Continental Flood Basalt (CFB) provinces typically erupt low-Ti (LT) and high-Ti (HT) lavas. LT lavas typically dominate the erupted volume in CFBs; however, their magmatic origin is not well understood. This is in part because CFB lavas are not primary magmas; rather they are differentiated products which result from large volumes of melt migrating and stalling in the lithosphere prior to eruption. In addition, the magmatic plumbing systems that feed continental flood basalts are complex networks of intrusive magma bodies in the crust undergoing frequent cycling of inputs (magma recharge, assimilation), outputs (fractional crystallization, eruption), and mixing/mingling of magmas. A temporal sequence of lava flows exposed at the surface does not represent an evolving magma chamber, but rather snapshots of individual plumbing system pathways at a given point in time. For this reason, geochemistry of lava flows observed in a stratigraphic context will result in an apparently inconsistent liquid line of descent. Developing a more complete understanding of the overall system that creates CFB provinces first requires insight into how lithospheric processes impact the geochemical evolution of these magmas, and a relatively complete flow sequence is needed to accurately evaluate how the magma system evolves over time. The Ethiopian LT flood basalt province is an ideal location to probe the lithospheric processes that impact CFB magmas because the flow-stratigraphy is well-preserved and the geochemical distinctions between LT basalt and HT basalt are well characterized. In this body of work, 190 lava samples were collected from the LT province and analyzed by a combination of methods including petrographic analysis, bulk rock geochemical analysis, and Sr-Nd-Pb-Hf isotope analysis. The approach of this dissertation is to use stratigraphic, petrographic, and bulk-rock geochemical datasets to first evaluate the role of lithospheric processes on the geochemical evolution of the LT province. Chapter 1 provides the theoretical framework on how magma evolves in the lithosphere and how different processes (fractional crystallization, assimilation, magma recharge, and magma evacuation) impact the liquid line of descent. Chapter 2 establishes the temporal framework, the petrographic taxonomy, and broader magmatic processes impacting the LT lavas from flood basalt initiation to termination. The results of this work establish 3 main divisions and 3 subdivisions within the LT province based on petrologic transitions observed in the stratigraphy. The transitions in the petrostratigraphic groups indicate a shallowing of the magmatic plumbing system over time. Chapter 3 evaluates the geochemical evolution of the LT basalts within the main phase of volcanism using the petrostratigraphic subdivisions determined in Chapter 2. Geochemical models are then used to constrain the depth of magma storage and relative contribution of recharge, evacuation, assimilation of crust, and fractional crystallization acting upon the plumbing system over time. After establishing the lithospheric imprint on the LT magma evolution, a multi-component melt source is evaluated using Sr-Nd-Pb-Hf isotopic data (Chapter 4). Mixing models and assimilation and fractional crystallization models illustrate that the LT magmas originated from a multi-component melt source which includes depleted upper mantle, a known regional plume component, and a lithospheric component. It is also shown that the relative contribution of each component varies over time, from flood basalt initiation toward flood basalt termination.
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