Head-to-tail transition of the Afar mantle plume: Geochemical evidence from a Miocene bimodal basalt–rhyolite succession in the Ethiopian Large Igneous Province

Abstract

Miocene rhyolites and basalts from the Ethiopian Large Igneous Province (LIP) crop out in north Shewa on the central Ethiopian plateau. The volcanics were emplaced at the onset of development of the Main Ethiopian Rift and postdate eruption of the Oligocene Ethiopian continental-flood basalt (CFB) province by ~ 15 Ma. Variations in major- and trace-element chemistry indicate that fractionation of the north Shewa parental magmas occurred at ~ 15 km depth; this is consistent with geophysical studies which have predicted the presence of gabbroic intrusions in the mid to upper parts of the underlying ~ 50 km thick crust. High-to-moderate [La/Nb] n (1.1–1.4) and Ce/Pb ratios (17–19.5) for the basalts, together with mantle δ 18O values for the rhyolites, suggest that the north Shewa magmas contain only minimal contributions of crustal melts. The formation of the Ethiopian CFB province has been associated with the sub-lithospheric impact of the Afar mantle plume ‘starting-head’ whereas recent volcanic activity in the Main Ethiopian rift has been linked to lithospheric extension and adiabatic decompression melting in the steady-state ‘tail’ of the plume. Rare-earth element (REE) inversion modelling of Oligocene, Miocene and Recent mafic magmas from the Ethiopian LIP has allowed us to determine changes in the amount of partial melting that accompanied the head-to-tail transition of the Afar plume. The results also provide information on the approximate depth of melting and mantle potential temperature and allow us to assess how these have varied both spatially and temporally. The REE inversion results suggest that, since the Oligocene, the amount of adiabatic decompression melting has decreased two-fold, from 13 to 7%, and the potential temperature of the Afar mantle plume has decreased from ~ 1450 to 1350 °C. The ~ 15 Ma north Shewa basalts formed by slightly lower degrees of partial melting (5%) than those in the present-day rift, presumably due to the presence of thicker lithosphere (60 km) prior to the main phase of rifting during which the lithosphere thinned to ~ 50 km at the rift axis.