Permo-Triassic evolution of the southern margin of the Central Asian Orogenic Belt revisited : Insights from Late Permian igneous suite in the Daheishan Horst, NE China

The Daheishan Horst in Jilin Province, NE China, is the key geological unit that links the Solonker-Xar Moron-Changchun and Changchun-Yanji sutures, which are generally interpreted to mark the zone of closure of the Paleo-Asian Ocean along the southeastern margin of the Central Asian Orogenic Belt (CAOB). Here we investigate a suite of volcanic rocks from Daheishan and the Doushantouzi syenogranite intrusion to gain insights into the Permian–Triassic tectonic evolution of the eastern segment of the Paleo-Asian Ocean. Zircon U-Pb data and zircon rare earth element (REE) patterns indicate that the igneous suite formed during the Late Permian (ca. 253–256 Ma) and underwent late-stage alteration during Early Triassic tectono-thermal events. The presence of older magmatic zircon grains in this igneous suite suggest multiple pulses of magmatism during the Permian in this region. The Daheishan volcanic rocks predominantly consist of intermediate-felsic rocks, including andesite, rhyodacite and rhyolite with minor basaltic lava. Geochemically, the basaltic rocks belong to calc-alkaline series, whereas the intermediate-felsic rocks classify as calc-alkaline to high-K calc-alkaline series. The Doushantouzi syenogranites belong to peraluminous I-type granites, with the A/CNK ratio between 1.06 and 1.15. All these rocks have an arc-like affinity with enriched light rare earth elements (LREE) and large ion lithophile elements (LILE; e.g. Rb, Ba and U) and depleted high field strength elements (HFSE; e.g. Nb, Ta and Ti). The Daheishan intermediate-felsic rocks and Doushantouzi syenogranites have higher SiO₂ contents but lower MgO contents and Mg# values and plot within the field of experimentally derived partial melts from metabasaltic rocks in MgO vs. SiO₂ diagram. These geochemical features, together with the positive ε_Nd(t) (+ 1.6 to + 4.6) and ε_Hf(t) (+ 1.53 to + 7.41) values of zircon grains, indicate that the primary magma of these intermediate-felsic rocks likely originated from the partial melting of a juvenile metabasaltic lower crust. In contrast, the basaltic lavas were probably derived from the partial melting of a depleted mantle wedge that was metasomatized by fluids from a subducted slab, as suggested by their low initial ⁸⁷Sr/⁸⁶Sr ratios and depleted ε_Nd(t) (+ 3.6 to + 4.4) values. These data, in conjunction with regional geological investigations, suggest that formation of this Late Permian igneous suite was related to the northward subduction of the Paleo-Asian oceanic plate beneath the Songliao-Xilinhot block and that the eastern segment of the Paleo-Asian Ocean did not close before the Late Permian.