高喜马拉雅中段基性捕虏体的变质作用及其构造意义

西藏南部聂拉木—樟木剖面出露的高喜马拉雅变质带主要由副变质片麻岩和花岗质片麻岩组成,其次为伟晶岩和淡色花岗侵入体,区域变质程度为角闪岩相。我们对其中的变质基性捕虏体进行详细的变质作用研究,内容包括变质矿物组合,矿物变质反应结构和变质作用的温度—压力条件分析。基性捕虏体中的石榴子石角闪片麻岩和斜长角闪片麻岩均保存了两期变质矿物组合。温度与压力计算结果表明,石榴子石角闪片麻岩早期变质阶段(M1)温度约为829 ℃,压力为7.3 kbar; 晚期(M2)变质温度为625 ℃,压力为4.3 kbar。斜长角闪片麻岩所经历的早期变质阶段(M1)温度约为776 ℃、压力约为10.6 kbar; 晚期(M2)变质温度超过692 ℃,压力为7.4 kbar。石榴子石角闪片麻岩和斜长角闪片麻岩捕虏体均记录了典型的顺时针P-T轨迹,表明高喜马拉雅变质带曾向北俯冲到下地壳深度,之后被抬升到地表剥蚀出露。变质基性捕虏体的研究说明高喜马拉雅结晶岩系经历过较高温度—压力的变质作用,支持了其沿着藏南拆离系和主中央逆冲断裂系向南挤出的大地构造模型。

Metamorphism of mafic enclaves in central greater Himalaya sequence and its tectonic implication

Greater Himalayan central gneiss mainly comprises of ampholite-face paragneiss and granite gneiss,with minor pegmatitic and leucogranitic intrusion.Detailed research on the mafic enclaves in the Greater Himalayan sequence has been conducted,which involves metamorphic mineral assemblage,reaction textures and P-T conditions.The enclaves,including garnet-amphibole gneiss and plagioclase-amphibole gneiss,which formed during early and latter decompression stages,respectively,have preserved different metamorphic mineral assemblages.Thermobarometric computation shows that the metamorphic conditions of the garnet-amphibole gneiss are 829 ℃ at 7.3 kbar for the early stage,and 625 ℃ at 4.3 kbar for the late stage,respectively.The metamorphic conditions of the plagioclase-amphibole gneiss are 776 ℃ at 10.6 kbar for the early stage,and >692 ℃ at 7.4 kbar for the late stage,respectively.These rocks have undergone metamorphism characterized by high temperature-presure and clockwise P-T paths,which indicates that Greater Himalayan metamorphic belt has been subducted northward into the lower crustal level and then was exhumed to the surface.This study can further support the tectonic model that Greater Himalayan Crystallization was extruded between South Tibetan Detachment System(STDS)and Main Central Thrust(MCT).