Petrogenesis of granulite facies rocks at Winding Stair Gap, North Carolina Blue Ridge

Granulite facies rocks in the thermal core of Paleozoic (Taconic) metamorphism are exposed in southwestern North Carolina in a 370 m long road cut at Winding Stair Gap. Lithologic units are primarily composed of metasedimentary and metavolcanic rocks. Prograde conditions of T = 750°- 775°C; P = 6.5-7.0 kb were determined from mineral stability fields and exchange geothermometry/geobarometry. Plutons emplaced within the Hayesville thrust sheet during the Taconic orogeny are the inferred heat source. Partitioning of water into anatectic melts of quartzo-feldspathic gneiss and garnet-sillimanite schist units resulted in _PH₂O ^ ^Ptotal the prograde event. An isobaric cooling path, from metamorphic peak to 600°C, may be related to extrusion of partial melts from Winding Stair Gap in the form of pegmatites and trondhjemite dikes emplaced at higher structural levels. Limited retrograde effects were facilitated by the release of water from crystallization of remaining in situ melts at T < 700°C. Structural mapping of Winding Stair Gap and the surrounding area indicates three periods of deformation occurred before, during, and after peak metamorphism. Reconstruction of the predeformation stratigraphy suggests five of the six lithologic units belong to the Tallulah Falls Formation or the upper part of the Coweeta Group. The sixth unit, an orthopyroxenite, me be related to structurally emplaced ophiolitic slices occurring elsewhere in the Hayesville thrust sheet. Mineral assemblages in the mafic granulites suggest the granulite facies rocks at Winding Stair Gap are different from Precambrian granulite facies rocks.