A narrow belt of tonalite-granodiorite-granite plutons and batholiths intruded the accretionary prism in southwestern Alaska about 60 m.y. ago, simultaneously with plutonism over 100 km north along the main arc axis. The presence of metasedimentary xenoliths, kyanite, and garnet within the intrusion exposed on the Kodiak, Shumagin and Sanak islands establishes the presence of a crustal component. Extremely high values of Δ18O, from +10.9 to +13.2 0/00, require a crustal origin for much of the oxygen in the intrusions. Open-system alteration has disrupted whole-rock Rb-Sr systematics in some samples from the Sanak pluton and Shumagin batholith. Mineral isochrons using unaltered minerals yield an age of 58.7¿1.2 m.y. and 87Sr/86Sri = 0.70534¿10 for the Shumagin batholith, and ages of 62.7¿1.2 m.y. and 87Sr/86Sri = 0.70523¿14 for the Sanak pluton. Comparison of the isotopic data for the intrusions with those of sedimentary rocks in the Kodiak and Shumagin formations requires that a low-Δ18O, low-87Sr/86Sr component be present as well. Mixing models combining 87Sr/86Sr and Sr contents of various kinds of mafic magmas with partial melts of metasedimentary wall rocks suggest that three types of mafic magmas can satisfactorily duplicate the oxygen isotope signature of the intrusions: (1) a mid ocean ridge (MORB)-like magma, (2) a magma derived by partial fusion of altered MORB, or (3) an arc basalt. The MORB model predicts abundances of Rb, Sr, Ba, and rare earth elements which are similar to those in the intrusions, while the altered MORB model does not. The arc magma model provides a better trace element match than the remelted MORB model, but is not as successful as the MORB model. These intrusions were preceded by a pulse of mafic to intermediate volcanism near or within the accretionary prism, recorded in the Paleocene Ghost Rocks Formation. This volcanism is probably the heat and mass source of the mafic component within the intrusions, and may be related to activity of the Kula-Farallon ridge approximately 60 m.y. ago.