Tectonic evolution of the Bayankhongor Ophiolite, Central Mongolia : implications for the Palaeozoic crustal growth of Central Asia
thesisposted on 15.12.2014, 10:39 by A. Craig. Buchan
The Bayankhongor ophiolite marks the closure of an ocean separating two microcontinents: the Baidrag complex with the Burd Gol accretionary complex to the south, and a northern continent which forms the basement for the Hangai region. Subduction was towards the SW with NE-directed ophiolite obduction onto a passive margin represented by the Dzag zone. Geochemical and Nd isotope studies of the ophiolitic rocks suggest that they were derived from a heterogeneous mantle source composed of a depleted N-MORB and enriched E-MORB component. A model for the tectonic setting of the ophiolitic rocks is presented in which the N-MORB rocks represent melts produced at a mid-ocean ridge, whilst the E-MORB rocks represent off-axis flows or melts produced at transform ridge intersections. Basalts from the Delb Khairkhan melange have island arc-like chemistry and provide evidence that the ophiolite may have been trapped within a supra-subduction setting prior to obduction. New 207Pb/206Pb zircon evaporation ages for granites and rhyolite dykes that intrude the ophiolite and its neighbouring lithotectonic units, suggest that the ophiolite was obducted at c. 540 Ma at the beginning of a collisional event that lasted until c. 450 Ma. The new data combined with that of previous studies indicate regional correlation of isotopic ages north-westward from Bayankhongor to southern Tuva. These data record oceanic crust formation at c. 570 Ma, followed by approximately 30 million years of subduction-accretion that culminated in obduction of ophiolites, collision related metamorphism, and magmatism in the period c.540-450 Ma. Correlation of isotopic-age data for the ophiolites of western Mongolia and southern Tuva suggest that the ophiolites define a major collisional suture in the Central Asian Orogenic Belt which helps define the southern and western margins of the Hangai continental block.