华南云开高州紫苏花岗岩及其两类石榴石的成因:岩石学和锆石U-Pb年代学证据  

Genesis of the Gaozhou charnockite and its two types of garnets of Yunkai massif, South China: Evidence from petrology and zircon U-Pb geochronology

作　　者：李超[1,2] 仝来喜 刘兆 黄小龙[1] LI Chao;TONG LaiXi;LIU Zhao;HUANG XiaoLong(State Key Laboratory of Isotope Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,China;University of Chinese Academy of Sciences,Beijing 100049,China;State Key Laboratory of Continental Dynamics,Department of Geology,Northwest University,Xi'an 710069,China)

机构地区：[1]中国科学院广州地球化学研究所,同位素地球化学国家重点实验室,广州510640 [2]中国科学院大学,北京100049 [3]西北大学地质学系,大陆动力学国家重点实验室,西安710069

出　　处：《岩石学报》2020年第3期871-892,共22页Acta Petrologica Sinica

基　　金：国家深地专项项目(2016YFC600204);中国地质调查局地质调查项目(DD2016343)联合资助。

摘　　要：云开地块早古生代的构造背景至今仍存有争论。对云炉-龙修一带出露的含石榴石紫苏花岗岩进行研究可为该区构造-变质演化提供重要制约。详细的岩相学研究表明紫苏花岗岩中存在两种成因的石榴石(石榴石Ⅰ和石榴石Ⅱ),并识别出该岩石保留了三阶段演化的矿物组合(M1-M3)。峰期前矿物组合(M1)由紫苏辉石变斑晶中的包裹体矿物石榴石+黑云母+斜长石+钛铁矿+石英组成。峰期变质-深熔矿物组合(M2)由基质中平衡共生的斑晶矿物石榴石Ⅰ+紫苏辉石+黑云母+石英+斜长石+钛铁矿构成。退变质矿物组合(M3)以紫苏辉石和黑云母以及钛铁矿边部降温生成石榴石Ⅱ+石英冠状体("红眼圈")和黑云母+石英后成合晶为特征。传统地质温压计、平均温压法及在NCKFMASHTO模式体系下的相平衡模拟结果表明,紫苏花岗岩峰期前矿物组合形成条件为720℃/7.0kbar(M1),峰期变质-深熔结晶条件为835~810℃/6.5~5.8kbar(M2),退变质条件为740℃/5.6kbar(M3)。岩相观察和P-T计算结果表明紫苏花岗岩的形成可能反映了一条顺时针的P-T轨迹,且以峰期前略微减压并伴随明显的升温和峰期后具有近等压冷却(IBC)过程为特征。LA-ICP-MS锆石U-Pb年代学结果显示锆石核部加权平均年龄为~431Ma,锆石边部加权平均年龄为~243Ma。我们认为峰期深熔作用和岩浆结晶石榴石Ⅰ的形成可能发生于早古生代(加里东期),而晚期退变质作用和变质重结晶石榴石Ⅱ的形成可能发生在早中生代(印支期)。本研究也表明云开地区含石榴石紫苏花岗岩的形成与早古生代扬子和华夏陆块碰撞后抬升过程有紧密联系,并遭受了早中生代印支期构造热事件的明显叠加。The tectonic background of Yunkai massif in the Early Paleozoic has still been controversial.Studying on the garnet-bearing charnockites cropping out in Yunlu-Longxiu area may provide important constraints on the tectono-metamorphism evolution of this region.Detailed petrological studies showed that there are two genetic types of garnets(garnetⅠand garnetⅡ)within the charnockite,and a three-stage evolution is also distinguished(M1-M3).The pre-peak assemblages(M1)are composed of garnet+biotite+plagioclase+ilmenite+quartz,which occur as inclusions within porphyroblastic hypersthene.The peak metamorphism-anatexis mineral assemblages(M2)consist of the equilibrated association of garnetⅠ+hypersthene+biotite+quartz+plagioclase+ilmenite in the matrix.The retrograde assemblages(M3)are characterized by garnetⅡ+quartz corona('red-eye socket'texture)after coarse-grained hypersthene,biotite and ilmenite,together with the formation of biotite+quartz symplectite.Thermobarometries,average P-T approaches and phase equilibrium modeling in the NCKFMASHTO system indicate the the three-stage evolutional P-T conditions of 720℃/7.0kbar(M1),835~810℃/6.5~5.8kbar(M2)and 740℃/5.6kbar(M3),respectively.The petrological observations and P-T calculations suggest a possible clockwise P-T evolution path,which was characteristic of a pre-peak minor decompression associated with obviously heating evolution,and subsequently,a post-peak nearly isobaric cooling(IBC)process.LA-ICP-MS zircon U-Pb age dating results show the two weighted mean ages of^431Ma for the cores and^243Ma for the rims.We propose that the peak metamorphism(M2)and the followed formation of magmatic crystallized garnetⅠmight occur in the Early Paleozoic(Caledonian),whereas the late metamorphism(M3)and formation of metamorphic recrystallized garnetⅡprobably occurred in the Early Mesozoic(Indosinian).This study also suggests that the formation of garnet-bearing charnockites in this region was tightly associated with the Early Paleozoic uplifting process after the c

关 键 词：云开地块 紫苏花岗岩 两类石榴石 相平衡模拟 锆石U-PB定年 P-T轨迹 

分 类 号：P578.947[天文地球—矿物学] P588.34[天文地球—地质学]