PETROFABRIC AND TIMING CONSTRAINTS ON THE EXHUMATION HISTORY OF THE BUCK CREEK-CHUNKY GAL ULTRAMAFIC/MAFIC COMPLEX AND ADJACENT CHUNKY GAL MOUNTAIN FAULT IN THE SOUTHERN APPALACHIAN BLUE RIDGE

The Buck Creek-Chunky Gal (BC) ultramafic/mafic complex lies adjacent to the Chunky Gal Mountain Fault (CGMF) within the southern Appalachian Blue Ridge. Evaluation of the post-peak metamorphic and structural history of the BC complex and CGMF provides new P-T-t-deformation constraints on their exhumation history.

Previous work indicates that meta-troctolites in the BC complex preserve high P/T anhydrous conditions and experienced localized hydration at and following peak metamorphism. In hydrated meta-troctolites, the dominant (S2?) foliation is defined by coarse edenitic amphibole (EdA), part of the peak (~1.2GPa, ~825^oC) hydrated assemblage. A few outcrops that preserve late structural fabrics reveal up to 5 planar fabrics defined by (EdA) and chlorite (Chl) of varying size and texture. Chl grains and recrystallized EdA/Chl domains define the dominant NNE-striking, steep foliation in these outcrops (S3?). Chl defines an E-W spaced cleavage (S4?). Two pre-S3 Chl fabrics transect relict compositional layering.

The deformation and metamorphic character of the CGMF is constrained via studies of its defining SSW-facing road exposure along US Hwy 64. The dominant (S2?) moderate/shallow foliation is deformed by steep NNE-striking high strain zones, defined by mylonite, ultramylonite, and brittle faults, with outcrop-scale normal offset, spaced several meters apart within biotite gneiss and at the contact with Chunky Gal amphibolite. Shear zones preserve a shallow lineation with dextral shear sense, and/or a down-dip lineation associated with a normal shear sense.

Preliminary PT estimates and EPMA monazite ages from a garnet mylonite yield monazite growth (cores) at peak metamorphism (~470 Ma, ~8.5 GPa, >720^oC), likely associated with dynamic recrystallization and possible fast-migrating grain boundaries in feldspar, strain-induced myrmekite growth, and plastic deformation of hornblende. Continued retrograde deformation is marked by local domains of foliated muscovite and muscovite fish, resulting from similar diffusion and dislocation creep mechanisms, facilitated by fluid flow. Later monazite growth (rims and small matrix grains, some with dextral matrix tails) at ~450 Ma, may be linked to this retrograde deformation at conditions of ~7.2 GPa, >580^oC.