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Nature of the crust beneath Afar: insight from geochemistry of rhyolites

Publié le 29 septembre 2022 Mis à jour le 29 septembre 2022
Prof Dereje Ayalew - 7 octobre 2022 - Local S.DB4.245 - 15h00

The volcanic suites in intra-continental plate rifts are strongly bimodal basalt-rhyolite type, with a distinct lack of intermediate compositions. A major problem in the petrogenetic studies of continental rift volcanic suites involves the origin of magmas more silica-rich than basalt. In the Ethiopian rift, basalts and rhyolites are clearly related by processes of fractional crystallization, combined with variable degrees of crustal contamination. We should expect the greatest degrees of crustal contamination in rhyolites generated with higher rates of crustal extension, as these are associated with higher geothermal gradient as a consequence of the emplacement of large volumes of high-temperature basaltic magma into the crust. In contrast, Quaternary Afar rhyolites, emplaced close to the active rift axis, exhibit mantle-like isotopic and trace element geochemical characteristics. We conclude that true continental crust is absent beneath Afar and, that the continental crust is stretched and progressively invaded by mafic dykes.
Le 7 octobre 2022
Campus du Solbosch