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Journal of Petrology Advance Access originally published online on November 16, 2006
Journal of Petrology 2007 48(1):185-218; doi:10.1093/petrology/egl061
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© The Author 2006. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

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

Jodie A. Miller* and Chris Harris

Department of Geological Sciences, University of Cape Town, Rondebosch, 7700, South Africa

RECEIVED JANUARY 30, 2006; ACCEPTED SEPTEMBER 20, 2006


  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 {varepsilon}Nd 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 {delta}18O values indicate that the rhyolite magmas had {delta}18O values between 5·3 and 6·7{per thousand}, consistent with derivation from a basaltic protolith with {delta}18O values between 4·8 and 6·2{per thousand}. The low-{delta}18O rhyolites (< 6·0{per thousand}) come from the same stratigraphic horizon and are overlain and underlain by rhyolites with more ‘normal’ {delta}18O magma values. These low-{delta}18O 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 {delta}18O 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 {delta}18O magma value. It is proposed that the low-{delta}18O rhyolites were derived by melting of earlier altered rhyolite in calderas situated to the east, which were not preserved after Gondwana break-up.

KEY WORDS: rhyolite; Lebombo; stable and radiogenic isotopes; low-{delta}18O magmas; partial melting

*Corresponding author. Present address: Department of Geology, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa. Telephone: 27 21 808 3121. Fax: 27 21 808 3129. E-mail: jmiller{at}sun.ac.za


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