Plume-lithosphere interaction and the origin of continental rift-related alkaline volcanism -: the Chyulu Hills Volcanic Province, southern Kenya

Geochemical data are presented for primitive alkaline lavas from the Chyulu Hills Volcanic Province of southern Kenya, situated some 100 km east of the Kenya Rift Valley. In addition to their primitive compositions, a striking and ubiquitous feature is a strong but variable depletion in K relative to other highly incompatible elements when normalized to primitive mantle values. Semi-quantitative models are developed that best explain the petrogenesis of these lavas in terms of partial melting of a source that contained residual amphibole (but not phlogopite). The presence of amphibole implies a source in the subcontinental lithosphere rather than the asthenosphere. It is suggested that the amphibole is of metasomatic origin and was precipitated in the lithospheric mantle by infiltrating fluids and/or melts derived from rising mantle plume material. A raised geotherm as a consequence of the continued ascent of the plume material led to dehydration melting of the metasomatized mantle and generation of the Chyulu Hills lavas. It is proposed that the Chyulu Hills Volcanic Province represents an analogue for the earliest stages of continental rift initiation, during which interaction between a plume and initially refractory lithosphere may lead to the generation of lithospheric melts.