6533b7d6fe1ef96bd12664eb

RESEARCH PRODUCT

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

subject

description

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 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 ( ɛ Nd( t )  = +7.6 to +4.7; initial 87 Sr/ 86 Sr 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.