| Summary: | Annette, Gravina, Duke, and southern Prince of Wales Islands are underlain primarily by Cambrian (and perhaps Proterozoic) through Triassic volcanic, sedimentary, plutonic, and metamorphic rocks. These rocks belong to the Alexander terrane, which is a coherent tectonic fragment that underlies much of southeastern (SE) Alaska, the Saint Elias Mountains of British Columbia, Yukon, and eastern Alaska, and coastal regions of west-central British Columbia. Geologic mapping combined with U-Pb (zircon) geochronologic studies have delineated the major geologic units and features of these islands, and contribute to our understanding of the geologic and tectonic evolution of the Alexander terrane. The oldest rocks recognized on Annette, Gravina, Duke, and southern Prince of Wales Islands consist of greenschist- and amphibolite-facies metavolcanic and metasedimentary rocks of the Wales metamorphic suite. These rocks are locally intruded by dioritic and granodioritic metaplutonic rocks which yield U-Pb apparent ages of approximately 540-520 Ma (Middle and Late Cambrian). Rocks in the Wales suite were therefore deposited, at least in part, prior to Late Cambrian time, but their maximum depositional age is not known. The Wales suite and associated metaplutonic rocks are intruded by large dioritic to granitic plutons which yield U-Pb apparent ages in the 475-425 Ma (Middle Ordovician-Early Silurian) range and are probably overlain by Lower Ordovician-Lower Silurian volcanic and sedimentary rocks of the Descon Formation. However, depositional contacts between the Descon Formation and the older metamorphic rocks have not been demonstrated. Deformation, metamorphism, and uplift of rocks in the Wales metamorphic suite occurred during a Middle Cambrian-Early Ordovician tectonic event which I have referred to as the Wales "orogeny." The term orogeny is used informally in this instance, as little is known about the regional and tectonic significance of this event. Ordovician-Early Silurian rocks on these islands are interpreted to ...
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