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 these rocks has mainly focused on geochemical analysis, with the goal to characterize their protolith and attempt to determine the pressure and temperature conditions of metamorphism associated with their formation (DeCourt, 2013; Owens, 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 = 0.26) to prolate (U = -0.71). High and low temperature magnetic susceptibility measurements indicate that one sample is paramagnetic and the other two are ferromagnetic. The low-field AMS results for the ferromagnetic samples cannot be interpreted in the context of amphibole fabric. We will collect high-field AMS data to separate out the ferromagnetic and paramagnetic signals for these samples, and orient the AMS and XRCT data in the geo-tectonic frame of reference to investigate the intensity and orientation of amphibole fabrics in relation to field-defined large-scale deformation features.