From the late Paleozoic to early Mesozoic era, the northern margin of the North China Craton underwent intense tectonic-magmatic processes due to the southward subduction and closure of the Paleo Asian Ocean. However, the subduction and postcollision transition mechanisms remain unclear. This study identifies two distinct episodes of bimodal magmatism in the Xiaohongshilazi ore district of central Jilin Province, Northeast China, and presents new geochemical, geochronological, and isotopic (Sr-Nd-Hf) data for the magmatic rocks, which provide insights into the evolution of the Paleo-Asian Ocean. The first Late Permian bimodal magmatism involved intermediate-mafic (259–258 Ma) and felsic (258–256 Ma) volcanic rocks. The intermediate-mafic rocks show geochemical traits characteristic of elevated MgO levels (4.00–8.46 wt%), moderate to high Mg# (51–64), and depleted Sr-Nd-Hf isotopic signatures (I_Sr = 0.704597–0.706031, ε_Nd(t) = 3.0–3.8, ε_Hf(t) = 7.5–11.0), which suggest that the rocks are sanukites, likely generated by partial melting (1%–10%) of a depleted, metasomatized mantle wedge influenced by subduction-related fluids. The contemporaneous felsic rocks display higher I_Sr values (0.705210–0.716821), negative ε_Nd(t) (？0.87 to ？0.82), and lower ε_Hf(t) values (3.45–10.75), which indicate their origin from the partial melting of the lower crust. The second phase of magmatism occurred in the Late Triassic, with diorite (ca. 220–218 Ma) and granite dikes (ca. 219–218 Ma) intruding the Late Permian volcanic sequences. The diorites belong to the alkaline series and exhibit moderately depleted isotopic compositions (I_Sr = 0.705099–0.706587, ε_Nd(t) = 0.5–1.0, and ε_Hf(t) = 8.59–10.01), which formed through the interaction of mantle-derived magma with crustal material. The granite dikes enriched in elements like Zr, Nb, Ce, Zn, and Ga are typical of A₂-type granites. These dikes have variable I_Sr values (0.704856–0.705487) and positive ε_Nd(t) (2.4–2.5) and ε_Hf(t) values (8.62–12.00), which suggest that they formed through magmatic underplating and partial melting of the lower crust. By synthesizing our findings with those of previous studies, we propose that the Late Permian bimodal volcanism occurred in an extensional setting resulting from slab breakoff, and the Late Triassic dikes developed in a postcollisional extensional setting driven by lithospheric delamination. Changes in Nd-Hf isotopic signatures from the Permian into the Late Triassic suggest a tectonic progression from subduction to slab breakoff, followed by postcollisional extension.

Article link: https://doi.org/10.1130/B38145.1