Stratigrphy and Petrography of Flood Basalt Succession at Debre Libanos Area Central Ethiopia
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Date
2022-12
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Addis Ababa University
Abstract
The study area is located in the Northwest Shewa, Oromia Regional State with in NW Ethiopian Plateau, specifically around Debre Libanos, in central Ethiopia, about 105 km, in the northwest of Addis Ababa. The studied stratigraphic successions are cut by Zega wedem River. The main objective of the study is to constrain stratigraphy and Petrography of flood basalt succession at Debre Libanos area. To accomplish the objectives, starting from the beginning various methods have been applied. The methods are; Literature review, field work, and petrographic (thin section) analysis of representative samples. A total of 18 petrographic samples from these stratigraphic units are studied by using a petrographic microscope. From field and petrographic perspective, the area comprises the major lava flows identified in this study are; Aphyric basalt, plagioclase-olivine phyric basalt, plagioclase phyric basalt, ignimbrite, plagioclase-olivine-pyroxene phyric basalt and olivine-plagioclase phyric basalt. The volcanic units are underlain by Mesozoic Sandstone formation. The basaltic lava flows are the dominant product with interlayered pyroclastic deposits. The majority of the samples are from Basaltic units and have a variety of textures ranging from aphyric to phyric to mildly porphyritic. Plagioclase and Olivine are the major phenocryst phases with a proportion of pyroxene and Fe-Ti-oxides. In some samples, the groundmass is made of microlites of the same phase with a glassy texture. This distinctive petrographic heterogeneity reveals difference in depth of fractionation and magma flux in the lithosphere. The presence of olivine-plagioclase-pyroxene phyric basalt indicates there is considerable depth of fractionation in the shallow plumbing system. The aphyric basalt flow unit reflects there is no fractionation of any mineral in the shallow crust this indicates that the rate of magma flux to the shallow plumbing system is high.
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Keywords
Debre Libanos Volcanics, Flood Basalt, Stratigraphy, Petrography.