Petrogenesis of the Swaziland and Northern Natal Rhyolites of the Lebombo Rifted Volcanic Margin, South East Africa期刊界 All Journals 搜尽天下杂志传播学术成果专业期刊搜索期刊信息化学术搜索

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

Authors:	Miller Jodie A; Harris Chris

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

Abstract:	The Jozini and Mbuluzi rhyolites and Oribi Beds of the southernLebombo Monocline, southeastern Africa, have geochemical characteristicsthat indicate they were derived by partial melting of a mixtureof high-Ti/Zr and low-Ti/Zr Sabie River Basalt Formation types.Compositional variations within the different rhyolite typescan largely be explained by subsequent fractional crystallization.The Sr- and Nd-isotope composition of the rhyolites is uniqueamongst Gondwana silicic large igneous provinces, having ${varepsilon}$ _Ndvalues 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 magmashad ${delta}$ ¹⁸O values between 5·3 and 6·7 ${per thousand}$ , consistentwith derivation from a basaltic protolith with ${delta}$ ¹⁸O values between4·8 and 6·2 ${per thousand}$ . The low- ${delta}$ ¹⁸O rhyolites (< 6·0 ${per thousand}$ )come from the same stratigraphic horizon and are overlain andunderlain by rhyolites with more ‘normal’ ${delta}$ ¹⁸O magmavalues. These low- ${delta}$ ¹⁸O rhyolites cannot have been produced byfractional crystallization or partial melting of mantle-derivedbasaltic material. The rhyolites have low water contents, makingit unlikely that the low ${delta}$ ¹⁸O values are the result of post-emplacementalteration. Modification of the source by fluid–rock interactionat elevated temperatures is the most plausible mechanism forlowering the ${delta}$ ¹⁸O magma value. It is proposed that the low- ${delta}$ ¹⁸Orhyolites were derived by melting of earlier altered rhyolitein calderas situated to the east, which were not preserved afterGondwana break-up. KEY WORDS: rhyolite; Lebombo; stable and radiogenic isotopes; low- ${delta}$ ¹⁸O magmas; partial melting

Keywords:	: rhyolite Lebombo stable and radiogenic isotopes low- ${delta}$ ¹⁸O magmas" target="_blank">gif" ALT="{delta}" BORDER="0">¹⁸O magmas partial melting
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