Asrat, Asfawossen (Professer)Hiluf, Hagos2019-04-042023-11-182019-04-042023-11-182018-05-30http://etd.aau.edu.et/handle/12345678/17571Continental flood basalts (Trap and shield basalts) of very large area and thickness, which occur in Ethiopia and Yemen are related to the Afro-Arabian continental break up. The current study deals with constraining the petrogenetic evolution of the mafic-felsic association of Alaje Formation in the Northwestern Ethiopian plateau, particularly at Aiba, Tigray, using detailed field mapping, petrographic analysis, as well as major and trace element geochemistry. The Alaje Formation is a succession of: rhyolitic tuff, olivine phyric basalt, rhyolitic ignimbrite, plagioclase phyric basalt, aphyric basalt. The basaltic rocks comprise phenocryst of plagioclase, clinopyroxene, Fe-Ti oxides and rare olivine phenocrysts. On average, the basaltic suite is plagioclase dominated, except the older basalt where olivine is dominant, and the younger basalt at Tsibet section where Fe-Ti oxide is dominant. The common phenocrysts of the silicic rocks are quartz and sanidine, though the banded rhyolitic lava is particularly quartz dominated. Although the basaltic rocks and the associated rhyolitic units have different phenocryst assemblages and textural relationship in between different unit of basalt and rhyolite, they do not show significant geochemical variation, while there is close genetic relationship between basalt and rhyolite. This is supported by the consistently constant similar trace element ratio Nb/Zr (0.09-0.14), Ce/Zr (0.20-0.29), La/Zr (0.10-0.012), Rb/Zr (0.05-0.14), Ta/Nb (0.05-0.07), La/Nb (0.85-1.05),Hf/La (0.21 0.26) and linear trend of two high incompatible trace element. Besides the geochemical data suggest that the basaltic units are more primitive than the associated rhyolites unites. However, the lower contents of MgO wt% (5.41 - 8.11) and compatible trace element content like Ni (19 – 151 ppm) and Cr (100-300 ppm) indicates that the basaltic units have undergone some fractionation except anomalous high Cr (670), Ni (421) of the lower basaltic unit. Furthermore, the rhyolites of the Alaji Formation have peralkaline affinity and show consistently similar geochemistry to the well-constrained rhyolitic rocks of Wegel Tena and Lima limo, which have been shown to be formed by low pressure crystal fractionation process. It is concluded that the rhyolites are derived by low pressure crystal fractionation of basaltic magma derived from enriched source component of the mantle.enContinental Flood BasaltsEthiopian PlateauEnriched SourceFractional CrystallizationPrimitive SourceCo-Genetic RocksThesis