Metamorphism and zircon U-Pb age of high-pressure mafic granulites in in Mian-Lüe suture zone,South Qinling orogen

Mianxian-Lueyang area is a representative of Mian-Lue tectonic melange belt. Two types of HP mafic granulites were recognized in Xujiaping region in the northern Mianxian County. According to petrographic study and P-T pseudosections calculated by THERMOCALC 3. 33,the peak metamorphic mineral assemblage of the first type granulite is associates with Grt 1 + Cpx + Pl 1 + Qz at the conditions of T = 800 ～ 860℃ ,P = 12. 4 ～ 14. 6kbar,and the retrograde assemblage is characterized by Grt 2 + Hbl + Pl 2 + Qz. The second type granulite is characterized by typical white eye-sockettexture with albite or oligoclase,indicating that Omp was once existed in the rock. Therefore,the peak metamorphic mineral assemblage of the second granulite might be Grt 1 + Omp( ?) + Qz or Grt 1 + Cpx( ?) + Pl + Qz,and the condition is estimated to be T = 775 ～ 900℃ ,P 19. 2kbar and T = 750 ～ 850℃ ,P = 16. 5 ～ 19. 8kbar, respectively. The rock was subsequently suffered two stages retrograded metamorphism,a granulite facies stage with mineral assemblage of Grt 2 + Opx + Hbl1 + Pl1 + Qz and an amphibolite facies stage with Grt3 + Hbl2 + Pl2 + Qz. The diversity initial composition might be the main factor that caused the different peak metamorphic mineral assemblages and conditions between the two types of the granulites. LA-ICP-MS zircon U-Pb dating obtained the metamorphic age of 214 ± 11Ma and the protolith age of 477Ma for the second type granulite,and the crystallization age of 215 ± 5Ma for the leucosome crossing the granulite,respectively. Combined with characteristics of trace elements,the age of 214 ± 11Ma might represents the amphibolite facies retrograded age. Thus a P-T-t path is established for these two types of HP mafic granulites,which reflects the process of subduction and collision of the Qinling orogen along the Mian-Lue suture zone at Late Triassic.