Whole-rock geochemistry of upper Paleozoic loessite, western Pangaea: Implications for paleo-atmospheric circulation

Average bulk (major and trace-element) chemical composition of upper Paleozoic loessite (lithified loess) from several widely scattered sites in western equatorial Pangaea (western North America) approximates Upper Continental Crust (UCC), but is more similar to the bulk composition of Precambrian basement coring uplifts of the Ancestral Rocky Mountains. However, distinct compositional signatures occur among the various loessite deposits, controlled primarily by differences in Al2O3, MgO, Na2O, K2O and Rb. Furthermore, values of the Chemical Index of Alteration (CIA) of these loessites are low (55–63, skewed toward lower values), closely matching values for North American loess and Precambrian basement. Although diagenesis can account for some of the compositional differences (e.g. local SiO2 or K2O enrichment), it fails to explain the major differences among the sites. Rather, these differences are best explained as a result of variations in regional basement that sourced the loess, coupled with transport (wind) pathways, which varied from zonal easterlies to monsoonal westerlies in this region of tropical Pangaea from late Carboniferous to early Permian time. The low CIA values generally reflect first-cycle derivation from regional basement uplifts of the ARM and, for younger deposits, a variable contribution from the suture zone uplifts and volcanism of the Appalachian–Ouachita–Marathon collisional belt. The variations in bulk chemical composition of the loessite, coupled with previously collected detrital zircon geochronology, suggest that these low-latitude loessites are not simply the result of deflation of mid-latitude erg deposits or coastal alluvial plain sediments. These data represent a first attempt at geochemical characterization of pre-Quaternary loess, and demonstrate the utility of loess geochemistry as a proxy for paleoatmospheric circulation in Earth′s deep-time past.