西藏南部拉萨地体约70Ma的岩浆作用与深部过程

Late Cretaceous magmatic belt formed during 80~65Ma in the Lhasa Terrane has been widely regarded as a good record of the geological process which occurred from the northward subduction of the Neotethyan oceanic slab to the early stage of the India-Asia collision. This project is planning to conduct systematical petrological, geochronological, elemental, and isotopic geochemical studies on 70Ma magmatic rocks from Gongbo' gyamda located southeast of the southern Lhasa Terrane, and perform mineralogy, petrology, geochronology and conventional geochemical analyses (e.g. major, trace elements, Sr-Nd-Pb-Hf and zircon Hf-O isotopes analysis). This project will explore the origin and petrogenesis of the Late Cretaceous magmatic rocks, trace the deep process involved in the generation of the magmatic rocks. This project is expected to provide not only crucial constraints on the deep process operating at the southern Lhasa Terrane during the Late Cretaceous, but also solid lines of evidence for the collision of the India-Asia at this time in terms of deep process.

西藏南部拉萨地体晚白垩世80~65 Ma发育的带状岩浆活动，被认为真实记录了由新特斯洋壳北向俯冲向印度－亚洲大陆碰撞转换的地质演化过程。在充分分析南部拉萨地体晚白垩世岩浆岩研究现状的基础上，本项目拟以南部拉萨地体工布江达县以南地区以往研究程度很低的、约70 Ma岩浆岩为重点对象，开展系统的岩石学、年代学、元素和同位素地球化学研究，通过Sr-Nd-Pb-Hf及锆石Hf-O测试，分析岩浆起源和岩石成因，探讨其深部过程。同时，通过对比南部拉萨地体与区域上其它出露晚白垩世80~65 Ma岩浆岩地区的岩石学和地球化学特征，可以从更大范围和更深层次上来探究这一时期岩浆活动的特点，进而为新特提斯洋板片俯冲及随后的印度－亚洲大陆碰撞提供新的约束。

西藏南部拉萨地体晚白垩世岩浆作用对研究新特提斯洋壳北向俯冲、消减以及印度-亚洲大陆碰撞等科学问题有重要意义。本项目以南部拉萨地体东段朗县-工布江达及相邻地区的岩浆岩为研究对象，在野外调研的基础上，开展了系统的岩石学、元素和同位素地球化学及高质量的锆石U-Pb年代学和锆石Hf同位素研究。主要的研究内容和结果包括以下几方面：（1）南部拉萨地体晚白垩世末期发育连续的岩浆作用。本项目获得的12件锆石U-Pb年代学数据表明，研究区发育晚白垩世以来的岩浆活动（98-40Ma），进一步证明南部拉萨地体不存在~80-65Ma的岩浆活动间歇期。（2）南部拉萨地体存在~70Ma的基性岩浆活动。通过对工布江达以南地区发现的~70Ma寄主花岗岩及镁铁质包体开展研究，识别出~70Ma的基性岩浆活动，来源于受俯冲板片流体交代后的地幔楔物质部分熔融。（3）南部拉萨地体晚白垩世末期（80-65Ma）的岩浆活动是新特提斯洋壳岩石圈北向正常俯冲的产物。项目成果为存在争议的南部拉萨地体晚白垩世末期岩浆作用成因和新特提斯洋壳岩石圈的北向俯冲提供了重要的岩石学和地球化学证据。