Hidden but ubiquitous: the pre-rift continental mantle in the Red Sea region

The extensive volcanism in the western part of the Arabian plate forms one of the largest Cenozoic alkali basalt provinces in the world where large lava fields with sub-alkaline to alkaline affinity are scattered from Syria and the Dead Sea Transform Zone through western Saudi Arabia to Yemen (Coleman et al. 1983). Most of volcanism took place after the emplacement of the Afar plume in Yemen (~30 Ma) and progressively propagated northward due to the lithospheric thinning related to the Red Sea rifting starting from 27-25 Ma (Bosworth and Stockli, 2016). However, few lava fields were emplaced during the Mesozoic, with the oldest volcanic activity as old as 200 Ma in the north Israel (Atlit- 1 and Haifa-1 drillholes) (Khon et al., 1993). Here, we report new results from volcanic pipes in the Marthoum area immediately to the east of Harrat Uwayrid where over a hundred pipes are aligned along NW-SE fractures in the Ordovician sandstone of the Saq Formation. The chilled vitric nature of these basalts suggests that the pipes are the result of phreatomagmatic explosions which occurred when the rising magma columns met the water table in the porous sandstone host. These lavas have Sr-Pb-Nd-Hf isotopic compositions that plot out of the field of the Cenozoic Arabian alkaline volcanism, being far more enriched in Nd-Hf and Pb isotopes than any lava ever reported in the Arabian plate. New K-Ar dating limits their age to 80 and 50 Ma, thus predating the emplacement of the Afar plume and the rifting in the Red Sea. Our findings indicate that these volcanic eruptions formed from melts generated by a low-degree partial melting of an enriched lithospheric source triggered by local variations in the asthenosphere-lithospheric boundary. This mantle source has a composition similar to the HIMU-like enriched isotopic component reported in eastern Africa Rift (Rooney et al., 2014) and considered to represent the lowermost lithospheric mantle of the Nubian shield. Although apparently hidden, this enriched deep lithospheric component is therefore ubiquitous and widespread in the cratonic roots of the Arabian and African lithospheric mantle, but variously mixed with melts derived from a depleted asthenosphere to produce a HIMU-like flavour dispersed in the Cenozoic Arabian alkaline volcanism.

Bosworth, W. and Stockli, D. Early magmatism in the greater Red Sea rift: timing and significance. Can. J. Earth. Sci., 53, 1158–1176, 2016.

Coleman, R. G., Gregory, R. T., Brown, G. F. Cenozoic volcanic rocks of Saudi Arabia. Saudi Arabian Deputy Minist. Miner. Resour., Open File Report, USGS-OF-03-93, pp. 82, 1983.

Khon, B. P., Lang, B. and Steinitz, G. ⁴⁰Ar/³⁹Ar dating of the Atlit-1 volcanic sequence, northern Israel, Israel J. Earth-Sci., 42, 17–28, 1993.

Rooney, T. O., Nelson, W. R., Dosso, L., Furman, T., Hanan, B. The role of continental lithosphere metasomes in the production of HIMU-like magmatism on the northeast African and Arabian plates. Geology, 42, 419–422, 2014.