Geochemical variation of Quaternary basaltic volcanics in the Turkana Rift, northern Kenya

Abstract

There are five Quaternary volcanic edifices along the axis of the Turkana Rift of northern Kenya. From north to south these are the Korath Range, North Island, Central Island, South Island and the Barrier. Each of these edifices occurs in a structurally distinct segment of the active Turkana Rift. Preliminary studies of these Quaternary volcanics show that crystalliquid differentiation is an important process in all centers, though there is evidence of magma mixing, multiple intrusive episodes and incorporation of earlier lava flows into younger eruptives. The least-fractionated lavas from each volcano have distinctive geochemical signatures, defined by Nb/Zr, Ba/Ti, Ba/Sr and similar ratios. The Korath Range is the most alkalic series, with high Nb/Zr and Ba/Ti. These differences are attributed in large part to melting of a veined mantle beneath the individual rift segments; assimilation of continental crust may be important in the North Island magmas. Elemental abundance patterns in the Korath Range and Central Range magmas are very similar to those for enriched or plume-type ocean ridge basalts. The source for these rift volcanics may not be fundamentally different than the veined mantle that has been postulated as the source for normal and enriched ocean-ridge basalts. Quaternary rift volcanism, much like volcanism along spreading ridges, is segmented both tectonically and chemically. Comparison of these five Quaternary centers to a few analyses of Tertiary lavas along the Turkana Rift indicate that magmas have been produced from similar sources and processes for most of the development of the rift, but that there is no persistence of a particular geochemical signature with time for each magmatic cell.