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 first-order 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 208 Pb- 232 Th age peak ca. 463 Ma. Monazite from the French Broad River alluvium suite yields a dominant 208 Pb- 232 Th 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 headwaters region. A minor zircon age population at 450 Ma, represented primarily by metamorphic zircon rims with Th/U Ordovician monazite ages and zircon rim ages correspond to Taconian metamorphism in the Eastern Blue Ridge province. The dominant monazite age mode (463 Ma) from tributaries is slightly older than the Ordovician metamorphic zircon age mode (450 Ma), which is accounted for by monazite growth primarily via prograde metamorphic reactions, and zircon growth by melt-forming reactions in migmatites at the thermal peak. The scattered middle to late Paleozoic zircon and monazite ages attest to the lack of significant thermotectonic and magmatic events of that age in the Southern Blue Ridge providing sediment to the French Broad River drainage system. This dearth of ages is consistent with the pattern of nonpenetrative late Paleozoic deformation, retrograde metamorphism, and scattered plutonism northwest of the Brevard fault zone. A reasonable source of Neoproterozoic zircon in alluvium or in Ashe–Tallulah Falls paragneisses is Neoproterozoic rift-related magmatic rocks. The rarity of Mesoproterozoic monazite compared to zircon contrasts with the marked abundance of monazite in Ashe–Tallulah Falls paragneisses, and emphasizes the responsiveness of monazite compared to zircon in regional metamorphism. The abundant Mesoproterozoic zircon was inherited from Mesoproterozoic basement lithologies by Neoproterozoic sediments, preserved through regional metamorphism and three phases of orogenesis, and persists in modern alluvium being shed by the orogen.