Scholarship of Discovery
Over the past decade, geochronology studies in the southern Appalachians have focused on zircon, monazite, and mica ages to confirm the occurrence of a major Taconic event (~460‐450 Ma) affecting the Western and Central Blue Ridge, followed by a younger NeoAcadian (~360‐345 Ma) event affecting the Eastern Blue Ridge and Piedmont. Peak conditions of granulite facies metamorphism are estimated at ~850°C (garnet‐biotite) and 7‐9 kbar (GASP) for sillimanite schists at Winding Stair Gap (WSG), but thermobarometric studies of metasedimentary rocks in the region are limited and consequently Pressure‐Temperature‐time‐Deformation (P‐T‐t‐D) paths are poorly understood. Many details of the orogenic processes in the region remain unanswered, such as the depth of burial, rates of burial, cooling, and exhumation of various terranes, and variation of ages along strike. Rutile provides a robust, high‐temperature U‐Pb geochronometer that forms during metamorphic reactions and is a key phase for applying several thermobarometers in the determination of P‐T conditions.
This study focuses on electron microprobe analysis of rutile‐bearing metapelitic schists in the Blue Ridge to obtain major and some trace element data and the generation of temperature constraints for the orogen. Initial sampling indicates that rutile‐bearing schists are somewhat sparse across the orogen, but have been confirmed in the Great Smoky Group of the Western Blue Ridge as well as in the Cartoogechaye and Cowrock terranes of the Central Blue Ridge, all of which were metamorphosed to high grades during the Taconic Orogeny. Rutile is found in both kyanite‐ and sillimanitegrade rocks, primarily as a matrix phase associated with biotite. Common assemblages in these rocks include garnet, biotite, muscovite, quartz, opaques, plagioclase, ±kyanite, ±sillimanite, and ±staurolite.
Zirconium‐in‐rutile thermometry performed on the samples generated varying results based on sample location. The thin sections from the Western Blue Ridge were consistent with one another and comparable to previously published temperature estimates of ~600°C, indicating method consistency. However, the samples of Shooting Creek schist from near the Appalachian Trail generated temperatures that were lower than expected, especially since the rutile grains exist as inclusions in the garnet and should represent an earlier, higher‐temperature assemblage. Finally, the temperatures for rocks near Winding Stair Gap were much lower than the published peak condition estimates for the area, possibly supporting similar findings from Chen et al. (2007) that temperature estimates may be consistently too low at extreme temperatures and/or pressures due to thermometer resetting.
Eccles, Kathryn A., "Trace Element Analysis of Rutile and Zr-in-Rutile Thermometry for Southern Appalachian Pelitic Schists" (2011). Honors Program Projects. Paper 10.