Paper No. 12
Presentation Time: 4:35 PM


CARRIGAN, Charles W.1, SCHKERKE, Chris2, ECCLES, Kathryn A.2 and EDDY, Tim2, (1)Dept. of Chemistry and Geosciences, Olivet Nazarene University, One University Avenue, Bourbonnais, IL 60914, (2)Dept. of Physical Sciences, Olivet Nazarene University, One University Avenue, Bourbonnais, IL 60914,

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.