Lithostratigraphy of the southeastern part of the Ethiopian flood basalt province

Abstract Fully preserved continental flood basalt stratigraphy provides a perfect window to comprehend the temporal evolution and geological history of plume-related volcanism, since continental flood basalt does not represent a primary magma, detail stratigraphic and phase equilibrium studies offer insight into the fractionation processes and plumbing system of the volcanism that leads to continental flood basalts. This study presents a new flow-by-flow petrographic modal description, major element and stratigraphic framework of Cenozoic rocks from the central part of the southeastern Ethiopian flood basalt province (the Chole section). Based on the petrographic variation and volcanic hiatus marker the section is divided in to: 1) lower flood basalt, 2) middle flood basalt and 3) upper flood basalt. The lower flood basalt is a transitional basalt with quartz-hypersthene and olivine-hypersthene normative alternate with narrow range of MgO (4.8-5.66 wt.%) and SiO2 (47.06-48.88 wt.%). The middle flood basalt is silica undersaturated basalt and plots towards the primitive magma composition with MgO (6.4-12.6 wt.%), whereas the upper flood basalt is quartz-hypersthene normative sub-alkaline tholeiitic basalt with SiO2 (49.73-53.73 wt.%) and MgO (4.16-6.49 wt.%). The Phase assemblage and major element demonstrate that the lower flood basalt in the province experienced low-pressure fractionation involving olivine + plagioclase + clinopyroxene with variable degree of recharge. A transition in modal mineralogy from plagioclase aphyric-phyric lower flood basalt to plagioclase-free olivine-pyroxene phyric middle flood basalt suggests a change in the magma plumbing system. Changes in modal mineralogy, decreasing flow thickness, and periodic eruption in the middle flood basalt are interpreted as the result of a significant decrease in magma production and magmatic flux to the lower and upper stalled magma reservoir. The lower stalled reservoir directly fed the middle flood basalt. The plagioclase aphyric-microcrystalline upper flood basalt results from low-pressure fractionation of olivine + plagioclase + clinopyroxene, and the basalt uncomformably overlies the middle flood basalt, implying changes in the parent magma and plumbing system.