Early Paleozoic metamorphic P–T–t evolution of the northern Cathaysia Block (SE China): Implications for thermal history and geodynamic setting

The tectono-thermal evolution of the Early Paleozoic orogeny in South China has been debated. In this contribution, detailed studies of petrography, mineral chemistry, phase equilibria modeling, conventional geothermobarometry, and zircon–monazite geochronology were conducted on metamorphic rock samples from the Zhoutan, Longyou, and Chencai groups in the northern Cathaysia Block, South China. All rocks record clockwise P–T paths, including the Pmax (pre-Tmax), Tmax, and post-peak retrograde stages. Sillimanite-garnet-mica gneiss from the Zhoutan Group is characterized by decompression and heating from the Pmax (0.97 GPa/710 °C) to the Tmax stage (0.70–0.85 GPa/795–815 °C). Garnet-amphibole gneiss from the Longyou Group and sillimanite-garnet-biotite gneiss from the Chencai Group experienced Tmax metamorphism at 0.90–1.00 GPa/865–880 °C and 0.90–1.00 GPa/805–820 °C, respectively. LA-ICP-MS zircon–monazite UPb dating yields 453–446 Ma for the retrograde metamorphism of the Zhoutan Group, ~457 Ma and ~ 432 Ma for the peak and retrograde metamorphism, respectively, of the Longyou Group, and 441–436 Ma for the peak metamorphism of the Chencai Group. This indicates that the Early Paleozoic crustal thickening event may have occurred 10–20 Ma earlier in the western region than in the eastern region of the northern Cathaysia Block. Integrated Early Paleozoic P–T–t data from the Cathaysia Block provide medium-pressure facies series conditions without the products of high-pressure facies series metamorphism, which are common in subduction-to-collision orogens. Based on the current metamorphic characteristics and other geological data for Early Paleozoic orogeny in South China, we infer that the current tectonic domains developing metamorphism–magmatism–deformation may not be the "first scene" of subduction-to-collision. The stress that caused the Early Paleozoic orogeny likely originated from the ancient plate boundaries of South China.