庐山星子变质穹窿中变形花岗伟晶岩脉中多期不同成因锆石的发现及意义  

Types and Genesis of Zircons from Deformational Pegmatite Dikes in Xingzi Metamorphic Dome, Lushan Area

作　　者：王继林[1,2] 何斌[3] 靳立杰 

机构地区：[1]中国冶金地质总局山东局测试中心,山东济南250014 [2]中国地质大学(武汉)地球科学学院,湖北武汉430074 [3]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,广东广州510640 [4]山东省第一地质矿产勘查院,山东济南250014

出　　处：《大地构造与成矿学》2015年第4期729-742,共14页Geotectonica et Metallogenia

基　　金：中国科学院知识创新项目(KZCXZYW128,KZCX1YW151);国家自然科学基金(41173037)联合资助

摘　　要：江西庐山地区星子变质穹窿核部出露有一套星子群中深变质岩系,其中片岩的峰期变质作用发生于142.6±1.5 Ma,结合其变质分带和点变质的特征推测星子群角闪岩相的变质可能与研究区中生代广泛的岩浆活动有关。然而,研究区出露的中生代岩体及岩脉侵位时间均晚于星子群中高级变质作用的峰期时间,故推测星子群的变质热源不是上述岩体,而可能来自更早期的侵入体。为了获得星子群中高级变质作用的热源信息,本文对星子变质穹窿中大量发育的变形花岗质伟晶岩脉进行了较为系统的锆石U-Pb年代学研究,发现其中存在有三期不同成因的锆石：（1）加权平均年龄为824±13 Ma的继承岩浆锆石核,其母岩可能为星子变质穹窿核部出露的新元古代观音桥片麻状花岗岩,这一年龄为该岩体的侵位时间提供了更加精确的限定；（2）加权平均年龄为140.5±1.7 Ma（MSWD=2.3）的简单岩浆锆石及相应同期形成的变质增生锆石,这一年龄代表了这期花岗伟晶岩脉的侵位时间,表明研究区确实存在与星子群变质时代同期的岩浆活动,这期岩浆活动直接导致了星子群的变质。另外,由于这期伟晶岩脉发生了强烈的韧性变形,研究区的构造变形事件应不早于岩脉的侵位,综合已有成果认为研究区的变形事件与岩浆活动同步；（3）得到大量年龄为～123 Ma的海绵状结构热液锆石,由于此类锆石是原生锆石经后期热液改造而成,因此其锆石形成年龄应早于实验得到的～123 Ma,可能也形成于～140 Ma岩浆活动期间。The Xingzi Group is located in the core of the Xingzi metamorphic dome, which experienced multistage metamorphism within which an amphibolites facies metamorphism was dated at 142.6±1.5 Ma. Field observations show that the metamorphism may have been related to the Mesozoic magmatism in the study area, but the available ages of the Mesozoic intrusions are all later than that of the Xingzi Group’s peak metamorphic time. In order to locate the possible the heat source for the metamorphism, we analyzed the zircons from the deformed pegmatite dikes that emplaced at the end of the Mesozoic magmatism. There are three kinds of zircons in the pegmatite dikes. The first kind of zircons show core-rim structure. The cores of the zircons are magmatic origin with weighted average age of 824±13 Ma, and were likely inherited from the Neoproterozoic Guanyinqiao granite, whereas the rims are metamorphic overgrowth which were dated at about 142 Ma. The second kind of zircons are magmatic origin showing simple internal structure, and are dated at 140.5±1.7 Ma. The age of the magmatic zircons indicates that the Mesozoic magmatic activity in the study area should not be latter than 140.5±1.7 Ma. Moreover, this age is consistent with the peak metamorphic time of the Xingzi Group. We thus consider that the Mesozoic magmatism caused the metamorphism of the Xingzi Group. In addition, the deformation of the pegmatite should be postdate its emplacement, we propose the deformation event in the study area maybe take place simultaneously with the magmatism. The third kind of zircons are hydrothermal in origin, commonly characterized by sponge texture, high content of U, and yielding U-Pb ages of about 123 Ma.

关 键 词：星子变质穹窿 花岗伟晶岩脉 锆石U-PB定年 岩浆活动 构造变形事件 

分 类 号：P597[天文地球—地球化学]