HR: 1340h
AN: V23C-1556    [Abstracts]
TI: P-T Evolution and Ages of Granulite Facies Rocks from the Winding Stair Gap, Eastern Blue Ridge
AU: * El-Shazly, A K
EM: elshazly@marshall.edu
AF: Dept. of Geology, Marshall Univ, 1 John Marshall Dr., Huntington, WV 25701,
AU: Loehn, C
EM: cloehn@vt.edu
AF: Dept. of Geosciences, Virginia Tech, 4044 Derring Hall, Blacksburg, VA 24061,
AB: The Winding Stair Gap, NW of Franklin, North Carolina, exposes schists, gneisses, granofelses, stromatic metatexites, and minor diatexites representing mostly pelitic and semipelitic rocks affected by granulite facies conditions. Characteristic mineral assemblages in mesosomes and melanosomes include: garnet (Gt) + biotite (Bt) + sillimanite (Sill) + plagioclase (Plg) + quartz (Qz), Gt+ hornblende (Hb) + Bt + Plg + Qz, and orthopyroxene (Opx) + Bt + Qz. Leucosomes are predominated by Plg + Qz, and contain Gt, kyanite (Ky), Sill, and retrograde muscovite (Musc). Moecher et al. (2004) concluded that these rocks followed a counterclockwise P-T path with peak conditions at 850-900°C, 8-9 kbar. However, the occurrence of relict Ky and staurolite overgrown by Gt and Sill, rutile included in Gt or rimmed by ilmenite, and texturally early Ky in leucosomes relative to intergranular fibrolite, all suggest that these rocks followed a clockwise P-T path in which dehydration melting of muscovite began in the stability field of Ky. Rinds of Plg + Qz + Musc separating Gt from Bt, and Bt inclusions in Opx suggest the onset of dehydration melting of Bt. Calculations using THERMOCALC and compositions of Gt and Bt separated by Qz or Plg (X_An = 0.35) suggest equilibration at 750 to 830°C, 7 to 9 kbar. This was followed by a stage of near isobaric cooling during which K- feldspar back reacted to Musc, and Sill + Bt replaced Gt. Two monazite crystals from a Bt + Gt + Sill + Qz + Plg schist, display patchy zoning for Th, Y, U, and Ca. Statistical analysis of microprobe age profiles across these grains yields three age populations of 353 ± 11 Ma, 450 ± 8 Ma, and 488 ± 8 Ma, with the oldest ages restricted to the grain interiors. A third monazite associated with Sill and Bt truncating Gt is almost homogeneous, and yields ages of ~348 ± 8 Ma. These data suggest that peak T metamorphism and partial melting in the eastern Blue Ridge was Taconic (~450 Ma) during which monazite crystals that record an earlier (~490 Ma) event, were partially resorbed and overgrown by new rims. The younger ages of ~350 Ma reflect monazite growth during near isobaric cooling probably effected by some melt extraction.
DE: 1037 Magma genesis and partial melting (3619)
DE: 1100 GEOCHRONOLOGY
DE: 3651 Thermobarometry
DE: 3652 Pressure-temperature-time paths
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting