Plume-lithosphere interactions in the generation of the basalts of the Kenya Rift, East Africa

Major and trace element and Sr-Nd-Pb isotopic data for mafic volcanic rocks are used to asses the number of mantle plumes contributing to the Tertiary -Holocene magmatism of the Kenya Rift Valley, current estimates of which va ry from none to three. Rocks ranging in composition from nephelinite to hyp ersthene-normative basalt have bee sampled from three lithospheric zones: t he Tanzanian craton, the craton margin reworked during the late Proterozoic , and the Mozambique mobile belt. The magmas are interpreted as the product s of variable degrees of partial melting within the spinel-garnet peridotit e transition zone. Trace element and isotopic compositions from all three z ones are broadly similar to those of oceanic island basalts, but there is c onsiderable compositional variation, which is related to a strong over prin t from the lithosphere on plume-derived melts, Sr and Nd isotopic ratios pr ovide the only clear distinction between magmatic rocks from the three lith ospheric domains. Within each setting, mafic magmatism has tended to become less silica undersaturated with time, and at any one locality magmatism ha s migrated towards the centre of the rift. Magmas may have formed as a resu lts of the infiltration of plume-derived melts into the base of the lithosp here. The extent of interaction of inferred plume melts with the lithospher e has not varied systematically in time or space. The plume component appea rs to be similar to the source of oceanic island basalts.