195-8 Timing of Granite Emplacement and Extension in the Southern Appalachian Eastern Blue Ridge: Evidence from Zircon U-Pb Geochronology and Hornblende 40Ar/39Ar Thermochronology

Session: Evolution of Orogenic Belts Through Time: Insights from Sedimentation, Deformation, Magmatism, and Metamorphism (Posters)

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Abstract:

The southern Appalachian Blue Ridge experienced multiple episodes of collisional deformation throughout the Paleozoic, which is recorded by the thrust faults, folds, and well-developed foliation that dominate the region. We also identified the presence of a large-scale, amphibolite-facies, extensional shear zone within the Eastern Blue Ridge north of Grandfather Mountain window. To understand the timing of emplacement of a foliated granite in the hanging wall of this fault, we carried out LA-ICP-MS U-Pb dating and cathodoluminescence (CL) image analyses of zircon within the granite. ²³⁸U/²⁰⁶Pb ages range from ~321 Ma to ~1.4 Ga. Zircon cores yield ²³⁸U/²⁰⁶Pb ages between ~1.1 and 1.4 Ga, which we interpret to be Grenville xenocrysts. Zircons which yield ²³⁸U/²⁰⁶Pb ages ~321 Ma have Th/U ratios ≤ 0.1, which we interpret as fluid assisted recrystallization or overgrowth during the Alleghanian orogeny. Kernel density estimates applied to concordant U-Pb ages reveal a dominant peak at ~350 Ma. Many of the zircons yielding ages of ~350 Ma are euhedral, contain oscillatory zoning and Th/U ratios ≥ 0.5, which suggests this age represents the timing of granite emplacement, rather than a high-grade metamorphic overprint. This age corresponds to hornblende ⁴⁰Ar/³⁹Ar ages from within the extensional shear zone, interpreted as the minimum possible age of faulting. If the timing of granite emplacement corresponds to the timing of extension, the granite formation may be associated with the extension along the fault, possibly representing the timing of decompression melting. The zircon ages also provide a minimum possible age for the event responsible for the foliation developed within the granite, which suggests the foliation developed no earlier than the Neoacadian orogeny. This work continues to build upon our understanding of the tectonic history of the Eastern Blue Ridge north of Grandfather Mountain window and how large-scale extension affected the now-exposed core of the Appalachian orogen. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9831

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