Permian–Triassic magmatic rocks in the Middle Gobi volcanic-plutonic belt, Mongolia: revisiting the scissor-like closure model of the Mongol-Okhotsk Ocean

As the youngest segment of the Central Asian Orogenic Belt, the Mongol-Okhotsk Belt is critical for understanding the final amalgamation of East Asia. However, whether the Mongol-Okhotsk Ocean closed in a scissor-like manner remains controversial, in part because the spatial and temporal change of the subduction-related magmatic records are not well constrained. This paper presents new zircon U–Pb ages and whole-rock geochemical data for the Permian–Triassic magmatic rocks in the Middle Gobi belt. Zircon U–Pb dating indicates that the Luus dacite, Luus andesite, East Monhhaan granite and South Monhhaan rhyolite yielded U–Pb ages of 272 ± 4 Ma, 221 ± 2 Ma, 256 ± 2 Ma and 229 ± 2 Ma, respectively. The Luus dacite and South Monhhaan rhyolite show enrichment in LREE and LILE and negative Nb, Ta and Ti anomalies, indicating genesis in a subduction zone setting. The Luus andesite shows adakite geochemical features and was likely formed from partial melting of subducted oceanic crust. The East Monhhaan granite is of peraluminous S-type granite. These Permian–Triassic magmatic rocks are likely formed in an active margin related to the southward subduction of the Mongol-Okhotsk Ocean. Based on available data, we suggest that there is no significant age difference from west to east for the subduction-related magmatic complex at the southern margin of the Mongol-Okhotsk Ocean. Various Late Triassic subduction-related records in central Mongolia indicate the western segment of the Mongol-Okhotsk Ocean was finally closed during Jurassic, nearly contemporary with final closure of the eastern segment. Thus, we argue that the Mongol-Okhotsk Ocean was not likely closed in a scissor-like pattern as previously thought.