Zircon ages of the Bayankhongor ophiolite mélange and associated rocks: Time constraints on Neoproterozoic to Cambrian accretionary and collisional orogenesis in Central Mongolia

Abstract

Central Mongolia is geologically characterized by close juxtaposition of an accreted oceanic terrane with an arc-microcontinent collision zone. We present new U–Pb zircon ages and geochemical data for the Bayankhongor ophiolite mélange from the oceanic terrane and for a syenite porphyry pluton from the arc-microcontinent zone, providing critical constraints on the regional evolution in late Neoproterozoic to early Cambrian times. An anorthosite (655 ± 4 Ma) associated with layered gabbro, a rodingite (metasomatized layered gabbro) (647 ± 6 Ma), and a high-level isotropic amphibole gabbro (647 ± 7 Ma) yielded the oldest zircon ages for the plutonic part of the ophiolite. A plagiogranite dike in the amphibole gabbro yielded an age of 636 ± 6 Ma, which is the youngest date obtained for the ophiolitic rocks. We suggest that the long duration (ca. 20 Ma) for formation of this plutonic sequence characterizes the sea-floor spreading evolution, and the Nd–Sr isotopic composition ( ɛ Nd( t) = +7.6 to +4.7; initial 87Sr/ 86Sr ratio = 0.70279–0.70327) points to a mid-ocean-ridge origin. The syenite porphyry, dated at 523 ± 2 Ma, records the terminal or post-collisional phase of orogeny. The Bayankhongor oceanic lithosphere experienced at least 92 Ma of drift between its formation and accretion.