羌塘中部白措花岗岩岩石地球化学特征、锆石U-Pb年龄和Hf同位素组成及其构造意义  

Petrogeochemical characteristics,zircon U-Pb ages and Hf isotopic composition of Baicuo granites in central Qiangtang Basin,and its tectonic significance

作　　者：李学仁 万友利[1,2] 王剑 LI Xueren;WAN Youli;WANG Jian(Chengdu Center of China Geological Survey,Chengdu,610081;Key Laboratory for Sedimentary Basin and Oil and Gas Resources,Ministry of Natural Resources,Chengdu,610081;Qiangtang Institute of Sedimentary Basin,Southwest Petroleum University,Chengdu,610500)

机构地区：[1]中国地质调查局成都地质调查中心,成都610081 [2]自然资源部沉积盆地与油气资源重点实验室,成都610081 [3]西南石油大学羌塘盆地研究院,成都610500

出　　处：《地质论评》2020年第5期1172-1185,共14页Geological Review

基　　金：国家自然科学基金资助项目(编号:41902138)的成果。

摘　　要：本文对羌塘中部白措花岗岩的研究表明,岩体是由花岗闪长岩和二长花岗岩同时侵位的复合岩基,锆石U-Pb年龄分别为213.8±1.3 Ma、210.0±1.1 Ma和208.1±1.4 Ma。白措花岗岩为硅过饱和的准铝质—弱过铝质岩石,Eu负异常明显,富集大离子亲石元素,亏损高场强元素,显示典型的壳源特征。εHf(t)为负值,主要集中于-10^-15,对应的Hf同位素二阶段模式年龄峰值为1.78 Ga,显示源区为古元古代古老地壳;同时,花岗岩中含有大量闪长质暗色包体,表明有壳幔岩浆混合作用,推测为同时期玄武质岩浆的底侵作用,在地壳深部诱发富硅质基底岩石重熔,快速喷发形成玄武岩—流纹岩双峰式组合,而花岗岩则是由玄武质岩浆在地壳充分熔融后,岩浆沿羌塘中部已有的构造破碎带侵入形成,并且在熔融过程中有镁铁质岩浆注入少量结晶的长英质岩浆中形成暗色包体。因此,花岗岩与双峰式火山岩是同一裂陷动力机制的产物。花岗岩的侵入预示了晚三叠世羌塘盆地构造岩浆活动的结束,之上开始了羌塘侏罗纪海相盆地的沉积。Objectives:The granites in central Qiangtang have important geological significance,which not only constraints the evolution of Longmuco—Shuanghu Suture belt,but also the onset of Qiangtang Jurassic marine basin.Combined with zircon geochronology,geochemistry and Hf isotope,we briefly discuss the geological significance of the Baicuo granites.Methods:Based on the field work,through the microscope observation,the zircon geochronology,the whole rock chemical analysis,Hf isotope analysis of Baicuo granites.Results:The Baicuo granites in central Qiangtang show that the pluton is a composite batholith placed by granodiorite and monzogranite.The zircon U-Pb ages are 213.8±1.3 Ma,210.0±1.1 Ma and 208.1±1.4 Ma,respectively.The Baicuo granites have typical characteristics of crust source,with enrichments of large ion lithophile elements(LILES)and depletion of high field strength elements(HFSEs),as well as exhibiting obvious Eu anomalies.TheεHF(t)is negative,mainly concentrated in-10^-15.The corresponding two-stage model age(TDM2)varies between 1.66 Ga and 2.17 Ga,which indicates that the source region is from the Paleoproterozoic ancient crust.The dark xenoliths maybe indicate magma mixing,which is caused by the injection of mafic magma into felsic magma.The Granites formed by basaltic underplating and full melting,intruded along the existed structural fracture zone in central Qiangtang.Conclusion:The granites and the coeval bimodal volcanic rocks are the products of the same rift dynamic mechanism.The intrusion of granites indicates the end of the tectonomagmatic activities in the Late Triassic Qiangtang basin,and the onset of the Jurassic Qiangtang marine basin.

关 键 词：锆石U-Pb年龄 地球化学 HF同位素 花岗岩 羌塘 

分 类 号：P597[天文地球—地球化学] P588.121[天文地球—地质学]