青藏高原西南部雄巴盆地中新世同期多类叠加岩浆作用与岩石圈结构反演

The Miocene magmatism in the southern Tibetan Plateau is essential in revealing the structure of the collision between India and Asia, as well as the extensional tectonics and early uplift of the plateau. This project is willing to study the Miocene synchronous multi-type magmatism in Xungba basin, far west of the Lhasa terrane in southwestern Tibet which were rarely of studies in the past. A systematic studies, including field geological observation and volcanic profile study, petrology, geochronology, major, trace, and isotopic geochemistry, will be carried out on the magmatic rocks in the Xungba basin. The U-Pb dating, trace element and Hf-O isotopes of the zircons from the rocks will be applied in studying the hidden magmatic records in the area. The temporal and spatial distribution, geochemical features of the ultrapotassic, potassic and adakitic rocks, which possibly originated from the lower crust or upper mantle will be verified, so as to further discuss the petrogenesis, and source-regions of the rocks. This research will also be promoted to recognize the homogeneity of the lower crust and upper mantle, from with the rocks were from via partial melting processes, in order to provide a petrological tracer for the subduction structure between Indian and Asian continents. The field linkage between the Xungba Miocene magmatism and the extension tectonics will be key clues for constraint of the initial time of the N-S faults and gardens, and the early uplift of the southern plateau. A thorough comparison of Miocene magmatism between Xungba and other Tethyan tectonic domain, such as from the Mediterranean, to Turkey, Iran, and Karakorum, and Tibet, will allowed us to achieve the general charactertics of the postcollisioinal magmatism that occurs in an orogenic belt.

青藏高原南部中新世岩浆作用对研究印度与亚洲大陆碰撞带结构、高原伸展构造与早期隆升等科学问题有重要意义。本申请拟在前期研究基础上，选择目前缺乏系统研究的拉萨地块西端雄巴盆地的中新世同期多类叠加岩浆作用，通过野外地质、岩石学、年代学和地球化学研究，注重多期锆石的U-Pb年代学、微量元素和Hf-O同位素研究和隐伏岩浆作用鉴别，确定盆地内和周边多类岩石的时空分布规律，鉴别从超钾质、钾质到埃达克质等各类幔源和壳源岩石的地球化学特征，探讨岩石成因和源区性质，反演岩石发生部分熔融的下地壳和上地幔物质组成的不均一性，为限制印度-亚洲大陆俯冲带结构提供岩石学证据；鉴别雄巴火山岩与伸展构造的紧密关系，力图为伸展构造的启动时间和高原西南部早期隆升等问题提供新资料；全面开展雄巴火山岩与邻区和地中海-土耳其-伊朗-喀喇昆仑-西藏整个特提斯构造域的中新世岩浆作用对比，归纳和阐述造山带碰撞后岩浆作用发生的一般规律。

青藏高原中新世碰撞后钾质超钾质岩浆作用广泛分布在拉萨地块，是揭示深部岩石圈性质、伸展作用与高原隆升时限、印度岩石圈俯冲等重要科学问题的关键证据。选取拉萨地块最西部的雄巴盆地中新世钾质和超钾质岩石叠加的火山岩剖面以及向东部断续出露在赛利普、扎布耶茶卡、麦嘎、当惹雍错和许如错、羊应乡等地的火山岩，开展了野外考察和采样，获得115套主微量元素、23个样品的锆石定年和微量元素、842个锆石Hf同位素、16套O同位素、47套Os同位素、58套Sr-Nd同位素、23套Pb和42套Mg同位素数据。在Geology、GCA、EPSL和Lithos发表了4篇英文论文和2篇《岩石学报》中文论文。主要成果包括：（1）确定雄巴盆地叠加产出的幔源超钾质和壳源钾质火山岩的喷发时间为早中新世~23 Ma；地壳混染作用和新特提斯洋壳沉积物（如洋底碳酸盐）交代作用对幔源超钾质岩浆形成有重要作用；高Sr/Y、La/Yb钾质中酸性火山岩可能起源于加厚下地壳部分熔融；幔源超钾质岩石和同期中酸性钾质岩石构成了类似双峰式火山岩组合，代表了加厚岩石圈地幔拆沉引发的岩石圈部分熔融作用。论文发表在2014年GCA，被SCI引用23次。（2）对拉萨地块超钾质火山岩锆石捕掳晶进行U-Pb定年、微量元素和Hf同位素分析，发现幔源超钾质火山岩中含有大量元古代以来锆石捕掳晶，显示90Ma、50Ma和20Ma的3个主要年龄峰值，记录了中部拉萨地块的三次大规模岩浆活动；锆石捕掳晶的(Dy/Yb)N和U/Yb比值从约55Ma开始系统升高，表明印度与亚洲大陆碰撞所引发了拉萨地块地壳自早古近纪以来的逐渐加厚过程。成果表明幔源岩石捕获锆石是反演地壳演化历史、多期隐伏岩浆记录和地壳组成的重要工具，也为碰撞后超钾质岩石经受过地壳混染作用提供了新依据。成果发表于2014年Geology，被SCI引用13次。（3）首次对拉萨地块碰撞后幔源超钾质火山岩进行了Mg同位素分析，结合Os-Sr同位素，进一步证实地壳混染作用的存在；超钾质岩石中具低187Os/188Os比值和更轻Mg同位素组成的样品受地壳混染影响较小，更能记录地幔源区特征，清楚显示出超钾质岩石地幔源区经受过镁碳酸盐交代作用。研究巧妙应用Os-Mg同位素的性质，对研究地幔碳酸盐交代与超钾质岩石成因、通过幔源富碱岩石Mg同位素示踪地球深部碳循环等都有重要意义。论文发表于2015年EPSL。