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

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 melange 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 (epsilon(Nd(t)) = +7.6 to +4.7: initial Sr-87/Sr-86 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. (C) 2009 Elsevier B.V. All rights reserved.