Geochemistry of 24 Ma basalts from northeast Egypt: Implications for widespread magmatism in northern Africa

Open Access
Author:
Endress, Chira Anneli
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
February 24, 2010
Committee Members:
  • Tanya Furman, Thesis Advisor
Keywords:
  • basalt
  • volcanism
  • Africa
  • Egypt
  • isotope geochemistry
  • mantle plumes
Abstract:
Basalts in the Cairo-Suez and Fayyum districts of NE Egypt represent an episode of magmatism ~24 Ma that corresponds to a time period of significant regional eruptive activity subsequent to emplacement of 30 Ma flood basalts attributed to the Afar Plume in Ethiopia and Yemen. New geochemical data provide insight into the thermochemical, isotopic, and mineralogical characteristics of the mantle beneath NE Egypt. The basalts are subalkaline with incompatible trace element abundances similar to those of ocean island basalts; their major element, trace element and isotopic geochemical features show little variation. They display rather flat primitive mantle-normalized incompatible trace element patterns, with notable positive Pb and negative P anomalies. Isotopic (143Nd/144Nd = 0.51274–0.51285, 87Sr/86Sr = 0.7049–0.7050) and trace element signatures (Ce/Pb = 16–22, Ba/Nb = 9–14, La/Nb = 0.9–1.0) are consistent with melting of a lithospheric source that has been slightly contaminated by continental crust during ascent and emplacement. The Pb isotopic ratios (206Pb/204Pb = 18.53 18.62, 207Pb/204Pb = 15.59–15.64, and 208Pb/204Pb = 38.80–39.00) are close to the range of those found in 30 Ma “C”- like Afar Plume-derived Ethiopian flood basalts, which are distinct from the more highly radiogenic, HIMU-type signature seen in basalts from other north African localities. Measured 207Pb/204Pb and 87Sr/86Sr values higher than those observed in the Ethiopian flood basalts and suites from the Red Sea and Gulf of Aden support trace element evidence for contamination, presumably through interaction with ancient upper crustal materials. These data allow us to develop a broad framework for understanding tectono-magmatic activity throughout northern Africa since the Eocene. Beginning ~35 Ma, widespread magmatism occurred across supraequatorial Africa in portions of Algeria, Libya, Chad, Sudan, Kenya, and Egypt, as well as further north into Italy. Available geochemical and isotopic data indicate that mafic lavas from Darfur, Sudan; Hoggar, Algeria; and Gharyan, Libya are similar to those from Turkana, Kenya, though no direct link between the N African lavas and the Kenya Plume has been made. A plausible model for widespread Tertiary volcanism suggests it was fed by shallow plumelets derived from chemically distinct regions of the margins of at least one large, heterogeneous plume rooted at the core-mantle boundary. The plumelets impinged against the base of the lithosphere, providing heat and HIMU-like metasomatic agents to the lithosphere in some regions and heat and “C”/Afar Plume-like material to the lithosphere beneath NE Egypt.