A metamorphic and structural transect in the Central Blue Ridge, Jackson and Macon counties, North Carolina

A Late Proterozoic metamorphic terrane in the central Blue Ridge of western North Carolina preserves a nearly continuous record of regionally metamorphosed high-pressure facies rocks ranging from amphibolite to granulite facies. A wide variety of lithologic types in the stratigraphic units (Copperhill Formation/Ocoee Series, Tallulah Falls Formation, and Otto Formation) of the study area allows documentation of metamorphic conditions across this boundary. Petrographic study indicates that the following metamorphic zones occur in the study area: (1) the first-sillimanite zone, (2) the second-sillimanite zone (first appearance of sillimanite + K-feldspar) in association with partial melting in pelites, and (3) the hypersthene zone (hornblende + orthopyroxene + clinopyroxene + garnet) in amphibolites. The second-sillimanite isograd is constrained along strike approximately 5 km northeast of Franklin, defining the northern extent of the granulite facies thermal core. Phase relations and mineral exchange equilibria place the assemblages of the amphibolite facies at 550-600 °C and 5.5-6 kbar, and assemblages of the granulite facies at 670-800 °C and >7.5 kbar. In addition, pre- to syn- (Hayesville) to post-metamorphic-peak (Soque River) faults juxtapose terranes of different metamorphic grade, allowing delineation of the timing of deformation relative to metamorphism. The presence of these thrust faults in the area emphasizes the effect that overthrusting had on burial depth and formation of high temperature and pressure metamorphic conditions.