Paleogene metamorphism of an accretionary flysch terrane, eastern Gulf of Alaska

Metamorphic history of Alaska

“…Metamorphic grade within the accretionary prism increases progressively to the east around the Gulf of Alaska where it culminates in the polymetamorphic rocks of the (informal) Chugach metamorphic complex (unit 78) of Hudson and Plafker (1982) that crops out in the eastern Chugach Mountains and the Saint Elias Mountains. Flysch on Sanak and Shumagin Islands underwent laumontitefacies metamorphism that was ascribed by Moore (1973) to burial.…”

“…A sequence of transitional greenschist-to amphibolite-facies rocks (unit 82) and amphibolite-facies rocks (unit 83) crops out along the eastern margin of the Gulf of Alaska in the Saint Ellas Mountains and Fairweather Range Hudson and others, 1977b;Hudson and Plafker, 1982). Protoliths are interpreted as being turbidites and tholeiite of Late Cretaceous age because of lithologic similarity with the flysch facies of the Chugach terrane (Plafker and others, 1977;Barker and others, 1985).…”

Geologic studies in Alaska by the U.S. Geological Survey, 1990

“…A U-Pb zircon date from one of these mylonitic plutons is 153±4 Ma (Plafker and others, 1989), but hornblende and white mica K-Ar dates from the metadiorites and metatrondhjemites in the region range from 149 to 112 Ma (Winkler and others, 1981;Plafker and others, 1989). The spread in K-Ar dates in this region probably reflects partial resetting of mineral dates during the Cretaceous event (described above) and (or) an Eocene thermal event that affected the accretionary prism to the south (Hudson and Plafker, 1982). The geochemistry of the Late Jurassic plutons shows that they are calc-alkaline (Hudson, 1983;Dodds and Campbell, 1988;Plafker and others, 1989).…”

Sandstone composition of the Valdez and Orca groups, Prince William Sound, Alaska

“…Laumontite-to mid-greenschist-facies rocks dominate in the western part of the study area; the eastern part contains mainly mid-to upper-greenschist facies, with the development of amphibolite facies locally north of Port Fidalgo and in a larger area east of the Copper River (Hudson and Plafker, 1982;Goldfarb and 1986; Sisson and others, 1986). The Late Cretaceous age of the group is based on a few widely scattered Campanian(?)…”