The Quaternary monogenetic Bayuda Volcanic Field, Sudan – Insights into mantle and crustal processes during magma petrogenesis

Abstract

The Holocene Bayuda Monogenetic Volcanic Field (BMVF) is located at the Great Bend of the river Nile in the north of Sudan. The analysed volcanic rocks are of basanitic to hawaiitic composition, characterising the BMVF as a sodic-alkaline volcanic field. Petrographic evidence documents olivine and clinopyroxene as the main fractionating phases as well as FC processes that were accompanied by assimilation of mantle and crustal rocks in addition to mingling and mixing processes. Utilising systematic differences in trace element variations, two primary magmas of Series A and B can be distinguished. Uniform normalised incompatible element patterns, e.g. LREE enrichment accompanied by HREE depletion, negative anomalies of K and Pb, and modelling based on partition coefficients of La, Dy and Yb in mantle residues, suggest partial melting of an amphibole-garnet lherzolite. The presence of amphibole provides a constraint, implying that the melt was derived from a metasomatised lithospheric mantle. The presence of the Pb-depletion (documented by Ce/Pb ratios of >30) suggests that all magmas are of HIMU-OIB type. This is supported by their uniform 87Sr/86Sr (0.703010–0.703347), 143Nd/144Nd (0.512869–0.512995), 206Pb/204Pb (19.787–19.895), 207Pb/204Pb (15.656–15.653) and 208Pb/204Pb (39.560–39.678) isotope characteristics. Additionally, we can describe the HIMU character of the NE African mantle with respect to 176Hf/177Hf (0.283006–0.283026). The mantle source of the BMVF magmas appears to have the composition of the common mantle “C” with an overprint by a HIMU component, resulting in higher radiogenic 206Pb/204Pb ratios above 19.5 (FOZO-HIMU-OIB type). We attribute this to the metasomatic character of the lithospheric mantle. The Hf/Nd isotope characteristics of the BMVF suggest that this metasomatism in the mantle source is most probably late Pan-African in age. When compared to circum-Mediterranean and regional volcanic fields, the high radiogenic Pb character of the BMVF indicates that their genesis cannot be related to magmatism associated with the activity of the Afar plume event and consequential magmatism along the Main Ethiopian Rift and the Red Sea Rift, as they are all of low radiogenic character.