Volcanic rocks of the Witwatersrand Triad, South Africa. II: Petrogenesis of mafic and felsic rocks of the Dominion Group

Abstract

A bimodal suite of volcanic rocks builds the bulk of the Dominion Group which, with an age of ∼ 2.72 Ga, is the oldest cover sequence overlying the granite-greenstone Archaean basement of the Kaapvaal craton in the western Transvaal, South Africa. The basic lavas are relatively rich in SiO 2 (50–58%) and aphyric and exhibit a large compositional range. This variation is typically tholeiitic in that it is characterized by strong enrichment of Ti, Fe, and V in differentiated lavas. The different amounts of relative enrichment of these elements allow the basic lavas to be subdivided into two groups. One group is characterized by evolution to low Ti/V ratios in evolved lavas and the other by high Ti/V ratios. For nearly all other elements the composition variation of the two groups is extremely similar and overlaps, although the high-Ti/V group contains the most evolved compositions and most of the primitive lavas (Mg-numbers > 60) belong to the low-Ti/V group. Although recognition of the two groups is based on geochemistry they are interlayered in a consistent pattern within the lava pile. Theoretical modelling indicates that the variation in both lava groups can be explained by low-pressure fractional crystallization of phases in a typical tholeiitic sequence. Primitive basalts of the low-Ti/V group show a large range in major element compositions and a more restricted range of incompatible trace elements and REE abundances. In particular, Fe 2O 3 (total) ranges from 9 to 13.5% and correlates positively with MgO, Cr, and Ni and negatively with Al 2O 5 and SiO 2. There are no correlations of Mg-number with incompatible trace elements such as Zr and Ce. These lavas also have LREE-enriched REE abundance patterns and negative ϵ Nd. These features cannot be explained by crustal contamination but are consistent with derivation by uniform amounts of melting of a mantle source which was heterogeneous with regard to major elements (particularly Fe, Al and probably Ca and Na) and uniformly enriched in incompatible trace elements at least 0.3 Ga before melting. The geochemical style of this enrichment is akin to that associated with modern subduction. This enrichment is associated with the development of a thick Archaean lithosphere, the existence of which is corroborated by diamond age and diamond inclusion thermobarometric studies. The geochemistry of the Dominion basic lavas differs significantly from those of the 3.4 Ga Barberton komatiitic and tholeiitic suites. This suggests that the changing pattern of crustal evolution from metastable ‘greenstone’ tectonic styles to stable ‘Proterozoic’ styles is reflected in the changing character of the mantle sources tapped by basic magmas. The porphyritic Dominion rhyodacites or rhyolites exhibit limited compositional variation and are genetically unrelated to the basic lavas. Nd-Isotopes and La/Th and Th/Co ratios suggest that they are partial melts of mafic crustal materials. Geochemical and petrological features of the Dominion suite are entirely consistent with eruption in a continental environment characterized by rifting or lithospheric thinning. This is consistent with some models for the tectonic development of the Witwatersrand Basin.