170-9 Two Metamorphic Events Recorded in Granulite Facies Rocks from the Winding Stair Gap, Central Blue Ridge

See more from this Division: Topical Sessions
See more from this Session: Strengthening Links between Metamorphic Conditions and Time: New Advances in High-Temperature Geochronology and Tracing P-T-t Paths of Metamorphic Terranes

Sunday, 5 October 2008: 3:30 PM
George R. Brown Convention Center, 310BE

Aley K. El-Shazly1, Clayton Loehn2 and Robert J. Tracy2, (1)Geology Department, Marshall University, Huntington, WV
(2)Geosciences, Virginia Tech, Blacksburg, VA
Abstract:
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.

See more from this Division: Topical Sessions
See more from this Session: Strengthening Links between Metamorphic Conditions and Time: New Advances in High-Temperature Geochronology and Tracing P-T-t Paths of Metamorphic Terranes