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 ɛNd values close to Bulk Earth (−0·94 to 0·35) and low, but more variable, initial 87Sr/86Sr ratios (0·7034–0·7080). Quartz phenocryst δ18O values indicate that the rhyolite magmas had δ18O values between 5·3 and 6·7‰, consistent with derivation from a basaltic protolith with δ18O values between 4·8 and 6·2‰. The low-δ18O rhyolites (< 6·0‰) come from the same stratigraphic horizon and are overlain and underlain by rhyolites with more ‘normal’ δ18O magma values. These low-δ18O 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 δ18O 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 δ18O magma value. It is proposed that the low-δ18O rhyolites were derived by melting of earlier altered rhyolite in calderas situated to the east, which were not preserved after Gondwana break-up.