Geodynamic setting of the south-east margin of Xing'an-Mongolian Orogenic Belt: Constraints from geochronology and geochemistry of the Permian volcanic rocks in Yanbian area, NE China

The south-east margin of the Xing'an-Mongolian Orogenic Belt (SM-XMOB) is a tectonic junction that is characterized by widespread Late Palaeozoic to Early Mesozoic igneous rocks. To determine the tectonic settings and provide new constraints on the regional tectonic evolution, petrological, geochronological, geochemical, and Hf isotope analyses have been conducted on the Permian volcanic rocks along the SM-XMOB. Zircon U-Pb geochronological data indicate that these rocks were formed at 271.4 +/- 3.2 Ma and 268.3 +/- 3.2 Ma during the Early-Middle Permian. Geochemical results suggest that the volcanic rock samples have 61.5-66.4 wt% SiO(2)and 0.98-1.71 wt% MgO, and belong to the calc-alkaline series, suggesting an affinity with fractionated I-type granite. They also exhibit positive zircon epsilon(Hf) (t) values ranging from + 8.6 to + 14.7, indicating that the primary magmas were dominantly derived from partial melting of Early Palaeozoic juvenile lower crustal. These volcanic rocks show an enrichment in large-ion lithophile elements (LILEs; e.g., Rb, Th, and U) and depletion in high-field-strength elements (HFSEs; e.g., Nb, Ta, Ti, and P), similar to that from an active continental margin setting. We therefore propose that the widespread Permian volcanic rocks in the SM-XMOB were closely associated with the southward subduction of the Palaeo-Asian Ocean (PAO) Plate to the lithosphere beneath the XMOB, and that the PAO had not closed prior to the Middle Permian in the SM-XMOB.