LATE PROTEROZOIC GRANITIC MAGMATISM ON SEWARD PENINSULA AND A BARENTIAN ORIGIN FOR ARCTIC ALASKA-CHUKOTKA

New U-Pb zircon age determinations of 676 +/- 15 Ma anti 681 +/- 3 Ma (upper intercept ages) for two granitic orthogneiss bodies on Seward P eninsula confirm a minimum age of Late Proterozoic for the lower Nome Group, a continental margin sequence in northwestern Alaska. Discordan ce 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 meta morphism during the Jurassic-Cretaceous Brookian orogeny. Age correlat ion of the Seward Peninsula bodies with Late Proterozoic orthogneiss b odies in the Brooks Range and eastern Siberia (Chukotka) provides an e arly paleogeographic link between these regions and indicates the exis tence of a Late Proterozoic magmatic belt in Arctic Alaska-Chukotka. T his magmatic belt is temporally distinct from known Late Proterozoic m agmatism in the North American Cordillera and Canadian Arctic Islands, but it is strikingly similar in age to pre-Caledonian magmatism in ea stern North America and in Svalbard. We suggest that the pre-Cretaceou s Arctic Alaska-Chukotka crustal fragment restores to the Barentian co ntinental margin and is thus exotic to western North America. Our reco nstruction supports models calling for a Canadian Arctic Islands trans form margin (rather than rotational models) during Cretaceous opening of the Canada Basin.