藏南吉隆地区大喜马拉雅结晶杂岩的新生代变质作用和构造意义

摘要

大喜马拉雅结晶杂岩位于喜马拉雅造山带的核部,其变质作用的研究是揭示碰撞造山作用和动力学的关键.本文对喜马拉雅造山带中部吉隆地区的大喜马拉雅结晶杂岩中的片麻岩和片岩进行了岩石学和锆石U-Pb年代学的研究,表明这些高级变质岩经历了高角闪岩相的变质作用和部分熔融,以及近等温降压的退变质过程.片麻岩和片岩的峰期变质矿物组合为斜长石+钾长石+黑云母+白云母+石英±石榴子石±钛铁矿和石榴子石+蓝晶石+斜长石+黑云母+白云母+钛铁矿+石英,晚期退变质矿物组合为斜长石+钾长石+黑云母+白云母+夕线石+石英±石榴子石和石榴子石+夕线石+斜长石+黑云母+白云母+钛铁矿+石英.相平衡模拟研究表明,岩石的峰期变质条件为685～ 755℃和9.5～ 12kbar,退变质作用条件为675 ～ 685℃和6～ 7.4 kbar.锆石U-Pb年代学表明,高级变质岩的部分熔融时间为38 ～ 16Ma,熔体结晶时间为17 ～ 15Ma.本次研究表明,大喜马拉雅结晶杂岩中、上部并没有普遍经历高压麻粒岩相变质作用,部分地区包括吉隆地区经历的是高角闪岩相变质作用.此外,本文在吉隆地区高级变质的大喜马拉雅结晶杂岩中识别出较低级变质的特提斯喜马拉雅岩系单元.结合已发表的藏南拆离断裂的主要活动时间,本文认为构造楔模型更适用于研究区喜马拉雅造山过程的解释.%The Greater Himalayan Crystalline Complex located at the core of the Himalayan orogenic belt is key to reveal the collisional orogenesis and dynamics.Here,we present the petrology and zircon U-Pb chronology of the gneisses and schists from the Gyirong area,central Himalayan orogen.The results show that these high-grade metamorphic rocks have experienced a peakmetamorphism of the upper-amphibolite facies and partial melting,and then a retrograde process of isothermal decompression.The peak-metamorphic mineral assemblages are plagioclase + K-feldspar + biotite + muscovite + quartz ± garnet ± ilmenite and garnet +kyanite + plagioclase + biotite + muscovite + ilmenite + quartz,and the retrograde mineral assemblages are plagioclase + K-feldspar +biotite + muscovite + sillimanite + quartz ± garnet and garnet + sillimanite + plagioclase + biotite + muscovite + ilmenite + quartz.Phase equilibria modeling shows that these high-grade metamorphic rocks have underwent peak metamorphic conditions of 685 ～ 755℃ and 9.5 ～ 12kbar,retrograde metamorphic conditions of 675 ～ 685℃ and 6 ～ 7.4kbar.Zircon U-Pb chronology indicates that the partial melting occurred at ca.38 ～ 16Ma,and melt crystallization at ca.17 ～ 15Ma.The present study elucidates that the middle-upper section of the Greater Himalayan Crystalline Complex haven't generally experienced high-pressure granulite facies metamorphism,but rather the upper-amphibolite facies metamorphism in some areas including the Gyirong area.A lower metamorphic unit of the Tethyan Himalayan Sequence is identified within the Greater Himalayan Crystalline Complex from the Gyirong area.Combined with the published motion time of the South Tibet Detachment,this study suggests that the tectonic wedge model is more suitable for the interpretation of the Himalayan orogeny in the study area.