Geochemical evidence of mantle reservoir evolution during progressive rifting along the western Afar margin

Tyrone O. Rooney (1) , Paul J. Mohr (2) , Laure Dosso (3) , Chris Hall (4)

1 Dept. of Geological Sciences
2 Tonagharraun
3 LDO - Domaines Océaniques
4 Department of Earth and Environmental Sciences [Ann Arbor]

The Afar triple junction, where the Red Sea, Gulf of Aden and African Rift System extension zones converge, is a pivotal domain for the study of continental-to-oceanic rift evolution. The western margin of Afar forms the southernmost sector of the western margin of the Red Sea rift where that margin enters the Ethiopian flood basalt province. Tectonism and volcanism at the triple junction had commenced by ∼31Ma with crustal fissuring, diking and voluminous eruption of the Ethiopian-Yemen flood basalt pile. The dikes which fed the Oligocene-Quaternary lava sequence covering the western Afar rift margin provide an opportunity to probe the geochemical reservoirs associated with the evolution of a still active continental margin. 40Ar/39Ar geochronology reveals that the western Afar margin dikes span the entire history of rift evolution from the initial Oligocene flood basalt event to the development of focused zones of intrusion in rift marginal basins. Major element, trace element and isotopic (Sr-Nd-Pb-Hf) data demonstrate temporal geochemical heterogeneities resulting from variable contributions from the Afar plume, depleted asthenospheric mantle, and African lithosphere. The various dikes erupted between 31Ma and 22Ma all share isotopic signatures attesting to a contribution from the Afar plume, indicating this initial period in the evolution of the Afar margin was one of magma-assisted weakening of the lithosphere. From 22Ma to 12Ma, however, diffuse diking during continued evolution of the rift margin facilitated ascent of magmas in which depleted mantle and lithospheric sources predominated, though contributions from the Afar plume persisted. After 10Ma, magmatic intrusion migrated eastwards towards the Afar rift floor, with an increasing fraction of the magmas derived from depleted mantle with less of a lithospheric signature. The dikes of the western Afar margin reveal that magma generation processes during the evolution of this continental rift margin are increasingly dominated by shallow decompressional melting of the ambient asthenosphere, the composition of which may in part be controlled by preferential channeling of plume material along the developing neo-oceanic axes of extension.

Mots clés: argon-argon dating; dike; flood basalt; geochronology; heterogeneity; igneous geochemistry; lava; magma; mantle structure; rifting; tectonics; trace element; triple junction; volcanic eruption; volcanism; afar

Domaines: Géochimie; Milieux et Changements globaux

Tyrone O. Rooney, Paul J. Mohr, Laure Dosso, Chris Hall. Geochemical evidence of mantle reservoir evolution during progressive rifting along the western Afar margin. Geochimica et Cosmochimica Acta, 2013, 102, pp.65-88. ⟨10.1016/j.gca.2012.08.019⟩. ⟨insu-00785364⟩