The chemical and isotopic compositions of sills at West Mageik Lake are constrasted with high-silica rhyolite erupted in 1912 at the nearby Valley of Ten Thousand Smokes to gain insight into the petrogenesis of evolved magmas in the Aleutian arc. A high-silica rhyolitic sill has an ϵNd value (+8.8) similar to an adjacent dacite porphyry stock, but the elevated Al, Sr, and 87Sr/86Sr of the rhyolite suggest that fractionation of a parental dacite magma was accompanied by significant contamination by Sr-rich sediments, probably of Paleozic age. The chemical and isotopic character of the West Mageik Lake rhyolite suggest that the system was of small volume, making it easily affected by crustal contamination. The fact that the 8 km3 of high-silica rhyolite erupted at the Valley of Ten Thousand Smokes shows only a slight Sr isotopic shifts from associated dacite and andesite suggests it was part of a larger system that could assimilate small masses of crust without having its composition significantly affected. ¿American Geophysical Union 1991