安多地区高压麻粒岩的发现及其意义

Identification and significance of high-pressure granulite in Anduo area,Tibetan Plateau

本文以班公湖-怒江板块缝合带所夹的聂荣微陆块中新近发现的高压麻粒岩为研究对象,通过岩石学、矿物学以及变质作用等的较详细研究,确定其早期矿物组合主要为石榴石、单斜辉石和斜长石等,后期发生了较强烈的退变质作用,出现大量角闪石和斜长石,并发育典型的"白眼圈"结构。利用相关的地质温压计估算其麻粒岩相变质峰期T=790～840℃,P=11～15kbar,角闪岩相退变质阶段T=700～730℃,P=4.6kbar。它可能是燕山期班公湖-怒江板块缝合带闭合过程的产物,也可能反映更早期的碰撞事件,进一步的研究工作将对班公湖-怒江板块缝合带的构造演化或者重新认识青藏高原特别是冈底斯地块的构造格局具有重要意义。

The high-pressure granulite(HP-granulite)newly found is hosted in Nierong microcontinent which is located in Bangong Co-Nujiang plate suture zone.Through the intensive study on petrology,mineralogy and metamorphism,we attribute that the HP-granulite was made up by the early mineral assemblages of garnet(Grt)+clinopyroxene(Cpx)+plagioclase(Pl)and the massive later retrograde mineral assemblages amphibole(Hb)+plagioclase(Pl).Garnets in high-pressure granulites are surrounded by plagioclase which forms the corona.Thermobarometric estimates based on equilibrium texture and mineral chemistry indicate that the early mineral assemblages formed at T=790~840℃,P=11~15kbar,and the later retrograde assemblages experienced near-isothermal decomposition to T=700~730℃,P=4.6kbar.It may be the product of the Yanshanian closure of Bangong Co-Nujiang plate suture zone or early collision event.Further study on the HP-granulite is of great significance to recognize the tectonic evolution of Bangong Co-Nujiang plate suture zone or re-recognize the tectonic framework of Tibetan Plateau,especially the Gangdese Terrane.