MORPHOLOGY AND COMPOSITION OF CHROMITE IN KOMATIITES FROM THE BELINGWE GREENSTONE BELT, ZIMBABWE

Chromite grains in nearly fresh Archean komatiites from the late Archean Belingwe greenstone belt (ca. 2.7 Ga), in Zimbabwe, have diverse morphologies: skeletal and dendritic chromite is restricted to spinifex zones, whereas euhedral grains occur throughout the flows. Systematic differences in chromite compositions ate present between the spinifex (iop) and cumulate (base) zones of the flows and between skeletal and euhedral chromite. Two stages of crystallization have been identified: an early euhedral core is mantled later by skeletal chromite. The core is enriched in Mg and Cr, but depleted in Fe, Ti and Mn relative to the skeletal rim, in a manner similar to variations in compositions of euhedral chromite from the top to the base of the flows. Accordingly, the euhedral chromite and the core of skeletal chromite in the spinifex zones could have formed at an early stage, in equilibrium with a large volume of magma, whereas the skeletal mantle could have formed rapidly from a small volume of evolved residual liquid after eruption of the magma. Thermometry based on the pair chromite-olivine may provide an estimate of the ambient conditions of the komatiitic liquids, Chromite in komatiites has uniform Al contents and characteristic compositional trends, distinct from chromite in mafic-ultramafic rocks from other geodynamic settings, thereby providing an independent means of discriminating its origin.

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