Petrogenesis and geodynamic significance of the volcanism of the Northern Ethiopian plateau

After the pioneering works of Morgan during the Seventies (Morgan, 1971) a renewed interest on mantle plumes has arisen in the last decades with controversial hypotheses on depths of plume provenance, triggering mechanisms, shape and size of the convective mantle, as well as relationships with hot spots, Large Igneous Province (LIP), and rift volcanism (Ernst & Buchan, 2001; Foulger et al., 2005; Foulger & Jurdy, 2007). In this regard, the Ethiopian-Yemen basaltic plateau represents a very convenient natural laboratory to study Continental Flood Basalts (CFB) and the tectonomagmatic processes that led to the formation of the Red Sea, Gulf of Aden and East African rift system, from Oligocene to Present. This area is centred on the Afar hot spot which appears unequivocally related to a deep plume originating in the lower mantle (Courtillot et al., 2003, Davaille et al., 2005). A multidisciplinary study has been developed on the Ethiopian Oligocene Plateau, integrating field data with detailed sampling of selected basaltic sections, geochemical and petrological results, as well as GIS processing, in order to provide new insights on the mantle plume region from which plateau basalts were generated. The integrated approach includes: 1. Digitalization, georeferentiation and projection in the UTM system WGS-84 Datum of the geological map 1:2.000.000 by Merla et al. (1973), and subsequent creation shapefiles for each geological formations related to the Oligocene CFB and a shapefile containing the UTM coordinates of the studied samples. The topographic values (z) were obtained by NASA Shuttle Radar Tomography Mission (SRTM) rasters. 2. Petrochemical analyses of samples collected along selected plateau sections and petrological classification allowed definition of a zonal arrangement of volcanics with Low-Ti tholeiites in the NW part and High-Ti transitional basalts (and picrites) in the SE sector of the Ethiopian plateau. Calculation of the erupted volumes of each magma-type was carried out. 3. Thermobarometric methods and the Niu & Batiza (1991) empirical model were applied to the various magmatypes in order to constrain their degree of partial melting (F), temperature (T), as well as initial (P ) and final (P ) pressures of melt generation in a mantle upwelling region. Calculations were carried out on selected representative near-primary magmas, i.e. in equilibrium with peridotite mantle sources. Petrogenetic modelling is also used to figure out the plume influenced mantle region from which northern Ethiopia CFB were generated.