Giordano F. a,b, D'Antonio M. a,b,*, Civetta L. a,b, Tonarini S. c, Orsi G. b, Ayalew D. d, Yirgu G. d, Dell'Erba F. b, Di Vito M.A. b, Isaia R. b
Genesis and evolution of mafic and felsic magmas at Quaternary volcanoes within the Main Ethiopian Rift: Insights from Gedemsa and Fanta 'Ale complexes
This paper presents the results of an investigation carried out on young volcanic rocks from the Gedemsa and Fanta ‘Ale volcanoes, located in the Main Ethiopian Rift, the site of an intense magmatism since Eocene-Oligocene. The earlier NW-SE direction of extension of the Rift, which generated NE-SW trending faults, rotated around E-W in Quaternary times, and produced the still active N to N-NE Wonji Fault System. The Gedemsa volcano is located in the central part of the Ethiopian Rift, about 100 km SE of Addis Ababa. It is characterized by a wide central caldera, about 8 km in diameter. The general stratigraphic sequence in the area includes, from base upwards, rift-floor ignimbrites, pantelleritic and subordinate trachytic pyroclastic deposits and lava flows and domes, and widespread basaltic deposits. The Fanta ‘Ale volcanic complex is located in the northern part of the Main Ethiopian Rift, where the Afar depression begins. It is characterised by a summit caldera which diameter is about 4 km. This volcano erupted trachytic and rhyolitic lavas, whereas the most diffuse unit is an ignimbrite related to the caldera collapse. Explosive activity has occurred inside and outside the caldera, forming tuff cones and thick pumice fallout deposits. The only mafic unit is represented by a basaltic eruption occurred in 1,870 AD. Historical eruptions and intense fumarolic activity are evidence for the persistence activity of the Fanta ‘Ale in this part of the Main Ethiopian Rift.New geochemical and Sr-Nd-Pb isotope data on representative samples from Gedemsa and Fanta ‘Ale volcanoes are presented and discussed in order to shed light on the genesis of mafic and felsic magmas, the genetic link between them, and their possible interaction with the local crust. Volcanic rocks show a typical mafic-felsic bimodal distribution with few intermediate terms (Daly Gap), as observed at regional scale along the Main Ethiopian Rift. Geochemical data and modelling suggest that magmas evolved mainly through fractional crystallization processes, accounting for the entire mafic-felsic compositional variation. However, Sr-Nd-Pb isotope data reveal also open-system evolution processes. The most differentiated, Sr-poor rhyolites suffered important low temperature contamination by shallow fluids of hydrothermal and/or meteoric origin. This affected mostly the Sr isotopic composition of whole-rocks, and much less that of separated feldspars that provide more reliable 87Sr/86Sr values. Mafic rocks, as well as the least contaminated felsic rocks, provide evidence for two components involved in the genesis and evolution of mafic magmas: a depleted component, carrying the isotopic composition of the Afar mantle plume, and an enriched component, likely Pan-African sialic lower crust, that might have been added in small amounts, about 2 %, to mafic magmas. The origin of the primary magmas is inferred to have occurred by 7 % partial melting of a mixed source region composed of depleted MORB mantle and enriched mantle.
Key-words: Main Ethiopian Rift, peralkaline magmas, mantle plume, crustal assimilation, isotope geochemistry