摘要
俯冲带是全球最大的物质循环系统,控制着壳幔相互作用、岛弧岩浆作用、成矿物质聚集和全球元素循环等诸多重要的地质过程。俯冲带流体作为板片向上覆地幔楔进行物质传输的重要媒介,在上述地质过程中扮演着关键角色。基于岛弧火山岩地球化学特征、高温高压实验、相平衡模拟,以及高压-超高压变质岩的研究,我们对俯冲带流体基本特征及其地球化学行为已经有了较为深刻的认识,然而,目前对俯冲带流体作用的复杂性,如多期流体作用,还缺少深入的理解。本文对西南天山高压-超高压变质带榴辉岩中一条含石英-绿帘石-铁白云石-硫化物的高压脉开展了详细的岩石学、地球化学研究。脉体中有绿辉石、铁白云石、金红石等特征高压矿物的产出,暗示其代表的流体作用发生于高压条件下。脉体与主岩榴辉岩界线截然,之间无蚀变带/过渡带/交代晕产出,说明主岩榴辉岩与迁移流体之间没有发生显著的水-岩交换反应。脉体矿物(如绿帘石、铁白云石、黄铁矿等)的多期生长特征及其成分差异研究揭示出至少三期流体作用。前两期流体作用均发生于高压条件下,特征如下:(1)两期流体分别以两期矿物组合(Ep^(1)-Dol^(1)-Py^(1)组合以及Ep^(2)-Dol^(2)-Py^(2)组合)为代表,在第二期流体作用下,第一期流体沉淀的矿物(如Dol^(1)等)发生了明显的溶解-再沉淀作用;(2)两期流体性质均以富Ca、Si,含CO_(2)、S为特征,第一期流体较第二期流体富Fe贫Mg,富Ni贫Co,富轻稀土元素(LREE);(3)两期黄铁矿的δ^(34)S值分别为-8‰和-5‰,暗示这两期流体均主要来源于俯冲沉积物,且第二期流体可能有更多俯冲洋壳或蛇纹岩脱水流体的参与;(4)两期流体作用下,碳酸盐矿物和硫化物显示出耦合共生的特点,暗示C、S在流体作用下可能具有协同演化(如溶解-迁移-沉淀)的关系。第三期流体作用以绿帘石+铁白云石反应生成方解石+绿泥石、绿帘石边部(与黄铁矿相邻处)发生成分改造为特征,流体作用发生在高压岩石折返的退变质阶段,记录了退变质流体的信息。本研究通过榴辉岩高压脉体中多期矿物生长的特征,揭示了俯冲带内多期流体作用,对深入理解俯冲带流体性质、流体来源、以及俯冲带关键挥发分元素的耦合演化信息,具有重要的科学意义。
Subduction zone is the largest material cycling system on the Earth,governing many important geological processes such as crust-mantle interactions,arc magmatism,ore metallogenesis,and global element cycling.Subduction zone fluids,as a crucial media for material transport from the slab to the mantle wedge,play a pivotal role in the mentioned geological processes.Based on studies involving geochemical characteristics of arc volcanic rocks,high-temperature and high-pressure(HP)experimental petrology,phase equilibrium simulations,and high-pressure to ultrahigh-pressure(UHP)metamorphic rocks,we have gained a profound understanding of the nature and geochemical behaviors of slab fluids.However,the complexity of subduction zone fluid processes,including multi-stage fluid activities,still lacks in-depth understanding.This paper presents a detailed petrological and geochemical study of a HP vein(containing quartz,epidote,dolomite,and sulfides)in eclogite from the Southwestern Tianshan HP-UHP metamorphic belt.The occurrence of HP index minerals such as omphacite,dolomite,and rutile in the vein suggests that the vein formed under eclogite-facies conditions.The sharp boundary between the host rock and the vein,without reaction selvage in between,indicates no significant fluid-rock interactions between eclogite and the infiltrating fluid.Polyphase growth of vein minerals reveals at least three stages of fluid infiltration.These vein minerals(such as epidote,dolomite,and pyrite)are with chemical discontinuity.The first two stages of fluid activities occurred under HP conditions,and detail characteristics are as follows:(1)The first stage of fluid is represented by mineral assemblages of Ep^(1)-Dol^(1)-Py^(1),while the second fluid by Ep^(2)-Dol^(2)-Py^(2).The first-stage minerals occurred dissolution-reprecipitation process with the second stage fluid,and then the second-stage minerals formed;(2)Both two-stage fluids are enriched in Ca,Si,CO_(2),and S,with the first being Mg-rich/Fe-poor,Ni-rich/Co-poor,and LREE-enriched compared to the second one;(3)Theδ^(34)S values of pyrite of the two stages are-8‰and-5‰.Theseδ^(34)S values suggest that the fluid is mainly derived from metasediments,with increased involvement of subducted oceanic crust or serpentinite in the second-stage fluid;(4)Carbonate minerals and sulfides exhibit coupled precipitation during the two-stage fluid activities,indicating that C and S may have a synergistic evolution(during dissolution-migration-precipitation).The third stage of fluid activity is characterized by the reaction of epidote and dolomite to form calcite and chlorite and the compositional alteration of epidote adjacent to pyrite,indicating the presence of a likely a retrograde metamorphic fluid during exhumation of eclogite.Based on the polyphase mineral growth characteristics of the HP vein in eclogite,this study reveals multi-stage fluid activity within subduction zones.Therefore,it is of great significance for understanding of subduction zone fluids,their sources,and the coupled C-S evolution within subduction zones.
作者
吕佳阳
李继磊
王钟瑞
马智佩
L JiaYang;LI JiLei;WANG ZhongRui;MA ZhiPei(State Key Laboratory of Lithospheric and Environmental Coevolution,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China;College of Earth and Planetary Sciences,University of Chinese Academy of Sciences,Beijing 101408,China)
出处
《岩石学报》
SCIE
EI
CAS
北大核心
2025年第1期183-197,共15页
Acta Petrologica Sinica
基金
国家自然科学基金项目(42122011)
中国科学院地质与地球物理研究所重点部署项目(IGGCAS-202204)
中国科学院青年创新促进会项目(Y2022030)联合资助.
关键词
西南天山
榴辉岩
高压脉体
俯冲带流体
多期流体作用
Southwest Tianshan
Eclogite
High-pressure vein
Subduction zone fluid
Multi-stage fluid activity
