Physical volcanology of the Rooiberg Group near Loskop Dam (Mpumalanga) South Africa

Abstract

The 2.06 Ga Rooiberg Group of South Africa, related to the greater Bushveld Igneous Complex forming event, is one of the most unique silicic large igneous provinces (sLIPs) of the Precambrian rock record. Akin to the Gawler Range Volcanics (Australia), the Trans-Pecos volcanic field (USA), the Sierra Madre Occidental (Mexico), Whitsunday province (Australia) and other silicic dominated LIPs, the Rooiberg Group is subaerial, dominated by voluminous silicic lava flows and formed in an intracontinental setting. The original extent of the Rooiberg Group is thought to be as much as ~200,000 km2, of which 50,000-67,000 km2 are remaining after erosion. The inferred eruption volume is ~300 000 km3. The Rooiberg Group, overlying the metasediments and metavolcanics of the Transvaal Supergroup on the Kaapvaal Craton, can be subdivided into four formations, which are in stratigraphical order: the Dullstroom, Damwal, Kwaggasnek and Schrikkloof Formation. The best outcrop conditions for the Rooiberg Group can be found in the Loskop Dam area in the Mpumalanga Province, ca. 120 km E of Pretoria and 52 km N of Middleburg, where three of the four formations can be encountered in the field. After extensive mapping in this area, a lithofacies analysis was initiated in order to provide for the first time a properly constrained and detailed set of the lithofacies types that can be encountered within the Rooiberg Group. Within the scope of the study area (Loskop Dam), eight lithofacies types could be identified, ranging from coherent lava flows and massive tuffs to cross-bedded sandstones and conglomerates. The lithofacies types can be grouped into syn-, and inter-eruptive lithofacies associations, thus, illustrating changes in time and space, as shown by intercalated products of effusive and explosive eruptions and clastic sediments characterising times of relative quiescence. The tectonic situation within the Kaapvaal Craton and the predominance of lava flows is seen as evidence for a majority of fissure eruptions in the Loskop Dam area. Thick pyroclastic units elsewhere within the extent of the Rooiberg Group, however, suggest the existence of larger volcanic features and related explosive eruptions, probably contemporaneous with the fissure eruptions. The new information gained in the course of this study enables the reconstruction of the geodynamic setting in which the Rooiberg Group formed. The felsic (dacitic, rhyodacitic, and rhyolitic) Rooiberg Group has a geochemical character consistent with within – plate setting. The REE and trace elements suggest the magma with origin consistent with partial melting of the crust.