FIRST CYCLE OR POLYCYCLIC? COMBINING APATITE AND ZIRCON DETRITAL U-Pb GEOCHRONOLOGY AND GEOCHEMISTRY TO ASSESS SEDIMENT RECYCLING IN THE SOUTHWESTERN MONTANA FORELAND BASIN, NORTH AMERICAN CORDILLERA

Even with the advent of multiple new geochronologic provenance techniques in recent decades, almost all provenance studies are still hampered by a fundamental limitation: recycling of older strata is often a significant concern but cannot be adequately addressed using current techniques. Detrital zircon is the dominant mineral used in provenance studies, but zircons are robust and almost always represent both first-cycle and polycyclic sediment contributions in a sandstone. To overcome these issues, we implement two relatively novel applications of detrital geochronology – U-Pb geochronology and trace and rare earth element geochemistry of detrital apatite – to take a multi-method approach of analyzing both mineral phases from the same sandstone. Our results indicate that zircon and apatite detrital age distributions from the same sandstone that are nearly identical, paired with apatite TREE geochemistry denoting an igneous source, are a key diagnostic indicator of sediment derived from shallowly-emplaced (<~500°C) or extrusive igneous rocks. Detrital age distributions that are similar but offset from one another such that the apatite peaks are younger than the zircon peaks and also young up-section, paired with apatite TREE geochemistry denoting an igneous source, implies deep exhumation of an igneous source through the apatite U-Pb closure temperature. The combination of detrital zircon and apatite U-Pb geochronology and TREE geochemistry also permits detection of recycled versus first-cycle components from metamorphic basement terranes. This study signifies a significant advance in provenance research by demonstrating the ability of combining detrital apatite and zircon U-Pb geochronology and geochemistry to decipher first-cycle versus polycyclic sediment from various types of igneous and metamorphic rocks. Our results show this methodology has potential applications and implications for all types of sedimentary systems, paleogeographic reconstructions, provenance interpretations, and tectonic reconstructions.