LATE PROTEROZOIC MAGMATISM IN ALASKA AND ITS IMPLICATIONS FOR PALEOGEOGRAPHIC RECONSTRUCTIONS OF THE ARCTIC ALASKA-CHUKOTKA PLATE

The Neoproterozoic to Paleozoic paleogeography of Seward Peninsula, Alaska has been investigated using U-Pb dating of zircons from intrusive and metasedimentary rocks. Gneisses and metavolcanic rocks from Arctic Alaska range from 750 to 540 Ma. Seward Pen. igneous rocks cluster at 670-680 Ma, but younger magmatism is represented by calc-alkaline granitic gneiss in the Kigluaik Mtns. at 565 ± 6 Ma and gabbro in the York Mtns. at 541 ± 6 Ma (2s).

LA-MC-ICPMS U/Pb method was used to determine detrital zircon ages of metasedimentary rocks. The oldest and least metamorphosed unit dated, the York Slate from the York Mtns., Seward Pen., contains coarse immature lithic metasandstone and is overlain by fossiliferous Early Ordovician carbonates. Of 99 grains analyzed, 20 grains comprise the youngest peak on a probability distribution diagram and have a mean of 543 ± 5 Ma. Other peaks are at 655 and 715 Ma. Youngest robust peaks from previously analyzed schists in the Nome Gp. are 583 ± 5, 523 ± 7, and 575 ± 5 Ma, with other main peaks at 590-630 and 680 Ma. Grenville-type ages (1.3-1.1 Ga) are present but not dominant. 94 zircons from paragneiss from the Kigluaik Mtns were analyzed with the main peak consisting of 13 grains and a weighted mean of 544 ± 5 Ma. The youngest peak (4 grains) yields an age of 355 ± 38 Ma. These grains have U/Th values indicating an igneous origin and their age is similar to Devonian granites in Arctic Alaska. The unconformity between 540 Ma and older intrusive rocks and overlying sedimentary cover rocks is only well-exposed on Wrangel Island, but can be inferred to have existed across all of Arctic Alaska based on the above relations.

Magmatism spanning 750-650 Ma is rare in Laurentia but is common in arcs at the margin of northern Siberia and in rifts in Barentsia. 565-544 Ma magmatism is not present on Barentsia but 560 Ma arc granites are found in Baltica associated with the Timanide orogeny at 600-550 Ma when Siberia and Baltica collided prior to the formation of the Urals. Pre-Cretaceous Arctic reconstructions put Arctic Alaska, northern Siberia, and Baltica/Barentsia in proximity. These data may allow for more precise reconstructions but also suggest that the Arctic Alaska-Chukotka plate was likely assembled by Early Ordovician time based on similar fauna and lack of magmatism and tectonism between 544 Ma and mid-Devonian.