Volcanic rocks of the witwatersrand triad, south Africa. I: Description, classification and geochemical stratigraphy

Abstract

The Witwatersrand triad contains thick volcanic sequences confined largely to the Dominion Group at the base and the Ventersdorp Supergroup at the top. These volcanic sequences are of late-Archaean to early-Proterozoic age and are amongst the oldest supracrustal volcanic sequences erupted onto the Archaean Kaapvaal craton. The volcanic rocks have suffered low-grade greenschist facies metamorphism but primary textures and, in some samples, primary mineralogies are well preserved. Major and trace element analyses of a large number of samples of volcanic rocks from boreholes in the Klerksdorp area, Western Transvaal, indicate that the chemical compositions of the lavas are largely unmodified by secondary processes and can be used for classification, stratigraphic studies and petrogenetic interpretation. The volcanics of the Dominion Group are a bimodal tholeiitic basalt-rhyolite association with considerable compositional variation from basalt to tholeiitic andesite within the basaltic suite. The Ventersdorp Supergroup is a weakly developed bimodal tholeiitic basalt-dacite suite. Basic rocks in the Klipriviersberg Group at the base and in the overlying Platberg Group are largely basaltic, whereas those of the Allanridge Formation at the top are largely tholeiitic andesites. Within the Klipriviersberg Group the more primitive samples occur at the top of the sequence. Phenocryst-rich porphyries of the Makwassie Formation in the Platberg Group are largely of dacitic composition and have textural features suggestive of an ash flow origin. A distinct geochemical stratigraphy is present within the volcanic sequences and application of discriminant function analysis and an empirical discrimination grid based on ratios of immobile elements P, Ti, and Zr allows for unknown samples to be placed within their correct stratigraphic unit.