PB ISOTOPES IN DETRITAL FELDSPARS CAPTURE SHIFTING PROVENANCE TRENDS DURING EVOLUTION OF THE MESOZOIC GOLD BEACH TERRANE, SW OREGON

Provenance research utilizing U-Pb geochronology of detrital zircons has been widely employed to test models of terrane displacement along the western margin of North America; however, zircon provides an incomplete picture of tectonomagmatic processes. Another promising tool is the Pb isotope system, which is controlled by time-integrated differences in the U/Pb and Th/U ratios of source regions. Detrital minerals containing Pb, but not U or Th, such as feldspar, preserve the initial Pb isotope composition of source magmas allowing for discrimination between crustal provinces. Previous bedrock Pb isotope studies have played a vital role in the characterization of crustal provinces in North America and as the database of Pb compositions continues to grow, provenance correlations become more apparent.

In this study, we measured Pb isotopes by LA-MC-ICP-MS in detrital orthoclase and plagioclase from Late Mesozoic sedimentary rocks in southwestern Oregon to test a hypothesis of northward displacement of the Gold Beach terrane. Ratios normalized to ²⁰⁴Pb show both unradiogenic and highly radiogenic populations, while Pb isotope compositions based on the ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb ratios further discriminate grains into four distinct groups. Pb from the Jurassic units (both basement and sedimentary rocks) of the Gold Beach terrane are most similar to sources in Arizona and northern Mexico. Detrital feldspars in Lower Cretaceous units have Pb compositions that match many of the accreted Paleozoic arc terranes in both the western US and Canada and are thus nonunique. However, two populations of feldspars in the Upper Cretaceous sandstone have Pb isotope signatures that mostly match the Sierra Nevada batholith. Together, these data are consistent with a northward shift of the terrane from the Jurassic to Late Cretaceous, complementing previous research that supports the displacement of crustal blocks along the western margin of North America.