Petrogenesis of the Swaziland and Northern Natal Rhyolites of the Lebombo Rifted Volcanic Margin, South East Africa

Abstract

TheJozini and Mbuluzi rhyolites and Oribi Beds of the southern Lebombo Monocline, southeastern Africa, have geochemical characteristics that indicate they were derived by partial melting of a mixture of high-Ti/Zr and low-Ti/Zr Sabie River Basalt Formation types. Compositional variations within the different rhyolite types can largely be explained by subsequent fractional crystallization.The Srand Nd-isotope composition of the rhyolites is unique amongst Gondwana silicic large igneous provinces, having eNd values close to Bulk Earth ( 0 94 to 0 35) and low, but more variable, initial Sr/Sr ratios (0 7034^0 7080). Quartz phenocryst dO values indicate that the rhyolite magmas had dO values between 5 3 and 6 7o, consistent with derivation from a basaltic protolith with dO values between 4 8 and 6 2o. The low-dO rhyolites (56 0o) come from the same stratigraphic horizon and are overlain and underlain by rhyolites with more ‘normal’ dO magma values.These low-dO rhyolites cannot have been produced by fractional crystallization or partial melting of mantle-derived basaltic material. The rhyolites have low water contents, making it unlikely that the low dO values are the result of post-emplacement alteration. Modification of the source by fluid^rock interaction at elevated temperatures is the most plausible mechanism for lowering the dO magma value. It is proposed that the low-dO rhyolites were derived by melting of earlier altered rhyolite in calderas situated to the east, which were not preserved after Gondwana break-up.