Paleoproterozoic oceanic subduction in the North China Craton: Insights from the metamorphic P–T–t paths of the Chicheng Mélange in the Hongqiyingzi Complex

Abstract

How and when modern-style plate tectonics started in the Earth’s history is one of the most important scientific issues. The Chicheng mélange in the north margin of North China Craton (NCC) presents a new insight into the Paleoproterozoic cold oceanic subduction and the beginning of modern-style plate tectonic regime. The mélange comprises blocks of retrograde eclogites, amphibolites and peridotites in a matrix of metasediments. The retrograde eclogites mainly contain garnet, amphibole and symplectites consisting of fine-grained plagioclase, clinopyroxene and amphibole. Three metamorphic stages were derived, involving the prograde, peak, and post-peak decompression stages. The P–T conditions for the prograde and peak stages range from ~17 kbar/660 °C to >22/~750 °C kbar based on mineral composition isopleths in P–T pseudosections, involving core–to–mantle increasing pyrope and decreasing grossular in garnet and Ti content in amphibole. The post-peak decompression is proposed to be isothermal under melt (fluid)-absent conditions, recorded a pressure of 8–11 kbar on the basis of symplectite assemblages. For the amphibolites pre-peak heating, peak temperature, and post-peak cooling metamorphic stages were derived. They experienced anataxis during this overprinting metamorphism when heated to above the solidus. The peak temperature conditions are constrained to be 5–7 kbar/780–830 °C on the basis of the contours of the maximum Ti in amphibole and anorthite of plagioclase in P–T pseudosections. Zircon U–Pb dating for the selected samples and a summary of published age data reveal that the eclogite facies metamorphism may have occurred at 1.85–1.80 Ga, and the eclogitic rocks exhumed to a lower crust level during or after subduction to be overprinted by multi-phase thermal events at ~1.66 Ga and 460–420 Ma with undefined conditions, and at 360–270 Ma under amphibolite facies. The eclogite facies metamorphism from the Chicheng mélange shows a geothermal gradient of ~9 °C/km, pointing to a cold oceanic subduction at late Paleoproterozoic that occurred along the northern margin of the NCC. The overprinting metamorphism with anataxis recognized from the amphibolites represents a low-pressure type with a geothermal gradient of ~35 °C/km, which was interpreted to occur at 360–270 Ma governed by extension.