恒山灰色片麻岩和高压麻粒岩包体及其地质意义

GREY TONALITIC GNEISS AND HIGH-PRESSURE GRANULITE INCLUSIONS IN HENGSHAN, SHANXI PROVINCE, AND THEIR GEOLOGICAL SIGNIFICANCE

本文将恒山群解体为以灰色片麻岩为主的中太古界和以表壳岩为主的晚太古界两部分,着重讨论下部灰色片麻岩及其中的包体,特别是高压麻粒岩残渣包体。此包体类似于榴辉岩,形成温压约为800～900℃,1300～1600MPa。发育精美的减压结构,表明高压麻粒岩转变到二辉麻粒岩,再转变为斜长角闪岩的过程。文中还讨论了灰色片麻岩及高压麻粒岩包体的来源,这将为研究太古代板块构造提供有用的线索。

This study enables us to divide the Hengshan Group into middle Archaean Dongzhuang supercrustal rocks intruded by Tuling-Linchang grey gneiss and later Archaean Qianzhuangwang and Zhujiafang supercrustal rocks intruded by Yixingzhai quartz diorite. The Tuling-Linchang grey gneiss is extremely similar to the typical Archaean grey gneisses in other places of the world in mineralogical composition, major, trace and rare-earth elements. Some high pressure granulite inclusions were found within the grey gneiss.They contain high grossular(24～29％) garnet and high Jd/Ts clinopyroxene, which were formed at about 800～900℃,1300～1600MPa. Exquisite symplectic intergrowth of hyperstheneplagioclase or hornblende-plagioclase around garnet shows a process of increase and decrease in pressure. The high-pressure granulite inclusions have been proved by REE petrogenetic modelling as a residue during partial meltting and formation of the grey gneiss.They have not been found in nearby surrounding racks.But along the southern margin of Chongli high-grade greenstone belt at 180km northeast away from Hengshan, we found an elongated high-pressure granulite zone with an age of 2.8Ga, just as the age of Tuling-Lingchang grey gneiss. So it is reasonable to suggest that, it might be the trace of the middle Archaean subduction zone similar to that of the recent plate tectonic model.