FORMATION OF BASALTIC - SILICIC CRUST ABOVE THE AFAR PLUME

The volcanically-seismically active Afar depression hosts the Red Sea - Gulf of Aden - East African Rift triple junction above a lower mantle-sourced upwelling, considered to be the youngest Afar mantle plume worldwide, beneath thick cratonic lithosphere. This Afar plume caused the eruption of a thick sequence of flood basalts ca 31-28 Ma and continues to the present in the Afar depression where rifting has progressed to continental rupture with bimodal basaltic-silicic magmatism.

We provide geochemical analysis of major, trace elements, and isotopes of Pb, Sr, Nd and Oxygen in fifty recent to 17 Ma mostly fissural erupted lavas from Gona on the western flank of the southern Afar rift and Dabbahu-Manda Harraro rift in the Afar depression. The lava chemistry reveals the processes occurring as magmas ascend from an enriched primitive mantle source through the thin plate to the surface in creating bimodal basaltic and rhyolitic groups. Nd, Pb and most of the Sr isotope data indicate a common source for both mafic and silicic magmas (ε_Nd=1.9-4.6, ⁸⁷Sr/⁸⁶Sr ~ 0.7045, ²⁰⁶Pb/²⁰⁴Pb = 18.2-18.6), with radiogenic Sr in some rhyolites. These isotopic data are compatible with a lower mantle plume source for the mafic magmas. Continental crustal contamination of the magmas can be ruled out on the basis of the aforementioned radiogenic isotopic data, normalized multiple trace element plots, and Ce/Pb ratios (~10) in both basalts and rhyolites, unlike typical mantle (~25) and continental crust (~4). Whole rock oxygen isotope ratios (δ¹⁸O) of basalts and rhyolites with plume-like Nd, Sr and Pb isotopes range from 6 – 6.5‰ with less than 0.8% LOI.

These data in combination, indicate that silicic magmas were produced by partial melting of previously emplaced basalts in the rift by the heat source from the plume below. This magma underwent fractional crystallization of plagioclase causing the observed strong negative Eu anomalies and the peralkaline nature of the silicic lavas. The silicic crust is volumetrically substantial (~35%) in the Dabbahu rift segment, as estimated by calibrating mapped data from Landsat imagery. This mechanism of generating bimodal basaltic-silicic magmas above the Afar plume may be applicable to grow early Hadean – Archean continental nuclei without requiring onset of subduction.