LOW TEMPERATURE AND DETRITAL 40AR/39AR THERMOCHRONOLOGY OF THE WESTERN BLUE RIDGE, FRENCH BROAD RIVER CATCHMENT AREA, NC

The western Blue Ridge (WBR) has a polymetamorphic history of Mesoproterozoic to Paleozoic events. Previous geochronologic studies in the WBR generally utilized high temperature geo- and thermochronometers (U/Pb zircon and monazite, ⁴⁰Ar/³⁹Ar hornblende) in high-grade and igneous rocks. However, these systems have tendencies to inherit and retain age signatures of earlier events in the WBR. We chose to determine the record of low-temperature metamorphism and cooling in medium- to low-grade metamorphic rocks in the WBR, as recorded by single crystals of muscovite and K-Feldspar, from basement and stream sediment samples collected in the area of the French Broad River (FBR) catchment in NC. Muscovite from basement rock samples in the catchment area yield single crystal ⁴⁰Ar/³⁹Ar ages that typically range from 315 Ma to 400 Ma. Easterly basement samples, of higher grade, within and near the Brevard Fault Zone, yield muscovite age distributions that are simple, with single modes of middle Carboniferous age. Basement samples collected near and west of Asheville are complex, and it is typical to find up to ~ 80 million years of variation in ages of muscovite crystals from a single hand sample. Similarly, microcline and orthoclase from Ky-grade rocks near the Brevard fault zone reflect Carboniferous metamorphism and cooling, whereas single K-feldspar crystals from a basement sample collected west of Asheville have initial incremental heating ages of ca. 270 Ma and ages up to ca. 1100 Ma for the high temperature increments. The ⁴⁰Ar/³⁹Ar age signature of detrital mineral samples collected along the WBR becomes dramatically more complex to the west, and our work on basement samples in the catchment shows the complexity is not only due to the increasing sediment input of local tributaries, but also to the complexity of polymetamorphism in low-grade metamorphic rocks west of the Brevard fault zone. Comparing the results of the present study to published data for high-temperature thermochronometers, it is remarkable that the low temperature record of K-feldspars can be used to characterize a greater range of meaningful history along this WBR transect. Our further study will focus on evaluating the tectonic significance of single-crystal ⁴⁰Ar/³⁹Ar ages for feldspar in the FBR area, and their use in detrital thermochronology.