Temporal and spatial variations in the chemistry of the Kenyan basic volcanics.

There has been active volcanism over a large part of western Kenya for the last 25 my. This has been accompanied by the development of the Kenyan Dome and Rift. The volcanics can be considered in terms of several space/time groupings based on their age and geographic position relative to the site of the present day rift valley. There is considerable variety in the petrography and chemistry of the volcanics. By examination of the more basic volcanics (> 4 wt. % MgO) it can be demonstrated that these variations are systematic through both time and space. As the alkalinity of the volcanics increases there is a sympathetic increase in the abundance of incompatible trace elements. Within the rift zone there is a decrease in alkalinity through time, and during the Miocene and Pliocene a north to south increase in alkalinity. Volcanics to the East and West of the rift zone are more alkaline than contemporaneous volcanics within the rift zone. The chemistry of the Kenyan Basic Volcanics is comparable to that of volcanics from ocean islands. North of the Kenyan Province, but associated with the same rift system, are the Ethiopian Volcanics, the chemistry of which is more akin to continental flood basalts. It is demonstrated that the chemical variety displayed by the Kenyan Volcanics can be produced by variable degrees of partial melting of and homogenous source mantle, and that this source must be garnet Iherzolite containing carbonate phases. The source must be enriched in Nb relative to other incompatible trace elements, but is not necessarily enriched in the light REE relative to the heavy REE. There may be variations in the Ba, P and Sr contents of the source. The trace element abundances of the source are similar to the source tapped by ocean islands. However, the ratios La/Nb and K/Nb show that unlike ocean islands the parental magmas to the Kenyan Volcanics are probably contaminated during transit through the sub-continental lithosphere. Geophysical and geotectonic evidence suggest the presence of a hotspot operating beneath Kenya. The variations in the chemistry, however, require a more sophisticated melting regime than that which would be produced a simple hotspot. It is possible that the volcanism is the result of partial melting in more than one melting regime and that these melting regimes have or have had a strong linear element to them, possibly related to upwelling of the mantle caused by passive stretching of the lithosphere