DIVERSITY IS A BEAUTIFUL THING: A TEST OF COMBINED 40AR/39AR MUSCOVITE, TH/PB MONAZITE AND U/PB ZIRCON AGES IN MODERN STREAM SEDIMENT TO CHARACTERIZE THE SOUTHWESTERN BLUE RIDGE

Combinations of different mineral chronometers are needed to outline complete P-T-t histories for even the simplest plutonic terranes. Thus, combinations of mineral chronometers, with differing responses to crystallization and thermal history, are needed to fully characterize the source(s) of clastic sediment, and to discern multiple sources vs. a single terrane with a polyphase history. Laser single crystal ⁴⁰Ar/³⁹Ar ages of muscovite (determined in ANIMAL with typical precision of ~ 0.5% and >100 ages per sample) from the trunk streams of the Etowah and Coosa rivers (in the southern end of the western Blue Ridge and part of the Talladega Belt of GA) are dominated by single modes of ca. 320 Ma that are skewed with ages that range up to ca. 360 Ma. A sediment sample from the trunk stream of the Coosawattee River (in the Murphy Syncline of GA) has muscovite with a single mode of 330 Ma that is similarly skewed with ages ranging up to ca. 400 Ma. Samples from the French Broad River (FBR; originally collected by Hietpas et al., Geology, 2010) yield muscovite age distributions that are dramatically different from the trunk stream in TN to some of its highest tributaries. Muscovite from a small tributary in NC (near the Brevard Fault Zone and the Pisgah National Forest) yields ages confined to a narrow range of ~310-322 Ma. In contrast, samples that are closer to the FBR trunk stream in TN are complex and yield multiple age modes, with some ages as old as ca. 420 Ma. (Note that ages of ca. 420 Ma are typical for Taconic muscovite in amphibolite facies rocks, that may have reached closure during cooling at rates of 5-10°C/m.y.) Comparing the ⁴⁰Ar/³⁹Ar muscovite ages of this study with the U/Pb monazite and zircon ages of Hietpas et al. (2010) for the same FBR samples, we find each isotopic system gives an indication of the complete Appalachian history. However, the ⁴⁰Ar/³⁹Ar results are biased toward younger ages that reflect the late Appalachian cooling and metamorphism, whereas the U/Pb ages of monazite and zircon are biased toward earlier events that crystallized those phases. We find the combined dataset accurately depicts an overall history of the southern Appalachians that began with incorporation of material from Grenville terranes in a polyphase orogen that has distinct Ordovician-Silurian, Devonian, and Carboniferous events.