The Magmatic Plumbing System of Quaternary Basalts in the Main Ethiopian Rift

Quaternary basalts from the Silti-Debre Zeyit Fault Zone, in the central Main Ethiopian Rift, provide an opportunity to investigate the magmatic plumbing system of continental rift basalts. We investigated olivine, clinopyroxene and feldspar phenocrysts and xenocrysts hosted in MgO-rich basalts to reveal information concerning the history of the host basalt en-route to the surface. These histories may be used to constrain some of the complex lithospheric processes that occur in continental rifting. Two end-members were identified with distinctly different magmatic ascent histories: one group that ascended rapidly to the surface and another that rose slowly or paused in the mid- to upper crust. The rapidly-ascending end-member is typified by the presence of skeletal olivine, dynamically recrystallised grains of olivine, kink-banding in clinopyroxene, and clinopyroxene crystallization pressures of 8-9 kbar. Electron microprobe analysis revealed reverse zoning of olivine xenocrysts in these lavas. The depths of clinopyroxene equilibration in rapidly-ascending basalts correspond to the base of the crust and indicate magma ponding at the crust-mantle boundary. In the shallowly-equilibrated basalts, the common stress features observed in the previous group were absent, and clinopyroxene equilibrium pressures were generally between 2 and 4 kbar. In these shallowly-equilibrated basalts, feldspar phenocrysts were pervasive and often nucleated on pre existing clinopyroxene and olivine xenocrysts. Olivine xenocrysts exhibit chemical re-equilibriation with the host magma. Evidence of rapidly ascending magmas in this area is consistent with surface morphology as explosive maars are observed along the Silti-Debre Zeyit Fault Zone. The presence of two basalt groups with distinctly different magmatic ascent histories and separated by a distance of less than 5 km argues strongly against the existence of large mid-crustal magma chambers in this region. Instead we propose a system of dykes and small magma chambers that traverse the lithosphere (e.g. Rooney et al., 2005). This petrographic model can be used in geochemical analysis of the regional basalts by constraining likely crustal contamination of those samples that have ascended slowly or ponded within the crust.