HR: 0800h
AN: V51B-0580    [Abstracts]
TI: Silicate Veining Above an Ascending Mantle Plume - Evidence from New Ethiopian Xenolith Localities
AU: * Rooney, T O
EM: tor102@psu.edu
AF: The Pennsylvania State University, Dept. of Geosciences, The Pennsylvania State University, University Park, PA 16802 United States
AU: Furman, T
EM: furman@geosc.psu.edu
AF: The Pennsylvania State University, Dept. of Geosciences, The Pennsylvania State University, University Park, PA 16802 United States
AU: Ayalew, D
EM: dereayal@geol.aau.edu.et
AF: Addis Ababa University, Dept. of Geology and Geophysics, Addis Ababa University, P.O. Box 1176, Addis Ababa, 0000 Ethiopia
AU: Yirgu, G
EM: yirgu.g@geol.aau.edu.et
AF: Addis Ababa University, Dept. of Geology and Geophysics, Addis Ababa University, P.O. Box 1176, Addis Ababa, 0000 Ethiopia
AB: Quaternary basaltic eruptions in the Debre Zeyit (Bishoftu) and Butajira regions of the Main Ethiopian Rift host Al-augite, norite and rare lherzolite xenoliths, xenocrysts and megacrysts. These explosive basaltic eruptions are located 20 km to the west of the main rift axis and are characterized by cinder cones and maars. The host basalt was generated as a small degree partial melt of fertile peridotite between 15 and 25 kb and host abundant Al-augite (Type II) xenoliths derived from pressures up to 10 kb. The central Main Ethiopian Rift lies in a transitional zone between the continental rifting of East Africa and the sea floor spreading associated with the Red Sea. Lithospheric and sub-lithospheric processes that occur during the transition from continental to oceanic magmatism may be investigated using these xenolith-bearing basalts. Neither carbonatitic nor hydrous (amphibole + phlogopite) metasomatism is evident in either the xenoliths or host basalts, suggesting that infiltration of silicate melts that produced Al-augite veining dominates the regional lower crust and lithospheric mantle. These veins are significantly hotter (200 - 300 $\deg$C) than the lherzolite wall rock they intrude suggesting the thermal influence of the Afar plume. Recent geophysical tomography indicates that this veining is pervasive and segmented, supporting the association of these Al-augite veins with the formation of a proto-ridge axis. Al-augite xenoliths and megacrysts have been observed in other continental rift settings such as Durango (Luhr, 2001) and Lake Baikal (Litasov, 2000), indicating Al-augite silicate melt metasomatism is a fundamental process associated with continental rift development.
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 1025 Composition of the mantle
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting