TEMPORAL AND SPATIAL CHANGES IN PALEOWINDS INFERRED FROM DETRITAL ZIRCON GEOCHRONOLOGY OF UPPER PALEOZOIC LOESSITE, WESTERN NORTH AMERICA

U-Pb geochronology of detrital zircons from upper Paleozoic loessite (western United States) provides preliminary data bearing on atmospheric circulation patterns within western equatorial Pangea. Loessite (lithified eolian silt) is increasingly recognized in upper Paleozoic strata of western Pangea. Zircon age spectra of six loessite samples representing four stratigraphic sections ranging from Middle Pennsylvanian (Desmoinesian) to Early Permian (Wolfcampian) ages vary significantly, reflecting changing provenances attributable to temporal and spatial shifts in winds. Zircons from two Desmoinesian samples (Arizona, Utah) show a dominant mode between 1800-1600 Ma, reflecting the Yavapai/Mazatzal terranes coring the Ancestral Rockies uplifts and suggesting northeasterly winds for both localities. Both samples also contain a secondary cluster of Grenville-age grains (1000-1300 Ma) and grains ranging from 440-370 Ma, reflecting a south-southeasterly source (e.g. Mexico/Texas). Age spectra for the four Wolfcampian samples (New Mexico, Colorado and Utah) differ from one another. The New Mexico loessite contains a large mode at 1700 Ma, which is missing in the Utah sample and minimal in the Colorado samples; this difference may reflect the locations of these samples on opposing sides of the "core" Ancestral Rockies uplifts within a strong westerly wind regime. Further, the age spectra of the two Wolfcampian samples from Colorado section exhibit significant provenance differences, suggesting a temporal shift in wind regimes. The younger sample contains zircon populations that suggest a return of significant north- and southeasterly winds, with less evidence for westerlies. Inferred easterly winds for Middle Pennsylvanian time matches predictions of zonal circulation models, and the presence of both northerly and southerly directions may reflect time-averaged fluctuation of the inter-tropical convergence zone (ITCZ). In contrast, our data indicate that monsoonal circulation and attendant westerly winds were established by earliest Permian time, but evolved during the Wolfcampian. Currently, we are matching the geochronologic data to more traditional geochemical and petrographic data to develop higher-resolution records of Late Paleozoic atmospheric circulation for western Pangea.