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Komatiite Flows from the Reliance Formation, Belingwe Belt, Zimbabwe: I. Petrography and Mineralogy |
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| Authors: | RENNER R; NISBET E G; CHEADLE M J; ARNDT N T; BICKLE M J; CAMERON W E |
| Institution: | 1Department of Earth Sciences Cambridge CB2 3EQ, UK 2Department of Geology, Royal Holloway, University of London Egham TW20 OEX, UK 3Department of Earth Sciences, The Jane Herdman Laboratories, Liverpool University Brownlow Street, Liverpool L69 3BX, UK 4Max Planck Institut fr Chemie Postfach 3060, Mainz, Germany 545 Tasmania Circle, Canberra, A.C.T., Australia |
| Abstract: | 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 Fo91.2 to Fo91.6, 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.
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