NEOPROTEROZOIC TECTONIC EVOLUTION OF THE OPHIOLITIC BAYANKHONGOR SUTURE ZONE, CENTRAL ASIAN OROGENIC BELT (CENTRAL MONGOLIA)

The amalgamation of the Central Asian Orogenic Belt (CAOB) in central Mongolia is represented by tectono-magmatic processes that accompanied Neoproterozoic ocean-basin closure and collision between the Khangai (to the northeast) and Baidrag (to the southwest) terranes along the ophiolitic Bayankhongor suture zone. The suture comprises a northeast-vergent thrust stack of (meta)sedimentary-matrix mélange units—from southwest to northeast, the Burd Gol, Delb Khairkhan, Haluut Bulag, and Dzag mélanges—that collectively bound a central ophiolite unit. Middle Paleozoic volcano-sedimentary and late Paleozoic sedimentary strata unconformably overlie or are in fault contact with the suture. The NW–SE-trending Bayankhongor ophiolite is 150–270 km long and ≤7 km wide and discontinuously exposes a complete pseudostratigraphic section including altered ultramafic rocks, layered and isotropic gabbros, sheeted dikes, pillow basalts, and pelagic sedimentary rocks—which previous researchers interpreted to reflect oceanic lithosphere generated via mid-ocean-ridge spreading, supra-subduction-zone spreading, or plume magmatism. New geologic mapping and structural analysis broadly contextualize petrologic and tectonic processes during and after crystallization and suturing. Zircon U/Pb geochronology for a plagiogranite intrusion from the ophiolite constrains its crystallization age to ~650 Ma. Youngest detrital-zircon-crystallization-age populations from mélange units young from south to north, possibly reflecting progressive terrane accretion. Ongoing mineral chemistry, whole-rock chemistry, and igneous-zircon geochronology for the Bayankhongor ophiolite will elucidate the tectonic setting in which it formed, improving regional-tectonic models for the CAOB.