Contrasting geochemistry of basalts from the Afar and East African mantle plumes
Abstract
Compositional differences in the present day characteristics of basalts derived from the Afar and East African (Kenyan) mantle plumes are well established, particularly for Sr and Nd isotopes. What is less clear is whether those differences existed during the earlier phases of plume activity or whether a single mantle source region can account for both magmatic systems. Basalts representative of the earliest phases of activity of the East African mantle plume outcrop in southern Ethiopia. They are of Eocene age (40-45Ma) and are typically transitional tholeiites with up to 10% MgO, modest REE fractionation and ɛNd values of 1-4. Comparison with experimental data implies melting at 2-3GPa pressure and a mantle potential temperature (T_p) of ∼ 1400^oC. By contrast, 30Ma high-Ti transitional tholeiites from the Ethiopian flood basalts, the earliest expressions of activity associated with the Afar mantle plume, are picritic or ankaramitic with up to 18% MgO, highly fractionated REE and eNd values of 6-8. They also have low Al_2O_3 contents and CaO/Al_2O_3 ratios which are consistent with melting at 4-5 GPa at a T_p of at least 1500^oC. The compositions of the southern Ethiopian basalts imply melting at depths that require lithospheric extension with b values of 1.6-1.8. The age of magmatism predates that of the earliest phase of extension in the Ethiopian Rift by at least 15Ma and it would appear that the tectonic episode that triggered this activity was related to the development of the NW-SE trending Kaisut graben of NW Kenya. This contrasts with the melting conditions for the generation of the northern basalts which do not require lithospheric thinning (β = 1), implying extension after plume impact. Collectively these differences imply derivation from distinct source regions with different geochemical and physical characteristics under different tectonic conditions that cannot be readily related to a single mantle plume.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....8800R