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

Abstract

The Ethiopian continental flood basalt province contains large volumes of felsic eruptive rocks (>6 × 10 4 km 3 ) 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 TiO 2 (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 87 Sr/ 86 Sr ratios (0.70357–0.70464) and generally negative initial ε Nd values (+0.3 to −1.4). In contrast, Wegel Tena rhyolites are characterized by relatively high TiO 2 (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 87 Sr/ 86 Sr ratios (0.70412–0.70469) and positive initial ε 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%).