东喜马拉雅地区高压麻粒岩岩石学研究及构造意义

将该区内的高喜马拉雅结晶岩划分为南部的角闪岩相岩石和北部的中低压麻粒岩相岩石,后者沿那木拉逆冲断层向南推覆于前者之上。高压麻粒岩相岩石仅以残余产出于后者,主要包括石榴石蓝晶石片麻岩和石榴石透辉石岩。前者的峰期矿物组合为石榴石+蓝晶石+三元长石+石英+金红石;后者的峰期组合为石榴石(铁铝榴石10_±钙铝榴石_>80)+透辉石+石英+方柱石+榍石(Al₂O₃为4%-4.5%).变质温压估计结果表明高压麻粒岩相岩石形成于大约1.7-1.8GPa,890℃,然后经历了近等温降压变质作用至0.5±0.1GPa,850±50℃。它们的原岩可能是大理岩及泥质岩。这表明在区内曾存在一高压麻粒岩带,那木拉冲断层可能是高喜马拉雅结晶岩内的一条重要的构造界线。

PETROLOGY OF HIGH PRESSURE GRANULITES FROM EASTERN HIMALAYA: IMPLICATIONS TO TECTONIC SIGNIFICANCE

The eastern Himalaya consists of Gangdise unit,Yarlung Zangbo unit,and Himalayan unit,each fault bounded.The Himalayan unit,a northernmost exposed part of Indian plate,mainly contains amphibolite facies rocks in the south and low-to-medium-pressure granulite facies rocks in the north.A noticeable thrust fault system in this unit is a series of moderately north dipping structures called as Namula Thrust that displaced the granulitic complex southwards over the amphibolitic complex.The high-pressure granulites only occur as relics in the granulite facies rocks and contain garnet-kyanite gneiss and garnet diopsidite.The peak stage assemblage of the garnet-kyanite gneiss includes garnet (core part)+kyanite+ternary feldspar+quartz+rutile.Spinel+albite+biotite and spinel+cordierite±orthopyroxene that are coronas around sillimanite and garnet are the retrograde products of this peak assemblage.Another peak assemblage includes Ca-rich garnet (CaO:32%-34%,Alm 10_± Grs >₈₀) and diopside (CaO: 22%-24%),meionite,quartz,and accessory Al bearing titanite (Al₂O₃: 4%-4.5%).Partial or complete breakdown of the Ca-rich garnets during post-peak metamorphism produced pseudomorphs and coronas consisting of fine- grained symplectic intergrowths of hedenbergite and anorthite.Thermobarometric estimates indicate the peak p≈1.7-1.8GPa, t≈890℃ and the retrograde assemblages experienced near- isothermal decompression to p=0.5±0.1GPa, t=850±50℃.The whole-rock compositions indicate that marble and pelite are candidates for the protolithes of these HP granulites.These facts suggest that:(1)sediments were transported to deep crust,equlibrated in a high-pressure granulite facies,and were emplaced into the upper crust quickly.During the fast rise of these rocks,the earlier high-pressure assemblages had been overprinted by the later low-to medium-pressure assemblages;(2)the Namula Thrust is an important tectonic boundary in this area.