SYNKINEMATIC PLUTON EMPLACEMENT DURING ALLEGHANIAN REGIONAL DEFORMATION: EVIDENCE FROM DETAILED GEOLOGIC MAPPING IN WESTERN NORTH CAROLINA

The Glenville, NC area preserves evidence of Taconic and Alleghanian deformation and metamorphism. Truncation of amphibolite and pelitic schist layers by the Whiteside pluton (~466 Ma) provides macroscopic evidence of early Taconian deformation (D₁). Inclusion trails defining S₁ in D₂ garnet and kyanite porphyroblasts, and polymorphic conversion of kyanite to sillimanite aligned parallel to S₂ fabric suggests intrusion of Whiteside accompanied D₂ Taconian regional deformation. These relationships confirm the traditional interpretation that the original deformation and metamorphism observed in the eastern Blue Ridge (EBR) of southwestern NC are Ordovician.

The Rabun and Walnut Creek plutons comprise part of the recently recognized Alleghanian plutonic suite exposed in the EBR of Georgia and North Carolina. Magmatic fabrics such as biotite-rich schlieren and foliation defined by aligned sub- to euhedral feldspar phenocrysts are locally preserved. Moderate to strong solid-state foliations defined by elongate quartz aggregates and anastomosing biotite folia occur near pluton margins. Parallelism of magmatic and subsolidus fabrics and concordance of solid-state fabrics with regional structural trends fit widely accepted evidence for synkinematic pluton emplacement with respect to Alleghanian regional deformation. Microstructural evidence including ductile bending and mechanical twinning of plagioclase grains, development of myrmekite, and both prism- and basal-slip systems simultaneously operating in quartz indicate deformation occurred at moderate to high temperatures following emplacement and solidification of these Mississippian (~335 Ma) plutons. Regional S₂ foliation is nearly completely transposed into S₃ between the intrusions. The terminal staurolite reaction is apparently reached within a thermal aureole extending ≤1km(?) around Rabun and Walnut Creek. Kyanite + staurolite porphyroblasts overgrew crenulated S₃ foliation during retrograde cooling.

Synkinematic pluton emplacement, presence of a thermal aureole, and truncation of the Rabun pluton by the Chattahoochee fault suggest Alleghanian orogenesis has modified the original orientations of Ordovician structures and locally obscured previous EBR metamorphic gradients.