Journal of Petrology Advance Access originally published online on July 24, 2008
Journal of Petrology 2008 49(8):1515-1547; doi:10.1093/petrology/egn036
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The Roles of Fractional Crystallization, Magma Mixing, Crystal Mush Remobilization and Volatile–Melt Interactions in the Genesis of a Young Basalt–Peralkaline Rhyolite Suite, the Greater Olkaria Volcanic Complex, Kenya Rift Valley
1Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK
2Institute of Geochemistry, Mineralogy and Petrology, University of Warsaw, AL. 
wirki i Wigury 93, 02-089 Warsaw, Poland
3US Geological Survey, 956 National Center, Reston, VA 20192, USA
4School of Geosciences, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK
5Department of Earth Sciences, Cespar, the Open University, Walton Hall, Milton Keynes MK7 6AA, UK
RECEIVED MARCH 13, 2008; ACCEPTED JULY 2, 2008
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Abstract |
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The Greater Olkaria Volcanic Complex is a young (
20 ka) multi-centred lava and dome field dominated by the eruption of peralkaline rhyolites. Basaltic and trachytic magmas have been erupted peripherally to the complex and also form, with mugearites and benmoreites, an extensive suite of magmatic inclusions in the rhyolites. The eruptive rocks commonly represent mixed magmas and the magmatic inclusions are themselves two-, three- or four-component mixes. All rock types may carry xenocrysts of alkali feldspar, and less commonly plagioclase, derived from magma mixing and by remobilization of crystal mushes and/or plutonic rocks. Xenoliths in the range gabbro–syenite are common in the lavas and magmatic inclusions, the more salic varieties sometimes containing silicic glass representing partial melts and ranging in composition from anorthite ± corundum- to acmite-normative. The peralkaline varieties are broadly similar, in major element terms, to the eruptive peralkaline rhyolites. The basalt–trachyte suite formed by a combination of fractional crystallization, magma mixing and resorption of earlier-formed crystals. Matrix glass in metaluminous trachytes has a peralkaline rhyolitic composition, indicating that the eruptive rhyolites may have formed by fractional crystallization of trachyte. Anomalous trace element enrichments (e.g. 
2000 ppm Y in a benmoreite) and negative Ce anomalies may have resulted from various Na- and K-enriched fluids evolving from melts of intermediate composition and either being lost from the system or enriched in other parts of the reservoirs. A small group of nepheline-normative, usually peralkaline, magmatic inclusions was formed by fluid transfer between peralkaline rhyolitic and benmoreitic magmas. The plumbing system of the complex consists of several independent reservoirs and conduits, repeatedly recharged by batches of mafic magma, with ubiquitous magma mixing.
KEY WORDS: alkali basalt; geochemistry; magma mixing; trace element
*Corresponding author. E-mail: r.macdonald{at}lancaster.ac.uk
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