Comparison of garbenschiefer fabrics and regional deformation features in the Chopawamsic terrane of Virginia

Garbenschiefer, a type of amphibolite with large, radiating, feather-like amphibole porphyroblasts among a finer-grained, recrystallized matrix of plagioclase and quartz, has been identified and mapped by Brown (1969) in the Chopawamsic terrane. Previous research on garbenschiefer in Virginia has focused on evaluating if "hornblende gneiss with pattern" identified by Brown (1969) could be considered garbenschiefer through mineralogy and textural observations as well as geochemical analysis to characterize the protolith of these rocks (DeCourt, 2013; Owens and DeCourt, 2014). This project aims to investigate orientations of amphiboles defining garbenschiefer fabrics, and determine their relationship to the larger tectonic history of the Chopawamsic terrane, in particular as a function of their proximity to large-scale deformation features. I hypothesize that garbenschiefer closer to shear zones or faults will have amphiboles that show a greater preferred crystallographic and shape orientation which would reflect greater strains, compared to garbenschiefer located farther away. Three samples were analyzed to assess the overall orientation and strength of the garbenschiefer fabric defined by amphibole. X-Ray Computed Tomography (XRCT) was used to characterize the shape preferred orientation of amphibole. Anisotropy of magnetic susceptibility (AMS) was used to measure the average crystallographic alignment of amphibole following the method of Biedermann et al. (2018). Amphibole grains in garbenschiefer samples for this study are subidiomorphic therefore, grain shape and crystallographic axes should be coincident. Results from the XRCT data using the SLD method (Ketchum, 2005) show that the samples range from oblate (shape parameter U modified from AMS, U = 0.06) to prolate (U = -0.62) for the amphibole fabric and oblate (U = 0.33) to prolate (U = -0.11) for the oxide fabric. Results from low-field AMS data show that samples range from oblate (U = 0.06) to prolate (U = -0.39) representing either the amphibole and/or oxide fabric. High and low temperature magnetic susceptibility measurements indicate that the AMS in one sample is dominated by paramagnetic minerals, and by ferromagnetic minerals in the other two samples. The low-field AMS results for the ferromagnetic samples cannot be interpreted as representing amphibole fabrics, but are rather interpreted as reflecting the shape preferred orientation of magnetite. Even though samples have different magnetic carriers, they both provide information about the three-dimensional geometry of mineral alignment in these samples. The magnetic data is interpreted using crystal shape data obtained by XRCT to ‘ground truth’ and investigate the significance of the magnetic fabric data.