Subduction-related oceanic crust in the Khantaishir ophiolite (western Mongolia).
Friday, 19 December 2014
Most of the oceanic crust is generated at mid oceanic ridges and only a minor portion results from magmatism related to subduction zones (i.e. back-arc basins or in incipient arcs). However it has been observed that several ophiolites preserve an oceanic crust displaying a subduction zone signature. Such a signature is also found in the Khantaishir ophiolite located in western Mongolia. This ~570 m.y. old ophiolite is ~480 km2 in size and displays a complete sequence, tectonically slightly dismembered during the emplacement process. The ophiolite exposes ~130 km2 of highly refractory harzburgitic mantle with local dunite channels and lenses. Towards its top the mantle is replaced by sub-horizontal km-wide discrete zones of pyroxenites situated either in the mantle or forming a crust-mantle transition zone overlain by gabbros. The crust is then composed of various gabbros and minor gabbronorite (both in part replaced by pyroxenites and/or cut by intermediate dykes), by a dyke/sill-complex and by pillow lavas. The entire ophiolite is re-equilibrated at lower greenschist facies conditions. Major and trace elements of the crustal rocks of the Khantaishir ophiolite show trends similar to those observed for the Izu-Bonin-Mariana subduction system. Mafic dykes/sills and pillow lavas of the Khantaishir ophiolite have overall basaltic-andesite compositions, resembling high-Mg andesites with an average SiO2 of 57 wt%. Their low TiO2 (<1.2 wt%) and Ti/V ratios of ~10 contrast with MOR suites, which are basaltic (average SiO2 ~50 wt%) with TiO2 values usually higher than 1.2 wt% and a Ti/V ratio between 20 and 50. This difference is prominent in Th/Yb and Nb/Yb, where the lavas from the Khantaishir ophiolite plot at higher Th/Yb than the typical MORB-OIB array. Moreover, some volcanics with nearly primitive compositions (XMg = 0.69, SiO2 = 56.5 wt%) have trace element patterns comparable with those displayed in high-Mg andesites and boninites from modern island arcs. This evidence suggests that the Kantaishir crust might represent the submarine initial stage of an incipient arc, probably when the preexisting oceanic crust is spread and incipient island arc crust is formed.