Triassic Deformation And Metamorphism Of Jiaobei Massif And Its Geological Significance

Jiaobei massif is located in the eastern part of North China Craton,mainly composed of the Paleoproterozoic Fenzishan group and Jingshan group.Previous geochronology study for this area was mainly focused on zircon.The metamorphism and deformation age was considered as Paleoproterozoic,because most geochronology data published was around 1.8 ? 1.9Ga.Zircon,Monazite and Rutile were separated from 2 mica schist samples and 5 garnet mica schist samples,which were collected from Jingshan group and Fenzishan group,were analyzed for U-Pb isotopic age.The zircon and monazite U-Pb ages were Paleoproterozoic(1869±72?1869±14Ma),and the Triassic U-Pb ages,were obtained from monazite(217.7±3.7?232.6±1.1Ma)and rutile(215.1±4.2?217.8±6.3Ma).It reveals that Fenzishan group and Jingshan group not only experienced the Proterozoic metamorphism,but also overprinted by Triassic deformation and metamorphism.The temperature range of the metamorphic process obtained by the rutile thermometry is from 658 to 680°C.It means that the Jiaobei massif involved in the Triassic Sulu-Dabie HP-UHP subduction-collision orogenic process,underwent Triassic amphibolite facies metamorphism and deformation.In addition,the study on the petrography of the garnet-mica schist in the Fenzishan group shows that the core mineral assemblage is chlorite+garnet+muscovite+ilmenite±quartz.It was reflected that initial stage of metamorphism,corresponding to the temperature and pressure conditions of ?530? and ?7.0Kbar;the mantle and rim mineral assemblage is garnet+muscovite+biotite+quartz+ilmenite±staurolite±chlorite,corresponding to the temperature and close pressure conditions of?570?and?8.0Kbar.The quartz rock fabric structure indicates that the deformation temperature is 550?650°C.The EBSD data collected are consistent with the visual interpretation of the shear direction,which can provides quantitative data support for the determination of the foliation inflection/intersection axes(FIA)preserved within porphyroblasts.According to the compositional mapping of the Y element of the in-situ monazite in garnet,the Y content decrease gradually from the core to rim.The 232.6±1.1Ma?229.5 ±3.7Ma monazite U-Pb ages correspond to the prograde metamorphic growth process of garnet.According to the data of geochronological and petrographic studies,the Jiaobei massif involved in the subduction-collision orogenic process of the Sulu UHP metamorphic belt,but its subduction depth is relatively shallow.This also provides evidence for the hypothesis of deep subduction erosion model of continent-continent collision for the formation of the Triassic Sulu HP-UHP metamorphic terrain,which mentioned that part of Jiaobei massif in the North China Craton dragged down together with Yangtze plate and also experienced the process of subduction and exhumation.