目(2013T60030)

摘　　要：榴辉岩及其围岩的变质作用和年代学研究是揭示超高压变质地体形成机制及演化过程的关键,但是作为榴辉岩围岩的副片麻岩,由于其矿物组合相对简单,缺少指示温压条件的特征矿物,不容易将其岩石学特征与变质年龄相联系。本文对柴北缘超高压变质带都兰地区3个含石榴石副片麻岩样品进行了岩石学及独居石电子探针原位U-Th-Pb化学法定年研究,获得3个样品中独居石的表面年龄分别为433±15.4 Ma(8S01,沙柳河剖面)、440±11.2Ma(8S55,都兰北带野马滩)和435±14.7 Ma(8S116,都兰南带)。其中沙柳河及都兰南带的两个样品年龄相似,与北带样品约有5 Ma的年龄差。3个测试样品中的独居石均表现为富集轻稀土元素,重稀土元素严重亏损,具有Eu的正异常,其中Eu/Eu*与寄主岩石石榴石中Ca含量存在耦合关系。岩石学研究及温压条件计算表明独居石为角闪岩相退变过程中变质重结晶生长而成。都兰南带独居石样品测试点年龄介于440～430 Ma时具有较高的Y含量,且Y与Eu/Eu*之间存在明显的负相关性,表明440～430 Ma之间都兰地区部分副片麻岩经历了角闪岩相退变质作用。The metamorphism and age of UHP eclogites and their country rocks constitute the key to understanding the formation and evolution of the UHP metamorphic terranes.However,because of simple mineral assemblages and lack of P-T sensitive minerals,it is difficult to relate the petrological characteristics with the metamorphic ages of the country rock paragneiss.This paper focused on the petrology and in-situ electronic microprobe monazite U-Th-total Pb dating of three paragneisse samples from Dulan terrane in North Qaidam UHP metamorphic belt.As a result,ages of 433 ± 15.4 Ma （8S01 from Shaliuhe cross-section）,440 ± 11.2 Ma （8S55 from Yematan in North Dulan） and 435 ± 14.7 Ma （8S116 from South Dulan） were obtained respectively.Monazites in paragneiss from Shaliuhe section and South Dulan yielded similar ages,which are different from ages of samples from North Dulan by about 5 Ma.The analyses of the three monazite samples from the Dulan terrane show that they are enriched in LREE and lack of HREE,characterized by positive Eu anomaly.The Eu/ Eu ＊ ratios of monazite have coupling relationship with Ca content in garnet in the matrix.In combination with petrological investigation,P-T calculation and monazite microprobe analysis,the authors hold that the monazite in this study grew or recrystallized under the arnphibolite facies metamorphic conditions during retrogression.The monazite analyses with ages from 440 Ma to 430 Ma in paragneiss from South Dulan are characterized by relatively high Y content.Furthermore,the Y content has an obvious negative correlation with Eu/Eu ＊,suggesting that part of the paragneiss in Dulan terrane had experienced amphibolite facies metamorphism during 440 ～430 Ma.

关 键 词：副片麻岩 独居石电子探针化学定年 柴北缘超高压变质带 都兰地体 

分 类 号：P588.345[天文地球—岩石学] P597.3[天文地球—地质学]