PETROLOGICAL CHARACTERISTICS OF THE HANTAISHIR OPHIOLITE COMPLEX, ALTAI REGION, MONGOLIA: COEXISTENCE OF PODIFORM CHROMITITE AND BONINITE

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.