GEOCHEMICAL CHARACTERISTICS OF THE ALKALINE BASALTS AND THE PHONOLITE -TRACHYTE PLUGS OF THE AXUM AREA, NORTHERN ETHIOPIA.

The Axum volcanic rocks constitute important deposits of trap (Oligocene) and post-trap (Miocene- to Pliocene) volcanism at the northern end of the great Ethiopian flood basalt sequence. The petrologic diversity of lavas erupted in this area is significant, ranging from basanites to tephrites and phonolites to trachytes. The variation in the concentration of major elements (Fe2O3, TiO2, CaO), rare earth elements (REE), and incompatible element ratios (Zr/Nb, Nb/Y) in the volcanic rocks of Axum demonstrates the heterogeneous character of their source region. Such heterogeneity can be interpreted by the involvement of a mantle reservoir to different degrees and mechanisms of partial melting. The geochemical data show that the Axum volcanic rocks represent two magma series, which we designate as the flood basalt sequence and the post-trap basalt sequence. The flood basalt sequence, which erupted contemporaneously with the Oligocene Ethiopian flood basalts, exhibit high TiO2(2.6 - 4.4 wt%), Fe2O3(13.4 - 17.4 wt%), and high Zr/Nb ratio (9 - 18). In contrast, the post-trap basalt sequence, which has a slight tendency towards the composition of the northcentral Ethiopian shield volcanoes (Guguftu shield volcano) and the central and southeastern Eritrean volcanic suites, exhibit low TiO2(2.0 - 2.6 wt%), Fe2O3(10.5 -14.6 wt%), Zr/Nb ratio (2.8 - 3.1), and high Nb (60 - 84 ppm), Th (3.9 - 7.2 ppm), and Nb/Y ratio (2.2 - 2.7). The acidic rocks, on the other hand, are indistinguishable from each other and values of their trace element ratio are comparable with the trace element ratios of the post-trap basalt sequence. The acid volcanics (phonolites and trachytes) might, therefore, have formed mostly through fractional crystallization of the post-trap basalt sequence magmas. [ABSTRACT FROM AUTHOR]