ALLEGHANIAN DEFORMATION IN THE GEORGIA AND NORTH CAROLINA EASTERN BLUE RIDGE: INSIGHTS FROM PLUTON AGES AND FABRICS

The eastern Blue Ridge (EBR; western Tugaloo terrane) in the southern Appalachians consists of the polydeformed and metamorphosed accretionary prism formed during Taconian orogenesis. Rocks comprising the Tugaloo terrane are dominantly biotite paragneisses (Ashe-Tallulah Falls Formation) with small ultramafic bodies, amphibolite, and rare Grenville basement orthogneisses. EBR rocks underwent Taconic deformation and metamorphism, were subsequently intruded by trondhjemitic to granodioritic plutons, and were deformed at amphibolite facies conditions at least once more. Differentiating fabrics resulting from specific deformation events related to distinct tectonic episodes is difficult, yielding ambiguous and often conflicting models of EBR tectonic evolution. The key to unraveling the timing of EBR tectonism lies within the fabrics and ages of the sparse granitic intrusions. Recent detailed mapping and structural analyses, combined with new geochronologic data, has shed new light on EBR plutonic history, and also EBR tectonic evolution.

Meso- and microstructural analyses of two EBR plutons (Walnut Creek and Rabun granodiorites) provide evidence for primary igneous textures, and both magmatic and tectonic fabrics. Some plagioclase or K-feldspar phenocrysts preserve concentric oscillatory zoning and/or lamellar growth twins, but most samples contain a moderate to nearly completely recrystallized texture indicating long periods at elevated temperatures. Macroscale magmatic fabrics include biotite schlieren, often oriented parallel to regional tectonic foliation, and rare alignment of feldspar phenocrysts. Aligned plagioclase phenocrysts with primary growth twins define a magmatic foliation in some samples. Phenocrysts are typically randomly oriented and foliation is defined by elongate quartz aggregates. Other tectonic fabrics in the intrusions include lenticular deformation twins in plagioclase, feldspar crystals with bent lattices, and ubiquitous subgrains in quartz. Relationships between magmatic and tectonic fabrics strongly suggest pre- to synkinematic pluton emplacement. New SHRIMP U-Pb (zircon) age data from these plutons indicate crystallization occurred at ~335 Ma, indicating deformation and metamorphism are Alleghanian.