Geochemistry of high-silica peralkaline rhyolites, Naivasha, Kenya rift valley

R. Macdonald*, G. R. Davies, C. M. Bliss, P. T. Leat, D. K. Bailey, R. L. Smith

*Corresponding author for this work

Research output: Contribution to JournalArticleAcademicpeer-review

Abstract

The Recent (<15000 y) volcanic complex of southwest Naivasha, Kenya, consists of mildly peralkaline (comenditic) rhyolite domes, lava flows, air fall pumices, and lake sediments, with minor, peripheral, basalts and hawaiites. The comendites are either aphyric or sparsely porphyritic, few samples containing >5 per cent phenocrysts. Phenocryst minerals are quartz-sanidine-ferrohedenbergite-fayalite-titanomagnetite-ilmenite-riebeckite-arfvedsonite-aenigmatite-biotite-zircon. Ferrohedenbergite and zircon are restricted to less peralkaline, and amphibole, aenigmatite, and biotite to more peralkaline, rocks.The comendites show unusually strong enrichment in Cs, F, Hf, Nb, Rb, REE, Ta, Th, U, Y, Zn, and Zr, and extreme depletion in Mg, Ca, Ba, Co, and Sr. REE patterns are moderately LREE-enriched, with large, negative Eu anomalies. Values of LIL/HFS element ratios, such as Th/Ta and Rb/Zr, are unusually high for peralkaline rhyolites, and are consistent with a substantial crustal component in the comendites. Parameters such as LREE/HREE and Zr/Nb ratios indicate that the Naivasha rhyolites represent several pulses of closely related, but subtly different, magmas. Sanidine/glass partition coefficients for Ba, Pb, Rb, Sr, U, and the REE are presented for one specimen.Major and trace element modelling, and feldspar-rock relationships, show that closed system crystal fractionation cannot alone account for the overall compositional variations in the comendites. A model involving partial melting of variable crustal source rocks and migration of dissolved volatile-metal complexes may be appropriate at Naivasha.

Original languageEnglish
Pages (from-to)979-1008
Number of pages30
JournalJournal of Petrology
Volume28
Issue number6
DOIs
Publication statusPublished - 1 Dec 1987

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