Detrital amphibole and biotite grains were extracted from five volcaniclastic sandstones of the Seymour Canal Formation (Late Jurassic-mid-Cretaceous Gravina belt, southeastern Alaska); forty of these grains were dated by fusion with the $$^{40}Ar/^{39}Ar$$ laser microprobe. Ar release characteristics suggest that the smaller biotite grains may have lost $$^{40}Ar$$ during regional tectonism, and thus their mid-Cretaceous to Late Jurassic whole-grain fusion dates are not reliable. Six additional large biotite grains were dated using laser-step-heating techniques. Results from the step-heated biotite grains, as integrated over the high-temperature steps, yield Middle Jurassic through Early Cretaceous dates ranging from $$159.0 \pm 14.2$$ to $$128.7 \pm 4.2 Ma$$. Ages for amphibole grains are generally less precise, although two-thirds yielded $$2\sigma$$ errors of < 10 m.y. The most precise amphibole grain dates cluster between 90 and 110 Ma (mid-Cretaceous), indicating that the amphibole grains were deriv...