CRYSTALLINE BASEMENT AGES, DETRITAL ZIRCON AGES, AND METAMORPHIC AGES FROM SEWARD PENINSULA: IMPLICATIONS FOR PROTEROZOIC AND CAMBRIAN-ORDOVICIAN PALEOGEOGRAPHIC RECONSTRUCTIONS OF THE ARCTIC-ALASKA TERRANE

SHRIMP U-Pb ages were obtained from igneous and metamorphic zircons from orthogneisses on Seward Peninsula, Alaska, at the Stanford/USGS facility. Most orthogneisses are Neoproterozoic and range from 700-565 Ma. Hen and Chickens orthogneiss has an inherited grain of 754 ± 4 Ma (206Pb/238U age, 1 grain; all errors are 2s) and an intrusive age of 695 ± 25 Ma (upper intercept; 14 grains). A Nome Group metavolcanic schist has inherited grains of 1631 ± 19 Ma (207Pb/206Pb age; 3 grains) and an eruptive age of 669 ± 23 Ma (upper intercept; 17 grains). A Nome Group orthogneiss north of Salmon Lake yielded a SHRIMP age of 678 ± 20 Ma (upper intercept, 14 grains), similar to a published TIMS age of 678 ± 4 Ma for the same sample. None of these samples has metamorphic zircon growth. The Thompson Creek orthogneiss, previously dated at 555 ± 15 Ma (TIMS upper intercept; Amato et al., 1994), had zircons with metamorphic rims growing on igneous cores. Core ages were 565 ± 6 Ma (average of three 206Pb/238U ages) or 583 ± 17 Ma (upper intercept, 9 grains) with Th/U>0.4. Rim ages were 94 ± 2 Ma (average of 5 206Pb/238U ages) with Th/U<0.1. A unit previously mapped as “metavolcanic gneiss” yielded only rounded detrital zircons with 207Pb/206Pb ages between 888 and 2027 Ma (16 grains). A garnet-bearing orthogneiss from the NW Kigluaik Mountains had igneous zircon ages of 96.4 ± 0.5 Ma (average of 6 206Pb/238U ages), igneous monazite core ages of 95.2 ± 1.3 Ma (average of 4 208Pb/232Th ages) and metamorphic monazite rim ages of 91.6 ± 2.3 Ma (average of 6 208Pb/232Th ages). Monazite from Hen and Chickens orthogneiss yielded monazite ages of 97.9 ± 1.8 Ma (average of 12 208Pb/232Th ages). Some rims yielded older ages than the cores. Previous U-Pb dating of three samples of detrital zircons from metasedimentary rocks from the Nome Group yielded youngest grains of mid-late Cambrian age, and a prominent peak on relative probability plots at 683 Ma. Magmatism of this age is rare in Laurentia. Other regions with similar aged magmatism include (1) Barentia/Baltica, east of Greenland,where magmatism associated with rifting of the eastern margin of Rodinia ranged in age from 690-600 Ma; (2) Siberia; and (3) Magmatism associated with subduction peripheral to the Gondwana supercontinent, known as the "peri-Gondwana" arcs (e.g. Avalonia-Cadomia-Florida) occurred from 820-600 Ma.