特提斯喜马拉雅带中段拉孜-康马地区早白垩世岩浆岩研究

Since Early Cretaceous, the Tibetan Himalaya was gradually separated from cratonic India by the means of forming a “Greater India Basin” (GIB) which existed between 120 and 70 Ma. The GIB hypothesis has become a subject extensively concerned because it challenges the classic collision model. No remnant of related ophiolite and/or arc-trench system are found to the south of the Indus–Tsangpo suture zone, thus the GIB hypothesis aroses drastic debates worldwide. Obviously, the key point to solve this dispute is to identify whether or not the GIB has oceanic nature. Based on our previous work of the project, a particular suit of magmatic rocks has been identified from Kangma to Lazi areas in the middle section of the Tethyan Himalaya. The timing of these magmatic rocks (118-115 Ma) was soon after the GIB formed, and spatial position of these magmatic rocks located in the northern margin of the India. For above reason, this project focused on these magmatic rocks in the Tethyan Himalaya; using zircon chronology, whole-rock major, trace elements, Sr-Nd-Re-Os isotopes and mineral chemistry as the main research methods to constrain source characteristics, material compositions, magmatic evolution, and spatial and temporal distribution, and discuss tectonic setting. This project could test the GIB hypothesis from the perspective of magmatic rocks, i.e. whether or not the presence of initial oceanic crust between the Tethyan Himalaya and cratonic India, thus providing petrological evidence for India–Asia collision process (classic collision vs. dual–collision collision).

早白垩世以来，特提斯喜马拉雅带与印度克拉通分离并形成大印度盆地(存续时间：120-70 Ma)，即“大印度盆地”假说；该假说由于挑战了传统的印度-欧亚板块碰撞方式而成为一个热点问题。由于在雅鲁藏布江缝合带以南缺少蛇绿混杂岩和岛弧-海沟产物，因此该假说引发了激烈争论。显然，论证大印度盆地是否具有洋盆属性是解决这一争论的关键。项目前期在特提斯喜马拉雅带中段拉孜-康马地区新发现一套独特的岩浆岩，其时代（118-115 Ma）处在大印度盆地形成不久,空间位置位于大印度北缘。鉴于此，本申请以这套岩浆岩为研究对象，通过锆石年代学、全岩地球化学、Sr-Nd-Re-Os同位素和矿物化学研究，约束源区性质、物质组成、岩浆演化、时空分布和探讨构造背景。这项研究有望从岩浆岩角度检验“大印度盆地”假说，即特提斯喜马拉雅带与印度克拉通之间是否存在初始洋壳，从而为碰撞模式（单阶段或者双阶段）提供岩石学证据。

依据项目设计，为探讨 “大印度盆地”假说；系统开展特提斯喜马拉雅带中段早白垩世双峰式岩浆岩的同位素年代学和岩石地球化学（主量元素和Sr-Nd-Hf-Os同位素）研究。锆石LA-ICP-MS U-Pb年代学结果表明这套基性岩浆岩时代集中在为118–115 Ma。区域上，这套早白垩世岩浆岩在特提斯喜马拉雅带西段同样发育。因此，认为这套早白垩世岩浆作用在整个特提斯喜马拉雅带都有出露，而不单单仅出现在东段措美或者西段仲巴等地区。从大地构造位置上，这套早白垩世岩浆岩出露在特提斯喜马拉雅带上。因此，综合以上两点考虑，建议把前人这套火成岩称为特提斯喜马拉雅火成岩省。研究区这套岩浆岩具有双峰式特征，即由基性岩（48.6–51.4 wt.%）和长英质岩石（63.0–70.0 wt.%）组成。总体上，出露在特提斯喜马拉雅带上的早白垩世岩浆岩，其岩性变化为早期的140–130 Ma 洋岛玄武岩（OIB）类型转变为晚期的118–115 Ma 洋中脊玄武岩（MORB）类型。因此，这种岩性变化可能表明特提斯喜马拉雅带逐渐经历了由大陆向大洋地壳转变的过程。本文研究成果虽不能解决特提斯喜马拉雅带与印度板块之间是否存在成熟的古大洋，但岩浆岩证据表明至少在118–115 Ma这个时期不存在洋盆，至于这种持续性的拉张是否最后导致洋壳的形成，还需要进一步的研究。项目资助发表国际SCI论文1篇，待发表1篇。项目投入经费27万元，支出19.4393万元，剩余经费7.5607万元。剩余经费计划用于本项目研究后续支出。