Magmatism in the Cenozoic Ethiopian rift zone the case against the Afar plume hypothesis

Abstract: Volcanism and rifting in the Afro-Arabian, i.e. Red Sea - Gulf of Aden - Ethiopian, region have often been attributed to a Tertiary plume centered on Afar, at the junction of the three rifts. Geological observations, and geochemical and isotopic studies of Oligocene (=32 Ma) to Holocene basic lavas covering the central Ethiopian rift zone (CERZ) do not support the plume hypothesis. In the African continent there are several examples of Mesozoic and Cenozoic swell and basin structures (800 - 1500 km across) which are more consistent with a thermal anomaly originating over a broad region rather than at a point source. Moreover, within the Cenozoic Ethiopian swell, domal structures (up to 300 km across) exist that appear to be discrete centers of volcanism and deformation, on a localscale. Extensive tholeiitic volcanism in the CERZ occurred from 32 Ma to 20 Ma and locally between 10 Ma and 8 Ma. Alkaline (alkalic + transitional) volcanics are the dominant type and exhibit systematic spatial and chemical relationships that do not extend to the tholeiites. The older (32-20 Ma) alkaline basalts were produced by higher extents of partial melting and at deeper levels than the younger ones. These have either EMI-PREMA or EMII-PREMA isotopic signatures. In contrast, tholeiites of various ages were derived from a homogeneous EMI type source, unique to these basalts. This, taken together with the observation that tholeiites are the relatively highdegree partial melts in the CERZ suggests that the tholeiites were directly derived from an anomalously hot mantle. The depletion of the highly relative to the moderately incompatible elements in both the alkaline and tholeiitic basalts with EMI isotopic signatures indicates the extraction of melt from these reservoirs previously and thus provides strong evidence for episodic transient thermal pulses and for the origin in the upper mantle of the Tertiary thermal anomaly. The incompatible element systematics of the alkaline lavas with the EMI -PREMA isotopic characteristics is similar to melts presumably derived from subduction-enriched subcontinental lithospheric mantle (SCLM), urging caution in the interpretation of such systematics. The nature, sequence and style of involvement of mantle reservoirs in the CERZ suggest a four-tiered enriched upper mantle beneath the northern Ethiopian rift zone.