Two phases of granulite facies metamorphism during the Neoarchean and Paleoproterozoic in the Qingyuan terrane, North China Craton

There are different understandings of the Neoarchean and Paleoproterozoic tectonic evolution of the North China Craton, so it is necessary to conduct detailed studies of its metamorphism. In this paper, intermediate granulites, garnet-orthopyroxene rocks, and metabasic dykes in the Qingyuan terrane of northern Liaoning were selected to carry out systematic studies of petrography, mineral chemistry, phase equilibria modeling, and zircon-monazite dating to document their metamorphic evolution and tectonic significance. Both intermediate granulites and garnet-orthopyroxene rocks are recognized to show two stages of granulite facies assemblages formed during the Neoarchean and Paleoproterozoic, respectively. The first phase plagioclase (Pl(1)) in the intermediate granulite has complex compositional zoning with X-An decreasing from core to mantle followed by increasing towards the rim, and in garnet-orthopyroxene rock the first phase garnet and orthopyroxene are enveloped between each other. The first phase granulite facies metamorphism shows anticlockwise P-T paths involving the pre-peak compression to the peak and post-peak decompressional cooling to a solidus. According to the stabilities of the observed peak assemblages in P-T pseudosections and the composition of the Pl(1) mantle, the peak P-T conditions are constrained to be 1. 0 similar to 1. 2GPa/890 similar to 1000 degrees C for the first phase metamorphism. In the garnet-orthopyroxene rocks, orthopyroxene is transformed into garnet during the pre-peak compression as revealed from the texture of garnet enveloping orthopyroxene, and vice versa, orthopyroxene is formed by consuming garnet during the post-peak decompression cooling as indicated by garnet enveloped by orthopyroxene. The second phase granulite facies assemblages of the intermediate granulites and garnet-orthopyroxene rocks are characterized by the symplectites or coronae of garnet + quartz and biotite + quartz clinopyroxene +/- K-feldspar. The metabasic dykes develop only the second phase assemblages of high-pressure granulite facies, and have clockwise P-T paths. According to the minimum X-An, in metamorphic plagioclase (Pl(b)), the maximum Ti content in amphibole, and the maximum X-py in garnet mantle, the peak P-T conditions are determined to be similar to 1. 15GPa/830 degrees C. Zircon dating shows that the first phase high-ultrahigh temperature granulite facies metamorphism occurred during 2. 49 similar to 2. 48Ga, representing the time of the post-peak cooling process, and the second phase high-pressure granulite facies metamorphism occurred at similar to 1. 83Ga. Monazite dating gave a post-peak retrograde age at similar to 1. 75Ga for the second phase metamorphism. Combined with other regional geological characters, we propose that the first phase high-ultrahigh temperature granulite facies metamorphism is controlled by the Archean unique vertical tectonics/sagduction, and the second phase high-pressure granulite facies metamorphism is related to the collision orogenic event along the northern margin of the North China Craton.

Automated summary

In this paper, detailed research on the metamorphism of the Qingyuan terrane in northern Liaoning was carried out, focusing on intermediate granulites, garnet-orthopyroxene rocks, and metabasic dykes. The results show that the intermediate granulites and garnet-orthopyroxene rocks underwent two stages of granulite facies assemblages formed during the Neoarchean and Paleoproterozoic. The first stage metamorphism followed an anticlockwise P-T path, while the second stage followed a clockwise path. The P-T conditions for these metamorphic stages were determined through mineral observations. Zircon and monazite dating further confirmed the timing of these two metamorphic events. Based on regional geological evidence, the first stage metamorphism is thought to be controlled by Archean vertical tectonics/sagduction, while the second stage is related to collisional orogenesis along the northern margin of the North China Craton.