INITIAL PRESSURE DETERMINATIONS FOR PELITIC SCHISTS IN THE SOUTHERN APPALACHIAN BLUE RIDGE

In recent years, geochronological studies have elucidated the timing of tectono-thermal events in the Southern Appalachians, shedding significant light on the accretionary history of the orogen. However, P-T-t conditions of these events are poorly understood, as few thermobarometric studies have been done. We are attempting to determine P-T conditions from garnet- & rutile-bearing pelitic schists, by employing Zr-in-rutile thermometry as well as garnet-rutile geobarometers. Our latest calculations involve pressure determinations using the GRAIL barometer via ThermoCalc. Samples from the western Blue Ridge consist of gar-ky-st schists near Bryson City, NC. Zr-in-rutile Ts yield consistent estimates of 589±18 & 600±26 °C, similar to previously published results in the area. The presence of kyanite indicates a minimum P of 5.5 kbar. Garnets in these rocks are almandine-rich (Alm₆₀Sps₂₀Pyp₁₅Grs₅) & show fairly flat patterns in major element zoning. Samples from the central Blue Ridge consist of higher grade, sillimanite-bearing rocks. Samples of the Shooting Creek schist contain garnets that are also almandine-rich, but also somewhat high in pyrope content, ranging from core (Alm₇₁Sps₂Pyp₂₄Grs₄) to rim (Alm₇₇Sps₃Pyp₁₆Grs₅). Fe-Ti oxide grains are dominantly ilmenite (Ilm91Hem7). The GRAIL reaction (Ilm + ALS + Qtz = Alm + Ru) is fully buffered in these rocks, & yields values of 7.9 kbar at 700 & 9.2 kbar at 850. Also in the central Blue Ridge, we sampled rocks at Winding Stair Gap, where Moecher et al. (2004) determined conditions of 850±20°C & 8.0±0.6 kbar by GARB & GASP thermobarometry. Our samples of the Shooting Creek Schist are similar to these values, but slightly higher. Samples from Winding Stair Gap contain garnets with Fe-Ti oxide inclusions that show significant exsolution. Dark laminae consist of ilm-rich material (Ilm₈₇Hem₉) found within dominantly hematite (Ilm₃₃Hem₆₅ to Ilm₂₅Hem₇₃) grains. Image analysis indicates that grains are ~20-25% dark, ilm-rich material & 75-80% hem-rich material. When reintegrated, the grains suggest original compositions of ~Ilm₄₂Hem₅₅. These compositions yield fairly low activities for ilm & consequently P determinations using GRAIL are unreasonably high, suggesting that the mineral pairings chosen are not in equilibrium & that further study is necessary.