DEFINING THE HAFNIUM ISOTOPIC SIGNATURE OF THE APPALACHIAN OROGEN THROUGH ANALYSIS OF MODERN FLUVIAL SEDIMENTS

Modern fluvial sediments are the product of erosion, weathering, and transport of bedrock within well-defined watershed boundaries, and their constituent grains may therefore record valuable information about the lithological and geochemical properties of geologic units within the upstream drainage area. Analysis of detrital zircon grains from major rivers in the eastern U.S. allows for the characterization of U-Pb-derived ages and Lu/Hf isotopic signatures for their respective watersheds within the Appalachian Orogen, each of which potentially contains multiple zircon-bearing geologic units. Six modern fluvial sediment samples, collected across a ~1500 km transect from Georgia to Connecticut and representing 170,000 km² of catchment area, reveal significant variations in U-Pb age fractions across the former Laurentian margin. However, their respective εHf signatures are largely consistent regardless of their geographic location. Mesoproterozoic (Grenville) zircons display a more limited range of εHf values (approximately 0 to 10 εHf units) compared to the more negative and variable values (-15 to +10) found in grains from the Paleozoic Taconic, Acadian, and Alleghenian orogenies. More negative εHf values in Paleozoic zircons may be genetically related to older Mesoproterozoic grains, as they are compatible with the continued isotopic evolution of a Lu/Hf array derived from the mantle between 1.2 and 2.0 Ga. Grains with crystallization ages coeval with the Taconic Orogeny show geographic and temporal trends consistent with existing hypotheses nature of plutonism in the Late Ordovician. This study shows that detrital zircon Hf isotope ratios can be complementary to detrital zircon U/Pb measurements for provenance analysis within the Appalachian region, but are of secondary importance and may be more useful in testing hypotheses related to the nature of magmatism and crustal generation through time.