The Mafic and Ultramafic Lavas of the Belingwe Greenstone Belt, Rhodesia

Abstract

The Belingwe Greenstone Belt (2.8 × 109 yrs old) contains a 7 km succession of mafic and ultramafic lavas and high-level intrusions which overlie a thin sedimentary formation, itself unconformable on a granitic basement. The lavas range in composition from andesites (4 per cent MgO) to peridotitic komatiites (32 per cent MgO). The mineralogy and textures of the most magnesian lavas demonstrate that they were extruded in a completely liquid state. If the source mantle had an MgO content around 40 per cent, then partial melts in the range 35 per cent to 55 per cent would be required to produce the most magnesian liquids observed. Chemical constraints on the petrogenesis of the ultramafic lavas allow estimates of source mantle composition. In particular, if the source had an MgO content around 40 per cent, then the overall source composition would be similar to that of garnet Iherzolite nodules in kimberlites. The calculated REE contents of the source are close to chondritic. If all the ultramafic lavas were derived from the same source then the variation in liquid composition may have been controlled by orthopyroxene as well as olivine during partial melting at depth. The evolution of the less magnesian komatiites, basalts, and andesites can be explained by lower degrees of partial melting of a common source, and by high-level fractionation of parent liquids similar to those extruded as ultramafic lavas. Physical constraints on the origin of the lavas imply derivation from a depth of 150 km or more, at temperatures of 1600–2000 °C.