Detrital zircon geochronology of the Alexander terrane, southeastern Alaska

Abstract

U-Pb analyses of 101 detrital zircon grains from Paleozoic and Triassic clastic strata of the Alexander terrane indicate that most detritus in the terrane was derived from intraterrane igneous rocks, although the occurrence of 1.0–3.0 Ga grains in a Lower Devonian sandstone indicates proximity to a continental region during the Silurian–Devonian Klakas orogeny. Because these Precambrian grains are not the same age as grains that were accumulating along the western margin of North America, it appears unlikely that the terrane was in proximity to the Cordilleran margin during Early Devonian time. Continental regions that could have shed grains of the appropriate ages, and that also record Silurian–Devonian orogenic activity, include the paleo-Pacific margin of Australia and the Scandinavian portion of Baltica. Existing paleomagnetic data are consistent with either paleoposition, whereas previous biogeographic studies suggest closer ties with Baltica.