西藏拉萨地块革吉石英闪长岩及镁铁质微粒包体成因及其对班——怒特提斯洋晚侏罗世构造演化的启示  

Petrogenesis of quartz diorite and mafic microgranular enclaves from Geji pluton in Lhasa terrance,Tibet and its implications for Late Jurassic tectonic evolution of Bangonghu--Nujiang Tethys Ocean

作　　者：彭勃 李宝龙[1,2] 赵拓飞 周磊 秦广洲[5] PENG Bo;LI Bao-long;ZHAO Tuo-fei;ZHOU Lei;QIN Guang-zhou(Institute of Mineral Resources,Chinese Academy of Geological Science,Beijing 100037,China;MNR Key Laboratory of Metallogeny and Mineral Assessment,Beijing 100037,China;College of Earth Sciences,Jilin University,Changchun 130061,China;Geochemistry Exploration Brigade of Sichuan Bureau of Exploration and Development of Geology and Minerals Resources,Deyang 618000,Sichuan,China;College of Earth Sciences,Chengdu University of Technology,Chengdu 610059,China)

机构地区：[1]中国地质科学院矿产资源研究所,北京100037 [2]自然资源部深地资源成矿作用与矿产预测重点实验室,北京100037 [3]吉林大学地球科学学院,长春130061 [4]四川省地质矿产勘查开发局化探队,四川德阳618000 [5]成都理工大学地球科学学院,成都610059

出　　处：《世界地质》2019年第3期690-707,共18页World Geology

基　　金：国家自然科学基金(41802103);中国博士后科学基金(2018M641434);中国地质调查局项目(DD20190167)联合资助

摘　　要：在班公湖—怒江结合带南缘,新识别出一条中-晚侏罗世岩浆岩带,其形成时代并非前人所认为的早白垩世,这一发现为重新认识班公湖—怒江特提斯洋俯冲极性与过程提供了新的资料。笔者以西藏阿里地区革吉岩体为研究对象,开展岩体地质及岩相学、锆石U-Pb年代学、地球化学及全岩Sr-Nd同位素研究。研究表明,革吉岩体岩性主要为石英闪长岩,岩体中普遍发育暗色的镁铁质微粒包体。革吉石英闪长岩及镁铁质微粒包体的锆石U-Pb年龄分别为153±2 Ma和150±2 Ma,两者在误差范围内一致,代表了同期岩浆事件的产物。地球化学特征显示革吉石英闪长岩属于钙碱性准铝质岩石系列,镁铁质微粒包体属于拉斑系列的准铝质岩石。镁铁质微粒包体与寄主石英闪长岩具有相似的稀土、微量元素配分曲线,但总体上微粒包体Nb、Ta、Zr和Hf等元素含量更低,而稀土总量更高。石英闪长岩样品(^87Sr/^86Sr)i值介于0.714 2~0.715 2之间,εNd(t)值为-12.0^-14.0;镁铁质微粒包体样品(^87Sr/^86Sr)i值为0.712 9~0.714 4,εNd(t)值介于-11.4^-13.8之间。综合岩相学、地球化学的特征,革吉岩体中的寄主石英闪长岩及其镁铁质微粒包体很可能是富集地幔来源的镁铁质岩浆与古老地壳物质熔融形成的长英质岩浆发生不同比例混合作用的结果。结合区域构造演化的研究,革吉岩体可能形成于班公湖—怒江特提斯洋南向俯冲的体制下,狮泉河多岛弧盆系统与古拉萨地块发生弧-陆软碰撞的构造背景。A Middle-Late Jurassic magmatic belt has been newly identified in the southern margin of the Bangonghu-Nujiang junction zone,its forming ages is not Early Cretaceous that was defined by the predecessors.This discovery provides new information for re-defining the subduction polarity and process of the Bangonghu-Nujiang Tethys Ocean.Taking the Geji pluton in Ali area of Tibet as a research target,the authors camed out studies on rock geology and petrography,zircon U-Pb dating,geochemistry and whole rock Sr-Nd isotopes.The results show that Geji pluton consists of quartz diorite,in which mafic microgranular enclaves(MMEs)are well developed.The zircon U-Pb ages of the Geji quartz diorite and MMEs are 153±2 Ma and 150±2 Ma,respectively,which are consistent within test error,representing contemporaneous magmatic event.Geochemical characteristics show that Geji quartz diorite belongs to a series of calc-alkaline metaluminous rock,whereas the MMEs are tholeiite series metaluminous rock.The Geji quartz diorite and MMEs have similar REEs and trace elements distribution patterns,but MMEs contain lower content of Nb,Ta,Zr,Hf and higher total rare earth.The(^87 Sr/^86 Sr)i values of quartz diorite are between 0.714 2 and 0.714 4,εNd(t)=-12.0^-14.0,and the MMEs have the(^87 Sr/^86 Sr)i values of 0.712 9~0.714 4 and theεNd(t)values of-11.4^-13.8.Combined with petrography and geochemical characteristics,the host quartz diorite and MMEs from Geji pluton are believed to be the results of mixing of mafic magma derived from enriched mantle with felsic magma derived from ancient crust in different proportions.Based on regional tectonic evolution,Geji pluton may be formed in the background of the arc-continent soft collision between Shiquanhe archipelagic arc-basin system and ancient Lhasa block,associated with the southward subduction of Bangonghu-Nujiang Thethys ocean.

关 键 词：班公湖—怒江特提斯洋 弧-陆碰撞 拉萨地块 革吉 狮泉河 

分 类 号：P595[天文地球—地球化学] P597[天文地球—地质学]