Textural analysis and plagioclase grain size distribution of the Rooiberg Group, Bushveld Igneous Complex, South Africa

Abstract

The Rooiberg Group, occurring within the Kaapvaal craton at ~2,06 Ga, is a silicic large igneous province (SLIP), is a precursor to, and is synonymous with the Bushveld Igneous Province (BIC), the world’s largest mafic body. The Rooiberg Group’s volcanic successions, namely the Dullstroom, Damwal, Kwaggasnek, and Schrikkloof formations, were previously interpreted as largely consisting of lava flows, ranging from low- and high-Ti basalts to high- Mg rhyolites. Lenhardt et al. (2017) challenged the dominant lava-flow interpretations and proposed the Rooiberg Group is dominantly composed of highly welded, lava-like ignimbrites. In this study, petrographical and textural analysis was undertaken, and plagioclase grain size distributions studied throughout the Rooiberg Group formations to bring to light a clearer understanding of the Rooiberg Group’s eruptional and emplacement styles, as well as the palaeoenvironment and volcanic origins. The Dullstroom and lower Damwal formations consist of lava flow sequences, whereas the upper Damwal, Kwaggasnek and Schrikkloof formations almost exclusively consist of ignimbrites originating from highly explosive pyroclastic eruptions, aligned with the increasingly siliceous composition of these formations. The Damwal, Kwaggasnek and Schrikkloof ignimbrites have various degrees of welding, ranging from nonwelded ignimbrites displaying classic pyroclastic textures, to highly welded, lava-like and rheomorphic ignimbrites, that have often been misinterpreted as lava-flows due to the lava-like textures. The various degrees of welding reflect the welding continuum, where different degrees of welding may be found within a single pyroclastic flow cooling unit. The Rooiberg Group volcanics were emplaced in an alluvial to lacustrine palaeoenvironment, originating from caldera ring-fissure eruptions. Grain size distributions of the different formations possibly reveal that the remaining Rooiberg volcanic exposures are distal deposits, where the youngest formations (Kwaggasnek to Schrikkloof) reflect increasingly distal deposits. The Rooiberg Group occurs over an extensive area, where outcrops occur according to “fragments” or “packages”, including the Nylstroom Package, Rooiberg Fragment, and Bothasberg Package.