滇西勐库退变质榴辉岩的P-T-t轨迹及地质意义

摘要

滇西双江县勐库地区的退变质榴辉岩经历了多期退变质作用的改造,早期的平衡共生矿物组合难觅踪迹.应用传统的石榴石-单斜辉石(GC)温度计、石榴石-单斜辉石-多硅白云母(GCP)压力计进行变质作用的PT条件估算存在许多不确定性.本文应用魏春景等(2009)依据MORB成分计算的PT视剖面图上多硅白云母、石榴石、绿辉石的成分随体系中PT条件的变化情况,估算了进变质的P=2.00～2.30GPa,T=420 ～ 460℃,相当于硬柱石蓝片岩相一硬柱石-蓝闪石榴辉岩相;峰期变质的P=3.35～4.46GPa,T=530 ～610℃,相当于硬柱石榴辉岩相;早期退变质的P=2.00～2.50GPa,T=470 ～ 540℃,相当于硬柱石-蓝闪石榴辉岩相;中期退变质的P=0.95 ～ 1.43GPa,T=700～750℃,相当于角闪石榴辉岩相-高压麻粒岩相.晚期退变质作用以出现大量的闪石类矿物为特征,可划分为3个阶段,并显示了持续的降温、降压过程.结合区域地壳演化进程,本文详细讨论了上述P-T-t轨迹的地质意义.%Early balanced paragenetic mineral assemblage in retrograded eclogites,which underwent more period remetamorphism,from Mengku area,Shuangjiang County of western Yunnan Province,is hardly found.It is uncertain to estimate pressure and temperature in early metamorphism by means of traditional garnet-clinopyroxene geothermometer and garnet-clinopyroxene-phengite geobarometer.In this article,the authors study compositions change of phengite,garnet and omphacite in the P-T pseudosection,which is calculated by Wei et al.(2009) based on MORB,with pressure and temperature change.By this way,the authors obtain P =2.00 ～ 2.30GPa and T =420 ～460℃ in the prograde metamorphism,P =3.35 ～4.46GPa and T =530 ～610℃ in the peak period metamorphism,P =2.00 ～ 2.50GPa and T =470 ～ 540℃ in the early period remetamorphism,P =0.95 ～ 1.43GPa and T =700 ～750℃ in the medium period remetamorphism,they are respectively equivalent to lawsonite blueschist facies to lawsonite-glaucophane eclogite facies,lawsonite eclogite facies,lawsonite-glaucophane eclogite facies and hornblend eclogite facies to high pressure granulite facies.Late period remetamorphism is characterized by a plenty of amphibolites.It can be divided three stages,and shows the continued cooling and pressure reduction process.Based on the research for regional crust's revolution,author discusses geological significances of the P-T-t path in detail.