Lavas and feeder dikes of two Quaternary volcanic complexes in southwest British Columbia provide evidence of an upper crustal syntectic origin for many dacite magmas. Contamination initially involves solid-liquid and liquid-liquid interdiffusion between the magma and <1-cm-wide, partially melted (≤60% ) margins of wall-rock xenoliths. Subsequent abrasion of mechanically unstable margins contributes both silicic melt and xenocrysts to the magma. Hybridization continues by elemental exchange between the magma and crustal liquid, where mechanical mixing produces a "meshlike" distribution of <1 mm rhyolite strands in the lava matrix. The magma may assimilate large quantities of crustal material through disaggregation of xenoliths within high-level subvolcanic dikes.