Date of Award
Master of Science
Robert D. Hatcher, Jr.
Harry Y. McSween, Jr, William M. Dunne
New field and analytical data collected in the eastern Blue Ridge (EBR) of southwestern North Carolina reveal a complex, polyphase thermotectonic history, and delimit the timing of previously unrecognized orogenic pulses in a portion of the southern Appalachian orogenic core. Detailed geologic mapping of the Glenville and Big Ridge 7.5’ quadrangles delineated the location of the Chattahoochee fault, a tectonic boundary separating the western Tugaloo terrane from distinct central Blue Ridge terranes.
Zircon geochronologic data for two EBR granitoids in the map area indicate emplacement occurred at ~335 Ma, making these plutons among the youngest recognized in the EBR. Bulk-rock geochemical analyses of the Walnut Creek pluton reveal similarities with the Whiteside Granite, and strongly suggest similar source regions for Ordovician and Mississippian trondhjemitic magmas. Field relationships and Al-in-hornblende barometry suggest intrusion into mid-crustal levels (~7 kbar) pre- to synkinematic with regional D3 deformation.
Host rock porphyroblast textures and microstructures indicate distinct tectonometamorphic histories for Cartoogechaye and Tugaloo terrane samples, providing support for distinct P–T–t–D paths. Cartoogechaye terrane samples reached significantly higher temperatures and pressures during Taconian orogenesis. Mineral assemblages and textures suggest pressures >9 kbar(?), and peak sillimanite II zone temperatures (≥750 ºC).
Tugaloo terrane samples in the southern Big Ridge quadrangle preserve pristine Taconian M2 mineral assemblages. Multi-equilibrium calculations using balanced, linearly independent reactions performed on a metapelite yielded near-peak metamorphic conditions of 703 ± 30 ºC and 9.0 ± 1.2 kbar (2 sigma). These conditions accompanied intrusion of the Ordovician Whiteside Granite.
A distinct thermal overprint is evident from metapelite samples collected ≤1 km from the Mississippian Rabun and Walnut Creek plutons. Evidence consists of psuedomorphs of muscovite ± sillimanite after staurolite at distances <0.5 km from the intrusions, and post-kinematic staurolite + kyanite porphyroblasts that cross cut all fabric at distances of ≤1 km. Plagioclase deformation mechanisms interpreted to have accommodated strain during shearing along the Chattahoochee fault include climb-accommodated dislocation glide and grain-boundary migration recrystallization. These microstructures are consistent with deformation temperatures >600 ºC. Geochemical signatures, xenocrystic zircons, and high–T shearing together strongly suggest Ordovician juxtaposition of the Tugaloo and central Blue Ridge terranes.
Stahr, Donald W., "Tectonometamorphic Evolution of the Eastern Blue Ridge: Differentiating Multiple Paleozoic Orogenic Pulses in the Glenville and Big Ridge Quadrangles, Southwestern North Carolina. " Master's Thesis, University of Tennessee, 2008.