Metamorphic evolution and Zircon ages of Garnet–orthoamphibole rocks in southern Hengshan, North China Craton: Insights into the regional Paleoproterozoic P–T–t history

Abstract

Garnet–orthoamphibole rock (GOR) from southern Hengshan of the North China Craton (NCC) shows a prominent porphyroblastic texture involving garnet, orthoamphibole, kyanite, sillimanite, chlorite, rutile and quartz, with or without talc, hornblende and staurolite. Garnet develops coronae or symplectite assemblages of cordierite, anthophyllite, ilmenite and quartz. The P–T and P–H2O evolution for three representative samples were determined by using pseudosections calculated with THERMOCALC. Generalized prograde paths are inferred from subtle garnet growth zoning, chlorite compositions and inclusion assemblages in garnet. The peak mineral assemblages are predicted to be characteristic of presence of garnet, gedrite, chlorite, kyanite and rutile under P–T conditions estimated to be 9–12kbar and 650–670°C. The post-peak history is characterized by isothermal decompression, developing coronae and symplectite assemblages under a mix of fluid-present and fluid-absent conditions. An idealized P–XMg [=Mg/(Mg+Fe) in mole] pseudosection drawn at T=655°C and a P–M(H2O) pseudosection at T=630°C suggest that the garnet-free cordierite–orthoamphibole rock (COR) can form from decompression of GOR assemblages when the rock is high-in XMg value or the decompression temperature is low. Zircon U–Pb dating yields a metamorphic age of 1964±25Ma (∼1.95Ga) and a protolith age of 2533±14Ma, where the former was interpreted to represent the prograde- or peak stages of metamorphism. This paper validates an argument that the amalgamation of the NCC via a crust thickening event occurred at ∼1.95Ga, followed by an isothermal decompression and a prolonged nearly isobaric cooling process from 1.90 to 1.78Ga.