Geology, Petrology, And Geochemistry Of The Basaltic Rocks Of The Axum Area, Northern Ethiopia

The Axum volcanic rocks constitute an important outcrop of trap basalts at the northern end of the great Ethiopian flood basalt province. They constitute about 5-480-m-thick sequences exposed around Axum and Shire regions covering a total area of about 900 km(2). Even though the Axum volcanic rocks were previously assigned to the low-Ti (LT) tholeiitic basalts of the northwestern Ethiopian volcanic province, our geochemical studies on rocks from this region reveal that these rocks are alkaline to transitional in composition with distinct geochemical characteristics. Their petrologic diversity is very significant, the lavas ranging from basanites to tephrites and trachy-andesites. The variation in the major element (TiO2 and Fe2O3) concentrations, incompatible trace elements, and the rare earth element (REE) concentrations and ratios (e.g., (La/Yb)n = 5.0-14.0) indicate a complex petrogenetic evolution of the flood basalts. The Axum volcanic rocks are entirely alkaline to transitional in nature and are classified here into the Lower Sequence (Low-Ti) basalts and the Upper Sequence (High-Ti) basalts. Both these sub-divisions are also reflected in their unique chondrite-normalized REE patterns and multi-element diagrams: the Lower Sequence basalts show enhanced REE fractionation and higher concentrations of the highly incompatible trace elements; whereas the Upper Sequence basalts exhibit less REE fractionation and relatively high concentrations of the moderately incompatible trace elements. The mineralogical and geochemical variations among the basalts of Axum provide clues about the complexity of the northern and northwestern Ethiopian flood basalts and their petrogenetic evolution.