The Late Carboniferous Khuhu Davaa ophiolite in northeastern Mongolia: Implications for the tectonic evolution of the Mongol–Okhotsk Ocean

The newly discovered Khuhu Davaa ophiolite is located in the Adaatsag suture zone, which marks the closure of the Mongol–Okhotsk Ocean in northeastern Mongolia. In this paper, we present new geochemical and SIMS zircon U–Pb data for the ophiolite to provide some constraints on the tectonic evolution of the Mongol–Okhotsk orogenic belt. Metagabbro and plagiogranite samples from the ophiolite yielded SIMS zircon U–Pb ages of 321 ± 4 Ma and 314 ± 3 Ma, respectively, which are interpreted to represent the formation age of the ophiolite. These data suggest that the Carboniferous Khuhu Davaa ophiolite is likely related to the contemporaneous Adaatsag ophiolite to the southwest, defining the suture zone of the Mongol–Okhotsk Ocean in northeastern Mongolia. Tholeiitic basalt samples from the ophiolite show N-MORB and E-MORB geochemical affinities but are enriched in Ba, Pb, and Sr, which may reflect an input of subduction-related fluids. Metagabbro samples exhibit HSFE depletion and LILE enrichment, which are diagnostic geochemical features of arc magmatism. Plagiogranites with low TiO2 contents appear to have formed through partial melting rather than fractional crystallization of a mafic crust in the subduction zone environment. Thus, we suggest that the Khuhu Davaa ophiolite resulted from generation of oceanic crust during the closure of the Mongol–Okhotsk Ocean and documents the initiation of subduction during the Late Carboniferous in northeastern Mongolia.