Volcanism of the Palaeoproterozoic Bushveld Large Igneous Province: The Rooiberg Group, Kaapvaal Craton, South Africa

Abstract

The volcanic rocks of the Rooiberg Group represent the uppermost unit in the Palaeoproterozoic Transvaal Supergroup and form one of the largest provinces of silicic volcanic rocks in the world. Although stratigraphically associated with the Transvaal basin-fill, the Rooiberg Group is petrogenetically linked with the larger Bushveld magmatic event for which emplacement was preceded by the extrusion of the vast Rooiberg lava flows in the northern part of the Kaapvaal Craton. Like many silicic-dominated Large Igneous Provinces (LIPs), which are increasingly recognised in the rock record, the Rooiberg Group volcanics are intracontinental, subaerial, and are dominated by voluminous lava flows. Originally, the Rooiberg Group is inferred to have covered an area of more than 200,000km2 of which, after erosion, an area of 50,000–67,000km2 remains. The lava flows form a stratigraphic succession up to 6km thick, and are divided into four formations in ascending order: Dullstroom, Damwal, Kwaggasnek and Schrikkloof. Due to a scarcity of reliable geochronological data, the temporal span of the Rooiberg Group is poorly understood. The Rooiberg Group consists of basaltic to rhyolitic lava erupted from fissural volcanism with estimated eruption temperatures of the rhyolitic lavas exceeding 1000°C. Minor explosive eruptions are represented by pyroclastic rocks, and subordinate sedimentary interbeds originated from sandy fluvial and lacustrine processes. The rocks are essentially undeformed and have not been buried so that their original textures are well preserved. The Bushveld Complex and the associated Rooiberg Group lava flows are proposed to have formed as a result of partial melting of subcontinental lithosphere and lower crust by a mantle plume. This thorough review of the geochronology, physical volcanology, and geochemistry of the Rooiberg Group enables construction of a geodynamic model.