作　　者： (田作林）; (康东艳）; (穆虹辰）;

机构地区： 中国地质科学院地质研究所,北京100037

出　　处： 《岩石学报》 2017年第8期2467-2478,共12页

摘　　要： 东喜马拉雅构造结南迦巴瓦杂岩由多种类型的高级变质岩组成,包括片岩、片麻岩、大理岩、石榴角闪岩和基性麻粒岩。石榴角闪岩呈透镜状产出在片麻岩中,可见不连续分布的规模不等的浅色体。石榴角闪岩由石榴石、角闪石、黑云母、斜长石、石英、金红石、钛铁矿和榍石组成,石榴石变斑晶可见由浅色矿物组成的"白眼圈"。岩相学、矿物化学和相平衡模拟表明石榴角闪岩经历了一条顺时针型的P-T演化轨迹,可划分为两个阶段:(Ⅰ)升温升压的进变质阶段,由石榴石和斜长石斑晶记录,峰期矿物组合为石榴石+角闪石+黑云母+斜长石+石英+金红石+钛铁矿。运用石榴石变斑晶边部和斜长石变斑晶边部成分在视剖面图上的投点确定出峰期温压条件为~11.5kbar、790℃,达到了高压麻粒岩相条件,并经历了部分熔融,产生至少9%的熔体;(Ⅱ)降温降压的退变质阶段,由石榴石边部"白眼圈"冠状体记录。运用平均温压法计算冠状体中黑云母+斜长石+角闪石+石榴石组合的形成温压条件为~7kbar、~750℃。该阶段金红石消失,熔体结晶,并与早期矿物发生回反应。在石榴石角闪岩的锆石中获得了从29.2Ma到10.2Ma的连续变质年龄。由于锆石通常在熔体结晶过程中生长,因此确定该组年龄代表石榴石角闪岩退变质年龄。本文和以前的研究结果表明,南迦巴瓦杂岩中的高温和中温麻粒岩相亚单元具有相似的降温降压P-T轨迹,但高温单元具有较高的变质压力条件,表明其俯冲到了更大的深度。 In the Eastern Himalayan Syntaxis,the Namche Barwa Complex consists of various types of high grade metamorphic rocks,including schists,gneiss,marble,garnet amphibolite and basic granulite. Garnet amphibolites occur as lens in gneiss and have various scales of discontinuous leucosomes. Garnet amphibolites are composed of garnet,amphibole,biotite,plagioclase,quartz,rutile,ilmenite and titanite. The garnet porphyroblasts are surrounded by the＂white corona＂consisting of light-coloured minerals. The petrographic observation,mineral chemistry and phase equilibria indicate that the garnet amphibolites have experienced a clockwise PT path which is subdivided into two stages. Stage Ⅰ is the prograde stage with increase in pressure and temperature,which was recorded by garnet and plagioclase porphyroblasts. The peak mineral assemblage was garnet ＋ amphibole ＋ biotite ＋ plagioclase ＋ quartz＋ rutile ＋ ilmenite. The peak P-T condition was estimated to be ~ 11. 5kbar and 790℃ base on the compositions of garnet rim and plagioclase rim. This indicates that the garnet amphibolites have experienced high-pressure granulite facies metamorphism and partial melting,generating about 9% of melt at least. Stage Ⅱ is the retrograde stage with decrease in pressure and temperature,which was recorded by the ＂white corona＂around garnet rim. On the basis of the average PT method,the P-T condition of biotite ＋ plagioclase ＋amphibole ＋ garnet in the corona was calculated to be ~ 7kbar and ~ 750℃. Rutile became unstable during this stage as well as the melt crystallized and reacted with the early minerals. The ages of metamorphic zircon in the garnet amphibolite are continuous from29. 2Ma to 10. 2Ma. Because the growth of zircon is related to the crystallization of melt in granulites,we prefer to interpret the age range to represent the cooling process during the retrograde stage. This study,together with previous investigations,shows that the high-and medium-temperature granulite sub-units of the Namche Barwa Co

关 键 词： 石榴角闪岩 轨迹 相平衡 锆石 定年 东喜马拉雅构造结