拉萨地块吉朗榴辉岩的岩石学研究及其对古特提斯洋壳俯冲折返过程的限定  

Petrological studies of Jilang eclogite in the Lhasa terrane and its constraint on the subduction and exhumation processes of the Paleo-Tethys oceanic crust

作　　者：申婷婷 张聪[1,2] 田作林 李杨[2] 杨经绥 SHEN Ting-ting;ZHANG Cong;TIAN Zuo-lin;LI Yang;YANG Jing-sui(Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources,Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China;Institute of Earth Science and Engineering,Shandong University of Science and Technology,Qingdao 266590,China)

机构地区：[1]自然资源部深地动力学重点实验室中国地质科学院地质研究所,北京100037 [2]山东科技大学地球科学与工程学院,山东青岛266590

出　　处：《岩石矿物学杂志》2018年第6期917-932,共16页Acta Petrologica et Mineralogica

基　　金：国家自然科学基金项目(41572051;41502039;41641015);中国博士后基金项目(2016T90011);中国地质科学院基本科研业务费(J1701;YYWF201702);中国地质调查局地调项目(DD20160023-01;DD20160022-01)~~

摘　　要：拉萨地块东部松多(超)高压榴辉岩记录了古特提斯洋俯冲及折返过程。松多榴辉岩带已发现松多、新达多、白朗和吉朗4个榴辉岩出露区,它们的峰期温压条件及变质p-T轨迹的研究对揭示拉萨地块古特提斯时期的俯冲及折返过程有重要意义。松多榴辉岩带东段吉朗榴辉岩的主要矿物为石榴子石、绿辉石、多硅白云母、角闪石、金红石、绿帘石、石英以及退变形成的后成合晶结构(透辉石+角闪石+斜长石)和少量的黑云母。石榴子石具有含丰富矿物包裹体的"脏"核和极少包裹体的"净"边,具有典型的进变质成分环带特征,从核部到边部镁铝榴石组分升高,锰铝榴石和钙铝榴石组分降低。石榴子石边部发育窄的角闪石+斜长石(An=28)组成的冠状体,表明石榴子石边部发生了后期角闪岩相退变质作用。通过变质相平衡模拟计算得到石榴子石以及多硅白云母记录的峰期温压条件为563℃、2. 4 GPa。结合岩相学特征,确定吉朗榴辉岩经历了4期变质演化阶段:(1)进变质阶段以石榴子石核部及其包裹体为代表性矿物组合;(2)峰期变质阶段矿物组合为石榴子石边部、绿辉石、多硅白云母、蓝闪石、硬柱石、金红石和石英;(3)早期退变质阶段以硬柱石分解产生绿帘石为特征;(4)晚期退变质阶段以绿辉石发育后成合晶和石榴子石生长冠状体为特征。认为吉朗榴辉岩为典型的低温高压榴辉岩,经历了顺时针p-T演化轨迹,折返过程为近等温降压过程。与松多带内其他(超)高压岩石相比,吉朗榴辉岩峰期温压条件较低,其围岩为变石英岩,区别于区内其他(超)高压榴辉岩的石榴子石白云母片岩及蛇纹岩围岩。推测吉朗榴辉岩来自于俯冲带浅部,由俯冲隧道中低密度沉积物裹挟折返。The （ultra-） high pressure eclogites from Sumdo area in eastern Lhasa terrane recorded the subduction and exhumation process of the Paleo-Tethys oceanic crust. Previous studies reported four main outcrops in Sumdoarea, i.e., Sumdo, Xindaduo, Bailang and Jilang. Studying the peak metamorphic conditions and p-T path of Sumdo eclogite is of great significance for revealing the subduction and exhumation mechanism of Paleo-Tethys Ocean. In this paper, the authors chose Jilang eclogite for a case study, which has a mineral assemblage of garnet, omphacite, phengite, hornblende, rutile, epidote, quartz and symplectite （ diopside ＋ amphibole ＋ plagioclase）, and minor biotite. Garnet has a ＂dirty＂ core with abundant mineral inclusions and a ＂clear＂ rim with less mineral inclusions, showing typical growth zoning. From the core to the rim, Prp content in garnet increases while Grs con- tent decreases, and the rim of garnets is wrapped by pargasite ＋ plagioclase （ An = 28） corona, showing an amphib- olite facies superimposition during retrogression, p-T pseudosection calculated with Domino contoured with isopleths of grossular and pyrope values in garnet and Si content in phengite constrained peak p-T conditions of Jilang eclogite as 563℃, 2.4 Gpa. Combined with petrographical observation, the authors recognized four stages of metamor- phism： prograde metamorphism represented by the core of garnet and mineral inclusions therein;① peak meta- morphism represented by the rim of garnet, omphacite, phengite, glaucophane, lawsonite, rutile and quartz; ② early stage retrograde metamorphism characterized by decomposition of lawsonite to epidote; ③ late stage retrograde metamorphism characterized by symplectites surrounding omphacite and cornona rimmed garnet. Jilang eclogite shows a clockwise p-T path, near isothermal decompression during exhumation, and is recorded as a high pressure, low temperature elcogite. Different from eclogites in other areas which are hosted by garnet-bearing mica schists

关 键 词：拉萨地块 榴辉岩 变质相平衡 古特提斯洋 洋壳俯冲 

分 类 号：P588.34[天文地球—岩石学] P581[天文地球—地质学]