Metamorphic and geochronological evolution of Paleoproterozoic high-pressure ultra-high-temperature pelitic granulite, Chicheng, northern Trans-North China Orogen

Abstract

Three half-meter-wide pelitic granulite interlayers are found within mafic, intermediate, and felsic granulites in Chicheng of the Trans-North China Orogen, they help to understand the assembly of the Eastern and Western blocks of the the North China Craton (NCC). Detailed petrological study shows that all have identical high-pressure mineral assemblages of garnet + kyanite + K-feldspar + plagioclase + quartz + rutile. Inclusions of mesoperthite in garnet indicate the former presence of ternary feldspar at peak conditions, which, in the matrix, have recrystallized. Kyanite is commonly surrounded by a narrow corona or chain of small garnets, enveloped by a zone of plagioclase and an outermost rim of potassium feldspar. Phase equilibria modeling using THERMOCALC yiels a peak metamorphic pressure of 1.6 GPa at 880 °C (M2a) and a peak temperature of 950 °C at 1.5 GPa (M2b). The ternary feldspar thermometer also indicates a peak temperature of 900–950 °C. The pelitic and mafic granulites exhibit the same peak metamorphic pressure, while the pelitic granulites show a 150 °C higher peak metamorphic temperature than the interlayered mafic granulites of 750–790 °C. Absence of sillimanite in the Chicheng pelitic granulite and orthopyroxene in the mafic granulite indicates a fast exhumation process of cooling, which is different from other high pressure rocks from the Trans-North China Orogen. Analysis of metamorphic monazite and zircon yields concordant SIMS U–Pb ages of ca. 1.91 Ga. This study suggests that the Chicheng HP granulite terrane was subjected to fast tectonic exhumation after the collision that occurred at ca. 1.91 Ga. The results provide a reliable metamorphic record for the final collisiion of the Paleoproterozoic Trans-North China Orogen in the NCC.