GEOTHERMOBAROMETRY ON MAFIC AND PELITIC GNEISSES FROM WINDING STAIR TO DELLWOOD, NC: FILLING THE GAP IN TACONIAN P-T DATA IN THE CENTRAL BLUE RIDGE

The most extensive exposures of Taconian granulite facies rocks in the southern Blue Ridge are at Winding Stair Gap (WSG; U.S. Hwy. 64 southwest of Franklin, NC). Previous studies there (Eckert et al. 1989; Moecher et al. 2004) deduced P-T conditions of 8-9 kbar/800-850C. Few other systematic thermobarometric data exist in the Central Blue Ridge (CBR) to evaluate variations in paleodepths along strike of the orogen.The hypersthene zone as mapped (Force, 1976) extends 15 km to the northeast past Franklin. Grt-bearing amphibolites and migmatitic metabasic rocks are the most common lithology suitable for thermobarometry in the CBR, but complex hornblende solid solutions and uncertainties regarding water activity make Hbl-Grt-Pl-Qtz thermobarometry more problematic than that in pelitic or mafic, pyroxene-bearing lithologies. New granulite-facies assemblages (opx-bearing) in mafic rock and pelitic gneisses have been analyzed to permit calibration of Hbl-Grt-Pl-Qtz thermobarometers for regional differential thermobarometry. WDS x-ray maps, compositional traverses, and WDS quantitative analysis on mafic (Opx-Cpx-Pl-Grt-Qtz-Hb-Ru) and migmatitic metapelitic gneisses (Grt-Bt-Pl-Qtz-Bt-Sil-Ky) between WSG and Dellwood were carried out to obtain appropriate mineral compositions for TWQ thermobarometry. Grt in mafic gneisses in the hypersthene zone between Franklin and Sylva retains subtle vestiges of prograde growth zoning. TWQ thermobarometry on mafic rocks yields T = 750-850C, consistent with WSG values, but significantly higher P (10-12 kbar). Migmatitic Ky-Sil gneisses 2 km northeast of WSG yield peak P-T consistent with previous studies. Sil-bearing metapelites near Sylva record P-T of 8kbar/700C. Hbl-bearing migmatitic metabasic rocks near Dellwood exhibit evidence for lack of complete equilibration in the form of chemically zoned Pl, Hbl, and Grt. Preliminary P-T values center on 8 kbar and 650C, consistent with previous studies in the Dellwood area. The P-T values are consistent with lower grade conditions to the northeast of the hypersthene zone but hot enough to form migmatites in a range of bulk compositions. Systematic application of Hbl-Grt-Pl-Qtz equilibria calibrated against pyroxene-bearing equilibria will be carried out in order to test for systematic variation in P-T using a single thermobarometric system.