Phase equilibria modelling and zircon U-Pb ages of the Paleoproterozoic high-pressure mafic granulites in the Jianping Complex and tectonic implications

The late Paleoproterozoic metamorphic evolution of the North China Craton (NCC) has been a controversial issue, and bears significant implications for the final cratonization of the NCC. The Jianping Complex (JPC) is located at the boundary between the Paleoproterozoic Trans-North China Orogen and the Eastern Block, and thus is an ideal site for studying the tectonic evolution of the NCC. Petrographic study reveals that mafic granulites exposed along the western margin of the JPC are characterized by the peak stage mineral assemblage of garnet + plagioclase + clinopyroxene + amphibole + ilmenite + quartz and post-peak growth of orthopyroxene/ amphibole + plagioclase + magnetite + quartz around peak-stage garnet. Two representative mafic granulite samples were selected for phase equilibria modelling using THERMOCALC 3.45. The modelling results, combined with petrographic observations, indicate that these mafic granulites recorded clockwise P-T paths involving near isothermal decompression (ITD) process or decompression with thermal relaxation. Isopleths of Ti content in amphibole and XAn in plagioclase define the peak metamorphic P-T condition as 13.0-13.5 kbar/850-900 degrees C. The post-peak final solidus assemblages of samples 19JP7 and 19JP32 were stable at the conditions of 7.5-8.0 kbar/750-800 degrees C and 10.5-11.5 kbar/880-920 degrees C, respectively. Metamorphic zircons from sample 19JP7 yield a metamorphic age of -1.82 Ga, while zircons from sample 19JP32 yield magmatic age of -2.1 Ga and metamorphic age of -1.85 Ga, representing the timing of emplacement and high-pressure metamorphism. Based on the metamorphic study and zircon U-Pb ages, we conclude that the Paleoproterozoic high-pressure metamorphism of the JPC resulted from the collision of the Eastern and Western Blocks along the Trans-North China Orogen at -1.85 Ga.