COMMON MINERALS AND COMMON PB ISOTOPES: UNCOMMON APPLICATIONS AND NEW METHOD DEVELOPMENT FOR DETRITAL K-FELDSPAR PROVENANCE ANALYSIS

Geochemical provenance studies use a combination of traditional and new analytical methods to characterize complex sediment source dynamics. One emerging provenance tool is the use of common-Pb isotopes in detrital K-feldspar grains. K-feldspars are less susceptible to extensive recycling than more durable mineral types such as zircon; they also are relatively abundant in clastic rocks, an advantage when working with limited sample material (e.g. core, well-cuttings). Lead isotope ratios in whole rocks and K-feldspar vary proportionally through time and by U/Th, and therefore are useful for fingerprinting gross crustal structure. The relationship between ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁴Pb falls along a somewhat predictable curve, depending on the system’s ²³⁸U/²⁰⁴Pb ratio through time.

New work presented here tests the use of detrital K-feldspar common-Pb isotopic ratios as a provenance indicator in sandstones from southern Utah. The well-studied Late Cretaceous Cordilleran foreland basin provides a robust detrital zircon U-Pb geochronology dataset, already integrated with stratigraphy, paleocurrent data, and sandstone petrography; however, some ambiguity regarding provenance interpretations remains, specifically the relative input of sediment from the more distal Mogollon Highlands and Cordilleran Magmatic Arc, versus the more proximal Sevier fold-thrust belt. Building on previous work (e.g. Tyrrell et al. 2012), we spot-analyzed ~30 detrital K-feldspar grains for each of four pilot samples. Preliminary data demonstrate analytical viability of the common-Pb isotope method, indicating a dominant population of Proterozoic-aged K-feldspars and a mixed Cretaceous arc signal. This supports previous interpretations suggesting sediment source terranes were located to the southwest, such as the Mogollon Highlands (1250-1900 Ma) and the Cordilleran Magmatic Arc (86-260 Ma). This work adds new insights for the development of Cretaceous Kaiparowits plateau deposits of southern Utah and promises new potential applications to other enigmatic basins.