Quaternary volcanic activity in the main Ethiopian Rift (MER)
|Location||International Geological Congress,oslo 2008|
|Author||Kersten, Franziska; Gloaguen, Richard; Pfaender, Joerg|
|Holding Date||08 October 2008|
The lithosphere beneath the MER has been modified significantly in the course of the still ongoing rifting process. Geophysical investigations revealed solidified magmatic intrusions as well as melt in shallow to midcrustal regions beneath the tectono-magmatic segments (TMS) aligned en echelon along the rift axis (Keranen et al., 2004; Whaler and Hautot, 2006). According to Keranen et al. (2004) and Kurz et al. (2007) Quaternary extension takes place exclusively in TMS where recent magmatism and fault activity is localised.
To investigate the nature and sources of volcanic activity and its interaction with the lithosphere in Ethiopia, we follow a combined approach of geochemical measurements, field observations and remote sensing. Geochemical analyses include major and trace element as well as isotope composition data from a bimodal suite of samples from eight volcanic centres within the rift valley (Ayelu, Hertali, Dofan, Fantale, Kone, Bosetti, Gedemsa and Aluto, from NE to SW). Rare earth-element (REE) patterns of the MER basalts, albeit exhibiting similar anomalies, intersect one another, indicative of an at least slightly heterogenous magma source region. Sole exceptions to this observation are cogenetic samples from Dofan and Fantale, both situated within the same TMS and thus likely to be fed from the same magma source.
Sr, Nd and Pb isotopic data attribute the MER basalts to an enriched mantle source reflecting both EM I and EM II-characteristics, while Afar basalts originate from a region influenced by enriched as well as depleted mantle end-members. Trace element modelling based on the assumption, that the sampled rhyolites were derived from the basalts by fractional crystallization of an ol-, cpx-, plag-assemblage (as suggested from petrographical observations) did not produce satisfying results. Crustal assimilation could be ruled out as well, indicating a rapid ascent of the magma through a well-developed magmatic plumbing system. We therefore hypothesize that the young sampled basalts are not the educts of their spatially related acidic counterparts. They could either have ascended through different pathways through the crust or used the same magmatic plumbing system, but at different times.
In addition to the geochemical evidence, field observations indicate that basaltic eruptions bound to the TMS are the most recent expression of rift-related magmatism and extension. Furthermore aerial photographs and satellite images of the axial portions of the rift reveal that basalts are erupted from fissures and various cinder cones, which are associated with or aligned along faults in the center of the segments whereas the mostly rhyolitic acidic volcanites are linked solely to stratovolcanoes, calderas and lava domes located near the segment tips.