The development of the Late Cenozoic alkali basaltic Marsabit Shield Volcano, northern Kenya

The Marsabit Shield Volcano has a surface area of 6300 km² and a volume of about 910 km³ of alkali basaltic lavas and pyroclastics to a maximum (summit) thickness of about 1200 m. Laterally extensive, fissure-sourced lava flow-units predominate with clinopyroxene, plagioclase ± olivine basal basla units followed by olivine basalts. Minor, youngest flows form narrow tongues from cone sources and are also olivine (± analcime) basalts. Cinder cones, block-and-ash cones and maars are concentrated in two belts controlled by fundamental crustal fractures. Hawaiian type volcanism commenced in the very late Pliocene followed by progressively more violent (Strombolian and Ultravulcanian) activity peaking at some stage in the Quaternary. Final vulcanism was relatively gentle and presently the volcano is extinct. The high concentrations of maars is unique to Marsabit in N. Kenya and due to the location of the Sheild over the major aquifer of the Chalbi Basin The volcanism, although contemporaneous with the tectonics of the Gregory Rift, is spatially separated and indicates a regional supply of mantle in N. Kenya. The lavas followed an alkali basalt fractionation trend from a periodotite (xenoliths present in maars) source. Little crust contramination of the source magma occurred, possiblly because of regional crustal tension during the Late Cenozoic in East Africa.