华北中部造山带西望山超高温基性麻粒岩的变质演化

Metamorphic evolution of ultrahigh-temperature mafic granulites from the Xiwangshan area, Trans-North China Orogen

华北中部造山带西望山地区基性麻粒岩的变质演化过程研究有助于揭示华北中部带前寒武纪变质基底的形成与演化历史。基于对宣化西望山地区基性麻粒岩的详细岩相学观察与矿物化学分析,识别出3期变质演化阶段:早期进变质阶段(M1)矿物组合为石榴石+单斜辉石+斜长石+金红石+石英;峰期变质阶段(M2)矿物组合为石榴石+单斜辉石+斜长石+角闪石+钛铁矿+熔体;晚期降压阶段(M3)矿物组合为石榴石+单斜辉石+角闪石+斜长石+钛铁矿+磁铁矿。对基性麻粒岩的相平衡模拟研究表明,峰期变质阶段(M2)的温压条件为940~1000℃,12.4~14 kbar;峰期后降压阶段的温压条件为660~760℃,5~6.2 kbar;构成一条降压冷却的顺时针P-T轨迹。本次研究表明西望山基性麻粒岩经历了高压-超高温变质作用。

An investigation of the metamorphic processes in the mafic granulites from the Xiwangshan area,Trans-North China Orogen(TNCO)provides new insights into the metamorphic evolution of the Precambrian basement of TNCO.According to petrographical observations and mineral analysis,three metamorphic stages were identified:the early prograde assemblage(M1)composed of garnet,clinopyrexene,plagioclase,rutile and quartz,the peak assemblage(M2)consisting of garnet,clinopyroxenec,hornblende,plagioclase,ilmenite and melt,and the post-peak decompression assemblage(M3)represented by garnet,clinopyroxene,hornblende,plagioclase,ilmenite and magnetite.The phase equilibrium modeling indicates that the temperature and pressure conditions of the peak metamorphic(M2)and the post-peak(M3)metamorphic stages of the mafic granulite are 940~1000℃,12.4~14 kbar,and 660~760℃,5~6.2 kbar,respectively.And that the M1-M2-M3 pressure-temperature path follows a clockwise P-T trajectory.The study suggests that the mafic granulites of the Xiwangshan area has experienced high-pressure ultrahigh-temperature metamorphism.