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

The Ethiopian continental flood basalt province contains large volumes of felsic eruptive rocks (>6 X 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 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 Sr-87/Sr-86 ratios (0.70357-0.70464) and generally negative initial epsilon(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 Sr-87/Sr-86 ratios (0.70412-0.70469) and positive initial epsilon(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%).