GEOCHEMISTRY OF LARGE EXPLOSIVE ERUPTIONS FROM THE MAIN ETHIOPIAN RIFT

The Main Ethiopian Rift (MER) is located in a geodynamic setting controlled by continental spreading, which has occurred since Eocene-Oligocene. In this area, the active tectonics has repeatedly favoured magma ascent along the rift axis resulting in intense volcanic activity characterized by a wide compositional range (from basalts to per-alkaline rhyolites with scarce intermediate products). During the Quaternary, numerous high-magnitude explosive eruptions fed by SiO2-rich magmas, blanketed extensive areas both within and beyond the Rift Valley, making their deposits important stratigraphic markers at a regional scale. These eruptions represent the culmination of various processes leading to the accumulation of large volumes of evolved magmas at upper crustal levels. Over the last decades numerous studies have been dedicated to the petrological and geochemical investigation of these ignimbrites, but several questions remain unanswered: 1) are the most evolved rhyolitic compositions a product of fractional crystallization processes from the associated transitional basalts or do they derive from melting of crustal lithologies due to thermal perturbations during basaltic underplating? 2) why are the intermediate compositions minor in the geological record? 3) What are the conditions that allow the development of peralkalinity in the SiO2-rich magmas? This thesis mainly focuses on the study of two large, low crystallinity, and compositionally heterogenous ignimbrites erupted from the MER: the Golja (~1.16 Ma) and Kencherra (~1.26 Ma) ignimbrites. The study of these units is particularly intriguing due to the presence of basaltic (plagioclase-hosted melt inclusions), rhyolitic and, above all, intermediate compositions coexisting within the same pyroclastic sequence. The study of these ignimbrites will shed light on the genetic relationship (if any) between the different magma compositions and on the pre-eruptive history of the magma bodies that fed these two large eruptions. In addition to the Goljia and Kencherra ignimbrites, in this thesis I expanded the geochemical analyses to distal deposits of other SiO2-rich and far travelled explosive eruptions sourced from the MER. The goal is to use these data to complement the stratigraphic reconstructions of volcano-sedimentary sections from the archeological area of Melka Kunture, situated to the west of the rift valley, where important archeological findings were identified in close association with tephra layers. The geochemical fingerprinting of these tuffs will support stratigraphic correlations between different sections and providing further constraints to its chronological interpretation.