Late Proterozoic-Cambrian metamorphic basement of the Alexander terrane on Long and Dall Islands, southeast Alaska

Geologic and U-Pb (zircon) geochronologic studies demonstrate that Long Island and southern and central Dall Island are underlain in large part by upper Proterozoic-Lower Cambrian metamorphic rocks of the Wales Group. The group comprises a lower assemblage of predominantly metadacite, metamorphosed to amphibolite and greenschist facies, and an upper assemblage of lower-grade basaltic metavolcanic rocks,thick marble layers, and interlayered calcareous metadacite, marble, metapelite, and metagraywacke. Rocks of both assemblages are intruded by metagranodiorite-diorite-gabbro suites, one of which (Kaigani orthogneiss) yields a concordant U-Pb (zircon) age of 554 ± 4 m.y. (Early Cambrian). Rocks of the upper Wales assemblage were thrust over Ordovician(?)-Silurian strata of the Descon Formation, possibly during the Middle Silurian-earliest Devonian Klakas orogeny. A younger normal(?) fault juxtaposes rocks of the upper Wales Group, and locally the Descon Formation and Heceta Limestone, over higher-grade rocks of the lower Wales Group. This poorly known structure may have been active during a Late Permian(?)-Triassic rifting event recorded along the northeastern margin of the terrane. The youngest major unit in the area is the Stripe Mountain granodiorite, which yields a concordant U-Pb (zircon) age of 114 ± 2 m.y.