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

How and when modern-style plate tectonics started in the Earth's history is one of the most important scientific issues. The Chicheng melange 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 melange 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 similar to 17 kbar/660 degrees C to > 22/similar to 750 degrees 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 degrees 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 similar to 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 melange shows a geothermal gradient of similar to 9 degrees 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 similar to 35 degrees C/km, which was interpreted to occur at 360-270 Ma governed by extension.