西南天山超深蛇纹岩和其包裹的榴辉岩岩石学及其折返机制研究

The exhumation mechanism of high-ultrahigh pressure rocks from oceanic subduction zone has been extensively debated. As one of the buoyant host rock that facilitating the exhumation of eclogites, the key role of serpentinites in the subduction channel need more further studies. This project will conduct systematic petrological, geochemical, geochronological and high-pressure experimental studies of serpentinites and enclosed eclogites from the HP-UHP metamorphic belt of southwestern Tianshan. Detailed petrological and geochronological studies combined with phase equilibrium simulation calculation of the enclosed eclogites will help us to infer their peak metamorphic conditions, P-T paths and metamorphic ages. High-pressure experiments will be carried out to determine the stability of 3.65Å phase and related phase transformation, and derive an estimation of the subduction depth for Tianshan serpentinites. On the basis of these studies, to determine whether the eclogites and serpentinites subducted deeply together, or the eclogites were captured by serpentinites during its exhumation and then exhumated together to the earth’s surface. On the other side, the metamorphism of many kinds of veins appeared in serpentinites is also studied, in order to discuss the fluid process after dehydration of serpentinites and the effects to the exhumation of eclogites made by the fluid.

洋壳俯冲带高压-超高压岩石的折返问题，一直存在争议。作为浮力折返机制之一的蛇纹岩在俯冲隧道中所起的作用值得深入研究。在前期研究的基础上，本项目拟进一步对西南天山超高压变质蛇纹岩和其中包裹的榴辉岩进行系统的岩石学、元素和同位素地球化学、地质年代学和高压实验的综合研究。通过矿物结构和成分分析，结合相平衡模拟计算以及锆石年代学的研究，确定包裹的榴辉岩的峰期变质条件、变质P-T轨迹和变质时代；通过3.65Å相高压实验研究，探讨其温压稳定范围，确定天山超高压蛇纹岩的折返深度。从而判断榴辉岩与蛇纹岩是一起经历深俯冲并折返，还是超深蛇纹岩在折返过程中捕获榴辉岩然后携带折返？此外通过研究蛇纹岩中各类脉体的变质过程，探索蛇纹岩变质过程中的流体行为以及这些流体对榴辉岩折返过程的影响。

洋壳俯冲带高压-超高压岩石的折返问题，一直存在争议。本项目通过研究西南天山包裹在蛇纹岩中的榴辉岩的岩石学和相平衡模拟计算及密度测试，得出榴辉岩的早期折返依靠自身浮力，后被蛇纹岩捕获并受其浮力共同折返。较低的地温梯度表明，榴辉岩和蛇纹岩可能并非来自于俯冲板片表面，而是来自于板片的内部；高压实验研究初步推断3.65Å相可能形成于30-45kbar，600-650ºC，这与实验合成的温压条件有很大差异，成分中Al和Fe的加入可能降低了其稳定温压范围，使其稳定的压力降低，温度升高；透射电镜研究叶蛇纹石的微观结构，获得了叶蛇纹石的晶体结构与温压条件之间的相关关系——P-T-m图解，从而为不含有特征变质矿物的变质蛇纹岩提供了可用的温压计；通过榴辉岩中电气石的研究，确定了侵入榴辉岩的富B流体来自于围岩蛇纹岩的部分脱水作用，而且这些大洋板片蛇纹岩是俯冲带中富B和重B的库，贡献于岛弧岩浆作用；全岩地球化学和矿物化学研究确定俯冲的西南天山UHP蛇纹岩具有弱的流体活动性元素（FME）富集，说明没有受到俯冲沉积物的影响，保留了原始地幔橄榄岩的特征，进一步说明蛇纹岩可能来自于俯冲板片中间位置，而不是板片表面。同时，钛硅镁石是HFSE的载体，B等流体活动元素随着脱水过程逐渐减少，但是仍旧通过变质作用部分转移到新的变质矿物中，从而带到地幔深部，实现壳幔元素循环。俯冲带高压-超高压蛇纹岩对于研究俯冲带折返机制、元素循环和流体过程具有重要的作用，是俯冲高压变质不可或缺的组成部分。