Late Proterozoic granitic magmatism on Seward Peninsula and a Barentian origin for Arctic Alaska–Chukotka

Abstract

New U-Pb zircon age determinations of 676 ± 15 Ma and 681 ± 3 Ma (upper intercept ages) for two granitic orthogneiss bodies on Seward Peninsula confirm a minimum age of Late Proterozoic for the lower Nome Group, a continental margin sequence in northwestern Alaska. Discordance of U-Pb data is attributed to Pb loss (lower intercepts of 117 ± 17 Ma and 124 ± 6 Ma) induced by high-pressure, low-temperature metamorphism during the Jurassic-Cretaceous Brookian orogeny. Age correlation of the Seward Peninsula bodies with Late Proterozoic orthogneiss bodies in the Brooks Range and eastern Siberia (Chukotka) provides an early paleogeographic link between these regions and indicates the existence of a Late Proterozoic magmatic belt in Arctic Alaska–Chukotka. This magmatic belt is temporally distinct from known Late Proterozoic magmatism in the North American Cordillera and Canadian Arctic Islands, but it is strikingly similar in age to pre-Caledonian magmatism in eastern North America and in Svalbard. We suggest that the pre-Cretaceous Arctic Alaska–Chukotka crustal fragment restores to the Barentian continental margin and is thus exotic to western North America. Our reconstruction supports models calling for a Canadian Arctic Islands transform margin (rather than rotational models) during Cretaceous opening of the Canada Basin.