Metamorphic Evolution And Tectonic Implication Of The High-grade Metamorphic Rocks From Manjinggou Area,the Huai'an Complex,North China Craton

Paleoproterozoic khondalite series are Al-rich pelitic grnulites that are supposed to preserve evidences about the early Precambrian tectonic events.Paleoproterozoic granulite facies rocks are widely distributed in the North China Craton(NCC).The Huai'an terrane,located within the northern segment of the Trans-North China Orogen(TNCO),a major Paleoproterozoic collisional belt in the central NCC expose mafic and pelitic granulites as well as TTG(tonalite-trondhjemitegranodiorite)gneisses,is located in an insected area of several precambrian geological units that do have significant effections on the exploration of the NCC tectonic mechanism.Here we investigate the pelitic granulites from this complex and identify four distinct mineral assemblages corresponding to different metamorphic stages.The prograde metamorphism(M1)is recorded by relict biotite and the compositional profile of Xca(grt)isopleths.The Pmax(M2)is distinguished by the Xca(grt)isopleths,which corresponds to the kyanite stable area with an inclusion mineral assemblage of Grt-c-(Ky)-Qz-Rt-Kfs-liq suggesting that the pressures were higher than 12 kbar with a temperature below 900?.However,kyanite is absent in thin sections suggesting its consumption during later stages.The Tmax metamorphism(M3)is characterized by the assemblage:Grt-m-Qz-Pl-Rt-Kfs-Sil-liq in the garnet mantle and also reflected in the compositional profile.Two-feldspar geothermometry yields a P-T range of 940?-950? and 9.5-10.5 kbar,indicating ultra-high temperature(UHT)metamorphic overprinting.The subsequent retrograde metamorphic stage(M4)is characterized by Grt-r-Bt-Sil-Kfs-Pl-Qz ± Rt ± Ilm with symplectites of Bt-Sil-Qz in the garnet rim suggesting garnet breakdown with P-T conditions estimated as 770?-840? and 6.5-8 kbar.LA-ICP-MS U-Pb analysis of metamorphic zircon grains yield two groups of ages at 1972.9 ± 8.1 Ma and 1873.3 ± 9.9 Ma.The P-T path of the studied pelitic granulite is quite different with the adjacent mafic granulite,but shares similar UHT metamorphic overpringting with the pelitic granulites from the Khondalite belt.We suggest that the protoliths of the Manjinggou HP-UHT granulites were deep subducted where they experienced HP metamorphism associated with the collision of the Ordos and Yinshan blocks at ca.1.97 Ga.Subsequently,the UHT metamorphic overprint occurred during the assembly of the unified Western and Eastern Blocks of the NCC along the TNCO at ca.1.87 Ga.