EAST AFRICAN RIFT VOLCANISM: GEODYNAMIC AND GEOCHEMICAL CHARACTERISTICS OF MAGMATISM

The East African Rift system (EARS), including the eastern and western rift, provides a natural laboratory to trace temporal, spatial and compositional evolution of bimodal volcanism and its implications on the continental rifting process, from plate stretching to rupture. This seismically and volcanically active triple junction hosts mantle-sourced mafic to silicic rocks with traces of impinged cratonic lithosphere, removed by multiple magmatic outpourings over a 45 million year period. In this paper, we summarize our petrological and geochemical (element and Sr, Nd, Pb, O isotopic) studies of the East African Rift magmatism. Geochemistry of two active Virunga volcanoes, Nyiragongo and Nyamuragira, 15 km apart, indicates production of alkalic ultrabasic lavas by partial melting of lower mantle-derived plume source, at deep to shallow mantle depths, respectively (Chakrabarti et al., 2009). A heterogeneous mantle plume beneath the Tanzanian craton can explain simultaneous but compositionally distinct volcanism in the adjacent Virunga volcanics, with magmas probably originating from different depths.

On the other hand, bimodal volcanics from Dabbahu-Manda Hararo (DMH) rift segment and Gona are consanguineous and can be explained by partial melting of hydrothermally-altered Afar plume-derived basalts, followed by fractional crystallization of the resulting rhyolitic magma in a shallow magma chamber. Temporal variability is evident from geochemical studies of the ~6 Ma to present day eruptions from Gona. Nd isotopic ratios distinguish the ~6 Ma lavas with MORB-like values (ε_Nd = 5 to 8.7), from the more recent volcanics with ε_Nd in the range of 1.9 to 4.6, suggesting involvement of the asthenosphere with the plume source. Our studies from the EARS include contributions from Arundhuti Ghatak, Ramananda Chakrabarti, Cynthia J.Ebinger, Robert T.Gregory, Jay Quade, Haibo Zou and Bastian Georg.

Reference

Chakrabarti, R., Basu, A.R., Santo, A.P., Tedesco, D., Vaselli, O., 2009. Isotopic and geochemical evidence for a heterogenous mantle plume origin, of the Virunga volcanics, Western rift, East African Rift System, Chemical Geology 259, 273-289.