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

摘要

The volcanic rocks of the Rooiberg Group represent the uppermost unit in the Palaeoproterozoic TransvaalSupergroup and form one of the largest provinces of silicic volcanic rocks in the world. Although stratigraphicallyassociated with the Transvaal basin-fill, the Rooiberg Group is petrogenetically linked withthe larger Bushveld magmatic event for which emplacement was preceded by the extrusion of the vastRooiberg lava flows in the northern part of the Kaapvaal Craton. Like many silicic-dominated LargeIgneous Provinces (LIPs), which are increasingly recognised in the rock record, the Rooiberg Groupvolcanics are intracontinental, subaerial, and are dominated by voluminous lava flows. Originally, theRooiberg Group is inferred to have covered an area of more than 200,000 km2 of which, after erosion, anarea of 50,000–67,000 km2 remains. The lava flows form a stratigraphic succession up to 6 km thick, andare 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 poorlyunderstood. The Rooiberg Group consists of basaltic to rhyolitic lava erupted from fissural volcanism withestimated eruption temperatures of the rhyolitic lavas exceeding 1000 ◦C. Minor explosive eruptions arerepresented by pyroclastic rocks, and subordinate sedimentary interbeds originated from sandy fluvialand lacustrine processes. The rocks are essentially undeformed and have not been buried so that theiroriginal textures are well preserved. The Bushveld Complex and the associated Rooiberg Group lava flowsare proposed to have formed as a result of partial melting of subcontinental lithosphere and lower crustby a mantle plume. This thorough review of the geochronology, physical volcanology, and geochemistryof the Rooiberg Group enables construction of a geodynamic model.