Zircon U-Pb geochronology and geochemistry of the volcanic rocks of Nanloushan Formation in Dachebeigou area, Jilin Province

The Dachebeigou area is located in the transitional zone between the Paleo-Asian Ocean and Paleo-Pacific tectonic domains. Detailed geological, petrographic, zircon U-Pb geochronological, and geochemical studies for the rhyolitic welded tuff of Nanloushan Formation in this area show that zircons extracted from the volcanic rocks are euhedral to subhedral, predominantly long-prismatic, and display clear oscillatory zoning, with Th/U ratios of 0.36-0.71 (> 0.4), indicating a magmatic origin. Zircon U-Pb dating yields a Middle Jurassic age of 174±1.4 Ma (MSWD ＝ 1.6). Geochemically, the rocks belong to the calc-alkaline series, characterized by significant enrichment in LILEs(K and Ba), LREE, and active incompatible elements(Th and U), and relative depletion in HFSEs(Nb, P and Ti). They exhibit high Mg^# values (0.47-0.57), with pronounced negative anomalies in Ta, Nb, and Ti. Combined with Nb/Ta and Ba/La ratios, it is suggested that the magma was derived from crust-mantle mixing. Furthermore, the Zr/Y and La/Yb ratios are generally consistent with those typical of an active continental margin setting. Combined with previous studies, it is concluded that the Middle Jurassic volcanic rocks of Nanloushan Formation in Dachebeigou area were formed in an active continental margin setting related to the subduction of the Paleo-Pacific Plate beneath the Eurasian Plate, providing key constraints on the tectonic evolution of the Paleo-Pacific Plate in this region.