Near-liquidus melting relations have been determined for a mafic, plateau-type, flood phonolite from the Kenya rift at 0.5, 0.7, 0.9, and 1.2 GPa, with H2O added through saturation, and at 0.7 GPa with H2O and CO2 added. Mixed-volatile experiments at 0.7 GPa delineate a near-liquidus multiple saturation of augite,andesine, phlogopite, oxides, and apatite at 1000 ¿C, XCO2=0.42, with calcic amphibole melting above 975 ¿C. The multiple saturation and phase assemblage are interpreted to indicate that plateau phonolites were in equilibrium with the residuum of a parental alkali basaltic composition at 0.7 GPa consisting of augite, andesine, titanomagnetite, and olivine (a product of incongruent melting of phlogopite and, possibly, amphibole). This evidence for lower crustal equilibration refutes suggestions that plateau phonolites are low-pressure differentiates. Their enormous volumes (about 50,000 km3), restricted eruptive period (14--11 Ma), uniform major element compositions, and the paucity of associated mafic-intermediate rocks also argue against a deep origin by fractional crystallization. A two-stage process for the origin of the phonolites is consistent with the thermal evolution of the rift. The lower crust was pervasively injected by alkali basaltic magmas during the period of voluminous eruption of early to middle Miocene basalts. Rising isotherms during rift evolution caused subsequent partial melting of this predominantly basaltic lower crust in the late Miocene, generating the plateau phonolites. ¿ American Geophysical Union 1995