Continental Flood Basalts and Mantle Plumes: a Case Study of the Northern Ethiopian Plateau

Abstract

New geochemical data integrated in a petrogenetic model indicate that the $30 Ma Northern Ethiopian continental flood basalts (CFBs) preserve a record of magmas generated from the centre to the flanks of a plume head, currently corresponding to the ' Afar hotspot' . Basaltic lavas appear zonally arranged with Low-Ti tholeiites (LT) in the west, High-Ti tholeiites (HT1) to the east and very High-Ti transitional basalts and picrites (HT2,TiO 2 4^6 wt %) closer to the Afar triple junction. Modelling provides estimates of the P^T^X conditions of magma generation showing that the Ethiopian CFBs could be generated in the pressure range 13^30 GPa at an approximate depth of 40^100 km from mantle sources that were increasingly metasomatized and hotter (1200^15008C) from west to east; that is, from the outer zones (LT) to the core of the plume head (HT2 ultra-titaniferous basalts and picrites). Metasomatizing agents can be envisaged as alkali^silicate melts that integrate various geochemical components (e.g. Ti and related high field strength elements, low field strength elements, light rare earth elements, H 2 O, noble gases, etc. ) scavenged and pooled along the plume axis, and derived from heterogeneous mantle materials mixed during the plume rise. This has significant implications for the current debate about mantle plumes, as the modelled compositionally and thermally zoned plume head (Texcess ! 3008C with respect to ambient mantle) is in accordance with seismic tomography and buoyancy flux, as well as geochemical characteristics, thus supporting a deep provenance of the Afar plume, which possibly originated in the transition zone or lower mantle.