新特提斯洋晚侏罗世岩浆作用:藏南白朗地体辉绿岩锆石U-Pb年龄和地球化学证据  

Late Jurassic magmatism in the Neo-Tethys Ocean:Evidence from zircon U-Pb ages and geochemistry of dolerites in the Bainang Terrane,southern Tibet.

作　　者：杨胜标 杨经绥[1,2] 李源 薄容众[2] 张健 李瑞保[4] 刘成军[4] 刘飞[1] 熊发挥[1] 陈晓坚[1] YANG Shengbiao;YANG Jingsui;LI Yuan;BO Rongzhong;ZHANG Jian;LI Ruibao;LIU Chengjun;LIU Fei;XIONG Fahui;CHEN Xiaojian(Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources,Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China;School of Earth Sciences and Engineering,Nanjing University,Nanjing 210023,Jiangsu,China;Tianjin Center,China Geological Survey,Tianjin 300170,China;School of Earth Sciences and Resources,Chang’an University,Xi’an 710054,Shaanxi,China)

机构地区：[1]中国地质科学院地质研究所自然资源部深地动力学重点实验室,北京100037 [2]南京大学地球科学与工程学院,江苏南京210023 [3]中国地质调查局天津地质调查中心,天津300170 [4]长安大学地球科学与资源学院,陕西西安710054

出　　处：《地质通报》2021年第8期1231-1246,共16页Geological Bulletin of China

基　　金：国家自然科学基金项目《西藏日喀则蛇绿岩构造变形变质序列的研究及对新特提斯洋演化的意义》(批准号:41772231);《蛇绿岩型铬铁矿的形成过程和深部地质作用》(批准号:92062215);中国地质调查局项目《拉萨-腾冲构造岩浆带区域地质调查》(编号:DD20190060);国家重点研发计划项目《大陆碰撞带深地幔过程与铬铁矿成因》(编号:2016YFC0600310);国家自然科学基金青年基金项目《新疆萨尔托海石英菱镁岩相关的金矿床成矿流体特征及其对矿床成因的限定》(批准号:41703053);中国地质科学院地质研究所基本科研业务费《阿尔巴尼亚布尔齐泽不同类型铬铁矿中的矿物包裹体特征及指示意义》(编号:J1903)。

摘　　要：藏南雅鲁藏布江缝合带代表印度板块和欧亚板块的碰撞界线,带中洋岛型基性岩构造归属还存在争议。雅鲁藏布江缝合带中段白朗地体为近北东—南西向展布的构造岩片,由放射虫硅质岩、硅质泥岩、泥页岩、凝灰岩、微晶灰岩、玄武岩、辉绿岩、辉长岩组成。辉绿岩和辉长岩呈岩脉或岩床侵入沉积地层中。测年结果显示,这些辉绿岩形成于晚侏罗世,锆石206 Pb/238 U年龄为150.3±0.8 Ma(n=39,MSWD=1.8)。地球化学特征表明,岩石富集轻稀土元素、大离子亲石元素(Rb、Sr、Pb)和高场强元素(Th、Nb、Ta、Zr、Hf和Ti),源区具有明显的石榴子石印记,是没有经历陆壳混染的洋岛型辉绿岩。这些特征与雅鲁藏布江缝合带晚侏罗世—早白垩世洋岛型基性岩相似。由此认为,这些洋岛型基性岩可能起源于新特提斯洋板内环境,代表了海山的残迹。结合区域地质资料,提出侏罗纪新特提斯洋地幔柱活动可能驱使大洋岩石圈向北俯冲到拉萨地体之下,并形成安第斯型大陆边缘,至早白垩世,先期俯冲的新特提斯大洋岩石圈与上覆板片解耦并向南后撤,诱发冈底斯弧前伸展形成现今保存在缝合带中的蛇绿岩。The Yarlung Zangbo suture zone(YZSZ)in southern Tibet marks the collision between the Eurasia plate and the Indian subcontinent,and the tectonic affinity of the ocean island-type(OIB-type)basaltic rocks within this suture remains controversial.The Bainang terrane in the middle segment of the YZSZ is a NE-SW-oriented tectonic slice,which is composed of radiolarian chert,siliceous mudstone,siliceous mudstone,shale,tuff,micritic limestone,basalt,dolerite and gabbro.The dolerite and gabbro are dikes or sills intruding into strata.The dolerites formed in the Late Jurassic,with a concordant zircon 206 Pb/238 U age of 150.3±0.8 Ma(n=39,MSWD=1.8).They are chemically characterized by LREE enrichment,variable enrichment of large ion lithophile elements(LILEs,e.g.Rb,Sr and Pb)and high field strength elements(HFSEs,Th,Nb,Ta,Zr,Hf,and Ti),indicating OIB affinity with little or no continental crust contamination.These features are similar to Late Jurassic-Early Cretaceous OIB in the YZSZ.It is suggested that these OIB rocks were derived from the intraplate environment within the Neo-Tethys and represented the remnants of seamounts.In combination with regional geological material,it is proposed that the Jurassic mantle plume in Neo-Tethys might drive the oceanic lithosphere northward subduction under the Lhasa terrane,and formed the Andean continental margin.During the Early Cretaceous,the Neo-Tethys oceanic lithosphere was decoupled from the overlying plate,and retreated southward to induce the Gangdese forearc extension to form the present ophiolites preserved in the YZSZ.

关 键 词：雅鲁藏布江缝合带 新特提斯洋 白朗地体 洋岛玄武岩 地幔柱 LA-ICP-MS锆石U-PB年龄 

分 类 号：P534.52[天文地球—古生物学与地层学] P588.12[天文地球—地质学]