Geochemical investigations support the petrogenesis of Kenya rift plateau-type flood phonolites (14--11 Ma) by partial melting of an alkali basaltic material at lower crustal pressures. High-pressure/high-temperature experiments on a natural plateau phonolite (Hay and Wendlandt, this issue) document multiple saturation of augite, andesine, titanomagnetite, and phlogopite at 0.7 GPa, 1000 ¿C, XCO2=0.42, with amphibole appearing at 975 ¿C. A least squares solution to major element modeling, involving subtraction of the compositions of near-liquidus augute, andesine, titanomagnetite, and olivine (Fo67; hypothesized product of incongruent melting of hydrous phases) from a Kenya Miocene alkali basalt composition, indicates that plateau phonolites can be derived by 15 wt % fusion of this hypothetical parental material (&Sgr;R2=0.07). Alkali basaltic magmas may have injected and/or underplated the lower crust in southern Kenya during prior rift-related basaltic volcanism (23--14 Ma). Bulk Earth values of (87Sr/86Sr)i and ϵNd near zero for four plateau phonolite samples are consistent with a mantle-derived parental composition. Three of these four samples reflect little, if any, postmelting modification; one sample may have evolved by fractional crystallization (high Rb/Sr, low Ba, Sr and Mg ♯). A fifth sample may show evidence of assimilation and fractional crystallization processes (elevated radiogenic Sr and Pb, large negative Eu anomaly, and low Ba, Sr, and Mg ♯). Much of the geochemical variation among plateau phonolite lavas, however, can be ascribed to melting of a predominantly alkali basaltic source with contributions from a lower crustal protolith. A mantle-derived source is also supported by Sr-Nd-Pb isotope data for the phonolites, which indicate that the alkali basaltic source can be described in terms of high U/Pb (HIMU) and enriched mantle (EM1 and EM2) components. ¿ American Geophysical Union 1995