Metamorphic Evolution Of The High Pressure Mafic Granulites From Xiwangshan Area,the Xuanhua Terrane,North China Craton

High pressure(HP)mafic-granulites are common in orogenic belts which commonly represent rocks formed as a result of tectonic events,such as deep crustal thickening or collision in convergent settings.The mineral assemblages and metamorphic structures preserved in these HP mafic-granulites provide significant clues that are useful in deciphering the metamorphic history of orogenic belts and tectonic evolution processes at convergent plate boundaries.HP mafic-granulites are widely distributed in Xuanhua area of the Trans-North China Orogen(TNCO).Understanding the metamorphic evolution of the Xiwangshan HP mafic granulites of Xuanhua area is crucial for the reconstruction of the precambrian metamorphic basement tectonic setting and evolution of the TNCO.In this paper,the Xiwangshan HP mafic-granulites are divided into coarse-grained and medium-fine grained through detailed petrography and mineralogy characteristics,and four-stage and three-stage metamorphic mineral assemblages are identified in coarse-grained and medium-fine-grained sample,respectively.In addition,we identified the melt and antiperthite in the samples for the first time.Petrographic observations combining with traditional geological thermobarometers and phase equilibrium modeling reveal that for coarse-grained sample:the prograde metamorphic stage(M1)is characterized by inclusions of garnet porphyroblasts in coarse-grained sample with metamorphic P-T conditions of 760?/11.5 kbar,the peak assemblage(M2)is composed of Grt+Cpx+Amp+Pl+Rt+melt+Ilm with the recorded P-T conditions of 990-1012 ?/14.1-14.7 kbar,the post-peak stage mineral assemblage(M3)contains Grt+Opx+Cpx+Amp+P1+Ilm with the inferred P-T conditions of 870-925 ?/6.8-8.5 kbar,and a later-retrogression stage(M4)is featured by coronas of Amp+P1 surrounding garnet with the P-T conditions of?5.2 kbar/-590?;For the medium-fine-grained sample:the M2 is composed of Grt+Cpx+Amp+P1+Rt+melt with the recorded P-T conditions of 1050-1080 ?/13.2-14.8 kbar,the M3 is composed of Grt+Opx+Cpx+ Amp+P1 with the inferred P-T conditions of 825-875 ?/5.7-7.3 kbar,and M4 is featured by coronas of Amp+P1 with the P-T conditions of?5.0 kbar/?550 ?.LA-ICP-MS U-Pb analysis of metamorphic zircon grains yields two groups of ages of 1853 ± 14 Ma and 1744 ±44 Ma.The study shows that the two types of HP mafic-granulites in Xiwangshan underwent the same metamorphic evolution with a high pressure and ultra-high temperature(HP-UHT)metamorphism during the peak stage and a clockwise P-T path of near-isothermal decompression,it is revealed that the HP mafic granulites may have similar metamorphic evolution history with the UHT pelitic granulites in the TNCO.These two groups metamorphic ages represent the post-peak retrograde metamorphism stage and the late amphibolite phase retrograde metamorphism stage,respectively.