Petrologic constraints on the source of fluid during mylonitization in the Blue Ridge province, North Carolina and Virginia, U.S.A.

Constraints on the source of fluid during retrograde mylonitization in and adjacent to the Blue Ridge province of the southern Appalachians are provided by (1) the geologic setting, (2) chemical changes in the mylonites relative to the host gneisses, (3) fluid inclusion studies and (4) oxygen and hydrogen isotope analyses of gneisses and mylonites. The Blue Ridge province was emplaced onto Paleozoic carbonates and late Precambrian clastic rocks during middle to late Paleozoic thrusting. Major element chemical data for mylonites from three localities (Fries, Virginia; Hot Springs, North Carolina, and Rosman, North Carolina) in the Blue Ridge province indicate large bulk rock losses of silica (and alkalis). Fluid inclusion studies in the Blue Ridge province suggest the fluids were dominated by CaCl₂MgCl₂ brines of variable salinity and similar in composition to fluids in the adjacent Valley and Ridge province. The variations in δ¹⁸O values for quartz and feldspar are consistent with an enrichment in feldspar due to interaction with a δ¹⁸O enriched fluid. These data together with the δ¹⁸O values of syntectonic quartz veins (11.8-14.4%.) indicate the mylonites exchanged with a fluid with a δ¹⁸O value of +3.7 to +9.2%. under open system conditions. The oxygen and hydrogen isotope data argue against direct input from a meteroic source, and the large silica losses in the mylonites imply a silica undersaturated fluid was involved in fluid-rock interaction. Relatively low temperature formation fluids expelled from the underlying carbonate bank (Knox Group) between 80 and 160 °C would have the required isotopic composition. Dilute metamorphic dehydration fluids may also have been expelled from the underlying clastic sedimentary rocks (Snowbird Group) at higher temperatures and mixing of these fluids with brines from the carbonate bank cannot be ruled out.