| High pressure mafic granulite carries plenty of information of the mid-lower crust,so it has been one of the hot topics in metamorphic geology.Our study mainly focuses on the high-pressure mafic granulite Womakeng,Chicheng,North Hebei province.We discover orthopyroxene in thin sections,which representing middle pressure granulite facies metamorphism.So it is necessary to redetermine the mineral assemblages and metamorphic stages,and to obtain more accurate temperature,pressure conditions and P-T-t path by means of phase equilibrium.There are three stage metamorphisms in working area,which are high pressure,middle pressure granulite facies and amphibolite facies in turn and the mineral assemblages are garnet+clinopyroxene+plagioclase+quartz,garnet+clinopyroxene+orthopyroxene+plagioclase+quartz+ilmenite and hornblende+plagioclase respectively.According to the results of mineral chemistry,the garnet in the samples is mainly almandine(XAlm=58-62).The garnet whether larger or smaller in size are relatively homogeneous,indicating that the garnet is in a state of equilibrium.Clinopyroxene is mainly diopside,and the plagioclase varies between andesine and oligoclase?An=22-31?.Based on the new mineral activity model fit for mafic rocks,we gain the metamorphic conditions by THERMOLCAL 3.46 as following:11.8±0.2kbar,780±20?;7.5±0.5kbar,750±50?;6.5±0.5kbar,550±50?,thus,we get a new clockwise P-T-t path with middle pressure granulite facies indexing by orthopyrexene.This study show that the high pressure mafic granulite first underwent isothermal decompression to middle pressure granulite facies metamorphism,and then get to amphibolite facies by nearly isobaric cooling.Which reflects the process of collisional orogenesis,exhumation and uplift,and is obviously different from the previous phase of the P-T-t path of direct retrograde to amphibolite facies from high pressure granulite.It reflects the progress of rapid uplift firstly,and then isobaric cooling. |
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