Evidence for extreme fractionation of peralkaline silicic magmas, the Boseti volcanic complex, Main Ethiopian Rift

Matrix glass and melt inclusions in phenocrysts from pantellerite lavas of the Boseti volcanic complex, Ethiopia, record extreme fractionation of peralkaline silicic magma, with Al2O3 contents as low as 2.3 wt.%, FeO* contents up to 17 wt.% and SiO2 contents ~65 wt.%. The new data, and published data for natural and experimental glasses, suggest that the effective minimum composition for peralkaline silicic magmas has ~5 wt.% Al2O3, 13 wt.% FeO* and 66 ± 2 wt.% SiO2. The dominant fractionating assemblage is alkali feldspar + fayalite + hedenbergite + oxides ± quartz. Feldspar - melt relationships indicate that the feldspar is close to the minimum on the albite-orthoclase solid solution loop through the entire crystallization history. There is petrographic, mineralogical and geochemical evidence that magma mixing may have been a common process in the Boseti rhyolites.