Publication Date:
2017-04-04
Description:
This paper presents the results of an investigation carried out on young volcanic rocks from the Gedemsa and Fanta
'Ale complexes, 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 characterized by a summit caldera of which the 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 onlymafic unit is represented by a basaltic eruption that occurred in 1870 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 showa typicalmafic–felsic bi-modal
distribution with fewintermediate terms (Daly Gap), as observed at regional scale along theMain Ethiopian Rift as
well as on the plateau. Geochemical data and modeling 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 mantle component, carrying the isotopic composition
of the Afar plume, and a crustal component, likely Pan-African sialic lower crust, that might have been
added in smallamounts, about 2%, tomaficmagmas. The origin of the primarymagmas is inferred to have occurred
by 7% partial melting of a mixed source region including both depleted and enriched mantle components
Description:
Published
Description:
130-144
Description:
1V. Storia e struttura dei sistemi vulcanici
Description:
JCR Journal
Description:
restricted
Keywords:
Main Ethiopian Rift
;
Peralkaline magmas
;
Mantle plume
;
Crustal assimilation
;
04. Solid Earth::04.08. Volcanology::04.08.99. General or miscellaneous
;
04. Solid Earth::04.08. Volcanology::04.08.05. Volcanic rocks
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
