The Archean komatiite-related Inyala Chromitite, southern Zimbabwe

Abstract

The chromitites at the Inyala mine, Zimbabwe, form part of a layered intrusion enclosed within the Archean granulite facies gneisses of the north marginal zone (NMZ) of the Limpopo belt. Chromitites occur within an olivine-orthopyroxene-chromite cumulate sequence which was derived from a komatiitic magma.An electron microprobe study and a comparison with the mineralogy of unaltered komatiites from the Belingwe greenstone belt, Zimbabwe, strongly suggest that some of the original olivine, chromite, and orthopyroxene mineral compositions are preserved. Olivine-chromite mineral pairs continued to equilibrate down-temperature until about 680 degrees C. Calculations based on the diffusion of Fe and Mg in olivine suggest that the cooling was rapid (several 1,000 degrees C/Ma), hence it is reasonable to expect igneous mineral compositions to be preserved. Calculations based on mineral-melt, Fe-Mg partitioning for olivine and orthopyroxene indicate that the parent komatiitie liquids varied in composition from 26 to 16 wt percent MgO. Thus some of the chromites crystallized from melts which are among the most magnesian komatiites known and which would have had an eruption temperature of ca. 1,520 degrees C.Orthopyroxenite cumulates, which are unusual in komatiitic sequences probably formed in response to assimilation of adjacent supracrustal rocks, possibly banded iron-formation. Oxygen activity calculations suggest that assimilation may have led to an increase in the oxygen activity in the melt which may be responsible for the crystallization of both chromite and orthopyroxene.