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引用本文:陈 梅1,2 田作林1 张 聪1 杨经绥1 黄 杰1,2. 拉萨地块松多超高压变质带含石榴石云母石英片岩的变质演化相平衡模拟[J]. 中国地质, 2015, (5): 1572-1587.
CHEN Mei1,2, TIAN Zuo-lin1, ZHANG Cong1, YANG Jing-sui1, HUANG Jie1,2. Phase equilibrium modeling for metamorphic evolution of garnet-bearing mica-quartz schist in Sumdo UHP metamorphic belt, Lhasa Block[J]. Geology in China, 2015, (5): 1572-1587(in Chinese with English abstract).
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拉萨地块松多超高压变质带含石榴石云母石英片岩的变质演化相平衡模拟
陈 梅1,2 田作林1 张 聪1 杨经绥1 黄 杰1,21,2
1. 中国地质科学院地质研究所,大陆构造与动力学国家重点实验室,北京 100037;2. 中国地质大学地球科学与资源学院,北京 100083
摘要:
提要:拉萨地块松多超高压变质带含石榴石白云母石英片岩为榴辉岩的围岩,岩石的矿物组合为石榴石、白云母、钠长石、绿泥石、石英及少量金红石、榍石。石榴石具有明显的成分环带,从核部到幔部Xprp= [Mg/(Mg+Fe+Mn+Ca)]缓慢升高,Xsps= [Mn/(Mg+Fe+Mn+Ca)]逐渐降低,表明石榴石从核部到幔部的成分记录了温度逐渐升高的进变质过程;幔部到边部,Xprp= [Mg/(Mg+Fe+Mn+Ca)]略微降低,Xgrs= [Ca/(Mg+Fe+Mn+Ca)]明显升高,Xsps= [Mn/(Mg+Fe+Mn+Ca)]先升高后降低,表明石榴石边部成分受到了退变质作用改造,呈现扩散环带的特征。利用Thermocalc变质相平衡计算软件在MnNCKFMASHO体系下计算出含石榴石云母石英片岩的P-T、P-M(H2O)视剖面图,结合石榴石镁铝榴石等值线、钙铝榴石等值线及饱和水含量等值线限定出含石榴石云母石英片岩的峰期变质条件为约27×105 kPa,523/580℃,对应的峰期矿物组合为(g-Jd-Cr-Law(+Phn+q/Coe+H2O)。石榴石核部到幔部成分记录了主要的进变质演化,结合饱和水等值线的变化,判断进变质阶段为升温升压的冷俯冲过程,岩石经历了蓝片岩相至榴辉岩相的变质演化;P-M (H2O)视剖面图及饱和水等值线反映了岩石在减压中的流体行为,通过其变化特征可以确定岩石在峰期之后先经历近等温降压的早期退变质过程,晚期降温降压的退变轨迹则由石榴石边部成分所确定,此过程中,岩石发生了角闪岩相至绿帘角闪岩相变质,并在后期经历了绿片岩相变质叠加。近等温降压的退变质过程反映了快速抬升的构造运动过程,早期硬玉转变为钠长石可能发生在这个阶段。对比含石榴石云母石英片岩与榴辉岩的P-T轨迹,峰期变质温压及变质演化特征,提出含石榴石云母石英片岩曾经历过高压变质,结合野外相互伴生的地质关系,认为该片岩与榴辉岩经历了相同或者相似的俯冲折返过程。
关键词:  拉萨地体  松多超高压变质带  含石榴石云母石英片岩  相平衡模拟  变质演化
DOI:
分类号:
基金项目:国家自然科学基金项目(41202034)、中国地质调查局面上项目(12120115026801,12120114061501)及中国地质科学院地质研究所基本科研业务费(J1518)资助。
Phase equilibrium modeling for metamorphic evolution of garnet-bearing mica-quartz schist in Sumdo UHP metamorphic belt, Lhasa Block
CHEN Mei1,2, TIAN Zuo-lin1, ZHANG Cong1, YANG Jing-sui1, HUANG Jie1,21,2
1.State Key Laboratory for Continental Tectonics and Dynamics,Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;2. School of Earth Science and Mineral Resources,China University of Geosciences,Beijing 100083,China
Abstract:
Abstract: The garnet-bearing mica-quartz schist of Sumdo UHP belt in Lhasa block occurs as country rocks of eclogite, and is mainly composed of garnet, muscovite, albite, chlorite, quartz and minor rutile and sphene. Garnet displays an obvious compositional zonation where Xprp increases from the core to the mantle and then decreases in the rim, whereas Xsps decreases gradually from the core to mantle, with the trend of declining following rising in the rim, indicating that garnet composition profiles from core to mantle have preserved the prograde growth zoning and were partially reset during retrogression. The model system MnNCKFMASHO was chosen to calculate P-T and P-M(H2O) pseudosections of the garnet-bearing mica-quartz schist. Garnet isopleth thermobarometry involved plotting compositional isopleths of garnet as contours on a P-T pseudosection, with the combination of contours of saturated H2O content, thus obtaining estimated peak P-T conditions of 27 kbar, 523/580 ℃ and peak mineral assemblages of g-jd-cr-law (+phn +q/coe+H2O). The compositional profile of garnet from the core to the mantle and contouring of the H2O content saturated indicate that prograde metamorphic evolution represents a cold subduction stage with heating with the increasing pressure, and the rocks experienced blueschist-facies to eclogite-facies metamorphism during this stage. P-M(H2O) pseudosections and isopleth of saturated H2O content could be used to assess evolution of mineral assemblages in terms of changes in water content during decompression, which shows that garnet-bearing mica quartz schist experienced an early isothermal decompression process and was then followed by a cooling with decompression evolution during the late stage. Amphibolite-facies to epidote-amphibolite-facies metamorphism occurred during early stage and was followed by greenschist-facies metamorphism. The isothermal decompression of garnet-bearing mica-quartz schist probably represents a fast tectonic exhumation. Albite was likely to replace early jadeite at this stage. A comparison with the P-T path and contact relationship in the field of garnet-bearing mica-quartz schist and eclogite shows evidently that garnet-bearing mica-quartz schist and the eclogite it hosted experienced similar subduction and exhumation processes.
Key words:  Lhasa block  Sumdo UHP metamorphic belt  garnet-bearing mica quartz schist  phase equilibrium modeling  metamorphic evolution