Early Carboniferous ophiolite discovery in central Mongolia: Expanding the record of subduction initiation in the Mongol-Okhotsk Ocean

The subduction initiation dynamics and southwestern extension of the Mongol-Okhotsk suture zone in Central Asia have been enigmatic due to a scarcity of robust ophiolitic records. This study presents an integrated analysis of petrography, geochemistry, and geochronology conducted on the newly discovered Erdendalai ophiolite in the Dundgobi Province of Mongolia. The ophiolite mainly comprises serpentinized peridotite, gabbro, anorthosite, and chert. Zircon U-Pb dating of an anorthosite sample yields a crystallization age of 350 ± 4 Ma (early Carboniferous), which identifies this ophiolite as the oldest documented ophiolite within the Mongol-Okhotsk Ocean realm. Geochemically, the gabbroic rocks exhibit normal mid-ocean-ridge basalt−like signatures with slight depletion in high field strength elements and enrichment in large ion lithophile elements, resembling forearc basalts. The anorthosites are interpreted to have crystallized from hydrous, arc-related tholeiitic magmas derived from boninitic precursors. These features collectively indicate that the Erdendalai ophiolite formed in a supra-subduction zone setting, specifically an intra-oceanic forearc, during the early Carboniferous. Detrital zircon age spectra from the enclosing Khoidgobi Formation schists show a youngest peak at ca. 302 Ma, constraining the accretion and emplacement of the ophiolite to the latest Carboniferous or later. Our findings not only confirm the southwestern extension of the Mongol-Okhotsk suture but also provide critical evidence for the early Carboniferous subduction initiation of the Mongol-Okhotsk Ocean, likely facilitated by the presence of a continental ribbon rifted during the ocean’s opening. The distribution of supra-subduction zone ophiolites with different ages along the Mongol-Okhotsk suture zone may document lateral propagation of subduction initiation.