Journal
JOURNAL OF PETROLOGY
Volume 50, Issue 2, Pages 323-359Publisher
OXFORD UNIV PRESS
DOI: 10.1093/petrology/egp001
Keywords
Kenya Rift; peralkaline rhyolites; mineral; glass partition coefficients; plumbing system; eruptive centres
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Funding
- NERC
- NERC [IMF010001] Funding Source: UKRI
- Natural Environment Research Council [IMF010001] Funding Source: researchfish
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The Greater Olkaria Volcanic Complex is a young (20 ka) multi-centred system in the central Kenya Rift Valley, mainly represented at outcrop by peralkaline rhyolites. The rhyolites show significant compositional variation; peralkalinity [mol. (Na2O K2O)/Al2O3] varies from 101 to 155, Zr contents from 442 to 3640 ppm and Rb contents from 262 to 1056 ppm. More peralkaline rhyolites were generated along multiple, but generally closely similar, liquid lines of descent by 75 fractional crystallization of alkali feldsparquartz-dominated assemblages from mildly peralkaline parental magmas, themselves probably derived by fractionation of trachytic magmas. Crystal fractionation took place at more than one upper crustal level. The rhyolite magmas were erupted from 13 centres, each having an eruptive history and geochemical evolution broadly similar to, but distinct from, those of the other centres. For most of the life span of the Olkaria system, almost the whole spectrum of peralkaline rhyolite compositions was erupted from vents in the complex at any one time. Apparent partition coefficients are presented for 34 trace elements in sanidine, fayalite, ferrohedenbergite, amphibole, biotite, ilmenite and chevkinite-(Ce). With the exception of certain values for Ba, Rb and Sr in sanidine, most coefficients vary systematically with whole-rock composition.
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