Browsing by Author "Belay, Endayen"

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    Volcanic-Stratigraphy and Petrography of flood Volcanics Along Jita River in North-Central Ethiopia
    (Addis Ababa University, 2020-06-22) Belay, Endayen; Ayalew, Dereje (Professor)
    The study area is located in the North Wollo zone within the NW Ethiopian volcanic plateau. The area provides a well-preserved volcano-stratigraphic sequence from lower basalt initiation to upper lava flow termination. The main objective of this study is to establish the petrovolcanic stratigraphy of the area and then finally to put the petrographic observations within the context of magmatic evolution or depositional history of volcanic products. To accomplish the research objectives, actual field observation, stratigraphic sampling and petrographic analysis methods were applied. The major volcanic units from bottom to top are aphyric-intergranular basalt, aphyric-trachy flow basalt, augite cumulophyric basalt, olivine-augite phyric basalt, Kfs vitrophiric rhyolite, augite phyric basalt, basaltic tuff, columnar-aphyric basalt, non-columnar-aphyric basalt, moderately welded rhyolitic tuff, Kfs phyric rhyolitic-ignimbrite, Kfs phyric rhyolite, columnar-aphyric basalt, thin layer basaltic agglomerate and slightly vesicular aphyric basalt. Petrographyically, these volcanic rocks have different mineral compositions and textures; this heterogeneity reveals that there is a variation in depth of mineral fractionation and magma flux in the lithosphere. The presence of Cpx cumulophyric, Ol-Cpx and Cpx phyric basalt flows in the lower flow pile suggests that there is a considerable depth of fractionation in the deeper crustal level. The ol-megacryst composition and iddingsite alterations also reflect a constant replenishing of new primitive magma recharging the deeper plumbing system. Similarly, the presence of paleosoils, Kfs vitrophyric rhyolite and basaltic tuffs are also indications of cyclicity of magma eruptive phases in both shallow and deeper crust plumbing system with variable magma influx rate. However, the upper basaltic groups are extremely dominated with plagioclase microphenocrysts marked by agglomeratic deposit and capped with vesicular aphyric basalt. These observations indicate that the plumbing system was fed by shallow reservoir with decreasing rate of magma flux. Furthermore, the felsic units have dominant Kfs phenocrysts implying that the rate of magma flux was decreased and crystallized at shallow crustal level. The flows have broad changes in mineralogy and eruptive cyclicity and this suggests that they were pulsed with a fluctuating magmatic influx along with complex plumbing systems and overtime fed by shallower magmatic plumbing reservoirs. Overall, the petrovolcanic- stratigraphy findings in this study provide a new insight into the magmatic evolution of the Ethiopian CFB provinces.