Chemical evolution of the Paleoproterozoic Rooiberg Group Kaapvaal Craton South Africa : new insights into the formation of a silicic large igneous province (SLIP)

Abstract

With an estimated erupted volume of 300,000 km3 and an areal extent of more than 200,000 km2, the Paleoproterozoic (2.06 Ga) silicic volcanic rocks of the Rooiberg Group (Kaapvaal Craton) in northern South Africa forms one of the largest and to the same time oldest silicic large igneous provinces (SLIPs) known. This large volume of rocks can be sub-divided into four formations: the Dullstroom, Damwal, Kwaggasnek and Schrikkloof Formations. The results of this study show that a clear chemostratigraphy (by using major elements such as TiO2, SiO2, Na2O, K2O, P2O2, MgO, and Fe2O3) can be established in the area north of Loskop Dam, dividing the rocks of the study area into the Damwal, Kwaggasnek and Schrikkloof formations. The studied rocks are characterized by aphanitic lavas bearing amygdales, spherulitic textures and flow-bands with some sedimentary and pyroclastic interbeds. The dacites could mainly be described as high-Mg felsites (HMF), whereas the rhyolites could be described as low-Mg felsites (LMF). The negative Eu anomaly, Nb and Ta values of the upper part of the Rooiberg Group range between 5.38-24.2 and 0.45-1.86 ppm, respectively, similar to crustal compositions. Furthermore, Nb/Ta values range from 10.91-14.83 (also similar to typical crustal compositions) while few samples from the Damwal Formation exhibit higher values of 15.13-16.02, similar to mantle-derived compositions. Tectonic discriminant diagrams show that the rocks used in this study evolved from fractional crystallization of a mafic liquid although all samples plot in fields with crustal signatures. Plot of ƐNd and 87Sr/86Sr show a mantle-derived origin for the upper part of the Rooiberg Group. However, ƐNd values of the upper part of the Rooiberg Group range between ~-10 to ~-6, typical of crustal composition or continental basalts formed in the crust. From the results, the Rooiberg Group exhibit both mantle (as observed in the Dullstroom and lower Damwal formations) and crustal signatures as exhibited by the Kwaggasnek and Schrikkloof formations. This is interprested as a result of the interaction of the thick crust and a shallow mantle source within the Bushveld Province during magmatism. Furthermore, similarities in geochemical signatures between the Rooiberg Group and selected SLIPs around the world suggest a similar origin for SLIPs by fractional crystallization of a mafic melt and melted (or assimilated) crustal material.