40AR/39AR ISOTOPIC EVIDENCE FOR LATE CARBONIFEROUS EMPLACEMENT AND EXHUMATION OF THE ASHE FORMATION ABOVE THE FRIES FAULT IN THE BLUE RIDGE OF NORTH CAROLINA

New ⁴⁰Ar/³⁹Ar analyses of minerals from rocks sampled along a NW-SE transect from the Fries fault to the Brevard fault in the Blue Ridge of NC, indicate a late Carboniferous age of emplacement and exhumation. ⁴⁰Ar/³⁹Ar cooling ages of amphibole, muscovite, and K-feldspar in the Ashe and Alligator Back formations in the hanging wall of the Fries fault indicate that metamorphic cooling from 500°C to 150 °C occurred ca. 360 to ca. 260 Ma. Inverse correlation ages in agreement with minimum age steps in 9 muscovite samples yield a SE trending age gradient from ca. 340 to ca. 325 Ma, while inverse correlation ages from amphiboles also yield a SE trending age gradient from ca. 360 to ca. 325 Ma. We interpret these ages to represent the time of cooling through the argon retention temperatures of 350 ± 50 °C for muscovite and 500 ± 50°C for amphibole.

In contrast, amphibole and K-feldspar cooling ages from Mesoproterozoic Grenvillian metamorphism in the footwall of the Fries fault indicate cooling from 500º to 150ºC from ca. 950 to ca. 260 Ma. Muscovite age spectra are complicated by the presence of mixed age populations. For example, one sample contained large randomly oriented muscovite and biotite grains truncated by a later foliation defined by intergrown muscovite and chlorite (S_n). The age spectra from this sample climbs from ca. 330 to ca. 540 Ma. We interpret the younger age as the maximum age of muscovite that defines S_n and the older age as the minimum cooling age of the coarse grained muscovite. A sample from within the Fries fault zone yielded an isotope correlation age of 329 ± 7 Ma. In view of the fine grained intergrowth of white mica and chlorite which define S_n, we interpret this middle Mississippian age as a crystallization age which is the time of S_n development and motion of the Fries fault. It is possible that the amphibole and muscovite age gradients within the Fries block are associated with folding of the duplex of the Grandfather Mountain Tectonic Window. The coincidence of Mississippian cooling ages in rocks in the hanging wall rocks of the Fries fault with the crystallization ages within the Fries fault provides strong evidence for the final emplacement of the Fries block in the Alleghanian orogeny.