Metamorphic evolution and SIMS zircon U-Pb geochronology of mafic granulite and amphibolite enclaves of the Pingyang trondhjemitic pluton, Fuping terrane, North China

Abstract

The discrete mafic granulite and amphibolite enclaves within the Pingyang trondhjemitic pluton, Fuping metamorphic terrane, middle Trans-North China Orogen, ubiquitously record three stages of metamorphic assemblages. For the mafic granulite enclaves, the prograde metamorphic assemblage (M1) is represented by mineral inclusions (clinopyroxene+hornblende+plagioclase+quartz±apatite) preserved in the garnet porphyroblast, the metamorphic peak (M2) assemblage consists of garnet and matrix minerals of clinopyroxene+orthopyroxene+hornblende+plagioclase+quartz±magnetite±ilmenite±zircon, and the retrograde (M3) stage is represented by symplectite assemblage of orthopyroxene+plagioclase±clinopyroxene±hornblende±quartz rimming the garnet porphyroblast. The metamorphic P-T paths of the mafic granulite enclaves are defined to pass from 560–750°C/6–9kbar (M1) through 740–880°C/11–14kbar (M2) to 720–800°C/6–8 (M3), indicative of a collisional orogenic process. The metamorphic P-T path of the amphibolite enclave experienced from 670–730°C/9.3–11.8kbar to 710–730°C/3.9–5.5kbar, further certify such metamorphic feature of an orogenic belt. SIMS U-Pb dating of zircons of the trondhjemitic gneiss yields the magmatic crystallization age of ∼2.52Ga, and both the trondhjemitic gneiss and the mafic granulite enclaves yield the metamorphic age of ∼1.89–1.86Ga. Therefore, it can be concluded that the Fuping terrane was involved in the late Palaeoproterozoic subduction-collision-uplifting tectonic process caused by amalgamation of the Eastern and Western Blocks of the North China Craton along the Trans-North China Orogen.