Comments on the Eu and Th geochemistry of the archaean continental crust of the Kaapvaal Craton, South Africa

Abstract

Chemical data from sedimentary and igneous rocks of the Kaapvaal Craton have been used to trace the occurrence of highly fractionated K2O-rich granites with pronounced negative Eu-anomalies back to the early Archaean. From the sediments of the Fig Tree Group (about 3.4 Ga-old) and granite pebbles in the overlying Moodies Group it was estimated that the igneous and metamorphic part of the continental crust at that time already contained up to about 35% of such granites. Their formation requires the presence of a thick continental crust to allow intracrustal partial melting with plagioclase as a residual phase, and the intrusion of the resulting granites at high levels within the crust. Save for some small remnants these Eu-depleted Archaean granites have since been removed by erosion. Shales of the 3.0 Ga-old Pongola Supergroup reflect only small source areas and contain considerable amounts of material derived from residual soils. The Eu-anomalies of the Pongola shales suggest up to 30% Eu-depleted granites in the source area. The shales of the Witwatersrand Supergroup (about 2.6 Ga-old) — with Eu-anomalies as low as 0.45 — reflect a source area already having typical post-Archaean crustal compositions. The Th-concentrations in South African shales support this conclusion and typical post-Archaean concentrations of above 10 ppm are found already in the early Archaean Sheba shales. Uranium and thorium mineralization has also occurred on the Kaapvaal Craton since the early Archaean. The South African late Proterozoic and Phanerozoic sedimentary record of Eu-anomalies and Th-concentrations confirms the trend observed on other cratonic areas and supports the concept of worldwide uniformity in crustal composition since the end of the Archaean. On the Kaapvaal Craton crustal growth and consolidation began 500–600 Ma earlier than in other parts of the world. The Kaapvaal Craton thus appears to represent a ‘cold spot’ on the face of the Earth.