Timing of Mesoproterozoic Magmatism and Metamorphism in the Vicinity of Mount Rogers, Southwestern Virginia and Northwestern North Carolina: Implications for the Nature of Grenville-Age Orogenesis in the Southern Appalachians

New geologic mapping indicates that Mesoproterozoic basement rocks exposed within the Stone Mountain thrust sheet at the northern end of the French Broad massif include multiple lithologic units that collectively preserve a nearly 300 m.y. record of episodic magmatism and metamorphism. The heretofore unrecognized assemblage includes an older suite of gneisses that was intruded by two series of granitoids. Protoliths for the oldest orthogneiss were intermediate in composition; SHRIMP U-Pb analyses of igneous zircon yield an age of ca. 1327 ± 7 Ma. Granitic magmatism occurred at ca. 1162 ± 4, 1161 ± 7 and 1054 ± 7 Ma, and included variably evolved, but generally silica-rich lithologies. Peak metamorphic conditions near the amphibolite- to granulite-facies transition are indicated by Plag + Hbd + Qz ± Cpx assemblages in the older rocks; whereas conditions corresponding to at least upper amphibolite facies are indicated by Plag + Hbd in mafic dikes that crosscut the youngest granitoid. In contrast to the ductilely deformed gneisses, mineral assemblages and fabrics in the granitoids record little macroscopic evidence of metamorphic processes. Cathodoluminescence imagery and SHRIMP-RG geochemical analysis of metamorphic zircon, combined with SHRIMP geochronologic analysis, indicate that compositionally distinct zircon was produced at ca. 1.17-1.12 Ga and 1.06-1.04 Ga. Application of Ti-in-zircon geothermometry suggests that both events occurred at temperatures ≥ 700 °C, consistent with conditions inferred from mineralogy. Spurious temperatures are indicated by anomalously Fe-rich rims. These results demonstrate that basement in the Mount Rogers area was affected by at least two orogenic pulses of Mesoproterozoic age that correspond temporally to periods documented in the Adirondacks and Canadian Grenville province. The data also underscore the utility of multidisciplinary studies of zircon using SHRIMP techniques to deduce important aspects of orogenic processes in polydeformed and metamorphosed rocks that have subsequently undergone pervasive retrograde recrystallization.