EVIDENCE FOR HIGH-TEMPERATURE DUCTILE ALLEGHANIAN DEFORMATION IN THE EASTERN BLUE RIDGE: IMPLICATIONS OF NEW STRUCTURAL, PETROLOGIC, AND GEOCHRONOLOGIC DATA FROM SOUTHWESTERN NORTH CAROLINA

Eastern Blue Ridge (EBR) plutons have been the focus of recent detailed structural and geochemical investigations that attempt to constrain the tectonic evolution of the eastern Laurentian margin during early (Taconian) to mid-Paleozoic (Neoacadian) orogenesis. This study examines the Walnut Creek Granodiorite (WCG), located ~15 km S of Sylva, NC. The WCG intruded polydeformed, amphibolite-facies rocks of the Ashe-Tallulah Falls Formation, and is the westernmost pluton exposed in the Chattahoochee thrust sheet of SW North Carolina. It is an equigranular, medium-grained 2-mica granodiorite–trondhjemite. Mesoscale igneous features preserved within the WCG include aplitic to pegmatitic dikes, and rare internal intrusive contacts with a parallel magmatic foliation. Most exposures display a moderate to strong solid-state foliation that was subsequently folded. Late NNW-striking, SW-dipping quartz-feldspar–filled shear zones cut earlier fabrics and record west-side-down (extensional) sense of motion.

Zircons were separated from one WCG sample for SHRIMP U-Pb analysis. All zircons are complex, containing single or multiple inherited cores overgrown by thick to thin, delicately oscillatory zoned, epitaxial magmatic rims. Significant textural variability exists in the zircons examined: (1) light to dark (in CL), rounded, unzoned cores surrounded by euhedral oscillatory zoned rims; (2) zoned cores truncated by later overgrowths; or (3) small, zoned inner cores truncated by outer cores displaying oscillatory zoning that is truncated by euhedral oscillatory zoned magmatic rims.

Seven of eleven microanalyses revealed a dominant core population of 1.3-1.0 Ga (Grenvillian), with three 1.5-1.4 Ga (Granite-Rhyolite Province?) ages, and one early Paleozoic core (440 Ma), similar to inheritance observed in other EBR plutons. Sixteen euhedral magmatic rims dated WCG crystallization. Ages range from ~345 to 290 Ma, with a small cluster at ~320 Ma, with some younger ages possibly related to partial lead loss. The strong tectonic fabric suggests high-temperature ductile deformation continued well after emplacement and crystallization of the WCG. An earlier (U-Pb) ID-TIMS age of 335 ± 2.8 Ma (B. Miller et al., 2002) on the Rabun Granodiorite, together with our new results, bring forth the strong possibility of previously underappreciated late Paleozoic high-temperature events in the EBR.