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 field mapping and a 200-m deep drill-hole, with excellent core recovery, demonstrated the existence of a suite of lava flows. Each major flow is approximately 10 m thick and characteristically exhibits chilled top and bottom margins, a spinifex zone dominated by random spinifex, a B1 zone, and a thick cumulate zone that typically composes two-thirds of the flow thickness. Preservation of olivine and pyroxene mineralogy is superb by Archaean standards, to the extent that even the tips of skeletal crystals survive. The matrix, although devitrified, is well preserved. Detailed study of two flows shows that skeletal grains from the 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, but rare large phenocrysts (approximately 5 vol. %) have maximum Fo contents of 93.6. The Fo contents of the cumulate olivines do not vary with stratigraphic height, implying that the cumulate zone formed rapidly, 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 from adcumulus growth within a cumulate pile formed by the gravitational settling of clusters of olivine crystals. Textural relationships indicate that the final part of the flow to start to crystallize was the lowermost part of the spinifex zone.