EVIDENCE FOR SYNCHRONOUS EXTRUSIVE AND INTRUSIVE BUSHVELD MAGMATISM

The intracratonic, 2.06 Ga volcanic rocks of the Rooiberg Group of sou thern Africa consist of nine magma types, varying in composition from basalt to rhyolite. Basalts and andesites, intercalated with dacites a nd rhyolites, are found towards the base; rhyolite is the chief magma composition in the upper succession. The absence of compositions inter mediate to the magma types and variations in major and trace element c oncentrations suggest that fractional crystallization was not prominen t in controlling magma compositions. REE patterns are comparable for a ll magma types and concentrations increase for successively younger ma gmas; LREE show enriched patterns and HREE are flat. Elevated Sr-i-rat ios and high concentrations of elements characteristically enriched in the crust suggest that the Rooiberg magmas were crustally contaminate d or derived from crustal material. Some Rooiberg features are related to the intrusive events of We Bushveld complex. Petrogenesis of both the Rooiberg Group and the mafic intrusives of the Bushveld complex is linked to a mantle plume, melting at progressively higher crustal lev els. The basal Rooiberg magmas have undergone a complex history of par tial melting, magma mixing and crustal contamination. Crustal melts ex truded as siliceous volcanic flows to form the Upper Rooiberg Group, s imultaneously intruding at shallow levels as granophyres. Crustally co ntaminated plume magma synchronously intruded beneath the Rooiberg Gro up to produce We mafic rocks of the Rustenburg Layered Suite. Granite intrusions terminated the Bushveld event. The Bushveld plume was short -lived, which conforms, together with other features, with younger, vo luminous plume environments.