变质岩中磷灰石变形与元素分布特征:以青藏高原东构造结变质岩为例  

Deformation and element redistribution characteristic of in-situ apatite in metamorphic rocks:A case study of metamorphic rocks in the Eastern Himalayan Syntaxis

作　　者：张岩岩 赵中宝[2,3] 杜瑾雪 梁凤华 毛小红[2] 田作林[2] 马绪宣[2,3] ZHANG Yanyan;ZHAO Zhongbao;DU Jinxue;LIANG Fenghua;MAO Xiaohong;TIAN Zuolin;MA Xuxuan(China University of Geosciences(Beijing),Beijing 100083,China;MNR Key Laboratory of Continental Dynamics,Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China;Jiangsu Donghai Continental Deep Hole Crustal Activity National Observation and Research Station,Donghai,Jiangsu 222300,China)

机构地区：[1]中国地质大学(北京),北京100083 [2]中国地质科学院地质研究所,自然资源部大陆动力学重点实验室,北京100037 [3]江苏东海大陆深孔地壳活动国家野外科学观测研究站,江苏东海222300

出　　处：《地质学报》2025年第4期1150-1168,共19页Acta Geologica Sinica

基　　金：中国地质科学院自然资源部深地科学与探测技术实验室基本科研业务费(编号JB2314);国家自然科学基金项目(编号41902211);中国地质调查项目(编号DD20221630)联合资助的成果。

摘　　要：变质变形岩石中副矿物的变形及元素分布特征能记录变质变形岩石的变质变形信息,特别是结合原位定年,可以更为精确地揭示岩石的变质变形历史。磷灰石是变质岩中常见的副矿物,伴随着磷灰石原位定年技术的快速发展,使其成为岩浆岩和矿床学研究中重要的定年矿物,在不同变质-变形过程中,磷灰石的变形机制与行为及其对元素扩散过程的制约效应等一系列重要问题还未得到解决。本研究选取喜马拉雅造山带中构造变形和变质作用最强的东构造结内的岩石开展研究。通过SEM和CL观测,显示麻粒岩中的磷灰石并未显示明显的成分环带,而糜棱岩化片岩中的磷灰石则呈现规律的明暗变化,也即成分变化。通过对薄片中磷灰石的展布特征统计发现两类变质岩中磷灰石的长轴方向近平行于面理的方向。EBSD组构分析显示麻粒岩中的磷灰石呈现出明显的粒内形变,而糜棱岩化片岩中的磷灰石几乎未发生粒内形变。EPMA对磷灰石成分面扫的结果显示麻粒岩中磷灰石的主量元素分布相对均匀,而糜棱岩化片岩中磷灰石的Si元素分布呈现明显的环带或者不均匀,这和CL图像的结果相一致。综合以上的磷灰石变形和元素分布特征可初步得出:(1)尽管麻粒岩中的磷灰石定向分布,参与了岩石的变形,但可能由于其变质温度较高,促使了元素的快速扩散,使元素分布趋于均一,但是后期的流体作用在沿着低角度颗粒边界或者近垂直于最大拉伸应力方向的裂隙发生了成分交代;(2)糜棱岩化片岩中磷灰石平行于糜棱面理排列,但其几乎未发生粒内形变,可能是由于形变温度较低(<450°),或者是由于糜棱岩化片岩的应变主要集中在石英和云母域,而相对强硬的磷灰石并未参与变形,其CL图像和部分主量元素分布图显示的成分变化可能是原生的,这预示着较低变质变形温度可能并未导致磷灰石The structural characteristics and element distribution within accessory minerals are crucial in documenting the metamorphicand deformational processes in rocks.In particular,in-situ geochronological dating of apatite has gained significant prominence,becoming vital for dating magmatic rocks and mineral deposits.Despite its significance,unresolved issues persist regarding the deformation mechanism,behavior,and restrictive impact on element diffusion in apatite during different metamorphic-deformation processes.This study focuses on rocks from the Eastern Himalayan Syntaxis,renowned for its intense tectonic deformation and metamorphism.Using SEM and CL observations,we identified notable differences inapatite within granulite and mylonitic schist.Granulite apatite lacks an apparent compositional structure,while mylonitic schist apatite exhibits distinct light and dark compositional changes.Statistical analysis of apatite distribution in thin sections revealed that the long axis of apatite grains in both metamorphic rocks aligns approximately parallel to the main foliation.Furthermore,EBSD fabric analysis unveiled clear intra-granular deformation in granulite apatite,whereas mylonitic schist apatite displayed minimal intra-granular deformation.EPMA scanning of apatite composition demonstrated a relatively uniform distribution of major elements in granulite,while silicon(Si) elements in mylonitic schist displayed evident zoning or unevenness,consistent with CL scanning results.Based on these observations,preliminary conclusions were drawn.Firstly,despite its directional distribution and intra-granular deformation,granulite apatitemay be influenced by higher metamorphic temperatures,promoting rapid element diffusion and uniform redistribution.Late-stage fluid activity metasomatized elements along low-angle grain boundaries or fractures perpendicular to maximum tensile stress.Secondly,mylonitic schist apatite,alignedwith mylonite foliation,exhibits minimal intra-granular deformation,likely due to lower deformation temp

关 键 词：喜马拉雅造山带 麻粒岩 糜棱岩化片岩 磷灰石粒内形变 磷灰石元素分布特征 

分 类 号：P588.3[天文地球—岩石学]