QUARTZ C-AXIS DEFORMATION THERMOMETRY CONSTRAINS RETROGRADE DEFORMATION WITHIN THE CHUNKY GAL MOUNTAIN FAULT, WESTERN NORTH CAROLINA

Quartz C-axis opening angle thermometry is used to determine the temperature of deformation in metamorphic rocks, using an empirically derived linear relationship between C-axis slip orientation and deformation temperature. Calibration equations were derived from both experimental and natural data. A recently developed calibration extends the relationship to higher deformation temperatures and documents a shift in slope with a change in mechanism from subgrain rotation (SGR) to grain boundary migration (GBM) at temperatures near 650°C.

This study estimates deformation temperatures from quartz-rich domains in mylonitic samples from The Chunky Gal Mountain Fault, Central Blue Ridge Province, western North Carolina. The fault zone preserves a range of deformation temperatures. Peak metamorphism within the shear zone is estimated at ~800^oC with evidence of ductile deformation of K-feldspar porphyroclasts. Muscovite fish swim in the mylonitic matrix and quartz microstructures include both SGR and GBM textures. All study samples (n=5) were collected from the same shear zone. Samples from other shear zones did not preserve a girdle pattern that could be used with this method. C-axis orientation data, collected using Electron Backscatter Diffraction (EBSD) data is plotted on stereonets, referenced to sample foliation and lineation. For samples with a c-axis girdle pattern, opening angles were measured from the net center between girdle limbs in each direction. We employed 3 different opening angle calibration equations.

Opening angles from shear zone samples range from 51° to 71°. Temperature estimates from these angles range from 400 °C to 580 °C and are similar for the 3 different calibrations. Deformation temperature estimates from the opening angle fall within the SGR-related calibration and are distinctly lower than peak metamorphic conditions. Our results may indicate that quartz fabrics document the late stages of shear zone movement.