Two Metamorphic Events Recorded in Granulite Facies Rocks from the Winding Stair Gap, Central Blue Ridge

Granulite facies rocks from the Winding Stair Gap, North Carolina, are predominated by quartzofeldspathic granofelses and pelitic schists that underwent partial melting. Mineral assemblages include: Garnet – biotite –sillimanite – plagioclase – quartz, Garnet – hornblende – biotite – plagioclase – quartz ± orthopyroxene ± clinopyroxene, and Orthopyroxene– biotite – quartz ± plagioclase. Garnet – bearing leucosomes predominated by plagioclase and quartz, contain kyanite overgrown by sillimanite.

Five textural generations of biotite and at least two generations of kyanite and sillimanite in pelitic schists support a polymetamorphic history for these rocks. During prograde metamorphism garnet overgrew staurolite, followed by dehydration melting of muscovite and biotite in the stability field of kyanite, before the kyanite = Sillimanite reaction was crossed. Compositions of coexisting garnet, hornblende, biotite , and plagioclase in quartzofeldspathic gneiss and a few garnet and biotite crystals separated by plagioclase in pelitic schists yield T of 750 – 900°C, 8 – 10 kbar, based on calculations using THERMOCALC. Similar results (890°C, 9.6 kbar) were obtained using orthopyroxene - garnet – biotite – plagioclase thermobarometry corrected for retrograde exchange for a quartzofeldspathic granofels, whereas calculations for most pelitic schists yielded significantly lower P-T conditions with large scatter. Following peak metamorphism, the rocks underwent a stage of near isobaric cooling during which K-feldspar was replaced by muscovite, and garnet by sillimanite + biotite.

Most monazite crystals from a pelitic schist display patchy zoning for Th, Y and U, with some matrix crystals having as many as 5 zones with microprobe dates of 290 to 500 Ma. Monazite inclusions in garnet and Y – rich cores of some monazite matrix crystals yield the oldest dates of 446 ± 8 Ma, whereas a few homogeneous matrix monazites yield dates of 361 ± 8 Ma. These data suggest that the central Blue Ridge terrane reached peak conditions during the Taconic, but was re-metamorphosed at lower temperatures? during the Acadian.