Abstract

Granulite-facies rocks in the thermal axis of Paleozoic (Taconic) metamorphism are exposed in southwestern North Carolina in a 370-m-long road-cut at Winding Stair Gap. Lithologic units are composed of metasedimentary and metaigneous rocks, some of which are now migmatites. Structural analysis of the exposure indicates that deformation occurred before, during, and after the peak of metamorphism. Interpretation of the predeformation stratigraphy suggests five of the six lithologic units may correlate with intervals in the late Precambrian Tallulah Falls Formation or the upper part of the late Precambrian or possibly early Paleozoic Coweeta Group. The sixth unit, an orthopyroxenite, may be related to presumed ophiolites elsewhere in the Hayesville thrust sheet.

Prograde metamorphic conditions of T =750–775 °C, P = 6.5–7.0 kbar were determined from mineral stability fields and cation exchange geothermometry-geobarometry. Partitioning of water into anatectic melts of quartzofeldspathic gneiss and garnet-sillimanite schist units resulted in PH2O < Ptotal during the prograde event. An isobaric cooling path, from the metamorphic peak to 600 °C, may be related to expulsion of partial melts that may be represented by pegmatites and trondhjemite dikes at higher structural levels. Limited retrograde effects were likely facilitated by water released from crystallization of the remaining in situ melts at T <700 °C.

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