Different melt source regions for the volcanics of the Bushveld large igneous province: New observations from MELTS modeling of the Palaeoproterozoic Rooiberg Group (South Africa)

Abstract

The volcanic Rooiberg Group represents the extrusive phase of the Bushveld Magmatic Province in South Africa, forming the roof as well as the floor of the mafic-ultramafic 2057 Ma Rustenburg Layered Suite. Typically, the volcanic rocks of the Rooiberg Group vary from mafic compositions in the oldest unit (the Dullstroom Formation) to felsic compositions in the younger overlying units (the Damwal, Kwaggasnek and Schrikkloof formations). The lower parts of the Dullstroom Formation (including a basal rhyolitic unit) occur beneath the Rustenburg Layered Suite (RLS) in the southeastern part of the Bushveld Province, whereas the remainder of the Rooiberg Group occurs above the RLS. In this study, petrographic descriptions, whole rock geochemistry and MELTS modeling are used to show that the Dullstroom rhyolite could have evolved from fractional crystallisation of a siliceous and magnesian mafic liquid such as the so-called B1 liquid, parental to the lower parts of the Rustenburg Layered Suite. Due to its unique position at the base of the otherwise andesitic to dacitic Dullstroom Formation, the focus of this contribution is on the Dullstroom rhyolite and a comparison thereof with the rhyolites of the upper formations. Consistent with previous studies, the new data generated in this study show that a clear distinction can be made between the rhyolite in the Dullstroom, and those of the Damwal, Kwaggasnek and Schrikkloof formations. The Dullstroom rhyolite exhibits higher MgO contents (1.41–1.87 wt%) compared to the distinctly ferroan rhyolites of the Damwal, Kwaggasnek and Schrikkloof formations (0.01–0.91 wt% MgO). Similarly, immobile trace elements such as Y and Nb range from 9.72 to 12.7 ppm and 4.43–4.53 ppm, respectively, for the Dullstroom rhyolite, and are significantly different to the upper rhyolites (Y - 12.6–87.2 ppm and Nb - 12.3–24.2 ppm) suggesting likely petrogenetic differences. MELTS modeling shows that the Dullstroom rhyolite could not have evolved from the same liquids that generated the rhyolites of the Damwal, Kwaggasnek and Schrikkloof formations. The modeling suggests that the Dullstroom rhyolite formed through ~20% assimilation of upper continental crustal rocks during fractional crystallisation of the B1 composition, and not from the low-Ti basaltic andesite, as previously proposed for the overlying rhyolites. The modeling aspects of this study provide evidence for different sources and melting-fractionation pathways throughout the evolution of the Bushveld Magmatic Province, consistent with characteristics recorded by the volcanic edifice of this large igneous province.