Geochronology and geochemistry of the Essimingor volcano: Melting of metasomatized lithospheric mantle beneath the North Tanzanian Divergence zone (East African Rift)

The North Tanzanian Divergence zone (NTD), at the southern end of the eastern branch of the East African Rift, is part of one of Earthʹs few currently active intra-continental rift systems. The NTD preserves a complex tectono-magmatic evolution of a rift in its early stage of activity. The oldest magmatism reported in the NTD is associated with the centrally located Essimingor volcano. Although major element oxides show narrow compositional variations suggesting fractional crystallization, trace element abundances and Sr–Nd–Pb isotopic data have complex distributions that require open-system processes. The more primitive samples (MgO > 9 wt.%) likely reflect partial melting of a metasomatized lithospheric mantle characterized by residual garnet, phlogopite and minor amphibole. The range of radiogenic Pb isotopic compositions indicates the presence of mixing between this source and the lithosphere of the western branch of the East African Rift (Toro Ankole and Virunga). Laser-incremental heating of selected samples gives 40Ar/39Ar ages that range from 5.76 ± 0.02 Ma to 5.91 ± 0.01 Ma, suggesting an age roughly 2 myr younger than previously reported.