Magnetic and Crystallographic Fabric Analyses of Amphibolite: A Proposed Methodology Applied to a Migmatite Dome

Abstract Mafic rocks are volumetrically and rheologically significant components of the mid‐to lower continental crust, yet tools to study their fabrics have not been well developed. We examine amphibolites exhumed from mid‐to lower crustal levels in a gneiss dome (Entia dome, central Australia) that display various strengths of mineral lineation and foliation associated with different deformation geometries. Combining petrofabric analysis (electron backscatter diffraction, EBSD) with magnetic fabric analysis (Anisotropy of magnetic susceptibility (AMS), we quantify relationships between AMS‐derived fabrics and crystallographic‐preferred alignment of fabric‐defining amphiboles. We combine single‐crystal AMS data with EBSD data to model amphibole textures and their expected magnetic anisotropy. We formulate a new EBSD‐derived petrofabric index, CA index , and correlate it with the calculated AMS shape parameter U . CA index values can then be estimated for natural samples using measured U values, leveraging both rapid but texturally low‐resolution AMS and texturally‐resolved but time‐ and analytically‐onerous petrofabric analyses to interpret petrofabrics from magnetic fabric data. In the Entia dome, we identify amphibole c‐fibers (L‐tectonite) in the high‐strain core of the dome, which reflect constrictional strains. In contrast, a‐fibers (S‐tectonites) are dominant near the dome margins and indicate flattening strains. Fabrics measured in different structural subdomains agree well with 2D and 3D numerical models of finite strain distribution in domal structures. Combining textural modeling, AMS measurements, and EBSD analyses allows investigation of previously unexploited records of ductile deformation and flow in amphibole‐bearing rocks. These results can be applied to a wide range of field‐based studies of tectonic and magnetic processes.