TECTONIC AND CLIMATIC EVENTS OF WESTERN PANGAEA RECORDED IN THE PROVENANCE AND GEOCHEMISTRY OF UPPER PALEOZOIC LOESSITE

U-Pb geochronology of detrital zircons within wind-blown siltstone (loessite) as well as whole-rock geochemistry enable constraints on changes in tectonic and climatic events in western Pangaea during the late Paleozoic. The late Paleozoic assembly of Pangaea led to a series of intra-plate uplifts, the Ancestral Rocky Mountains (ARM) and associated basins, particularly within western Pangaea. Additionally, long-term climatic changes in the form of the development of monsoonal circulation, and shorter-term climatic changes in the form of glacial-interglacial fluctuations affected western Pangaea. Our present dataset of detrital zircon geochronology includes 27 loessites, reworked loessites and paleosols ranging in age from Desmoinesian to Wolfcampian and spanning across western equatorial Pangaea. The data indicate that, during the Pennsylvanian, silt was derived from regional sources, including the ARM uplifts (Yazapai-Mazatzal basement) as well as a secondary source from uplifted terranes in present-day Mexico (early Paleozoic ages). However, by Wolfcampian time, the silt provenance shows increased mixing, with a large influx of silt reflecting Grenvillian and Neoproterozoic basement, likely signaling input of sediment from the Appalachian orogen. The presence of this Appalachian signal, however, is tempered spatially by the development of the western Pangaean monsoon by earliest Wolfcampian time. Whole-rock geochemistry of the loessites support these interpretations. On shorter-time scales, a number of coupled loessite and capping paleosols exhibit different detrital zircon age spectra, indicating atmospheric circulation changes accompanied the facies changes and likely reflect glacial-interglacial climate change; these inferences are again supported by whole-rock geochemistry and sedimentology of the facies pairs.