Eruptive history and magmatic evolution of Corbetti Caldera in the Main Ethiopian Rift

The East African Rift (EAR) is dominated by peralkaline volcanism, making it an ideal location to study such typically rift-related volcanism and the processes behind the evolution of these enigmatic magmas. Over 120 million people live within a 100 km radius of a volcano within the EAR, and with the ongoing development of geothermal projects associated with large silicic centres, investigating the nature of past volcanism becomes vital for understanding these systems and informing future hazard assessments. The Main Ethiopian Rift (MER) forms part of the EAR and hosts 59 Holocene volcanoes exhibiting diverse eruptive products. This thesis focuses on one of these centres, a restless caldera in the southern section of the MER: Corbetti caldera.
Through fieldwork, and geochemical and petrological investigations of peralkaline erup- tive products, this thesis sheds light on Corbetti’s eruptive history, magmatic processes, and, by comparison with literature data, the variation in peralkaline magmas originating across the MER. This thesis shows that Corbetti has followed a similar evolutionary pattern to other calderas in the MER. It provides a comprehensive account of Corbetti’s eruptive history, presents a relative chronology of activity, constrains the age of the Biftu Tuff cone to < 7375 ± 54 cal BP and delimits several new distinct lava flows from Chabbi. This work shows that over the last 2.3 ka Corbetti has experienced at least one eruption every 300-400 years. These eruptions have almost exclusively been peralkaline rhyolites, with only a single basaltic eruption found (reported and documented here for the first time). Detailed geo- chemical investigation reveals the monotony of erupted compositions and elucidates how peralkaline magmas are derived from alkali basalts through progressive fractional crystalli- sation. Water content in the alkali basalts (∼0.5-1.2 wt.%) and peralkaline magmas (∼ 7 wt.%) are estimated, and storage depths constrained using barometry (50-190 MPa) and RhyoliteMELTS modelling (150-250 MPa). The distinct lack of geochemical and petro- logical diversity of peralkaline magmas at Corbetti is in stark contrast to the rest of the rift. This thesis reviews the geochemical variation in peralkaline products across the MER and investigates the mineralogical controls on observed liquid lines of descent, with a focus on the Qz-Or-Ab-Ns-Ac residua system. Distinct trends at the alkali-feldspar quartz cotectic are identified, influenced by the proportion and composition of the fractionating feldspar phase and the presence or absence of Na-rich clinopyroxene. This thesis also presents evidence from some MER centres showing a shift in the location of the quartz-feldspar co- tectic, which may be in response to variations in storage depth or concentration of halogens, particularly F, in the melt.
This work presents new insights into Corbetti’s past eruptions, underlining potential hazards associated within this actively uplifting caldera. It also enhances our understanding of the generation and storage of peralkaline magmas and highlights the homogeneity of Corbetti’s magmas compared to the diverse range of peralkaline compositions observed across the rift. These findings contribute to hazard assessments and further our knowledge of rift-related volcanism.

Keywords: Main Ethiopian Rift
Volcanic systems
Corbetti Caldera
Peralkaline Volcanism
Rift Volcanism
Advisor: Mather, T
Pyle, D
Fontijn, K