Vinsamlegast notið þetta auðkenni þegar þið vitnið til verksins eða tengið í það: http://hdl.handle.net/1946/34512
Petrological and geochemical data are reported for a bimodal magmatic sequence of transitional basalts to peralkaline rhyolites from the Abaya volcanic area in the Southern Main Ethiopian Rift. The collection and analysis of a comprehensive dataset of rock samples aims to lay a foundation to understand processes of melt generation, storage and evolution beneath the rift and develop a source to surface model of the magmatic plumbing system in the sub-rift region. An attempt to characterize the nature of the heat source below the geothermal system is also presented.
Major and trace element geochemistry of basaltic lavas are consistent with melting of a primitive mantle, with partial melt fractions of 3–4% in the garnet lherzolite stability field. Sampled basalts reflect the eruption of homogeneous magma batches rising from a deep crustal storage zone at 21 to 25 km depth (~5.8 to 6.8 kbar) and temperatures ~1150°C, with short-term or no storage within shallow magma chambers. The different composition of basaltic lavas are related to fractionation processes during storage and ascent, with dominant mineral assemblage of plagioclase, clinopyroxene, olivine and iron-titanium oxides.
Petrological and geochemical evidence demonstrate that peralkaline rhyolites, pitchstones and ignimbrites are genetically related to basalts through prolonged fractional crystallization (up to 85%) within shallow magma chambers beneath the Salewa Dore–Hako Graben, located on the western boundary of Abaya. These chambers are located at shallow levels between 4 to 7 km depth (~1 to 2 kbar) and temperatures ~800°C; undergoing complex differentiation involving crystal fractionation of alkali feldspar, iron-titanium oxides, fayalite, amphibole and other accessory phases. Other ongoing processes in these shallow reservoirs include injection of basic magma, magma mixing and reaction with the country rocks. The eruptions of peralkaline domes and mafic scoria cones in the Holocene are closely associated with extensional structures belonging to the Wonji Fault Belt. The bimodal composition of the volcanic system indicates the presence of a dual petrogenetic mechanism with a primary mantle-derived basaltic system linked to crust-derived silicic system, obtaining heat and magma from the basaltic system.
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