Continental large igneous provinces are among the largest magmatic events on Earth, characteristically manifesting at the surface as stacks of monotonous flood basalt lavas that form through a dynamic pattern of eruptive pulses separated by hiatuses. Mantle plumes are typically considered responsible for generating the large volume of magma required, however, the geometry of these upwellings is poorly constrained. Two distinct flood basalt episodes are evident in East Africa-an Eocene Initial Phase and an Oligocene Traps Phase-both considered related to the African Large Low Shear Velocity Province. The initial interaction between material rising from the African Large Low Shear Velocity Province and the African lithosphere manifests as the Eocene Initial Phase, centered on southern Ethiopia and northern Kenya. In chapter 2 we present a geographically well-distributed geochemical dataset comprising the flood basalt lavas of the Eocene Initial Phase to refine the regional volcano-stratigraphy into three distinct magmatic units: (1) the highly-alkaline small-volume Akobo Basalts (49.4-46.6 Ma), representing the initial phase of flood basalt volcanism derived from the melting of lithospheric-mantle metasomes, (2) the primitive and spatially restricted Amaro Basalts (45.2-39.58 Ma) representing the early main phase of flood basalt volcanism derived from the melting of the upwelling thermochemical anomaly, and (3) the spatially extensive Gamo-Makonnen magmatic unit (38-28 Ma) representing the mature main phase of flood basalt volcanism that has undergone significant processing within the lithosphere resulting in relatively homogeneous compositions. The focused intrusion of these main phase magmas over 10 m.y. preconditioned the African lithosphere for the localization of strain during subsequent episodes of lithospheric stretching. The focusing of strain into the region occupied by this continental LIP may have contributed to the initial extension in SW Ethiopia associated with the East African Rift. Chapter 3 presents a stratigraphically well-constrained series of 54 flood basalt flows from the Eocene Initial Phase of magmatic activity in East Africa. These flows, and more importantly, their crystal cargo, permit temporal insight into the development of one the youngest and best-preserved continental large igneous provinces. The stratiform mafic lavas exposed in northern Kenya consist of alternating aphyric and plagioclase-rich lava packages consistent with periods of eruption punctuated by volcanic hiatus, where magmas stall and crystallize plagioclase at medium to shallow crustal levels. Plagioclase compositions (n = 545) exhibit little intra-crystal or intra-sample compositional diversity. Intra-crystal equilibrium calculations for Sr and Ti indicate internal chemical equilibrium, requiring storage at high temperature over a prolonged time interval (10,000-100,000 years). Using a series of seven interlinked partial crystal fractionation under equilibrium conditions models, we replicate the observed stratigraphic patterns in plagioclase composition. We find that the balance between recharge and evacuation, and diffusive equilibration within a shallow magmatic system, controls the composition of plagioclase in these flood basalts. We conclude that the shallow fractionation system modulates eruptive cycles and thus constitutes a critical component in studies of continental large igneous provinces. Chapter 4 presents the first chemo-stratigraphic section of the continuous series of flood basalts from the Eocene Initial Phase. Building upon the petro-stratigraphic framework developed in chapter 3, we explore how magma flux influences erupted lava compositions at Lokitaung in northern Kenya. We find that the geochemical variability within the lava flows parallels the existing petro-stratigraphic framework-stratigraphic trends in lava compositions have inflection points at the boundaries between petro-stratigraphic units. Incompatible trace element profiles of Lokitaung lavas show a common pattern consistent with a common parental magma composition, suggesting the observed geochemical heterogeneity between flows is a function of magma differentiation within the continental lithosphere. We interpret the variability in lava compositions as reflecting open-system magma differentiation processes and use the mass balance equations to construct numerical models to resolve the petro- and chemo-stratigraphic variability the lava compositions at Lokitaung. We find that the three volcano-stratigraphic units differ in terms of magma recharge: the Lower Basalts represent the initiation of the system (first 200m of stratigraphy) and are the period of highest magma recharge; the Middle Basalts, though the main phase of flood basalt activity (500 m thickness), are a period of lower magma recharge; the Upper Basalts represent the termination of flood basalt activity (uppermost 64 m), and are the period of the least magma recharge. This cycle of activity parallels similar petrographic observations from the Oligocene Traps Phase of the NW Ethiopian Plateau therefore illustrating a similar progression in the development of a continental flood basalt magma plumbing system at both locales.
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