An extensive belt of plutons and related dikes along the southern Alaska margin was emplaced into the accretionary prism anomalously close to the paleotrench and far seaward of a coeval magmatic arc. Numerous workers have attributed the near-trench magmatism to early Tertiary subduction of the Kula-Farallon Ridge beneath North America. We present new major, minor, and trace element data for a dike swarm in the Seldovia Quadrangle, southern Kenai Peninsula, that is part of the near-trench Sanak-Baranof belt. The dikes are generally calc-alkaline and cover the entire compositional spectrum from basalts to rhyolites. The basalts and andesites of obvious mantle (I type) origin compositionally grade into dacites and rhyolites having a significant sedimentary (S type) component. Trace element modeling suggests that the dikes evolved from parental magmas, some possibly as refractory as island arc tholeiites, through assimilation of sedimentary material coupled with fractional crystallization. The dikes appear to be products of interaction between older sedimentary rocks at the base of the accretionary prism and basaltic magmas that were emplaced by the subducting ridge (slab window). I-type basalts and basaltic andesites are products of mafic liquids that were only slightly contaminated by sedimentary material, whereas more extensive assimilation produced S-type dacites and rhyolites. This idea is supported by the geographical distribution of dikes within the forearc. Basalts, andesites, and low-silica dacites are found throughout the accretionary complex, whereas high-silica dacites and rhyolites only occur at some distance inboard from the paleotrench where the sedimentary column is thicker and more assimilation was likely to have taken place. We suggest that the subducting spreading center produced basaltic liquids that became more refractory closer to the paleotrench such that subducted portions of the ridge formed parental melts having the composition of island arc tholeiites. The basaltic liquids were injected into the overlying accretionary prism where variable assimilation of sedimentary material produced the wide range of dike compositions. ¿ 2000 American Geophysical Union