Stratigraphy and Petrography of Volcanics from the Western Margin of the Central Main Ethiopian Rift: the Case of Furi Volcano
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Date
2023-09
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Addis Ababa University
Abstract
Furi Volcano is located near the Western Margin of the Central Main Ethiopian Rift. The location of the area is 26_Km southwest of Addis Ababa. The study's primary goal is to establish volcanic stratigraphy and petrography of Furi Volcano and Escarpment. Various techniques were used to achieve the research objectives, including observation in field, remote sensing, petrographic analysis, and sampling from the study area. Additionally, the study attempts to integrate these data in order to relate them to regional studies investigations on geochronological and magma emplacement behaviour about the Main Ethiopian Rift. The main lithological units exposed in the research area from bottom to top are; partially welded rhyolitic tuff, welded rhyolitic tuff, rhyolite, aphyric basalt, scoria, trachytic basalt, andesite, welded andesitic tuff, welded trachyte tuff, massive to porphyritic trachyte lava. These volcanic rocks are petrographically diverse, with a wide range of mineral compositions and textures; their heterogeneity indicates that magma flux into the lithosphere and depth of fractionation has varied. Total of 25 petrographic samples are studied under the petrographic microscope. The rhyolites and ignimbrite (Welded Rhyolitic Tuff) samples are made of alkali feldspar, quartz, and plagioclase feldspars and are moderately to very porphyritic with up to 10–35% phenocrysts. The felsic units' predominance of alkali feldspar phenocrysts indicates a lower rate of magma flux. The presence of plagioclase, cpx and olivine in basalt reveals the fractionation happened at shallow upper crust, In contrast, the Olv + Cpx mineral assemblages in basaltic lavas demonstrate that crystal fractionation occurred at a deeper crustal level or at the crust-mantle boundary. The upper part of the volcanic unit is dominated by plagioclase and sanidine phenocrysts, intermediate rock, and trachyte lava. The total thickness of volcanic units in the study area is 700m, partially welded rhyolitic tuff to rhyolite (350m), basalt to trachytic Basalt (150m), and andesite to porphyritic trachyte (200m). This indicates that there is a higher abundance of felsic rocks compared to basic rocks. Overall, the study contributes knowledge of the geological history of the western margin of the Central Main Ethiopian Rift with a focus on the Furi Volcano.
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Keywords
Furi Volcano, Volcanic Stratigraphy, Petrography, Western Margin, Trachyte