Magmatic activity across the East African North Tanzanian Divergence Zone

Volcanism across the North Tanzanian Divergence Zone (NTD), part of the East African Rift System, occurred episodically from the late Miocene to Recent. Here, we present a summary of previously published K-Ar and 40Ar/39Ar ages, new 40Ar/39Ar ages, and geochemical and Sr-Nd-Pb isotopic analyses on samples collected from several volcanoes distributed across the NTD: Burko, Monduli, Tarosero, Ketumbeine, Gelai, Kerimasi and Meru. The locus of volcanism over this period progressed from the southwestern portion of the NTD to the north and east, with a main pulse occurring at about 2.3 Ma, possibly marking the inception of a main rifting event. We model the source of the NTD volcanic rocks as a metasomatized subcontinental lithospheric mantle that includes minor and variable amounts of garnet and amphibole. REE data indicate variations in residual garnet content, consistent with varying depth of melt separation. Radiogenic isotopic data show systematic variations requiring the involvement of up to three components. Two alternative but not exclusive tectonic scenarios are proposed: one requiring the involvement of contributions from recent plume-related fluids, and one explaining the observed geochemical variations by melting of a lithosphere layered by multiple metasomatic events.