Metamorphic Evolution And Zircon U-Pb Geochronology Of Granulite Within Pingquan,North Hebei Provience

Pingquan granulite is mainly distributed in Fenghuangling,Beiliang,Dahuaying,Mayiliang and other places,it is mainly outputting in the form of a lenticel,which is diffenent sizes.Its surrounding rock is granite gneiss.Pingquan granulite is mainly divided into three categories: low pressure granulite,medium pressure granulite and high pressure granulite.According to the petrographic characteristics and the calculation of temperature and pressure conditions,The metamorphic evolution of the granulite in Pingquan is divided into three stages: 1)the mineral assemblage in the formation stage of original rock of high-pressure granulite in Pingquan is clinopyroxene(Cpx)+ orthopyroxene(Opx)+ plagioclase(Pl).The temperature at this stage is 790-805℃.2)The mineral assemblage of low-pressure granulite during the stage of metamorphic granulite facies is clinopyroxene(Cpx)+ orthopyroxene(Opx)+ plagioclase(Pl)+ quartz(Qz).The mineral assemblage of medium-pressure granulite during the stage of metamorphic granulite facies is garnet(Grt)+ clinopyroxene(Cpx)+ orthopyroxene(Opx)+ plagioclase(Pl)+ quartz(Qz)The temperature and pressure of medium-pressure granulite facies is T=660-740°C,P=0.92-0.99GPa;The mineral assemblage of high-pressure granulite during the stage of metamorphic granulite facies is garnet(Grt)+ clinopyroxene(Cpx)+ plagioclase(Pl)+ quartz(Qz)The temperature and pressure of high-pressure granulite facies is T=700-880℃,P=1.06-1.45GPa;The mineral assemblage of the facies of retrograde metamorphic amphibolite is amphibolite(Amp)+ plagioclase(Pl)± garnet(Grt)+ quartz(Qz).The temperature and pressure in the high amphibolite facies with garnet is T=650-720°C,P=0.73-0.84 GPa,and the temperature and pressure value in the amphibolite facies without garnet is T=610-640°C,P=0.35-0.42 Gpa.The P-T trajectory of the metamorphic evolution of the Pingquan granulite is clockwise,indicating that the high-pressure granulite in Pingquan experienced a underthrusting and collision of the plate,followed by a turnback process.According to the occurrence and petrographic characteristics of the granulite in Pingquan,the original rock of the granulite in Pingquan should be gabbro.Because the depth of the subduction of the original rock is different,the rock of the metamorphic stage is retained,so Pingquan should be in a subduction zone.The zircon U-Pb geochronology shows that the weighted mean age of Pingquan high-pressure granulite is 1861±35 Ma,which represents the age of Pingquan high-pressure granulite rock.