ABSTRACT
Magmatic activity is widespread in Ethiopia. Oligocene to Pliocene flood basalts, ignimbrites and shield volcanoes cover an area of several hundred thousand square km. Recent to active basaltic to rhyolitic volcanism occurs within the Ethiopian Rift Valley and the Afar depression. Major and trace element studies, together with preliminary Sr isotopic investigations indicate that rhyolitic magmas from both the plateau and rift sequences have been formed by Fractional Crystallization (FC) and Assimilation plus Fractional Crystallization (AFC) processes, from basaltic parental magmas. Both plateau and rift basalts have undergone complex evolutionary processes, which have substantially modified their trace element composition. Mixing between basaltic magmas and associated acid liquids is responsible for the geochemical modifications of plateau and rift basalts. Hybrid mafic magmas display variable ratios of elements such as Rb/Ba, Rb/Zr, etc., which have an incompatible behaviour in mafic systems. Interaction between acid and mafic melts is also indicated by the widespread occurrence of xenocrystal phases (e.g. sanidine, quartz, etc.) in several basaltic rocks. Bulk crust assimilation processes have affected rift basalts.
The present data clearly indicate that many mafic magmas in the Ethiopian plateau and rift valley do not represent primary melts and, in most cases, their composition has been significantly modified by evolutionary processes. This places serious problems to the identification of mantle reservoirs and stresses the need for detailed investigations on single suites in order to discriminate between source and shallow level geochemical signatures.
