Microstructures and Geochemical Variations in Phyllonites of the Meadow Fork Fault Zone, Western Blue Ridge, North Carolina

Decollements detach thrust sheets at their bases from the underlying footwall typically along a relatively easy flowing stratigraphic level. However many thrust belts contain crystalline basement thrust sheets. In the western Blue Ridge of the Southern Appalachians, rocks such as the Max Patch Granite are converted to phyllonitic mylonites along the detachment zone of the Meadow Fork fault zone. In traverses across the thrust and local shear zones, quartz displays evidence of ductile deformation where as feldspar undergoes grain-size reduction by antithetic microfaults followed by a chemical breakdown to sericite, chlorite and epidote due to infiltration of a hydrothermal fluid, similar to the following reactions: 3K-feldspar + 2H+ = muscovite + 2K+ + 6SiO2 (1); K-Spar + Oligoclase + 0.5H2O + 3.5H+ = Epidote + Muscovite + 11SiO2 + 4Na+ (2). Protomylonties and mylonites of the Max Patch Granite collected from the Meadow Fork Fault Zone, showed significant grain size reduction and crystalplastic deformation. Based on microstructural observations and mineral assemblages rocks of the Max Patch experienced greenschist facies metamorphism. In protomylonite samples located in low grade conditions (300-400oC), quartz displayed minor dislocation creep and feldspar grains behaved brittlely. In mylonites experiencing medium grade conditions (400-450oC), quartz behaved very ductile with elongation and recrystallization, while feldspar still behaved rather brittle. The protomylonites and mylontes are enriched in elements such as SiO2, K2O, Zr, Y, and Rb and depleted in CaO, Na2O, MgO, Fe2O3, TiO2 and V compared to their protolith gneiss. The enrichment in K2O was due to the breakdown to K-feldspar to potassium-rich muscovite. The immobile element enrichments are attributed to enrichment in residual phases such as zircon and minor epidote in the mylonites. In the formation of phyllonites in the western Blue Ridge, mechanisms such as cataclasis and solution of feldspar and dislocation creep were important processes.