| Summary: | The Sanak-Baranof belt (SBB) extends for over 2,000 km along the southern Alaskan coastline and the plutons systematically young from 63 Ma in the west to 47 Ma in the east. On Yakobi and Chichagof Islands in Southeast Alaska, these plutons intrude the Sitka Graywacke that is correlated with the Late Cretaceous Valdez group of the Chugach-Prince William terrane (CPW). The intrusion into these forearc accretionary wedge sediments, in conjunction with the presence of ophiolite complexes in the CPW, has led many to invoke the subduction of a spreading ridge. However, the location of the spreading ridge and the latitude for the formation and accretion of the CPW terrane are debated. On Yakobi and Chichagof Islands and in Glacier Bay National Park, several mapped igneous bodies, gabbroic to granitic in composition, are analyzed in this study. Petrographic mineral identification, bulk rock geochemistry, uranium-lead dating, and hafnium isotopes suggest that six of the ten plutons dated in this study are part of the SBB, including two plutons on the Yakutat Block. These plutons include the Granite Islands pluton (52.3 ± 0.5 Ma), Mirror Harbor pluton (51.6 ± 0.5 Ma), Lost Cove pluton (51.5 ± 0.4 Ma), Yakobi Island pluton (50.7 ± 0.5 Ma), Graves Harbor pluton (48.3 ± 0.5 Ma), Desolation Valley pluton (48.7 ± 0.4 Ma), and the Grand Plateau Glacier pluton (48.9 ± 0.4). Two non-SBB plutons, the Squid Bay and Lituya Glacier plutons, yielded ages of 42.4 ± 0.4 and 39.3 ± 0.4 Ma, respectively. A large pluton-sized body in the Yakutat mélange, informally named the Mt. Fairweather megaclast, yielded a Jurassic age (194.4 ± 1.7 Ma).
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