西南天山含硫化物榴辉岩相脉体中成矿元素的来源及活动性研究

Subduction fluids are one of the hot topics in current geological researches. The formation of arc-associated ore deposits is closely related to the partial melting of mantle wedge triggered by the slab-derived fluids. The eclogite-facies veins occurred in high-pressure (HP) metamorphic belt is the direct record of these subduction fluids. The sulfide-bearing HP veins indicate that the subduction fluids may transport ore-forming elements (OFEs), however, few study focuses on the origin of fluids and metals, as well as the type and mobility of OFEs. Thus, based on microprobe and Nano-SIMS approaches, this project is devoted to promoting petrological and geochemical investigations of two types of eclogite-facies veins from southwest Tianshan HP metamorphic belt, including mineral major- and trace-element analysis, whole-rock major- and trace-element analysis, fluid inclusion analysis, as well as in situ sulfur isotopic analysis and mapping of sulfides. The project is planning to clarify the subduction fluid behaviors, trace the origins of fluids and OFEs, study the mobilizations of OFEs during fluid-rock interactions in subduction zone, and discuss the contributions of ore-bearing subduction fluids to the formation of arc-related ore deposits. This study will be conducive to understanding the nature of subduction fluids, and enrich the origins of ore-forming materials for arc-associated ore deposits.

俯冲带流体是国际地学领域的研究热点之一，其交代上覆地幔楔引发的部分熔融与岛弧地区的矿床成因有着密切的联系。产出于高压–超高压变质带中的榴辉岩相脉体直接记录了俯冲带深部流体活动的信息。含硫化物的高压脉体暗示俯冲带流体可以携带成矿元素进行迁移，但对于流体中成矿元素的来源、流体携带成矿元素的种类和能力等方面，一直缺乏系统的针对性研究。本申请拟以西南天山高压变质带中两类榴辉岩相脉体为研究对象，利用电子探针和纳米离子探针等分析技术，对脉体及主岩进行单矿物主微量元素分析、全岩主微量元素及Sr-Nd同位素分析、流体包裹体分析、硫化物原位S同位素分析和面扫描等详细的岩石学和地球化学研究。以期查明俯冲带流体性质和成矿物质的来源，确定俯冲带深部流体–岩石相互作用过程中成矿元素的活动性，并探讨含矿俯冲带流体对岛弧矿床成矿物质的贡献。项目的实施有助于加强对俯冲带流体性质的理解，并丰富岛弧成因矿床的成矿物质来源。

俯冲带流体在制约岛弧岩浆作用及相关矿床的形成方面扮演着关键的角色，因此理解俯冲带流体的性质及其携带的成矿元素的来源具有重要意义。本项目通过对西南天山高压榴辉岩精细的变质作用研究，反演了俯冲隧道内多期俯冲-折返的过程，建立了该变质带精细的俯冲-折返构造模型，为俯冲带内流体活动和成矿元素运移构建了大地构造背景。通过对变质沉积岩、蓝片岩、榴辉岩及其相关脉体中特征矿物的研究，查明了含硫相在各类岩石中的赋存状态，建立了俯冲洋壳的氧逸度随深度增加而下降的变化规律；厘定出俯冲带内深部还原性和浅部氧化性两类流体类型，为成矿元素活动搭建了氧逸度环境框架。通过质量平衡计算揭示出板片流体对俯冲带岩石中金属元素的淋滤作用。通过对俯冲板片的主要岩性（变质沉积岩、变质基性岩和蛇纹岩）的全岩S含量和S同位素测试，勾勒出俯冲带内的主要S储库的地球化学性质。通过不同脉体中硫化物的原位S同位素特征，反演出不同储库脱水释放流体的同位素特征，判定流体及成矿元素的来源，对岛弧岩浆过程中S的同位素特征做出了关键制约。