缅甸密支那早白垩世钾玄质岩石成因及其大地构造意义  

Petrogenesis of the Early Cretaceous Shoshonitic Rocks in Myitkina of Myanmar and its Tectonic Implications

作　　者：张靖祎 彭头平[1,3] 范蔚茗[3,4] 董晓涵 刘兵兵 高剑峰[5] 梁细荣[1] 陈林丽[1] ZHANG Jingyi;PENG Touping;FAN Weiming;DONG Xiaohan;LIU Bingbing;GAO Jianfeng;LIANG Xirong;CHEN Linli(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;CAS Center for Excellence in Tibetan Plateau Earth Sciences,Beijing 100101,China;Institute of Tibetan Plateau Research,Chinese Academy of Sciences,Beijing 100101,China;State Key Laboratory of Ore Deposit Geochemistry,Institute of Geochemistry,Chinese Academy of Sciences,Guiyang 550081,Guizhou,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 [3]中国科学院青藏高原地球科学卓越创新中心,北京100101 [4]中国科学院青藏高原研究所,北京100101 [5]中国科学院地球化学研究所,矿床地球化学国家重点实验室,贵州贵阳550081

出　　处：《大地构造与成矿学》2021年第3期553-569,共17页Geotectonica et Metallogenia

基　　金：国家自然科学基金重点项目(92055207);第二次青藏科考项目(2019QZKK0702);国家自然科学基金重大项目(41490613);国家自然科学基金面上项目(41672058)联合资助。

摘　　要：缅甸地处喜马拉雅造山带东段,紧临喜马拉雅东构造结。作为喜马拉雅造山过程深部动力学在浅部的响应,该区发育了大量晚中生代‒新生代岩浆岩,这些岩浆岩是揭示新特提斯演化过程及缅甸不同地块与青藏高原内部各块体之间大地构造属性的重要窗口。但是,目前该区域研究程度非常低,特别是东缅密支那地区几乎是空白。本文首次报道缅甸密支那地区的一套二长岩,锆石U-Pb年龄表明其形成时代为早白垩世(~130 Ma)。岩石地球化学特征显示,这套岩石具有高的SiO2(58.92%~63.73%)、全碱(Na_(2)O+K_(2)O=7.94%~10.14%)、K_(2)O含量(3.88%~6.14%)、K_(2)O/Na_(2)O值(>1),以及富集LILE、LREE和亏损Nb-Ta-Ti特征,类似典型的钾玄质岩石。结合其Sr-Nd同位素组成以及低Sr/Y、La/Yb值和高Y、Yb含量,暗示密支那钾玄质二长岩来源于受俯冲流体/溶体交代的含金云母岩石圈地幔。综合区域上研究资料,认为密支那钾玄质岩浆形成于洋内俯冲的岛弧环境,是俯冲板块回撤或者断离后软流圈上涌导致富集岩石圈地幔部分熔融的产物。因此,大地构造上,密支那洋并不是青藏高原班公湖‒怒江洋的南延,而是新特提斯洋的重要组成部分。Myanmar is located in the east of the Himalaya orogenic zone and proximal to the Eastern Syntaxis.Voluminous late Mesozoic-Cenozoic magmatic rocks developed in Myanmar,which are the products of the Himalaya orogeny.The origin and tectonic regime of these igneous rocks are critical for the understanding of the geodynamic evolution of the Neo-Tethys and the tectonic correlation of the different terranes in between Myanmar and Tibetan Plateau.However,little attention has been paid to these igneous rocks,particularly for those in Myitkyina of Myanmar.In this study,a suite of shoshonitic rocks were first recognized,which are characterized by high SiO2(58.92%–63.73%),total alkali(Na_(2)O+K_(2)O=7.94%–10.14%),K_(2)O(3.88%–6.14%),and K_(2)O/Na_(2)O ratios(>1)together with the strong enrichment of LILE and LREE and depletion of Nb,Ta and Ti.LA-ICP-MS zircon U-Pb dating results show that they were emplaced at~130 Ma.The above signatures coupled with their depleted Sr-Nd isotopic compositions,low Sr/Y and La/Yb ratios,and high Y and Yb contents indicate that the Myitkyina shoshonitic rocks were derived from partial melting of the enriched,phlogopite-bearing lithospheric mantle metasomatized by the subduction-related fluids/melts.Combined with other geological data,we propose that the Myitkyna early Cretaceous shoshonitic rocks were generated in an oceanic arc in response to the slab rollback or breakoff.It is thus noted that the Myitkyina Tethyan Ocean was not closed before~130 Ma,and the southern continuation of the Bangonghu-Nujiang Meso-Tethys was an important component of the Neo-Tethys.

关 键 词：早白垩世 钾玄质岩石 交代富集岩石圈地幔 洋内弧 密支那 

分 类 号：P581[天文地球—岩石学]