Petrology, phase equilibria modelling and zircon U-Pb geochronology of garnet-bearing charnockites from the Miyun area: Implications for microblock amalgamation of the North China Craton

Abstract

The North China Craton (NCC) was amalgamated through Late Neoarchean amalgamation of seven microblocks along zones of ocean closure represented by granite-greenstone belts (GGB). Typical basement rocks of the Zunhua GGB between the Qianhuai and Jiaoliao microblocks are exposed in the Miyun area of the NCC. Here we report petrology, mineral chemistry, metamorphic P–T estimation and zircon U-Pb geochronology of newly identified garnet-bearing charnockites from the Miyun area. The prograde mineral assemblages are composed of quartz + biotite + clinopyroxene + plagioclase occurring as inclusions in garnet. The peak metamorphic mineral assemblages are identified as garnet + clinopyroxene + orthopyroxene + quartz + plagioclase + K-feldspar + ilmenite ± hornblende whereas the retrograde assemblages are characterized by the breakdown of garnet and formation of quartz + biotite association, together with K-feldspar moats around garnet and magnetite in the matrix. The prograde metamorphic P–T condition is defined at <770 °C and < 9.2 kbar. The peak metamorphic P–T conditions of the charnockite are constrained by a combination of quantitative pseudosection modelling and conventional geothermobarometry, and the results show peak conditions of 835–905 °C and 9.1–11.4 kbar indicating relatively high temperature and high pressure metamorphism. The zircon grains in the garnet-bearing charnockite display clear core-mantle-rim texture, with partial melting event recorded at 2499 ± 12 Ma and subsequent Early Paleoproterozoic retrograde cooling and decompression stage at 2463 ± 11 Ma. The clock-wise P–T trajectories inferred from mineral reaction textures and quantitative P–T pseudosection modelling, in combination with the widespread Late Neoarchean to Early Paleoproterozoic metamorphic ages (2525–2446 Ma) reported from elsewhere in the NCC suggest that the collisional assembly of microblocks and initial cratonization might have occurred during the Neoarchean-Paleoproterozoic transition.