Lawrence, L., Spandler, C., Hilbert-Wolf, H.L., Mtelela, C., Stevens, N.J., O'connor, P.M., and Roberts, E.M. (2022) Radiogenic isotope record of magma genesis and lithospheric geodynamics of the Rukwa Rift Basin, Tanzania, from mid Mesozoic to present. Chemical Geology, 608. 121040.

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Abstract

The Western Branch of the East African Rift System has experienced multiple episodes of basin development and intraplate alkaline volcanism since the Jurassic, however the geodynamic processes and lithospheric evolution involved in this protracted geological history remains poorly defined. Here, we present Sm-Nd, Lu-Hf, and Rb-Sr isotopic data of igneous minerals for three magmatic episodes that coincide with basin reactivation in the Rukwa Rift of southwestern Tanzania, respectively represented by: (1) the Jurassic to Cretaceous Panda Hill carbonatite; (2) the late Oligocene phonolitic-carbonatitic Nsungwe Formation tuffs; and (3) the Miocene-recent bi-modal volcanism of the Rungwe Volcanic Province. Of these, the Nsungwe Formation tuffs offer a discrete record of the early phases of lithospheric disturbance associated with modern rifting of the Western Branch. Alkaline magmas erupted prior to the Miocene are isotopically distinct (epsilon Nd +0.5 to +1.5 and Sr-87/Sr-86 0.7041) from modern Rungwe lavas and likely originate from a lithospheric mantle that experienced enrichment during the Pan-African Orogeny (similar to 550-700 Ma). Consistent with previous studies, our results indicate that Rungwe Volcanic Province magmas were generated from a mixed, isotopically-enriched component of the lithospheric mantle that may have developed during the ca. 1.0 Ga Irumide Orogeny. The observed change in the source region of rift lavas though time is consistent with progressing melting of a compositionally layered lithosphere. Combined with existing geophysical and sedimentological data, we hypothesise that the lithosphere beneath the Rukwa-Malawi-Usangu junction of the Western Branch experienced an episode of destabilisation and foundering (lithospheric drip), during the early stages of East African Rift development.

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