Two types of eclogite were exposed in the Xindaduo area from the western section of Sumdo eclogite-bearing metamorphic belt,located in the central part of Lhasa Terrane,which are glaucophane-bearing eclogite and bimineralic eclogite.The general mineral assemblage of the glaucophane-bearing eclogite is garnet + omphacite + glaucophane + phengite,with minor epidote,hornblende,quartz and rutile.Garnet in the glaucophane-bearing eclogite shows no compositional zonation.Glaucophane in the matrix were mostly retrograded into barroisite,rimmed by hornblende + quartz symplectites.The mineral assemblage of the bimineralic eclogite is garnet + omphacite + quartz,with minor epidote,hornblende,rutile,ilmenite and sphene.Garnet porphyroblast shows typical growth zoning,with Xpy and X~ increasing from core to the inner rim,and then decreasing to the outer rim,coupled with the variation of the XAlm· Besides,pargasite + albite corona formed at the rim of garnet,showing an amphibolite facies overprintation during retrogradation.In order to constrain the metamorphic conditions of Xindaduo eclogites,two different thermodynamic techniques were applied:traditional geothermobarometer and P-T pseudosections calculated with Domino.The estimated peak P-T conditions of glaucophane-bearing eclogite are 615 ±5℃,33 ±0.5kbar.The peak temperature of the bimineralic eclogite is estimated to be 630 ± 10℃ at P ＜27kbar.In addition,the results of phase equilibrium simulation show that:(1) The glaucophane-bearing eclogite underwent an initially isothermal decompression process,characterized by lawsonite + talc dehydration to produce glaucophane and epidote.In the initial-late retrogression period,lawsonite consumed to produce internal fluid and barroisite and then formed hornblende + quartz symplectites rimming glaucophane.Pargasite + albite corona at the rim of garnet and sphene overgrowing rutile also occurred at this stage.(2) The bimineralic eclogite records the processes from the prograde metamorphic stage to the peak metamorphic stage,and to the final retrograde metamorphic stage by garnet composition and mineral assemblages.Petrological studies of two types of eclogite in the Xindaduo area,combined with the previous data from literature of the Sumdo region,show that the eclogites inside the Lhasa terrane originated from typical oceanic subduction zone,further prove the existence of the Paleo-Tethys Ocean between south and north Lhasa terrane.The coexistence of different types of eclogites in the same area was probably caused by the complex tectonic mechanism in the orogenic belt.%位于拉萨地块中部松多榴辉岩带西端的新达多地区出露两种类型榴辉岩:含蓝闪石榴辉岩和双矿物榴辉岩.含蓝闪石榴辉岩主要矿物组合为石榴石、绿辉石、蓝闪石、多硅白云母及少量的绿帘石、角闪石、石英、金红石.石榴石不具有成分环带结构,蓝闪石存在于基质中,边部大多退变为冻蓝闪石并普遍发育有角闪石和石英的后成合晶.双矿物榴辉岩的主要矿物组合为石榴石、绿辉石、石英及少量的绿帘石、角闪石、金红石、钛铁矿、榍石.石榴石具有典型的进变质环带特征,从核部到边部镁铝榴石和钙铝榴石组分先升高后降低,铁铝榴石组分变化与之耦合,石榴石边部发育角闪石和钠长石的冠状体,推断石榴石记录了进变质的生长过程后又受到了退变质改造.结合传统温压计和变质相平衡模拟两种温压计算方法对榴辉岩的峰期变质条件进行限定,得到含蓝闪石榴辉岩的峰期温压条件为:615±5℃,33±0.5kbar;双矿物榴辉岩的峰期变质温度为630±1O℃,压力不超过27kbar.变质相平衡模拟计算结果显示:(1)含蓝闪石榴辉岩经历了退变初期近等温减压过程,这一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为主要特征;中晚期退变质阶段以大范围硬柱石消失后局部富余流体的消耗在蓝闪石边部形成冻蓝闪石以及蓝闪石边部发育后成合晶为特征,部分石榴石边部的韭闪石冠状体大致也发生在这一过程;(2)双矿物榴辉岩则记录了从进变质生长阶段到峰期变质阶段,最后再到退变质演化阶段的完整变质过程.结合前人对松多榴辉岩的工作,对新达多地区新近发现的两类榴辉岩的岩石学研究表明:拉萨地块内部的榴辉岩为典型的大洋俯冲带产物,代表了古特提斯洋盆的存在.俯冲折返过程中复杂的构造机制使得不同类型榴辉岩在同一地区出露共生.
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