Ultrahigh Temperature Metamorphic Record of Pelitic Granulites in the Huangtuyao Area of the Huai’an Complex, North China Craton

Abstract

Pelitic granulite from the Huangtuyao area, occurrs in the Huai’an Complex, is located in the Trans-North China Orogen of the North China Craton. On the basis of petrolography, mineral component, and phase equilibrium modeling studies, the P-T conditions and mineral assemblages of pelitic granulites can be divided into four metamorphic stages: the prograde metamorphic stage M1 defined by the stable mineral assemblage of Grt1 (garnet core)+Pl+Bt+Kfs+Qz+Rt, the peak pressure Pmax stage M2 indicated by Grt2 (garnet mantle)+Kfs±(Ky)+Rt+Qz+Liq (melt), peak temperature Tmax stage M3 characterized by Grt3 (garnet rim)+Sill+Pl+Kfs+Qz+Ilm+Liq, and retrograde stage M4 represented by Grt (in matrix)+Kfs+Sill+Bt+Pl+Qz+Ilm. By using the THERMOCALC V340, the P-T conditions are estimated at ~13.8−14.1 kbar and ~840−850 °C at stage M2, and 7–7.2 kbar and 909–915 °C for the Tmax stage M3, indicating an ultra-high temperature (UHT) metamorphic overprinting during decompression and heating process after high pressure granulite facies metamorphism. The mineral assemblages and their P-T conditions presented a clockwise P-T trajectory for the Huangtuyao pelitic granulites. The major metamorphic events at ~1.95 and ~1.88 Ga obtained by the zircon U-Pb dating suggest that pelitic granulites from the Huangtuyao area has undergone HP granulite metamorphism which probably occurred in the prograde metamorphism and related to the collision between the Ordos and the Yinshan blocks, and afterwards UHT metamorphism is related to crustal extension after continental-continental collision.