Abstract

The volcaniclastic sediments and nephelinite lavas of the Mashikiri Formation, Lebombo Group, have been well documented in the Pafuri region and in the Sabi region, Zimbabwe. The well-exposed section along the Olifants River in the Kruger National Park provides cogent additional information. The Shishwayini Beds at the base of the succession represents volcaniclastic deposits on the Tshipise Sandstone Member of the Clarens Formation. The rest of the Mashikiri Formation in the Olifants River section is nephelinite in which nepheline is accompanied by olivine, clinopyroxene, devitrified glass, katophorite and opaque oxides. Chemical modelling shows that the nephelinites were derived by partial melting of mantle peridotite enriched in incompatible trace elements and by subsequent fractionation of pyroxene and olivine. Crustal contamination did not play a significant role in derived magma composition. The Olifants River section defines a third source of undersaturated parent magma distinct from the two sources previously identified from outcrops to the north by other authors.

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