--> Applications and Method Development for Detrital K-feldspar Common-Pb Provenance Analysis

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Applications and Method Development for Detrital K-feldspar Common-Pb Provenance Analysis

Abstract

The use of common-Pb isotopes in detrital K-feldspar grains is an emerging geochemical provenance tool. K‑feldspars are less susceptible to extensive recycling than more durable mineral types such as zircon; they are also 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-feldspars 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. Method development presented here includes establishing new common-Pb K-feldspar standards: Mary’s Vale (Utah), Little Cottonwood Stock (Utah), and Fish Canyon Tuff (Colorado), as well as continued use of Shap Granite (Ireland). Each of these are dissolved and analyzed for precise Pb concentrations and isotopic values and thus constrain our unknown laser ablation data. Additionally, we present a sediment source K-feldspar Pb-Pb database for North America from published literature, allowing us to understand isotopic ranges for most of the tectonic provinces recorded in the Cretaceous foreland basin. Data presented here test the use of detrital K-feldspar common-Pb isotopic ratios as a provenance indicator in sandstones from southern Utah. Building on previous work (e.g. Tyrrell et al. 2012), we spot-analyzed detrital K-feldspar grains for samples of the John Henry Member of the Straight Cliffs Formation. The well-studied Late Cretaceous Cordilleran foreland basin provides a robust detrital zircon U-Pb geochronology dataset, 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. Our data demonstrate viability of the common-Pb isotope method, indicating a dominant population of Proterozoic-aged K-feldspars and a weak Cretaceous arc signal. This supports previous interpretations suggesting sediment source terranes were located to the southwest, such as the Mogollon Highlands (1250-1900 Ma) but contradicts those attributing large volumes of sediment as sourced from the Cordilleran Magmatic Arc (86-260 Ma), posing new questions about sediment routing systems in this basin.