Magnetic Fabric Techniques are used to Characterize Deformation of Deep Crustal Granulites of the Arunta Block Central Australia

Granulites of the Capricorn Ridge shear zone of the Arunta Inlier of central Australia record deformation at 776 ± 38 °C and 800 MPa (~ 30 km depth). The planarity and continuity of compositional bands that parallel the mesoscopic foliation vary spatially, recording variations in finite strain magnitude within the lithologically heterogeneous shear zone. As is typical of granulite facies terranes, strain markers are absent. We have investigated the degree to which magnetic foliation and lineation track strain in different rock types by comparing magnetic anisotropy to field fabrics, with the goal of quantifying strain across this deep crustal shear zone. Samples collected along a 700m transect include mafic (most common), porphyroclastic, charnockitic, and quartzofeldspathic granulites. The field foliation generally strikes ESE and dips steeply to the SW and lineation plunges steeply to the SSE in all rock types. We measured anisotropy of magnetic susceptibility (AMS) and anisotropy of remnant magnetization (ARM) to determine the shape and orientation of the magnetic anisotropy ellipsoids of these deep crustal rocks. The mafic granulites' ARM principle directions show a tighter grouping than do the same samples' AMS principle directions. The other rock types have a tighter grouping of data points with AMS; however, the difference between the ARM and AMS ellipsoids is not as substantial as that exhibited by the mafic granulite samples. The maximum AMS axes of the porphyroclastic, charnockitic, and quartzofeldspathic granulites plunge steeply to the SSE, similar to the observed field lineation. In contrast, the mafic granulites generally show a shallowly plunging, NW-trending maximum ARM direction. The mafic granulite ARM ellipsoids have a relatively consistent anisotropy orientation, but shapes range from highly oblate to triaxial to slightly prolate. The charnockite AMS ellipsoid exhibits a consistent triaxial shape. The porphyroclastic and quartzofeldspathic granulite AMS ellipsoid shape varies across the transect. The charnockites generally show the highest degree of anisotropy whereas the mafic granulites show the lowest. Field observations and magnetic fabrics are consistent with lower crustal strain localization within the charnockites relative to the mafic granulites. These observations suggest lithology-specific deformation processes, fundamentally reflecting rheology, are responsible for the magnetic fabrics in the shear zone.