Metamorphic P–T paths for the Archean Caozhuang supracrustal sequence, eastern Hebei Province, North China Craton: Implications for a sagduction regime

Abstract

Archean supracrustal rocks of amphibolite-facies occur as enclaves within granitoids gneiss domes and belts between domes, representing collision or sagduction regimes. In order to distinguish between tectonic regimes using metamorphic patterns, systematic data on metamorphic evolution and zircon age dating are presented for the Caozhuang supracrustal sequence of eastern Hebei Province, China, which occurs as enclaves in gneisses. A garnet biotite gneiss records a P–T path involving pre-peak isobaric heating to peak conditions at 780–800 °C and 10–11 kbar (medium-P/T type), followed by decompression to 5–6 kbar. Two Mg/Al-rich schists from the same locality show low P/T conditions of 6–7 kbar/690–750 °C and 4–7 kbar/750–780 °C, similar to that of the decompression stage for the biotite gneiss. Zircon age dating suggests that the amphibolite-facies metamorphism occurred at ∼2.49 Ga, coeval with TTG magmatic activities. Combined with a summary of the metamorphism from Kaapvaal and Pilbara Cratons, we concluded that Archean amphibolite-facies supracrustal rocks occurring as enclaves and belts show different metamorphic patterns. The former exhibits the metamorphic patterns the same as those recovered for the Caozhuang rocks, characteristic of pre-peak isobaric heating to 600–800 °C at 10–12 kbar followed by isothermal decompression with the overprinting of medium- and/or low-P/T assemblages in different enclaves. The latter shows medium- or high-P/T peak conditions mostly lower than 600–650 °C followed by decompression, lacking the pre-peak heating. These metamorphic patterns could be good indicators of a sagduction regime.