青藏高原拉萨地块松多蓝闪石榴辉岩的变质演化：相平衡及变质作用P-T轨迹  

Metamorphic evolution of glaucophane eclogites from Sumdo,Lhasa block of Tibetan Plateau: Phase equilibria and metamorphic P-T path

作　　者：杨现力[1,2,3] 张立飞[2] 赵志丹[1] 朱弟成[1] 

机构地区：[1]地质过程与矿产资源国家重点实验室、中国地质大学地球科学与资源学院,北京100083 [2]北京大学地球与空间科学学院、造山带与地壳演化教育部重点实验室,北京100871 [3]中铁工程设计咨询集团有限公司,北京100055

出　　处：《岩石学报》2014年第5期1505-1519,共15页Acta Petrologica Sinica

基　　金：国家973项目(2011CB403102);国家自然科学基金项目(41273044、41225006、41121062);中国地质调查局工作项目(1212011121260、1212011121066);111计划(B07011);教育部创新团队发展计划(IRT1083)联合资助

摘　　要：青藏高原拉萨地块松多蓝闪石榴辉岩主要矿物组合为石榴石、绿辉石、蓝闳石、绿帘石／斜黝帘石，及少量的金红石、石英、多硅白云母和普通角闪石。石榴石具有成分环带，从核到边Xpy，升高，Xgr降低，部分石榴石外边缘受退变质改造影响，形成富Xgr的成分带。利用NCKMnFMASHTO体系中的P-T视剖面图，结合石榴石边部最大Xpy，等值线和多硅白云母最大Si-含量值确定了松多榴辉岩的峰期变质条件为30±0．6kbar和610±6℃。石榴石核部到幔部成分环带记录的相对平缓的P-T轨迹反应了岩石早期经历了以加热升温为主、轻微加压的缓慢俯冲过程，地温梯度为7～8℃／km。石榴石幔部到边部成分环带，结合多硅白云母最大Si含量等值线模拟了以缓慢升温、快速增压为特征的P—T轨迹，反应了岩石由早期的缓慢俯冲进入到后期的快速俯冲阶段，地温梯度由7～8℃／km减小到5～6℃／km。峰期之后的榴辉岩经历了早期近等温减压的变质过程，以硬柱石和少量滑石的脱水反应生成蓝闪石和绿帘石（约22—23kbar）为主要特征。其后的晚期退变质阶段以硬柱石消失后局部成分域内由富余流体的消耗形成冻蓝闪石（约16kbar）以及蓝闪石和绿辉石边部发育后成合晶为特征（11～12kbar），石榴石边部的韭闪石冠状体和金红石边部生成的榍石退变边也大致发生在该阶段。榴辉岩近等温减压的变质过程可能代表了早期的构造快速抬升过程。松多榴辉岩带可能代表了青藏高原拉萨地块内一条新的大洋型高压一超高压变质带，大约266Ma的榴辉岩相变质时代说明在拉萨地块内部可能存在过一个二叠纪的古特提斯洋盆。The general mineral assemblage of glaucophane eclogite from Sumdo, Lhasa block of Tibetan Plateau is garnet ＋ omphacite ＋ glaucophane ＋ epidote/clinozoisite, with minor rutile, phengite, hornblende and quaretz. Garnet porphyroblasts show compositional zonation with Xpy increase and Xgx decease from the core to the rim. Part of garnet has an Xgr rich outer rim due to retrogression. We calculated the P-T pseudosection in the system of NCKMnFMASHTO, and estimated a peak metamorphic condition of 30 ± 0. 6kbar and 610 - 6℃ based on the isopleths of the maximum Xpy of garnet and maximum Si content of phengite. The moderate slope of the P-T path recorded by the composition of garnet from the core to the mantle indicate that the rock experienced heating with less pressure increasing in the early stage of slow subduction. The geothermal gradient is about 7 - 8℃/kin. However, the composition of garnet from the mantle to the rim, combined with the Si content of phengite give out a slow heating but fast increasing pressure P-T path, which means it went into a fast subduction stage, and the geothermal gradient decreased to 5 - 6℃/km. After peak metamorphism, the eelogite went into an early isothermal decompressional process, characterized by lawsonite ＋ minor talc dehydrated to produce glaucophane and epidote （ at about 22 - 23kbar）. In the late retrogression period, lawsonite completely disappear and barrosite formed due to the enrichment in fluids in some area （ about 16kbar）. Sympleetite generally occur at the rim of glaucophane and omphacite （about 11 - 12kbar）, and pargasite corona in the rim of garnet, titanite overgrowing rntile also formed at such stage. The isothermal decompression of eclogite probably represent a fast tectonic exhumation. Above all, Sumdo eelogite maybe represent a new oceanic HP-UHP metamorphic belt in Lhasa block of Tibetan Plateau, and the metamorphic age of the eclogite which is about 266Ma suggest that there might be a palaeo-ocean in Permian in the central part of Lh

关 键 词：榴辉岩 相平衡 P-T轨迹 拉萨地块 青藏高原 

分 类 号：P588.348[天文地球—岩石学]