西阿尔卑斯造山带白片岩形成过程中的流体交代作用

Whiteschist in Western Alps is a special kind of ultrahigh-pressure metamorphic rocks. It is characterized by the mineral paragenesis of kyanite and talc, and extreme enrichment of Mg but strong depletion of Na, Ca, Fe, Rb, Ba and Sr. Previous studies conclude that the whiteschist is formed by fluid metasomatism of metagranite during continental deep subduction. However, it is still in hot debate on the origin of metasomatic fluid and its composition, the exact timing of metasomatism, and the transport mechanism for such fluid in the continental subduction zone. To address these questions, this project will carry out a detailed study of petrology and geochemistry for the whiteschist and its country rock metagranite in Western Alps. Intended investigations major include Mg-O isotope analyses of whole-rock and minerals, and zircon in-situ U-Pb dating,trace elements and Hf-O isotope analyses. These will be performed on the basis of systematic petrographic observations as well as mineral microanalysis of major and trace elements. Taken together, we expect to place new constraints for the origin and composition of metasomatic fluid, elucidate the exact timing of fluid metasomatism and its influence on the elements and isotopes of accessory minerals. Furthermore, we will explore the mechanism for fluid-rock interaction at the deep subduction zone and the consequences of fluid-rock interaction for the building of continental collision orogen.

西阿尔卑斯造山带出露的白片岩是一类独特的超高压变质岩石。它含有蓝晶石和滑石的特征矿物组合，具有异常富集Mg和强烈亏损Na、Ca、Fe、Rb、Ba和Sr的地球化学特征。尽管前人研究一致认为白片岩的形成是大陆深俯冲过程中流体交代变花岗岩的结果，但是对流体来源、流体组成、交代时限和交代流体的运移机制等方面的认识都存在很大争议。针对这些科学问题，本项目拟对西阿尔卑斯造山带白片岩及其围岩变花岗岩进行系统的岩石学和地球化学研究。主要通过全岩和矿物的Mg-O同位素分析以及锆石原位U-Pb定年、微量元素和Hf-O同位素分析，结合系统的岩相学观察和矿物微区主微量元素分析，明确制约白片岩形成过程中交代流体的来源和性质，查明交代作用发生的时代以及副矿物在元素和同位素上的记录，进而深入探讨陆壳俯冲带深部流体-岩石相互作用的方式及其与大陆碰撞造山带形成的关系。

西阿尔卑斯造山带出露的白片岩是一类独特的超高压变质岩石。它具有异常富集Mg和强烈亏损Na、Ca、Fe和大离子亲石元素(如Rb、Ba和Sr)的地球化学特征。这些样品为我们研究大陆俯冲带深部流体交代作用提供了良好对象。本项目以西阿尔卑斯造山带Dora-Maira地体白片岩及围岩变花岗岩为主要研究对象，开展了岩相学、全岩主微量元素和O-Mg-Li同位素、矿物微区主微量元素、矿物氧同位素以及锆石学等方面的研究，取得了多项重要创新性成果。厘清了白片岩的原岩为二叠纪S型花岗岩，澄清了关于其原岩属性的长期争议；发现白片岩具有高温岩石中最重的Mg同位素组成(δ26Mg最高达+0.72‰)，揭示了白片岩形成过程中交代流体来自进变质过程中板片-地幔楔界面大陆俯冲隧道中富滑石蛇纹岩的脱水作用；发现白片岩具有极轻的Li同位素组成特征(δ7Li低至-6.4‰)，暗示俯冲带深部蛇纹岩来源流体可能是轻Li同位素的重要储库。这一系列研究结果揭示了蛇纹岩在地球壳幔元素循环中的重要性，对理解地幔楔以及岛弧岩浆的Mg-Li同位素组成具有重要意义。同时，结合对苏鲁造山带超高压岩石的对比研究，深入制约了大陆俯冲带深部流体的组成特征、矿物的响应行为以及流体-岩石相互作用的方式。