40Ar/39Ar Ages of Feldspar and Muscovite from the Source and Detritus of the French Broad River, North Carolina

Abstract

As the westernmost metamorphic belt of the Appalachians, the Blue Ridge has been the subject of many geochronology studies. The Blue Ridge experienced high-grade deformation and peak metamorphism during Taconic orogeny, followed by a low-grade metamorphic overprint during the Acadian orogeny. The Alleghanian orogeny is the last collisional stage of the Appalachians and associated regional metamorphism and ductile deformation is documented along most of the Piedmont and the Carolina Slate belt. There is still debate, however, as to the extent of Alleghanian metamorphism in the western Blue Ridge. This concern is made more difficult to evaluate because previous work generally did not characterize the history of low-temperature metamorphism of the Blue Ridge in the region between western North Carolina and Tennessee. To address the cooling history of the Blue Ridge, samples were collected in the area of the French Broad River catchment in North Carolina. Single crystals of muscovite from basement and stream sediment samples and K-feldspar from the basement, were dated in this project to avoid the ‘inherited’ ages often associated with high-temperature geochronometers. Muscovite from basement rock samples in the catchment area yield single crystal 40Ar/39Ar ages that typically range from 315 Ma to 400 Ma. K-feldspar crystals from basement rocks yield single crystal ages as young as ca. 270 Ma and up to ca. 1100 Ma. Results for the two mineral phases show similar age distribution patterns: easterly basement samples, within and near the Brevard fault zone, yield Carboniferousage distributions characterized by simple, single modes, but basement samples collected near and west of Asheville are more complex. The incremental heating experiments for K-feldspar show variable discordance. The variation in ages of microcline and orthoclase crystals from a single basement sample collected west of Asheville covers a wide range of 800 million years from late Mesoproterozoic to Permian. Comparing the results of the present study to published data for higher temperature thermochronometers (e.g., U-Pb ages for zircon, 40Ar/39Ar ages for hornblende and micas), it is remarkable that the low temperature record of K-feldspars can be used to characterize a greater range of cooling history rather than the higher temperature thermochronometers. The 40Ar/39Ar age signature of detrital muscovite mineral samples collected along the French Broad River catchment becomes dramatically more complex within lower grade rocks downstream (northwest). Our work on basement samples in the catchment shows the complexity is not only due to the increasing sediment input of various local tributaries into the trunk stream, but also to the intra-sample complexity of polymetamorphic history recorded in the metamorphic rocks west of the Brevard fault zone.