The majority of felsic rocks from composite centers in the southernmost Cascades have geochemical and Sr, Nd and Pb isotopic ratios that suggest derivation by partial melting of lower crust that is compositionally similar to calc-alkaline basalts observed in the region. Only a few felsic rocks have δ¹⁸O and Pb isotopic compositions that indicate interaction with the upper crust. Mineralogical and geochemical differences among the felsic magmas result primarily from melting under variable f(H₂O) and temperature conditions. Partial melting under low f(H₂O) and high temperature conditions leaves an amphibole-poor residuum, and produces magmas that have orthopyroxene as the most abundant ferromagnesian phenocryst, relatively low silica contents, and straight rare earth element patterns. Partial melting under higher f(H₂O) and lower temperature conditions leaves an amphibole-rich residuum, and produces magmas that have amphibole ± biotite phenocrysts, relatively high silica contents, and pronounced middle rare earth element depletions. These conclusions are consistent with published thermal models that suggest that reasonable volumes of basaltic magma emplaced beneath large composite centers in the southernmost Cascades can serve as the heat source for melting of the lower crust. Melting of the lower crust under variable f(H₂O) conditions is likely to result from differences in the H₂O contents of these basaltic magmas.