青藏高原主要地质端元的锂同位素组成及其地质应用研究

Understanding the petrogenesis of post-collisional Cenozoic K-rich volcanic and porphyritic rocks can provide important constraints on the architecture of the lithosphere, the mechanisms of plateau uplift, and the geodynamics and history of crustal growth since the India–Eurasia continental collision. Although there have been a large amount of studies on these K-rich rocks, their petrogenesis and geodynamic setting remain controversial. Lithium isotope, as a “non-traditional stable isotope”, has become one of the most fast-developing fields in the study of crust-mantle interaction. One of the most important features is that lithium isotope undergoes negligible fractionation during magmatic processes, suggesting that the lithium isotopic compositions of mantle-derived magmas may directly record the compositions of their source materials. Consequently, the lithium isotope method is an effective tracing means to resolve the petrogenesis of post-collisional Cenozoic K-rich rocks, Tibetan plateau. However, the traditional lithium isotopic compositions of various reservoirs cannot explain the lithium isotopic compositions of Cenozoic K-rich rocks, Tibetan plateau. Given the unique geological conditions of Tibetan plateau, the lithium isotopic reservoirs of the main geological endmembers in Tibetan plateau should be established. This study will improve and complement the traditional lithium isotopic compositions of various reservoirs. This research will be carried out on the post-collisional Cenozoic K-rich volcanic and porphyritic rocks in Tibetan plateau. Source characteristics of these K-rich rocks and their deep-seated processes will be investigated to understand their petrogenesis by modeling the lithium isotopic compositions of the different endmembers.

理解后碰撞新生代富钾火山岩和斑岩的成因对青藏高原岩石圈结构、隆升机制以及地壳生长过程和动力学背景能提供重要限制作用。尽管前人对富钾岩石做了大量研究工作，但其岩石成因和动力学背景存在很大争议。作为一种“非传统稳定同位素”，壳–幔相互作用过程的锂同位素研究已成为近年来国际上研究热点之一。其重要特征是在高温的岩浆作用过程中不发生有意义的分馏，幔源岩浆锂同位素组成能直接记录源区物质组成，这对解决青藏高原富钾岩石的成因是一种有效的示踪手段。然而，传统的“主要地球化学储库的锂同位素组成”难以解释青藏高原富钾岩石的锂同位素组成。这就需要立足青藏高原地质实际，建立“青藏高原主要地质端元的锂同位素储库”，从而发展和完善传统的“主要地球化学储库的锂同位素组成”。本项目选择青藏高原富钾火山岩和斑岩开展应用研究，通过不同端元锂同位素组成数值模拟，揭示富钾岩石起源演化及深部过程，探讨其岩石成因，具有重要的理论意义。

作为一种“非传统稳定同位素”，壳–幔相互作用过程的锂同位素研究已成为近年来国际上研究热点之一。其重要特征是在高温的岩浆作用过程中不发生有意义的分馏，幔源岩浆锂同位素组成能直接记录源区物质组成，这对解决青藏高原富钾岩石的成因是一种有效的示踪手段。然而，传统的“主要地球化学储库的锂同位素组成”难以解释青藏高原富钾岩石的锂同位素组成。这就需要立足青藏高原地质实际，建立“青藏高原主要地质端元的锂同位素储库”，从而发展和完善传统的“主要地球化学储库的锂同位素组成”。据此，在确定了印度上下地壳的Li同位素组成基础上，该项目又确定了新生下地壳、软流圈地幔和拉萨上地壳的Li同位素组成以及印度下地壳和拉萨上地壳的Mg同位素组成。.印度下地壳δ7Li值（−4.4 ‰~−0.1 ‰）明显轻于印度上地壳δ7Li值（+0.9 ‰~+5.6 ‰），印度上地壳高的δ7Li值是由于印度下地壳释放的高δ7Li流体造成的，而印度下地壳低的δ7Li值是由于残余印度下地壳部分熔融形成的。印度下地壳δ26Mg为−0.70 ‰~−0.03 ‰。新生下地壳δ7Li值（+0.8 ‰~+6.6 ‰），与EMI/EMII地幔的相似，归因于大陆岩石圈地幔的部分熔融，受到俯冲洋壳和沉积物的不同比例流体的交代。软流圈地幔δ7Li值为−2.0 ‰~2.0 ‰，受到俯冲沉积物的交代。拉萨上地壳δ7Li为−2.4 ‰~−0.7 ‰，δ26Mg为−0.11 ‰ ~−0.76 ‰。.在此基础上，通过Li同位素系统研究，证实了印度下地壳俯冲到拉萨地块之下和青藏高原中部存在残余大洋板块，发现了西藏驱龙埃达克质岩岩浆分异和热液过程中存在锂同位素分馏，探讨了四川甲基卡硬岩型Li矿床富集机理。.此外，综述了U、Ca、Mg和Li同位素最新研究进展，尤其是在铀矿床和碱性岩-碳酸岩型稀土矿床研究中的应用，旨在展示金属同位素在关键金属矿床研究中的良好应用前景。