Granulites at Winding Stair Gap, North Carolina; the thermal axis of Paleozoic metamorphism in the Southern Appalachians

Granulite-facies rocks in the thermal axis of Taconic metamorphism consist of metasedimentary and meta-igneous units, some of which are now migmatites. Deformation occurred before, during, and after the peak of metamorphism. The lithological units may correlate with the upper part of late Precambrian or possibly early Palaeozoic formations elsewhere. Prograde metamorphic conditions of T 750-775 degrees C, P 6.5-7.0 kbar were determined from mineral stability fields and cation-exchange geothermometry-geobarometry. During the prograde event, partitioning of water into anatectic melts of quartzofeldspathic gneiss and garnet-sillimanite schist units resulted in PH (sub 2) O < P (sub total) . Pegmatites and trondhjemite dykes may be related to expulsion of partial melts on an isobaric cooling from the metamorphic peak to 600 degrees C. Limited retrograde effects may be facilitated by water released from crystallization of the remaining in situ melts at <700 degrees C.