CHEMICAL VARIATION OF CHROMITE IN THE ULTRAMAFIC CUMULATES OF THE GREAT SERPENTINITE BELT, UPPER BINGARA TO DOONBA, NEW-SOUTH-WALES, AUSTRALIA

Compositional variation for disseminated chromite in olivine-dominated cumulate rocks from the ophiolite sequence of the Great Serpentinite Belt, in New South Wales, Australia, is best displayed by divalent cat ions. X(Fe2+) of chromite included in cumulus olivine is higher than t hat of chromite found entirely within postcumulus plagioclase and clin opyroxene. These variations result from subsolidus re-equilibration be tween chromite and olivine at the grain scale and give an inverse corr elation between X(Fe2+) and grain size of the chromite. Chromite inclu ded entirely within clinopyroxene is similar in composition to that lo cated entirely in plagioclase; both types are largely unaffected by su bsolidus reactions. A range of X(Fe2+) for intergrain chromite located entirely within plagioclase, which is nearly twice as large as that s hown by chromite associated with olivine, is inherited from the magmat ic stage. Chemical differences between concentrated and disseminated t ypes of chromite reflect initial differences in conditions of crystall ization for the two types.