LATE PROTEROZOIC–PALEOZOIC EVOLUTION OF THE ARCTIC ALASKA CHUKOTKA TERRANE: EVIDENCE FROM ZIRCON AGES AND ND GEOCHEMISTRY

The Seward Peninsula of NW Alaska is part of the Arctic Alaska–Chukotka (AAC) terrane, a crustal fragment with an uncertain origin and pre-Mesozoic evolution. U-Pb zircon ages from on deformed igneous rocks indicate felsic volcanism at 870 Ma, coeval with early breakup of eastern Rodinia. Orthogneisses on Seward Peninsula yielded numerous 680 Ma U-Pb ages. The AAC terrane has pre-Neoproterozoic basement based on Mesoproterozoic Nd model ages from 870 Ma and 680 Ma igneous rocks and detrital zircon ages between 2.0–1.0 Ga in overlying cover rocks. Minor magmatism occurred in the Devonian.

U-Pb dating of detrital zircons in metamorphosed Paleozoic siliciclastic cover rocks to this basement revealed a major zircon age population at 700–540 Ma with prominent peaks at 720–660 Ma, 620–590 Ma, 560–510 Ma, 485 Ma, and 440-400 Ma. Devonian and Pennsylvanian age peaks are present in the samples with the youngest zircons. These data show that the Seward Peninsula is exotic to western Laurentia because of the abundance of Neoproterozoic detrital zircons that are rare or absent in lower Paleozoic Cordilleran continental shelf rocks. Maximum depositional ages include Latest Proterozoic, Cambrian, Ordovician, Silurian, Devonian, and Pennsylvanian. These overlap with conodont ages from Seward Peninsula carbonates.

The distinctive features of the AAC terrane include Neoproterozoic felsic magmatic rocks intruding 2.0–1.1 Ga crust overlain by Paleozoic carbonates and siliciclastic rocks with Neoproterozoic detrital zircons. The Neoproterozoic ages are similar to those in the peri-Gondwanan Avalonian-Cadomian arc system, the Timanide orogen of Baltica, and other circum-Arctic terranes that were proximal to Arctic Alaska prior to the opening of the Amerasian basin in the Early Cretaceous. Our Neoproterozoic reconstruction has the AAC terrane in a position near Baltica, northeast of Laurentia, in an arc system along strike with the Avalonian-Cadomian arc terranes. Previously published faunal data indicate Siberian and Laurentian links by Early Ordovician. The geologic links between the AAC terrane with eastern Laurentia, Baltica, peri-Gondwanan arc terranes, and Siberia from the Paleoproterozoic to the Paleozoic help constrain paleogeographic models from the Neoproterozoic to the Mesozoic opening of the Arctic basin.