Metamorphism And Geochronology Studies Of High-pressure Granulite And Amphibolite In Jiaobei Terrane,North China Craton

The Jiaobei terrane is located in the southern section of the Jiao-Liao-Ji Belt(JLJB),which is one of the most significant Precambrian cores in the Eastern Block of the North China Carton.The mafic granulite inclusions occur in TTG gneisses and are associated with high-pressure(HP)felsic granulites and amphibolites.They constitute an important part of Precambrian metamorphic basement.These TTG gneisses recorded important information on the peak metamorphic HP granulite stage and subsequent exhumation related stages.The P-T conditions of mafic granulites have been re-evaluated by combining petrographic observations,mineral chemistry data using phase equilibria modelling and thermobarometer techniques.Five metamorphic stages were derived from the mafic granulite,containing two high grade metamorphic stages of HP and ultrahigh-temperature(UHT)metamorphism.The early prograde pre-peak(M1)stage is characterized by mineral inclusions in garnet cores,such as the assemblage clinopyroxene+plagioclase±quartz±K-feldspar,which is stable at?9-10 kbar/740-770?.Based on the observed mineral assemblage of garnet mantle+clinopyroxene+rutile+plagioclase+quartz and matrix-plagioclase,the peak pressure(M2)conditions constrained were 15-17.6 kbar/907-1040?.In addition,conventional thermobarometers document maximum P-T estimates of 15-16.3 kbar/999-1006?.The post-peak(M3)stage characterized by symplectites of clinopyroxene+orthopyroxene+plagioclase+ilmenite was determined using CaM2 in clinopyroxene and XAn in plagioclase,formed by garnet decomposition reaction at 2.4-7.2kbar/960-1055?.The late growth of hornblende ± quartz ± biotite represents the isobaric cooling stage(M4),which is identified to be 2.2-6 kbar/821-867?,based on mineral compositions including CaM2 in clinopyroxene,Ti in hornblende and Ti in biotite.Late sub-greenschist facies stage(M5)represented by the mineral assemblage actinolite-prehnite-chlorite-epidote-albite-(calcite)-titanite is defined to have occurred at 0.51-1.57 kbar and 300-316?.The mafic granulites show a clockwise P-T path with maximum P-T conditions higher than those estimated in previous studies.Two metamorphic stages were preserved in the felsic granulite.The peak(M2)stage is featured by the assemblage garnet+antiperthite+quartz+ilmenite with P-T condition of 14-16.5 kbar/985-1077?,calculated by phase equilibrium modeling.Temperatures of 1041-1081? were estimated using two feldspar geothermometer.The post peak stage(M3),represented by the assemblage garnet+orthopyroxene+K-feldspar+biotite+plagioclase+quartz+ilmenite yields a P-T range of 7.8-9 kbar/889-902?.Petrological and mineralogical studies show that the amphibolite recorded three stages of metamorphism:the peak stage(M1)is characterized by the mineral assemblage hornblende+plagioclase+titanite.The temperature and pressure range calculated by pseudosection modeling is T=660-715? and P=6.5-7.1 kbar.The mineral assemblage at the post-peak stage(M2)is actinolite+epidote+plagioclase+chlorite+albite,suggesting temperature and pressure conditions of 537-630?/4.1-5.8 kbar.The mineral assemblage of a retrograde stage(M3)is albite+prehnite+chlorite+calcite,represented by prehnite-and calcite-veins,and indicating a late prehnite-pumpellyite facies metamorphism.Cathodoluminescence(CL)imaging,trace element and U-Pb dating of zircons from the mafic HP granulites in the JLJB recorded characteristics that indicate three Paleo-proterozoic metamorphic events.The peak pressure metamorphic event occurred at 2.0-1.9 Ga(207Pb/206Pb ages)and the decomposing process of ages at 1.86-1.84 Ga.These events were followed by a retrograde amphibolite facies metamorphic event related to the post-orogenic extension at the age of 1.76-1.74 Ga,representing the exhumation of the granulite to the upper crust level.From zircon CL images and respective U-Pb isotopic dating of felsic granulites,two metamorphic events were yielded,one of which represents a near peak stage at?1.88 Ga whereas a retrograde event occurred at 1.86-1.83 Ga.Amphibolite was studied using LA-ICP-MS U-Pb geochronology of zircon,combined with CL imaging.A total of 21 zircons investigated,characterized by a faint luminescence and a weakly oscillatory zoning document relatively low Th/U ratios(0.06-0.43).The morphology and internal structures of zircon suggest that their growth was associated with partial melting.They yield 206Pb/238Pb weighted mean ages of 1845± 23Ma(MSWD=0.35).After the magmatic formation of the mafic rock,it has experienced granulite facies metamorphism,and subsequently an anataxic event occurred during the collisional convergence of the Jiao-Liao-Ji orogenic belt at?1.85Ga.Afterwards the investigated amphibolite experienced amphibolite facies metamorphism and two later episodes of retrograde metamorphism during the lift of the belt.The combination of results derived from the three types of rocks,mafic and felsic granulites and amphibolite,construct a complete clockwise P-T-t path,which is a characteristic of a typical collisional orogenic belt.The discovery of UHT metamorphism deepened the understanding of the formation and evolution of the entire JLJB.