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

The Belingwe Greenstone Belt (2.8 x 10⁹ yrs old) contains a7 km succession of mafic and ultramafic lavas and high-levelintrusions which overlie a thin sedimentary formation, itselfunconformable on a granitic basement. The lavas range in compositionfrom andesites (4 per cent MgO) to peridotitic komatiites (32per cent MgO). The mineralogy and textures of the most magnesianlavas demonstrate that they were extruded in a completely liquidstate. If the source mantle had an MgO content around 40 percent, then partial melts in the range 35 per cent to 55 percent would be required to produce the most magnesian liquidsobserved. Chemical constraints on the petrogenesis of the ultramafic lavasallow estimates of source mantle composition. In particular,if the source had an MgO content around 40 per cent, then theoverall source composition would be similar to that of garnetIherzolite nodules in kimberlites. The calculated REE contentsof the source are close to chondritic. If all the ultramaficlavas were derived from the same source then the variation inliquid composition may have been controlled by orthopyroxeneas well as olivine during partial melting at depth. The evolutionof the less magnesian komatiites, basalts, and andesites canbe explained by lower degrees of partial melting of a commonsource, and by high-level fractionation of parent liquids similarto those extruded as ultramafic lavas. Physical constraints on the origin of the lavas imply derivationfrom a depth of 150 km or more, at temperatures of 1600–2000°C.     

Komatiite Flows from the Reliance Formation, Belingwe Belt, Zimbabwe: I. Petrography and Mineralogy

The 2.7-Ga Reliance Formation of the Ngezi Group, Belingwe GreenstoneBelt, Zimbabwe, contains extremely fresh komatiite lavas. Detailedfield mapping and a 200-m deep drill-hole, with excellent corerecovery, demonstrated the existence of a suite of lava flows.Each major flow is 10 m thick and characteristically exhibitschilled top and bottom margins, a spinifex zone dominated byrandom spinifex, a B1 zone, and a thick cumulate zone that typicallycomposes two-thirds of the flow thickness. Preservation of olivineand pyroxene mineralogy is superb by Archaean standards, tothe extent that even the tips of skeletal crystals survive.The matrix, although devitrified, is well preserved. Detailedstudy of two flows shows that skeletal grains from the spinifexzone have maximum Fo contents of 91.4. The Fo contents of microphenocrystsfrom the cumulate zone range from Fo_91.2 to Fo_91.6, but rarelarge phenocrysts ( 5 vol.%) have maximum Fo contents of 93.6.The Fo contents of the cumulate olivines do not vary with stratigraphicheight, implying that the cumulate zone formed rapidly, by accumulationof transported crystals. The cumulate zones contain 42–57%modal olivine and display reverse size grading of the olivinemicrophenocrysts. This grain-size variation is believed to resultfrom adcumulus growth within a cumulate pile formed by the formedby the gravitational settling of clusters of olivine crystals.Textural relationships indicate that the final part of the flowto start to crystallize was the lowermost part of the spinifexzone. Reprint requests.