GEOCHEMISTRY AND ORIGIN OF EARLY PROTEROZOIC VOLCANIC-ROCKS FROM THE TRANSVAAL AND SOUTPANSBERG SUCCESSIONS, SOUTH-AFRICA

The early Proterozoic Transvaal (TR) and Soutpansberg (SB) successions in South Africa are composed of sediments and variable amounts of volcanic rocks. Volcanic rocks are chiefly tholeiitic in character and in some formations range from mafic to felsic in composition. Except for Machadodorp basalts, all volcanics exhibit Nb-Ta depletion relative to Th and REE. Machadodorp basalts are relatively depleted in incompatible elements and some exhibit flat to depleted LREE patterns like MORB. Andesites and more felsic volcanics in the TR succession can be related to associated basalts by shallow fractional crystallization. In terms of incompatible trace-element ratios, two and perhaps three mantle sources are required for TR-SB basalts. These sources may be part of an ancient subcontinental lithosphere beneath the Kaapvaal Craton which was variably enriched with a subduction-zone component during one or more episodes of arc collision prior to 3.0 Ga. © 1990.