Phase equilibria modelling and zircon U-Pb geochronology of Paleoproterozoic mafic granulites from the Chengde Complex, North China Craton

Abstract

The Chengde Complex is located in the northern margin of the Trans-North China Orogen (TNCO) of the North China Craton (NCC), and consists mainly of tonalitic-trondhjemitic-granodioritic (TTG) gneisses, garnet-bearing amphibolites and granulites. Mafic granulites are widely distributed as enclaves or boudins within the Chengde Complex. Three representative mafic granulite samples (CD95, CD84, and CD07-2) with different reaction textures were selected for decoding the metamorphic evolution of the Chengde Complex, through an integrated investigation of petrography, mineral chemistry, phase equilibria modelling and zircon U-Pb dating. Petrographic observations and phase equilibria modeling suggest that these mafic granulites have experienced granulite facies metamorphism with characteristic of clockwise P-T paths. Their metamorphic peak P-T conditions were constrained to be 8.4–12.9 kbar/ 850–920 °C. Zircon U-Pb dating for sample CD95 gives an inherited age of ∼ 2.55 Ga as well as a metamorphic age of ∼ 1.84 Ga, while the age of ∼ 2.51 Ga may represent a metamorphic age or mixed age. Zircons in sample CD84 yield three age groups of ∼ 2.52 Ga, ∼ 2.46 Ga and ∼ 1.83 Ga, interpreted to represent the captured zircon grains from country TTG rocks, metamorphic age of the late Neoarchean, and metamorphic age of the Paleoproterozoic, respectively. In sample CD07-2, zircon cores and rims recorded metamorphic ages of ∼ 1.84 Ga and ∼ 1.81 Ga, respectively, which may represent metamorphic peak timing and the later cooling event. Combined with the zircon HREE patterns and previous investigations, we conclude that these mafic rocks in the Chengde Complex experienced the Paleoproterozoic granulites facies metamorphism. The clockwise P-T paths of mafic granulites in the Chengde Complex imply an orogenic process of thickening followed by cooling and uplift-due to the assembly of the Eastern Block and the Western Block.