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Journal of the Geological Society; 2003; v. 160; issue.1; p. 47-56;
DOI: 10.1144/0016-764901-169
© 2003 Geological Society of London

Original Article

Crustal contribution to the genesis of Ethiopian plateau rhyolitic ignimbrites: basalt and rhyolite geochemical provinciality

Dereje Ayalew & Gezahegn Yirgu

Department of Geology and Geophysics, Addis Ababa University, PO Box 1176, Addis Ababa, Ethiopia (e-mail: dggaau@telecom.net.et)

The Ethiopian continental flood basalt province contains large volumes of felsic eruptive rocks (>6 x 104 km3) overlying, and sometimes interlayered with the upper parts of the flood basalt sequence. Rb–Sr isochron ages (30.60 ± 0.79 Ma for Lima Limo and 30.17 ± 0.54 Ma for Wegel Tena rhyolite sequences) coincide with the short, well-defined continental flood basalt pulse, indicating rapid emplacement of the rhyolites. Lima Limo rhyolites are characterized by relatively low TiO2 (0.3–0.5%), Th (5–7 ppm) and Nb (48–68 ppm) concentrations, high Rb/Nb (1.7–3.0) and La/Nb (1.4–2.9) values, non-radiogenic initial 87Sr/86Sr ratios (0.70357–0.70464) and generally negative initial {varepsilon}Nd values (+0.3 to –1.4). In contrast, Wegel Tena rhyolites are characterized by relatively high TiO2 (0.5–1.0%), Th (10–17 ppm) and Nb (73–115 ppm) abundances, low Rb/Nb (0.8–1.3) and La/Nb (0.8–1.2) values, low initial 87Sr/86Sr ratios (0.70412–0.70469) and positive initial {varepsilon}Nd values (+4.0 to +5.7). The origin of the rhyolite suites is attributed to low-pressure fractional crystallization of basaltic magmas, similar in composition to the exposed flood basalts, combined with assimilation of mafic crustal rocks. Assimilation–fractional crystallization modelling suggests that the genesis of Lima Limo rhyolites involved higher amounts of contamination by crustal material (<14%) relative to those of Wegel Tena (<6%).


Keywords: Ethiopian volcanic province, Oligocene, petrogenesis, rhyolites, fractional crystallization, assimilation.




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