The Belingwe greenstone belt in southern Zimbabwe contains one of the most well preserved Archean volcanic successions in the world. The komatiites in this succession have been studied in great detail, but the associated basalts have received much less attention. A detailed study of these basalts in the Zeederbergs Formation has revealed the existence of a previously unrecognised lava type which has important implications for the petrogenesis of the suite. The Zeederbergs Formation and the underlying Reliance Formation form the 2.7 Ga Ngezi Group volcanics, which are underlain by thin, generally shallow water sediments of the Manjeri Formation. These in turn rest unconformably on 3.6 Ga and 2.9 Ga granitoid gneisses in the east and on older (2.9 Ga) greenstones in the west. The nature of the basal contact of the Ngezi Group volcanics on the sediments of the Manjeri Formation has been the subject of recent controversy, and is interpreted here as comformable. The type section of the Zeederbergs Formation in the Ngezi River is logged and described in detail for the first time. Combined with correlation of geochemical marker horizons in other sections this has led to a re-evaluation of the vertical thicknesses of the Zeederbergs Formation - estimated here to be approximately 3km. Study of the geochemical stratigraphy has revealed a horizon of basalts with low Zr/Nb and high CaO/Al<SUB>2</SUB>O<SUB>3</SUB> compared to the rest of the formation. The basalts in this horizon are called Type II (as opposed to the Type I basalts which make up the majority of the formation). Examination of the petrography of the Zeederbergs Formation basalts has revealed that no subdivision into different rock types on petrographic grounds is practical. The lavas are generally fine grained, sparsely phyric and altered to hydrated low greenschist assemblages. The 'spheroids' in the lavas are considered in some detail, and are thought to represent products of spherulitic devitrification.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:641891 |
| Date | January 1996 |
| Creators | Brake, Chris |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/12669 |
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