DETRITAL ZIRCON GEOCHRONOLOGY AND PROVENANCES OF QUARTZOSE METASEDIMENTARY ROCKS FROM PARAUTOCHTHONOUS NORTH AMERICA, EAST-CENTRAL ALASKA

Detrital zircons (DZ) in quartzite and quartz semischist from four lithotectonic units of parautochthonous North America in east-central Alaska (Healy schist, Keevy Peak Formation, and Sheep Creek Member of the Totatlanika Schist in the northern Alaska Range, and the Butte assemblage in the northwestern Yukon-Tanana Upland) suggest consistent provenance for these units across a broad region over a difficult-to-constrain time interval. All but one sample have dominant DZ populations of 1.9–1.8 Ga and a subordinate population of 2.7–2.6 Ga. The outlier sample (one of three from Healy schist) has abundant 1.6–0.9 Ga DZ, subordinate 2.0–1.8 Ga and 2.7–2.5 Ga populations, and the youngest minimum DZ age population of 1.0 Ga. Although maximum depositional ages for the samples, based on minimum DZ age populations, are Paleoproterozoic for all but the anomalous sample, the following correlations indicate the possible range of stratigraphic ages of our samples. DZ from the anomalous Healy schist sample resemble zircon ages in Cambrian Adams Argillite and Devonian Nation River Formation in eastern Alaska. DZ from the remaining samples are similar to (1) other peri-Laurentian units in east-central Alaska; (2) the Snowcap assemblage in Yukon, basement of the allochthonous Yukon-Tanana terrane; (3) Neoproterozoic to Ordovician Laurentian passive margin strata in southern British Columbia; or (4) Proterozoic Laurentian Sequence C strata of northwestern Canada. Sourcing of DZ from the Paleoproterozoic Great Bear Magmatic Zone in the Wopmay orogen and its Archean precursors could explain the ³1.8 Ga DZ ages and the whole-rock arc trace-element signatures of quartz semischist samples, as could recycling of strata derived thereof. DZ in lithologically similar metasedimentary rocks in the parautochthonous North America unit and the Yukon-Tanana terrane have overlapping Archean and Proterozoic DZ, but the latter can be discriminated by having mid-Paleozoic DZ that are absent from all parautochthonous samples. Depositional age constraints for the parautochthonous strata consist of crosscutting Late Devonian orthogneiss and sparse Devonian to Cambrian fossils, suggesting that lower Paleozoic, rather than Precambrian, stratigraphic ages are more likely.