Trace-element data relevant to the origin of trachytic and pantelleritic lavas in the East African Rift system

Determination of the trace-elements Ba, Ce, La, Nb, Rb, Sr and Zr have been made on lavas from six trachytic and pantelleritic volcanoes in the Kenyan and Ethiopian Rifts. Consideration of these data shows that Ce, La, Nb and Zr have behaved as truly residual elements. In the peralkaline suites examined, plotting other chemical parameters against a residual-element such as Zr appears to be informative. The individual residual-element ratios are constant at any one volcano suggesting that the salic and basic lavas are geochemieally related and that the trachytes and pantellerites do not have an origin independent of the associated basalts.Different values of individual residual-element ratios characterize different groups of volcanic centres. In the northern part of the Kenyan Rift, progressive change in these ratios with time can be traced.In the salic lavas, Ba and Sr show patterns of extreme depletion relative to the alkali basalts and these data suggest that the salic rocks are the result of protracted crystal-fractionation.The relative volumes of the extruded lava-types are discussed and it is suggested that the trachytic and pantelleritic centres developed above cupolas of salic magma situated on top of a large basaltic reservoir. It is likely that the preponderance of salic rocks and the scarcity of lavas of intermediate composition are not criteria by which one can rule out an origin of differentiation by crystal-fractionation.