KOMATIITE FLOWS FROM THE RELIANCE FORMATION, BELINGWE BELT, ZIMBABWE .1. PETROGRAPHY AND MINERALOGY

The 2.7-Ga Reliance Formation of the Ngezi Group, Belingwe Greenstone Belt, Zimbabwe, contains extremely fresh komatiite lavas. Detailed fie ld mapping and a 200-m deep drill-hole, with excellent core recovery, demonstrated the existence of a suite of lava flows. Each major flow i s approximately 10 m thick and characteristically exhibits chilled top and bottom margins, a spinifex zone dominated by random spinifex, a B 1 zone, and a thick cumulate zone that typically composes two-thirds o f the flow thickness. Preservation of olivine and pyroxene mineralogy is superb by Archaean standards, to the extent that even the tips of s keletal crystals survive. The matrix, although devitrified, is well pr eserved. Detailed study of two flows shows that skeletal grains from t he spinifex zone have maximum Fo contents of 91.4. The Fo contents of microphenocrysts from the cumulate zone range from Fo91.2 to Fo91.6, b ut rare large phenocrysts (approximately 5 vol. %) have maximum Fo con tents of 93.6. The Fo contents of the cumulate olivines do not vary wi th stratigraphic height, implying that the cumulate zone formed rapidl y, by accumulation of transported crystals. The cumulate zones contain 42-57 % modal olivine and display reverse size grading of the olivine microphenocrysts. This grain-size variation is believed to result fro m adcumulus growth within a cumulate pile formed by the gravitational settling of clusters of olivine crystals. Textural relationships indic ate that the final part of the flow to start to crystallize was the lo wermost part of the spinifex zone.