Abstract
The recently discovered high, plume-like 3He/4He ratios at Rungwe Volcanic Province (RVP) in southern Tanzania, similar to those at the Main Ethiopian Rift in Ethiopia, strongly suggest that magmatism associated with continental rifting along the entire East African Rift System (EARS) has a deep mantle contribution (Hilton et al., 2011). New trace element and Sr-Nd-Pb isotopic data for high 3He/4He lavas and tephras from RVP can be explained by binary mixing relationships involving Early Proterozoic (+/- Archaean) lithospheric mantle, present beneath the southern EARS, and a volatile-rich carbonatitic plume with a limited range of compositions and best represented by recent Nyiragongo lavas from the Virunga Volcanic Province also in the Western Rift. Other lavas from the Western Rift and from the southern Kenya Rift can also be explained through mixing between the same endmember components. In contrast, lavas from the northern Kenya and Main Ethiopian rifts can be explained through variable mixing between the same mantle plume material and the Middle to Late Proterozoic lithospheric mantle, present beneath the northern EARS. Thus, we propose that the bulk of EARS magmatism is sourced from mixing among three endmember sources: Early Proterozoic (+/- Archaean) lithospheric mantle, Middle to Late Proterozoic lithospheric mantle and a volatile-rich carbonatitic plume with a limited range of compositions. We propose further that the African Superplume, a large, seismically anomalous feature originating in the lower mantle beneath southern Africa, influences magmatism throughout eastern Africa with magmatism at RVP and Main Ethiopian Rift representing two different heads of a single mantle plume source. This is consistent with a single mantle plume origin of the coupled He-Ne isotopic signatures of mantle-derived xenoliths and/or lavas from all segments of the EARS (Halldorsson et al., 2014).
Highlights
The East African Rift System (EARS)—from southern Tanzania and Lake Malawi, where the African continent begins to break apart, northward to the Afar Triple Junction, where the on-land Danakil Depression meets the oceanic spreading ridges of the Red Sea and Gulf of Aden (Figure 1)—provides exceptional opportunities to study geologic processes controlling continental rifting and eventual ocean formation
We propose further that the African Superplume, a large, seismically anomalous feature originating in the lower mantle beneath southern Africa, influences magmatism throughout eastern Africa with magmatism at Rungwe Volcanic Province (RVP) and the Main Ethiopian Rift representing two different heads of a single mantle plume source
Closer inspection of the Sr-Nd isotope characteristics (Figure 5A) reveals that RVP samples fall along curved trajectories formed through binary mixing between sample TAZ09-12, which falls within the ∼middle of the field for Early Proterozoic lithospheric mantle, and one of the melilitebearing nephelinites from Nyiragongo volcano in the Virunga Volcanic Province (VVP) (Vollmer and Norry, 1983a,b)
Summary
The East African Rift System (EARS)—from southern Tanzania and Lake Malawi, where the African continent begins to break apart, northward to the Afar Triple Junction, where the on-land Danakil Depression meets the oceanic spreading ridges of the Red Sea and Gulf of Aden (Figure 1)—provides exceptional opportunities to study geologic processes controlling continental rifting and eventual ocean formation. An outstanding feature of the EARS is the diverse and extensive magmatism associated with the various facets of rifting along its strike, making it a prime target to study the role of magmatism in continental break-up, one of the major issues regarding ocean formation. Results of investigations on this topic far suggest that mantle plume-type melting provides the bulk of magmas associated with rifting in the EARS, suggesting active, in this case plume, magmatism plays a significant role in ocean formation (e.g., Rogers, 2006; Furman, 2007). The number and nature of plumes involved in rifting in the EARS are controversial. Chakrabarti et al (2009) propose a heterogeneous mantle plume supplying the Virunga Volcanic Province (VVP) in the Western Rift.
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