Petrogenesis of the Swaziland and Northern Natal Rhyolites of the lebombo rifted volcanic margin, South East Africa

Abstract

The Jozini 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 Sr- and Nd-isotope composition of the rhyolites is unique amongst Gondwana silicic large igneous provinces, having ε Nd values close to Bulk Earth (-0.94 to 0.35) and low, but more variable, initial 87 Sr/86 Sr ratios (0.7034-0.7080). Quartz phenocryst δ 18 0 values indicate that the rhyolite magmas had δ 18 0 values between 5.3 and 6.7‰, consistent with derivation from a basaltic protolith with δ180 values between 4.8 and 6.2‰. The low-δ180 rhyolites (<6.0%;) come from the same stratigraphic horizon and are overlain and underlain by rhyolites with more 'normal' δ180 magma values. These low-δ180 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 δ180 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 δ180 magma value. It is proposed that the low-δ180 rhyolites were derived by melting of earlier altered rhyolite in calderas situated to the east, which were not preserved after Gondwana break-up. © 2007 Oxford University Press.