羌塘雀莫错始新世石英二长岩的年代学、地球化学与岩石成因  

Geochronology, Geochemistry and Petrogenesis of the Eocene Qoimaco Quartz-monzonites in Qiangtang Area

作　　者：曾纪鹏 王强[1] 欧权 齐玥 郝露露 孙鹏 王军 陈怡伟 ZENG Jipeng;WANG Qiang;OU Quan;QI Yue;HAO Lulu;SUN Peng;WANG Jun;CHEN Yiwei(State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, Guangdong, China;University of Chinese Academy of Sciences, Beijing 100049,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,广东广州510640 [2]中国科学院大学,北京100049

出　　处：《大地构造与成矿学》2018年第1期150-162,共13页Geotectonica et Metallogenia

基　　金：中国科学院前沿重点项目(QYZDJ-SSW-DQC026)、国家自然科学基金项目(41630208)、深地专项(2016YFC0600407)、广东人才项目(2014TX01Z079)和广州地化所“135”项目(135TP201601)联合资助.

摘　　要：正长岩以及富碱的石英二长岩常常被认为起源于富集地幔的熔融。本文报道了起源于增厚陆壳熔融的石英二长岩。雀莫错岩体是分布在羌塘北部(青藏高原中部)的雀莫错(湖)东北部雀莫山上的一个侵入体,前人认为其由正长斑岩组成,形成时代不确定(45~23 Ma)。近期,我们对该侵入体进行了详细的野外地质调查和室内岩石学、地球化学以及年代学研究。雀莫错侵入岩主要由石英二长岩组成,激光锆石U-Pb测年显示,该岩体的侵位时代为始新世(41.71±0.29 Ma),与区域上大面积始新世火山岩同期。雀莫错侵入岩高硅(SiO_2=65.12%~66.71%)、富碱(Na_2O+K_2O=9.08%~9.71%)、富钾(K_2O=5.50%~5.92%)和高铝(Al_2O_3>14.79%),同时高Sr(1874~2001μg/g),亏损重稀土Yb(1.24~1.34μg/g)和Y(14.4~15.7μg/g),高Sr/Y(124~136)和La/Yb(67~74)比值,富集大离子亲石元素(LILEs),亏损高场强元素(HFSEs),具有不明显的Eu负异常、Sr正异常和略高的Mg~#(47~56),与区域上大面积的同期埃达克质火山岩类似。岩石具有非常均一的(^(87)Sr/^(86)Sr)i同位素比值(~0.7069)和ε_(Nd)(t)值(-2.6^-2.8)。结合区域地质、岩石和构造资料,认为雀莫错侵入岩形成于印度-欧亚大陆汇聚诱发的高原中部挤压阶段:挤压导致陆内俯冲,俯冲地壳发生部分熔融,岩浆在上升的过程中与地幔橄榄岩发生小规模反应,然后岩浆上升侵位形成了该区侵入岩。Syenite and alkali-rich quartz-monzonite are generally considered to be derived from partial melting of the enriched mantle.This paper reports quartz-monzonites derived from partial melting of thickened continental crust.The Qoimaco pluton is located at the Qoima Mountain,Northeast of the Qoimaco Lake in the northern Qiangtang(central Tibetan Plateau).The pluton was regarded as mainly consisting of syenite porphyries with formation age of45to23Ma.Our recent detailed field geological investigation,petrological,geochemical and geochronological studies demonstrated that the pluton mainly consists of quartz-monzonite,and has zircon U-Pb age of41.71±0.29Ma,simultaneously with the Eocene volcanic lavas in the area.The rocks have high SiO2(65.12%–66.71%),Al2O3(>14.79%),and alkali(Na2O+K2O,9.08%–9.71%),K2O(5.50%–5.92%),Sr(1874–2001μg/g),and Sr/Y(124–136)and La/Yb(67–74)ratios.The rocks are characterized by depleted heavy rare earth elements(e.g.,Yb=1.24–1.34μg/g),Y(14.4–15.7μg/g)and high field strength elements,enriched large ion lithophile elements,and negligible Eu and Sr anomalies and slightly high Mg#values(47–56).The major and trace element features of the rocks are similar to those of the contemporary adakitic volcanic lavas in the area.In addition,the rocks have very uniform initial87Sr/86Sr ratios(~0.7069)andεNd(t)values(from?2.6to were generated through partial melting of the subducted crust during intra-continent subduction,which subsequently interacted with the mantle peridotite during ascent,in the compression setting in the central Tibet triggered by India-Asia continental collision.

关 键 词：石英二长岩 埃达克质岩石 地壳增厚 始新世 羌塘 青藏高原 

分 类 号：P597[天文地球—地球化学] P595[天文地球—地质学]