POIKILITIC CHROMITE IN KOMATIITIC CUMULATES

Chromite is a widespread accessory mineral in olivine-rich cumulates derived from komatiitic lavas. The distribution and crystal habit of chromite is related to the degree of differentiation of the parent magma as reflected in the composition of cumulus olivine. Cumulates with olivine forsterite content greater than 93 mol percent typically contain no chromite at all, while chromite forms clusters of disseminated euhedral grains in cumulates with forsterite less than 91 mol percent. In the forsterite 91-93 interval, chromite may develop lobate interstitial habits. In the Six Mile Well ultramafic complex in the Yakabindie region of the Agnew-Wiluna greenstone belt, and in other olivine-rich komatiitic units within this belt, chromite shows well developed poikilitic textures enclosing olivine. The absence of intercumulus silicate phases in these rocks and the low normative chromite content of the parent magma make it impossible for these chromite grains to have crystallised from intercumulus trapped liquid. These rocks must be cotectic chromite-olivine adcumulates, owing their unusual texture to differing relative rates of nucleation and growth of chromite and olivine, crystallising together in-situ. This observation provides further evidence for a cumulus origin for oikocrysts in layered intrusions, and casts doubt on the usefulness of cumulus terminology.