P-T-T PATH CONSTRAINTS DURING DEFORMATION WITHIN THE CHUNKY GAL MOUNTAIN FAULT, CENTRAL BLUE RIDGE, NORTH CAROLINA

The Chunky Gal Mountain Fault (CGMF) is located within the Central Blue Ridge province of the Southern Appalachians. This terrane-bounding fault lies adjacent to the Buck Creek-Chunky Gal mafic-ultramafic complex (BCC), an ocean crustal fragment that experienced peak (Taconian) conditions of ~ 825°C, 1.2 GPa. At the type locality, the CGMF is characterized by several focused shear zones along the boundary between BCC amphibolites and garnet-biotite or sillimanite-garnet-biotite gneisses. CGMF samples preserve complex physical and chemical textures that may help constrain the emplacement history of the BCC.

A biotite gneiss ultramylonite (sample 11L3), with a shallow lineation and sinistral asymmetry includes garnet, K-feldspar, plagioclase, and muscovite porphyroclasts and accessory ilmenite, rutile, and monazite. EPMA monazite geochronology indicates a range of ages for monazite growth. The ages of cores of larger monazite grains and those preserved as garnet inclusions cluster around 470 Ma. The small matrix monazites and low Y rims of larger grains display asymmetry consistent with growth during mylonite formation and cluster around an age of ~ 450 Ma.

Complex garnet zoning in 11L3 and other CGMF samples suggests preservation of prograde zoning (increasing Mg/decreasing Ca) similar to large garnets at Winding Stair Gap (WSG) where peak granulite facies are documented. However, a distinct increase in Ca and Mn toward the rims, which has not been previously reported in the area, is distinctive in 11L3 and other CGMF rocks.

We estimate peak metamorphic conditions of ~ 800°C and > 0.8 GPa, similar to conditions at WSG, using TWQ thermobarometry and Theriak-Domino pseudosections. Retrograde temperatures of ~ 600°C are estimated from Garnet rim/adjacent biotite. Preliminary P-T-t modeling of garnet zoning (Gibbs program) suggests that prograde growth is characterized by a slight increase in temperature and distinct decrease in pressure. The increased Ca toward the rim produces a nearly isothermal and sharp increase in pressure. This may indicate significant loading at or near peak temperature conditions, possibly resulting from thrusting across the GCMF.