U-PB GEOCHRONOLOGY OF NEOPROTEROZOIC BASEMENT ROCKS AND PALEOZOIC COVER SEQUENCES ON WRANGEL ISLAND AND IN ARCTIC CHUKOTKA

Wrangel Island and the Arctic coast of Chukotka are key localities for studying the geologic history and paleogeographic affinities of the central portion of the vast Arctic Alaska Chukotka terrane (AAC). Like the Alaskan part of the AAC, these areas experienced Neoproterozoic magmatism, spanning ~710-620 Ma (versus 980-560 Ma in Alaska). U-Pb zircon crystallization ages of magmatic rocks in the Wrangel Complex cluster around 710-700, 675 and 630-620 Ma. On the northern Arctic coast of Chukotka, 675 and 620 Ma ages have been documented in the Velitkinay metamorphic complex (69N, 177E).

Strata deposited unconformably above the Wrangel Complex are most likely Devono-Mississippian (Miller et al., 2010). U-Pb analyses of detrital zircons (DZ) from four sandstones and three quartzite clasts from a conglomerate in the basal strata overlying the Wrangel Complex yield predominantly 710-620 Ma ages, with age peaks that mimic basement ages. The youngest zircons (in both cobbles and sandstones) are ~580 Ma, notably much older than the suggested depositional age of enclosing strata.

In Arctic Chukotka, identification of the basement-cover contact is obscured by Cretaceous magmatism and deformation. In the Velitkinay massif, quartz-rich meta-sedimentary rocks (depositional age unknown) are structurally above basement (nature of contact with basement uncertain). DZ age ranges 415-700 Ma and 0.8-1.8 Ga with a max. prob. age peak at 520-550 Ma are observed in three samples from the structurally deepest units. A structurally ½ km higher sample yields similar spectra with a second max. prob. age peak ~970 Ma. Two other samples from Velitkinay yield similar results to published results from strata dated Carb.-Perm. on Wrangel (Miller et al., 2010). Three samples from metasedimentary rocks (age uncertain) in the Alyarmaut uplift (69N, 166E) yield DZ results that closely match Velitkinay. Another Alyarmaut sample beneath Triassic strata yields 250-300 Ma ages.

Stratigraphic and DZ evidence suggest the oldest Wrangel sediments are locally (rift) derived from the Wrangel Complex basement, with a transition to more distal and mixed DZ age inputs occurring during Carboniferous time. Arctic Chukotka strata, by contrast, yield well-mixed spectra with peaks characteristic of circum-Arctic orogenic events.