Insights into southern Appalachian tectonics from ages of detrital monazite and zircon in modern alluvium

Ages of detrital monazite and zircon from alluvium collected from the French Broad River drainage basin, an orogen-crossing main trunk river, and alluvium in firstorder tributary streams, provide an unconventional perspective for examining the regional tectonic and metamorphic history of the southern Appalachian orogen (eastern United States). The French Broad River system samples migmatitic Ashe-Tallulah Falls suite paragneisses (with inferred Neoproterozoic clastic protoliths) of the Eastern Blue Ridge and western Inner Piedmont, Mesoproterozoic basement orthogneisses, numerous Paleozoic metaplutonic gneisses, and tectonite equivalents of these lithologies in the Brevard fault zone. Middle Ordovician ages dominate the monazite age spectrum. Monazite from tributaries has a dominant ²⁰⁸Pb-²³²Th age peak ca. 463 Ma. Monazite from the French Broad River alluvium suite yields a dominant ²⁰⁸Pb-²³²Th age mode ca. 450 Ma, but differs from the tributaries in having scattered Mesoproterozoic, Silurian- Devonian, and Carboniferous ages. Electron microprobe total Th-U-Pb chemical ages for selected tributary monazite grains also analyzed by ion microprobe reveal additional monazite growth events (i.e., metamorphic reaction) at 480-475 Ma and 445-440 Ma. Tributary and French Broad River zircon age spectra are dominated by Mesoproterozoic and Ordovician grains. Most Ordovician zircon from the French Broad River has Th/U > 0.1 and is most likely derived from the Henderson orthogneiss (447.6 ± 5.4 Ma), the largest pluton in the French Broad River head waters region. A minor zircon age population at 450 Ma, represented primarily by metamorphic zircon rims with Th/U < 0.05 on magmatic Mesoproterozoic zircon cores, is present in tributaries and samples of migmatitic Ashe-Tallulah Falls suite paragneiss.