Study on metamorphic evolution and graphite crystallization of graphite-bearing granulites in Dajinbu,Pingdu,Jiaobei
The Jingshan Group displaying characteristics typical of khondalite series is located within Jiaobei Terrane,which is the southern section of the Jiao-Liao-Ji belt,and the rocks of the Jingshan Group have generally experienced high amphibolite facies or granulite facies metamorphism.The Dajinbu area in Pingdu is within the Pingdu-Laixi graphite metallogenic belt,and the graphite ore occurs in the Xucun Member of the Douya Formation,which is part of the Paleoproterozoic Jingshan Group.Research results of petrography,mineralogy,petrology,geochemistry,laser Raman spectroscopy and chronology of the graphite-bearing pelitic granulite in the Dajinbu crystalline graphite mining area show that the granulite has undergone three metamorphic evolution stages(M1-M3).During the peak metamorphic stage,the conditions reach high pressure(HP)-ultra high temperature(UHT).The mineral assemblage in the early metamorphic stage(M1)is incomplete due to the influence of UHT metamorphism.The peak stage(M2)is marked by the disappearance of biotite,with the mineral assemblage of Grt+Kfs+Pl+Rt+Qz+Liq,and the temperature and pressure is in a range of 1010～1050℃ and 1.09～1.20GPa.During the retrogressive stage(M3),biotite reappears and the mineral assemblage is Bt+Grt+Kfs+Pl+Rt+Qz,the temperature and pressure conditions of this stage is estimated to be 810～843℃ and 0.68～0.74GPa.The graphite-bearing pelitic granulite has a clockwise P-T path.The graphite samples generally exhibit high degree of crystallinity,and their Raman spectra characteristics are in line with those formed under granulite facies metamorphism.The high-crystallinity graphites primarily crystallize during the transition from the peak stage(M2)to the retrogressive stage(M3).It is obvious that the degree of graphite crystallinity in the graphite-bearing granulite from the Dajinbu crystalline graphite mining area is closely related to the metamorphic temperature of the host granulites.The weighted mean age of the metamorphic zircons is 1830±18Ma,which is consistent with the age of anatexis in the Jiao-Liao-Ji belt or Jiaobei terrane.This age also represents the age of crystallization and mineralization of high crystallinity graphite in Dajinbu area.Revealing of the relationship between graphite crystallization and metamorphism of the host rock is of great significance for the exploration and prospecting of high-quality crystalline graphite deposits.
Graphite-bearing pelitic granuliteGraphite crystallizationUltra-high temperature metamorphismP-T pathJiao-Liao-Ji belt
万方数据
维普
