U-Pb geochronology of mid-Cretaceous and Tertiary plutons along the western edge of the Coast Mountains, Revillagigedo Island, and Portland Peninsula, southeast Alaska

Abstract

U-Pb geochronologic and field studies demonstrate that steeply dipping, highly deformed, sheet-like tonalitic to granodiorite plutons along the western flank of the Coast Mountains in the Ketchikan, Alaska, region were emplaced in two distinct episodes, between 105 and 90 Ma and ca. 65 and 50 Ma. A nondeformed tonalitic body that postdates deformation yields an Oligocene age. U-Pb ages from zircon further suggest that mid-Cretaceous plutons are younger northeastward. In the Ketchikan region, magmatism migrated northeastward and probably reflects a migration of the mid-Cretaceous arc between 105 and 90 Ma. Proterozoic inheritance in zircon indicates the presence or provenance of old crustal rocks. Thus, the assimilation of old crustal material played a role in the petrogenesis of the Coast Plutonic Complex. Mid-Cretaceous plutons intrude all basement constituents on the western flank of the Coast Mountains, including juvenile, mantle-derived material of the Alexander terrane and reworked older continental crustal fragments of the Taku terrane, confirming that the two terranes must have been adjacent at the time of mid-Cretaceous arc magmatism.

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