Permian tectonic evolution of the eastern section of the northern margin of the North China Plate: Constraints from zircon U-Pb geochronology and geochemistry of the volcanic rocks
华北板块北缘东段二叠纪的构造属性:来自火山岩锆石U-Pb年代学与地球化学的制约

LA-ICP-MS zircon U-Pb dating and geochemical data have been obtained for the volcanic rocks from the Daheshen and Guanmenzuizi formations in the middle-eastern parts of Jilin Province, with the aim of constraining the tectonic nature of the eastern section of the northern margin of the North China Plate in Late Paleozoic. Zircons from the volcanic rocks in the Daheshen and Guanmenzuizi formations are euhedral-subhedral in shape and display fine-scale oscillatory zoning and striped absorption (basaltic andesite) as well as high Th/U ratios (0.31~1.56), implying their magmatic origin. LA-ICP-MS zircon U-Pb age data indicate that the dacite and rhyolites from the Daheshen Formation in Huadian County formed in the Early Permian (279±3Ma~293±2Ma), whereas the basaltic andesite and basalt from the Guanmenzuizi Formation in the Hunchun and Tumen areas formed in the Early Permian (275±7Ma) and Late Permian (250±5Ma), respectively. The volcanic rocks from the Daheshen Formation have SiO2=64.9%~75.4%, Mg#=0.21~0.57, belonging chemically to medium- to high-K calc-alkaline series, and display an enrichment in LREEs and LILEs and depletion in HFSEs (such as Nb, Ta, and Ti) and P, similar chemically to those from an active continental margin setting. Their initial Hf isotopic ratios and Hf two-stage model ages range from +0.9 to +10.37 and from 785Ma to 1240Ma, respectively, suggesting that their primary magmas could be mainly derived from partial melting of the Meso-Neoproterozoic accretted lower crust. The Early Permian basaltic andesites from the Guanmenzuizi Formation in Hunchun area, belonging chemically to medium-K calc-alkaline series, are poor in SiO2 (53.4%~53.7%) and HFSEs, rich in Al2O3 (16.4%~16.8%) and LILEs, and display low REE abundances and flat REE pattern, similar to those from an island arc setting. The primary magma of the basaltic andesites could be mainly derived from partial melting of the depleted mantle wedge metasomatized by the subducted slab-derived fluid. The Late Permian basalts from the Guanmenzuizi Formation in Tumen area have SiO2=48.7%~49.6%, Mg#=0.64~0.72, and are enriched in LREEs and LILEs, and depleted in HREEs and HFSEs, similar chemically to those from a volcanic arc setting. Combined with its Nd isotopic ratios (+6.01), we consider that its primary magma could be derived from partial melting of a depleted lithospheric mantle. Taken together, we conclude that the subduction of the Paleo-Asian oceanic plate happened in both the eastern section of the northern margin of the North China Plate (in middle part of Jilin Province) and the southwestern margin of the Khanka Massif from Early Permian to Late Permian.