Petrology,Metamorphic P-T Paths and Zircon U-Pb Ages for Paleoproterozoic Mafic Granulites from Xuanhua,North China Craton  

作　　者：Guidong Wei Fanmei Kong Hao Liu Xiaoman Wang Yancheng Zhang Xiaohan Liu 

机构地区：[1]Shandong Key Laboratory of Depositional Mineralization and Sedimentary Minerals,Shandong University of Science and Technology

出　　处：《Journal of Earth Science》2019年第6期1197-1214,共18页地球科学学刊（英文版）

基　　金：supported by the National Natural Science Foundation of China (No. 41761144061);the Shandong Provincial Natural Science Foundation (No. ZR2016DM04);the University Students Innovation Program of SDUST (No. 2015TDJH101)

摘　　要：The studied mafic granulites are located at Xiwangshan,Xuanhua region in the north of the Trans-North China Orogen(TNCO),occurring as lens within tonalite-trondhjemite-granodiorite(TTG)gneisses in the eastern part of the Xiwangshan area.The rocks contain the representative granulite-facies minerals such as garnet,clinopyroxene,orthopyroxene,plagioclase,amphibolite,rutile and quartz,and also well-developed melt pseudomorph and antiperthite.Although the prograde metamorphic stage(M1)cannot be retrieved due to rare preservation of pre-peak-stage mineral associations,three distinct mineral assemblages that formed in different metamorphic stages can be identified,based on petrography and mineralogy characteristics.The peak stage(M2)is characterized by Grt2+Cpx2+Amp2+Pl2+Rt+melt pseudomorphs,and a post-peak decompression stage(M3)contains a mineral assemblage of Grt3+Opx3+Cpx3+Amp3+Pl3,while a later-retrogression stage(M4)is featured by coronas of Amp4+Pl4 surrounding garnet.By calculating metamorphic P-T conditions using THERMOCALC,stage M2 was constrained to be 13.2–14.8 kbar and 1050–1080℃,and M3 recorded P-T conditions of 5.7–7.3 kbar and 825–875℃,while M4 yielded P of^5 kbar and T of^660℃,consistent with amphibolite facies metamorphism.Taking into account of all these petrological data,we propose that the mafic granulite experienced a high-pressure(HP)and ultra-high temperature(UHT)granulite-facies metamorphism during the peak metamorphism,which was accompanied with a clockwise P-T path.U-Pb dating of metamorphic zircons in the granulites yields two groups of ages at 1853±14 and 1744±44 Ma,respectively.We suggest that the older age corresponded to the HP-UHT metamorphism,while the younger age represented an retrograde metamorphic event during cooling.The studied mafic granulites are located at Xiwangshan,Xuanhua region in the north of the Trans-North China Orogen（TNCO）,occurring as lens within tonalite-trondhjemite-granodiorite（TTG） gneisses in the eastern part of the Xiwangshan area. The rocks contain the representative granulite-facies minerals such as garnet,clinopyroxene,orthopyroxene,plagioclase,amphibolite,rutile and quartz,and also well-developed melt pseudomorph and antiperthite. Although the prograde metamorphic stage（M1） cannot be retrieved due to rare preservation of pre-peak-stage mineral associations,three distinct mineral assemblages that formed in different metamorphic stages can be identified,based on petrography and mineralogy characteristics. The peak stage（M2） is characterized by Grt2+Cpx2+Amp2+Pl2+Rt+melt pseudomorphs,and a post-peak decompression stage（M3） contains a mineral assemblage of Grt3+Opx3+ Cpx3+Amp3+Pl3,while a later-retrogression stage（M4） is featured by coronas of Amp4+Pl4 surrounding garnet. By calculating metamorphic P-T conditions using THERMOCALC,stage M2 was constrained to be 13.2–14.8 kbar and 1 050–1 080 ℃,and M3 recorded P-T conditions of 5.7–7.3 kbar and 825–875 ℃,while M4 yielded P of ～5 kbar and T of ～660 ℃,consistent with amphibolite facies metamorphism. Taking into account of all these petrological data,we propose that the mafic granulite experienced a high-pressure（HP） and ultra-high temperature（UHT） granulite-facies metamorphism during the peak metamorphism,which was accompanied with a clockwise P-T path. U-Pb dating of metamorphic zircons in the granulites yields two groups of ages at 1 853±14 and 1 744±44 Ma,respectively. We suggest that the older age corresponded to the HP-UHT metamorphism,while the younger age represented an retrograde metamorphic event during cooling.

关 键 词：ultrahigh-temperature metamorphism phase equilibrium modeling MAFIC GRANULITE Trans-North China OROGEN North China CRATON 

分 类 号：P58[天文地球—岩石学]