俯冲隧道内不同深度流体的性质与来源：以西南天山为例

Subduction channel at the interface between the convergent plates is not only an effective pathway for exhumation of deeply-subducted crustal rocks, but also represents a place with extremely strong fluid activities. Fluids in the subduction channel play a key role in understanding subduction process, element migration, crust-mantle interaction, global element cycling and other important geological processes. However, current research is concentrated in the fluid activities within oceanic crust or underlying serpentinite, and little is known about those happened in the subduction channel at the top of the the subducting slab. Taken Southwestern HP-UHP metamorphic belt as an example, this project is devoted to promoting petrological and geochemical investigations of veins and alteration halos that representing fluid activities at different depths of the subduction channel. Based on the petrographic observation, the trace-element compositions of bluk-rock and minerals, as well as fluid inclusions will be studied to identify the fluid chemistry and properties under different P-T conditions in the subduction channel. The traditional bluk-rock Sr-Nd isotope and the emerging in-situ mineral isotope analysis will be used to trace the fluid sources and their mixture processes. In addition, the metasomatism process and element mobilization during fluid-rock interaction in the subduction channel will be also performed. This study will be conductive to understanding the nature of subduction fluids, reconstructing the geochemical frame of the fluid activity in the subduction channel, and deciphering its implications on arc magmatism and metallogenesis.

汇聚板块界面之间的俯冲隧道不仅是深俯冲地壳岩石向上折返的有效通道,也是流体活动最为强烈的场所，对其研究有助于理解俯冲带深部过程、元素迁移、壳幔相互作用和全球物质循环等重要地质过程。然而，目前俯冲带流体的研究多集中于俯冲板片内部（洋壳或下伏蛇纹岩），对俯冲隧道内自深到浅流体活动的系统性研究匮乏。本项目拟以西南天山高压-超高压变质带为例，采集记录俯冲隧道不同深度处流体活动的脉体、蚀变带样品，在岩相学观察的基础上，采用全岩/矿物主微量元素、流体包裹体测试等方法查明俯冲隧道中不同温压条件下流体的成分和性质；利用传统的全岩Sr-Nd同位素和新兴的单矿物原位微区同位素分析示踪不同深度处流体的来源与混合比例；并解析隧道内流体-岩石的交代过程及元素迁移情况。项目的实施有助于加强对俯冲带流体性质的理解，重建俯冲隧道不同深度的流体作用模型，研究成果在揭示俯冲带流体对岛弧岩浆、成矿作用的制约方面有重要科学意义。

板片流体不仅制约了俯冲带内部高压-超高压变质作用和中源地震的发生，在控制壳幔相互作用、岛弧岩浆作用和全球元素循环方面也扮演着关键的角色。目前虽然对俯冲板片的脱水机制、流体作用和元素循环有了一定认识，但在俯冲带尤其是俯冲带隧道不同深度流体的性质和来源方面还缺乏深入理解。本项目对西南天山高压-超高压变质带的蓝片岩、榴辉岩及其相关的脉体开展了详细的矿物学、岩石学、热力学模拟、微量元素地球化学和同位素地球化学的研究，查明了俯冲板片深、中、浅部流体的氧化还原特征，基本勾勒出俯冲隧道不同深度流体性质的框架。主要研究进展如下：（1）系统梳理了俯冲带不同深度岩石氧逸度及其演化特征，通过热力学模拟计算出不同深度下板片流体的硫含量特征，搭建了俯冲带不同深度流体的地球化学特征框架；（2）查明了俯冲带深部不同岩性释放流体的δ34S变化范围，提出了利用δ34S值作为示踪俯冲带内不同来源流体新的地球化学指标；（3）查明了俯冲隧道中部（50-60km）流体的高氧逸度性质，揭示该深度富Mg流体对折返岩石的蓝片岩相改造特征；（4）识别出俯冲隧道浅部（30-40km）流体的高氧化性特征，揭示俯冲带弧前深度释放富Cu-SO2的流体；（5）首次从板片角度给出全面、定量的俯冲带S循环解析，精确限定了流体诱导的板片脱硫效率（6.3%）、脱硫类型（还原相）和脱硫δ34S（负值）。本项目对认识俯冲带不同深度流体性质、有效区分流体来源、揭示俯冲带S循环演化模型、以及进一步理解岛弧岩石高氧逸度来源及其δ34S等地球化学特征具有重要的科学意义。