Abstract:
Petrological and geochemical examination shows that the Hantaishir ophiolite complex of the Altai region in Mongolia contains two large ultramafic massifs (the Taishir and Naran massifs), a sheeted dyke complex, and a serpentine mélange zone containing blocks of chromitite and peridotites. The ultramafic massifs consist mainly of harzburgite and dunite with a small amount of podiform chromitite. Cr/(Cr+Al) atomic ratios of chromian spinels are high in the peridotites and sheeted dykes (>0.7), and in the chromitites (>0.8). Fo contents of olivines in the peridotites span a relatively narrow range (91.8-93.0). Most of the sheeted dykes are boninite-type high-Mg andesites. The coexistence of podiform chromitite and boninite in the Hantaishr ophiolite complex suggests that the ophiolite formed in an intra-oceanic subduction zone, from a depleted mantle source.
