Search for: All records

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 parameterU.CA_indexvalues can then be estimated for natural samples using measuredUvalues, 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. 

Abstract In orogens worldwide and throughout geologic time, large volumes of deep continental crust have been exhumed in domal structures. Extension‐driven ascent of bodies of deep, hot crust is a very efficient mechanism for rapid heat and mass transfer from deep to shallow crustal levels and is therefore an important mechanism in the evolution of continents. The dominant rock type in exhumed domes is quartzofeldspathic gneiss (typically migmatitic) that does not record its former high‐pressure (HP) conditions in its equilibrium mineral assemblage; rather, it records the conditions of emplacement and cooling in the mid/shallow crust. Mafic rocks included in gneiss may, however, contain a fragmentary record of a HP history, and are evidence that their host rocks were also deeply sourced. An excellent example of exhumed deep crust that retains a partial HP record is in the Montagne Noire dome, French Massif Central, which contains well‐preserved eclogite (garnet+omphacite+rutile+quartz) in migmatite in two locations: one in the dome core and the other at the dome margin. Both eclogites recordP ~ 1.5 ± 0.2 GPa atT ~ 700 ± 20°C, but differ from each other in whole‐rock and mineral composition, deformation features (shape and crystallographic preferred orientation, CPO), extent of record of prograde metamorphism in garnet and zircon, and degree of preservation of inherited zircon. Rim ages of zircon in both eclogites overlap with the oldest crystallization ages of host gneiss atc.310 Ma, interpreted based on zircon rare earth element abundance in eclogite zircon as the age of HP metamorphism. Dome‐margin eclogite zircon retains a widespread record of protolith age (c.470–450 Ma, the same as host gneiss protolith age), whereas dome‐core eclogite zircon has more scarce preservation of inherited zircon. Possible explanations for differences in the two eclogites relate to differences in the protolith mafic magma composition and history and/or the duration of metamorphic heating and extent of interaction with aqueous fluid, affecting zircon crystallization. Differences in HP deformation fabrics may relate to the position of the eclogite facies rocks relative to zones of transpression and transtension at an early stage of dome development. Regardless of differences, both eclogites experienced HP metamorphism and deformation in the deep crust atc.310 Ma and were exhumed by lithospheric extension—with their host migmatite—near the end of the Variscan orogeny. The deep crust in this region was rapidly exhumed from ~50 to <10 km, where it equilibrated under low‐P/high‐Tconditions, leaving a sparse but compelling record of the deep origin of most of the crust now exposed in the dome.