恒山西段石榴石角闪岩和麻粒岩的变质作用、PT轨迹及构造意义

METAMORPHISM, PT PATHS AND TECTONIC SIGNIFICANCE OF GARNET AMPHIBOLITE AND GRANULITE FROM HENGSHAN,NORTH CHINA CRATON

恒山西段广泛发育石榴石角闪岩和麻粒岩包体，石榴石斑晶的生长环带都完好地保留，甚至晚期的麻粒岩相变质条件也未使其破坏，显示了快速的构造过程和有限的热松弛。石榴石核心保存了早期高压角闪岩相的矿物组合和矿物化学特征，变质作用条件为６００—６５０℃和大约１１ ＧＰａ。随后，石榴石麻粒岩的变质温度和压力继续上升，达到高压麻粒岩相，大约为８１０℃和１２５ ＧＰａ。两类岩石变质作用的晚期都经历了特殊的减压升温过程，显示典型的顺时针ＰＴ 轨迹。因此可以推断，恒山西段在太古宙末或早元古代很可能经历了某种形式的大陆俯冲或碰撞过程，并且随后发育逆冲和一系列壳内变形剪切等。

Garnet amphibolites and granulites are widely distributed as enclaves within migmatitic felsic gneisses and mylonites in west Hengshan,North China craton.Garnet growth zones are well preserved in both amphibolites and granulites.In amphibolites,garnets show Ca,Mn content decrease and Fe content increase from core to near rim.In the granulites,garnet zones are more complicated than in amphibolite:from core outward,Mg increase markedly,Ca increase little,Mn and Fe decrease;from mantle approaching rim,Mg and Ca decrease,Fe increase(Fig.2).The two contrast garnet zoning trends in granulites are correspond respectively to cpx and pl intergrowth outside garnet and opx-amp-pl-cpx symplectite intergrowth arround garnet.The garnet growth zoning has not been reconstructed by the subsequent progress metamorphism,even up to granulite facies.That requires rapid metamorphic and tectonic process. Mineral assemblage of early metamorphic stage has been recognized in the core of garnet porphyroblasts in amplhibolites and granulites,that is composed of grt(core)-pl-rut-ilm-qtz±amp.The early high-P amphibolite facies metamorphic conditions of 600—650℃ and ～1.1 GPa have been estimated from geothermobarometers of grt-pl-rut/ilm-qtz assemblage.After the early episode,both the metamorphic temperature and pressure of garnet granulite kept on increasing till high-pressure granulite facies condition of about 810℃ and 1.25 GPa,which has been calculated from compositions of garnet mantle and cpx-pl intergrowth outside garnet.The late phase of metamorphism shown pressure decreasing and temperature increasing,up to 710℃—1.0 GPa and 880℃—1 05 GPa for garnet amphibolite and granulite respectively,which has been estimated from assemblages arround garnet porphyroblasts.Then,typical clockwise metamorphic PT paths of garnet amphibolite and granulite have been constructed from these calculations.Both rocks show a decompressional process with increasing temperature in the late stage of metamorphism. In conclusion,west Hengshan probably subjected to some type of continental subduction or collision and then thrust in the end of Archaean or early Proterozoic.