Magmatic evolution of the Boku Volcanic Complex, Main Ethiopian Rift
Abstract
The Boku volcanic complex is a Quaternary center situated on the axial segment of the Main Ethiopian Rift
(MER), located 92 km South-east from Addis Ababa. The main objective of this study is to understand the
magmatic evolution of the volcanic complex and to develop a model to answer some outstanding questions
related to bimodal products of rift related volcanism, using geological mapping, petrographic and geochemical
approaches. The Boku complex is characterized by two main phases of activity: pre-caldera/caldera forming
eruptive activity and post-caldera eruptive activity. The volcanic stratigraphy consists from bottom to top of a
sequence of rhyolitic lava flows, pumice flows, welded ignimbrite, pumice fall, rhyolitic lava dome, obsidian
flow, lower basaltic lava flow, ash flow, basaltic scoria and upper basaltic lava flows. The lithologic varieties
together with the geochemical results indicate that the Boku eruptive products are bimodal in composition; no
intermediate compositions are found. The mafic rocks are transitional to weakly subalkaline basalts while the
silicic rocks are predominantly peralkaline rhyolites. These two groups of rocks are co-genetic and related to
each other by fractional crystallization processes starting from mantle-derived basaltic magma with a small
component of crustal contamination. The available geophysical, geochemical and field data suggest that the
evolution of the evolved silicic center which hosts a bimodal rock distribution can be explained as a result of
prolonged stagnation of transitional basaltic melt (sourced from the mantle) at relatively high pressures, where
these evolve to intermediate compositions. The transitional basaltic melts can occasionally erupt to the surface
along weakness lines such as faults that infrequently cut the lower part of the shallow reservoir, generating
basalts from the intra-caldera and lateral eruptive centers. We consider the intermediate magma is mechanically
trapped at mid-crustal depths because of its higher crystal load (ca. 50%). Silicic magma is formed at shallow
depth by prolonged fractional crystallization of the intermediate magma and minor assimilation of crustal
material. These silicic magmas generate both effusive and explosive eruption products in the overall stratigraphy
of the volcanic complex.
Fichier principal
Zafu+Tadesse+et+al+(accepted)+-+Boku+Magmatic+evolution.pdf (6.16 Mo)
Télécharger le fichier
Origin | Files produced by the author(s) |
---|