PLIO-PLEISTOCENE VOLCANIC CENTERS IN THE ETHIOPIAN HIGHLANDS, DISTINGUISHED USING MAJOR AND TRACE ELEMENT ABUNDANCES FROM TEPHRA LAYERS IN THE TURKANA BASIN

Slightly over 4 Ma, crustal extension accompanied by plume generated thermal erosion at the base of the lithosphere formed the present day Turkana Basin located in Southern Ethiopia and Northern Kenya. These processes resulted in thinning, subsidence and sedimentation into the newly formed basin. Basaltic magma of theoliitic composition ponded at the base of the thinned crust eventually erupted as a flood basalt covering the thin layers of sediments in the basin. Rifting continued to the present, resulting in one of the most active basins in the region, filled with sediments that include many volcanic ashes and mammalian fossils such as some of the most famous hominid fossils discovered so far. While the Turkana Basin was forming, tectonic and volcanic activities in the Ethiopian highlands were actively producing the Main Ethiopian Rift and igneous rocks most notably a large volume of silicic rocks. Evidence of this silicic volcanism is well preserved in the sediments within the Turkana basin in the form of distal tephra layers. Today, in the Turkana basin over 130 chemically and stratigraphically distinct volcanic ashes between 4.1 and 0.7 Ma have been identified. Several of these are known from many localities quite distant from the Turkana Basin. Chemical analyses of glass separated from these tephra layers were used to identify tuffs derived from the same eruption and correlate the different formations of the Turkana basin with other basins in the region. Here, relations between the compositions of tephra from individual eruptions are used to group tephra that were probably erupted from the same source. Major and incompatible trace element abundances and ratios show variable values, which allow several compositionally distinct magmatic groups to be distinguished. Twelve groups of tephra are established on the basis of more extensive ash layers found in the basin. Each of these groups is present in the section over a restricted stratigraphic interval, corresponding to only a few hundred thousand years. Representatives of more than one group are found in the same stratigraphic interval, showing that more than one source volcano were active at any given time.