Petrogenesis of Bimodal Basalt-Rhyolite Suite from Debre Sahil/ Guna Area, Southeastern Ethiopia

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Addis Ababa University


The Debre Sahil volcanic rocks are part of southeastern province, eastern shoulders of Northern MER. The area is located approximately 302 Km from Addis Ababa. The main objective of the research study was to understand the petrogenetic evolution of bimodal volcanic suite. To achieve the objectives, starting from the beginning different methods have been applied. The methods are; field observation, petrographic data analysis, and geochemical data analysis. The major volcanic rock units are pyroxene- olivine phyric basalt (upper basalt), olivine-pyroxene phyric basalt (lower basalt), white rhyolite, rhyolitic ignimbrite and glassy rhyolite. The petrographic study show that the mafic volcanic product of the area is characterized by olivine, clinopyroxene, plagioclase and Fe-Ti oxide crystal phases with microlite groundmass, whereas the rhyolitic rocks are dominated by alkali feldspar, plagioclase, quartz and lithic fragments with glassy groundmass. The geochemical analysis results indicate the Debre Sahil rocks are bimodal in composition, mafic and silicic, lacking intermediate composition. The mafic rocks are alkaline basalt and the silicic rocks are dominantly peralkaline rhyolites. Furthermore, these results indicate that the Debre Sahil volcanic rocks are sourced from spinel bearing peridotite with some residual garnet mantle source. Moreover, the basalt-rhyolite suite is proved to be genetically related to each other by fractional crystallization process. Selected trace element ratios such as Zr/Sm (25.3- 34.15), Zr/Hf (34.12-40.73), Nb/Ta (15.96-21.23), La/Nb (0.77-0.96), Nb/La (1.04-1.29) and Ba/Nb (10.27-21.67) indicates that the study samples are comparable with primitive mantle values, and thus are originated from enriched mantle source or OIB type source.



Debre Sahil Bimodal Suite, Fractional Crystallization, Crustal Contamination, Mantle Melting Condition (Magma Generation)