DECIPHERING THE PRESSURE-TEMPERATURE-TIME-DEFORMATION HISTORY OF THE EASTERN BLUE RIDGE: INTEGRATED PETROLOGICAL, GEOCHRONOLOGICAL, AND STRUCTURAL ANALYSES FROM SOUTHWESTERN NORTH CAROLINA

The eastern Blue Ridge (EBR) of southwestern North Carolina is comprised of polydeformed paragneisses and schists metamorphosed to amphibolite facies that were invaded by trondhjemitic to granodioritic plutons during two or three distinct magmatic pulses spanning the Paleozoic. Recent interest has focused on granitoid geochemistry and geochronology, but no quantitative estimates of intrusion depth exist, and near-peak P–T estimates of adjacent host rocks are lacking. The scarcity of temporally constrained P–T data severely limits our ability to develop accurate tectonic models of the EBR during much of its Paleozoic orogenic history. This study utilizes regional and pluton–host rock structural relationships, detailed metamorphic petrology, and incorporates new precise pluton ages to obtain temporally-constrained P–T estimates, allowing us to piece together portions of the P–T–t–D path of western Tugaloo terrane rocks. Tugaloo terrane samples in the southern Big Ridge quadrangle preserve pristine Taconian M₂ mineral assemblages that grew during regional D₂ deformation, coeval with intrusion of the Whiteside Granite (~466 Ma). Multi-equilibrium calculations using balanced, linearly independent reactions yielded near-peak metamorphic conditions of 703 ± 15 ºC and 9.0 ± 0.6 kbar. These conditions imply this portion of the Tugaloo terrane was near the base of crust of normal thickness (~34 km) during Middle Ordovician orogenesis. The Mississippian Rabun and Walnut Creek plutons intruded during regional D₃ deformation and produced a distinct thermal overprint on existing M₂ assemblages in a ≤1 km-wide thermal aureole. Evidence consists of Ms ± Sil psuedomorphs after St at distances ≤0.5 km from the intrusions, and post-kinematic St + Ky porphyroblasts that cross cut all fabric at distances between 0.5 and 1 km. Samples of hastingsitic amphibole-bearing trondhjemite porphyry collected at the NE margin of the Rabun pluton allowed application of the Al-in-hornblende barometer. Temperatures of 720 ± 40 ºC obtained from Pl–Hbl thermometry were used in the temperature-corrected Al-in-Hbl barometer calibration of Anderson and Smith, and yielded crystallization pressures of 6.8–7.3 ± 0.6 kbar (~26 km). This depth estimate is consistent with Ky + St porphyroblast growth and indicates the region was exhumed to the middle crust by ~335 Ma.