INSIGHTS INTO SOUTHERN APPALACHIAN METAMORPHISM FROM AGES OF DETRITAL MONAZITE AND ZIRCON IN MODERN ALLUVIUM AND BEDROCK SOURCES

Detrital monazite and zircon for alluvium collected from the French Broad River (FBR) and first order tributaries provide an unconventional perspective for examining the metamorphic history of the southern Blue Ridge. Tributaries sample migmatitic Ashe Metamorphic Suite (AMS) paragneiss and Grenville orthogneiss; the FBR collects primarily eastern Blue Ridge alluvium, but headwaters collect Brevard Fault zone and Western Inner Piedmont (WIP) alluvium. Monazite from tributaries is remarkably uniform in age; 61 grains have a weighted mean ion probe Th-Pb age of 463 ± 2 (2σ; MSWD = 0.7). Monazite from FBR alluvium yields a dominant age mode at 450 Ma, with scattered Mesoproterozoic, Siluro-Devonian, and Carboniferous ages. Both sets of Middle Ord. monazite age distributions appear to consist of more than one growth event. Electron probe Th-U-Pb chemical ages for selected tributary monazite grains also analyzed by ion probe, reveal additional monazite growth events at 480-475 Ma and 445-440 Ma. Grenville modes dominate zircon ages for tributaries, river alluvium, and AMS paragneiss. A minor age mode at 450 Ma, represented primarily by metamorphic zircon rims (Th/U < 0.5), is present in tributaries and samples of AMS gneiss. Most Ord. zircon from the FBR has Th/U > 0.1 and is probably derived from Ord. plutons (e.g., Henderson gneiss is 450 ± 5 Ma). All Ord. monazite and zircon rim ages correspond to the time of regional Sil-grade Taconian metamorphism in the EBR. The weighted mean age of the dominant monazite age mode (463 Ma) from tributaries is slightly older than the dominant Ord. metamorphic zircon age mode (450 Ma), indicating monazite growth via prograde reactions prior to peak T, and zircon growth by melt-forming reactions in migmatites at peak T. The rarity of Precambrian monazite compared to the abundance of Precambrian zircon contrasts with the marked abundance of monazite in AMS gneisses, emphasizing the responsiveness of monazite compared to zircon in regional metamorphism. The overwhelming abundance of Precambrian zircon, even in migmatitic AMS gneisses, reflects the remarkable zircon fertility of Grenville basement lithologies, which was inherited by Neoproterozoic sediments, preserved through regional metamorphism and three phases of orogenesis, and preserved in modern alluvium shed by the orogen.