The geology and geochemistry of the Ngezi Group volcanics, Belingwe greenstone belt, Zimbabwe

Scholey, Stephen Philip (1992) The geology and geochemistry of the Ngezi Group volcanics, Belingwe greenstone belt, Zimbabwe. University of Southampton, Doctoral Thesis.

Abstract

Field, petrographic and geochemical studies focusing on the well-preserved, ca. 2.7 Ga high-magnesian (in part komatiite) volcanics of the kilometre-thick Reliance Formation in the Belingwe greenstone belt, southern Zimbabwe, have identified key stratigraphic, structural and magmatic relationships which have important implications for their tectonic setting and petrogenesis. The Reliance and overlying, mafic to intermediate Zeederbergs Formations (< 6.5 km thick in total), together termed the Ngezi Group volcanics, are underlain by thin (generally ca. 100m), shallow-water sediments of the Manjeri Formation. These in turn rest with demonstrable unconformity on ca. 3.6 Ga and 2.9 Ga granitoid gneisses in the east and on older greenstones (the ca. 2.9 Ga Mtshingwe Group) in the west. The stratigraphic contiguity of the basal sedimentary and overlying volcanic units of the Ngezi Group has been assumed in previous studies inferring the eruption of komatiites through pre-existing continental crust, despite the occurrence of a sheared contact between them. Detailed correlation within the Reliance Formation has shown that the lower 170-200 m of the volcanic succession is absent from the eastern central section (strike length ≥10 km) of the 80 km-long, roughly north-southtrending Belingwe belt. This overlies the Shabani Ultramafic Complex, which was possibly a high-level magma chamber feeding the Ngezi Group lavas, but also coincides with the known extent of the detachment zone at the Manjeri-Reliance contact. Although topographically-controlled omission of the lowermost lavas due to uplift associated with high-level intrusion cannot be excluded, their structural removal is favoured. Geometric constraints prefer that this resulted from tectonic emplacement of the volcanics rather than accommodational slip during the synclinal folding of the belt. A four-fold stratigraphic sub-division of the Reliance succession based on variations in petrography, composition and flow morphology includes Basal Mafic (120 to 160 m thick), High-Magnesian Basalt (ca. 240 m), Komatiite (0 to over 180 m) and Upper Mafic (largely volcaniclastic; 200-350 m) Members. The physical volcanology of flow units is commensurate with composite `lava-plain' facies generated by episodic channelised- and periodically-switching sheet-flow at low rates of eruption. Peak activity is represented by thick (typically ca. 25-75 m), predominantly cumulate, open-system channel and well-differentiated `lava pond' facies in the upper part of the High-Magnesian Basalt Member. Kromatiites were erupted only during the waning stages of eruption.

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