Rifting of a Continent: the North Tanzanian Divergence Zone, East African Rift System

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• Sara Mana (Rutgers University)
• Thursday 24 May 2012, 17:00-18:00
• Harker II, Geology Department.

If you have a question about this talk, please contact Yves Moussallam.

The role of magmatism in continental rift initiation and evolution is of much debate. Our research focuses on a section of the magmatic-rich eastern branch of the East African Rift in Northern Tanzania that depicts the complex early stage of tectono-magmatic rift evolution. This area, the North Tanzania Divergence (NTD), is currently volcanically active with a magmatic history that initiated in the Miocene, prior to documented extension. Some of the NTD volcanoes are among Earth’s largest (Kilimanjaro, Ngorongoro), and have produced a diverse array of lavas from basalt to rhyolite, trachyte, nephelinite to phonolite and carbonatite. Their distribution is widespread, both N-S along the rift axis and E-W across the valley floor and onto the adjacent rift margins.

The oldest NTD magmatism is recorded at the centrally located Essimingor volcano. We report new 40Ar/39Ar ages, major and trace element analyses and Sr-Nd-Pb radiogenic isotopic signatures on well-located lavas representing the observable variation in lithology and stratigraphy from the S and SW slopes of Essimingor. Laser-incremental heating 40Ar/39Ar analyses of whole rock, matrix and nepheline separates yield plateau ages ranging from 5.76±0.02 Ma to 5.91±0.01 Ma.

Essimingor major element data define narrow compositional variations consistent with fractional crystallization. Open system processes of mixing or contamination are inferred from an increase in Sr isotopic values with indices of fractionation. Ce/Pb varies over a large range (59 to 7), the lower end of which implies crustal assimilation that overprints the mantle signature. An FC versus AFC process has therefore been modeled. The Sr-Nd-Pb isotopic values indicate the involvement of a HIMU -like component. Trace element abundances of the more primitive samples (MgO >9 wt%) suggest partial melting of a metasomatized lithospheric mantle peridotite characterized by the presence of residual garnet and phlogopite combined with minor amphibole and apatite. The coexistence of garnet and phlogopite in the source suggests melting at ~80–150 km depth, consistent with the base of the lithosphere in the eastern branch identified using Rayleigh wave tomography (120-160 km; Weeraratne et al. 2003) and indicating that Essimingor represents the initial phase of lithospheric removal.

Ongoing analysis of younger NTD volcanoes should help constrain the timing and location of the progressive lithospheric thinning during early rifting.

This talk is part of the Cambridge Volcanology series.

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• Harker II, Geology Department

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