THE NATURE AND TIMING OF ACADIAN DEFORMATION IN THE SOUTHERN APPALACHIAN BLUE RIDGE CONSTRAINED BY THE SPRUCE PINE PLUTONIC SUITE, WESTERN NORTH CAROLINA

South of the Grandfather Mountain window, granodioritic plutons of the Spruce Pine Plutonic Suite intrude the amphibolite facies Ashe Metamorphic Suite. Associated pegmatite dikes are commonly transposed into the regional metamorphic foliation but are also found crosscutting the foliation. Many of the larger plutons are moderately to strongly foliated. These observations suggest that the Spruce Pine Plutonic Suite may have been emplaced syntectonically. A field and microstructural study combined with new radiometric ages of rocks from the Spruce Pine Plutonic Suite address this issue and may constrain the nature and timing of Acadian deformation in the southern Blue Ridge. The Chalk Mountain pluton, near Spruce Pine, North Carolina, is locally strongly foliated and yields a zircon U-Pb age of 377.7 ± 2.5 Ma. The foliated rock has been subjected to extensive high-temperature solid-state deformation, as indicated by grain-boundary-migration recrystallization, recrystallized tails on plagioclase porphyroclasts, core-and-mantle structures, myrmekite, anastomosing folia, and a weak S-C fabric. The only microstructural evidence of a preserved magmatic fabric in this exposure is the rare alignment of plagioclase twins, subparallel to the foliation. The relationship between the solid-state and magmatic fabrics suggests pre- to syntectonic emplacement. The microstructural evidence indicates deformation of the Chalk Mountain pluton at temperatures above 500 °C, which occurred after crystallization at 377 Ma. A sheared Spruce Pine pegmatite boudin in the Burnsville fault, approximately 18 km northwest of the Chalk Mountain pluton, was previously dated also at 377 Ma. This post 377 Ma event may be responsible for the widespread amphibolite facies metamorphism observed in this part of the Ashe Metamorphic Suite. Because no amphibolite facies metamorphism has been associated with the Alleghanian orogeny, this deformation is presumably related to Acadian events.