LOW-TEMPERATURE COOLING HISTORY OF THE BLUE RIDGE PROVINCE, NORTH CAROLINA

New K-feldspar ⁴⁰Ar/³⁹Ar cooling ages help constrain the low-temperature cooling history of the Blue Ridge in the vicinity of Mt. Rogers. Previously reported ⁴⁰Ar/³⁹Ar cooling ages of amphibole-muscovite pairs show that the Western Blue Ridge (WBR) cooled slowly from the Early Neoproterozoic to Early Cambrian times whereas the Eastern Blue Ridge (EBR) reached peak amphibolite-facies metamorphic conditions during the Ordovician Taconic orogeny (consistent with monazite crystallization ages, Hietpas et al., 2010) and then experienced intra-block dynamic exhumation through the middle Mississippian. However the time at which the rocks of the EBR and WBR reached thermal equilibrium below ~350°C is not well constrained.

A suite of K-feldspar bearing rocks was collected along a NW-SE transect from Mt. Rogers to the Brevard Fault zone (BFZ) in order to better constrain the low-temperature cooling history of Blue Ridge Province. K-feldspar separates were analyzed via the step-heating method where all of the samples yielded climbing age spectra. High temperature portions of these spectra show 1) disparate Early Paleozoic cooling of the WBR through 250 ± 50 °C and 2) Pennsylvanian cooling of the EBR through 250 ± 50 °C. Low temperature (150 ± 50°C) portions of the age spectra show ages decreasing from Late Pennsylvanian to Early Jurassic across the entire Blue Ridge. This indicates that the relative positions of the rocks were modified after the cooling ages were fixed in the minerals below ~150°C. This would allow the age gradient to be exposed at present erosional levels. Possible mechanisms for preservation of this age gradient are tilting of the Blue Ridge Province, NW-side down along a normal fault west of Mt. Rogers or differential erosion in the Blue Ridge due to sedimentary loading of the eastern Appalachians and bowing of the crystalline rocks.