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Quaternary peralkaline silicic rocks and caldera volcanoes of Kenya

Summary The late-Quaternary trachytic caldera volcanoes of the Kenya rift provide an unrivalled opportunity for studying the mechanisms of evolution of large peralkaline volcanic complexes and assessing the fundamental problems of magma genesis and chemical differentiation. In a northern set of centres (the Barrier, Emuruangogolak, Silali and Paka) basalt is a major component and mugearites, while scarce, may also be present. Caldera collapse was possibly of Kilauean type. Geochemical variations and the relationships between eruptive rocks and suites of plutonic nodules are consistent with fractional crystallization as the dominant differentiation mechanism in these centres. The southern basalt-absent centres (Menengai, Longonot and Suswa) show Krakatoa-style collapse. The pyroclastic sequences, especially syncaldera ash-flow deposits, indicate the ubiquity of striking compositional zonations within the magma chambers. Chemical variations in these centres have been ascribed to complex interplays of side-wall crystallization, magma mixing and liquid-state differentiation processes, especially involving volatile complexing. The ultimate origin of the trachytes at these centres is still debatable; crystal fractionation and partial melting of basalt are both viable mechanisms. The different development of the two types of centre may be related to the growth of a volatile-rich cap, which profoundly influences the types of differentiation mechanism.