Major and Trace Element Compositions of Basaltic Lavas From Western Margin of Central Main Ethiopian Rift: Enriched Asthenosphere vs. Mantle Plume Contribution

Abstract

Major and trace element data are presented for basaltic lavas from western rift margin of central main Ethiopian rift located at Kella area to investigate the processes involved in the petrogenesis of the erupted magmas and the nature of mantle source compositions. Kella area is composed of Quaternary (<1.6 Ma), Miocene (10.6-8.3 Ma) and Oligocene basalts (30-29 Ma) ranging from alkaline to tholeiitic in composition. The geochemical variations of basaltic samples from Kella area exhibit two compositionally distinct basaltic groups. The Oligocene tholeiitic basalts display low MgO (5.29-6.11 wt.%), TiO2 (2.15-2.47 wt.%), P2O5 (0.28-0.34 wt.%), and high ratio of CaO/Al2O3 (0.68-0.72) and La/Nb (0.89-1.10). Whereas Quaternary and Miocene alkaline basalts display high MgO (7.40-8.86 wt.%), TiO2 (2.4-2.53 wt.%), P2O5 (0.44-0.52 wt.%) and low ratio of CaO/Al2O (0.62-0.66) and La/Nb (0.71-0.76). The contrasting incompatible element ratios (e.g., K/Nb, La/Nb, Rb/Zr and Zr/Nb) between tholeiite and alkaline basalts reflect differences in their mantle sources. Major and trace element variations, therefore, reflect the involvement of two geochemically distinct mantle sources in the petrogenesis of Kella basaltic lavas: i) the Oligocene tholeiite basaltic melts derived from enriched asthenosphere mantle source (E-MORB) and ii) the Miocene and Quaternary alkali basaltic melts show close similarity with ocean island basalts (OIBs) geochemistry, and this end member ascribed to the arrival of Afar plume head. The geochemical modeling reveals that the Oligocene tholeiite basaltic melts produced by equilibrium melting with 3-5 % degree of partial melting in spinel lherzolite mantle source, whereas the alkali basalts are produced with ~2% degree of partial melting within spinel-garnet lherzolite transition zone mantle sources.