FABRIC ANALYSIS OF STRAIN PARTITIONING IN A GRANULITE FACIES SHEAR ZONE, ARUNTA INLIER, CENTRAL AUSTRALIA

Deformation in granulite-facies shear zones is poorly understood relative to deformation at shallower crustal levels. We have studied a ~200 m wide shear zone formed at 710 ± 50 °C, at depths of 20-30 km, in the Arunta Inlier of central Australia. The area contains three main rock types whose distribution varies among structural units. Metasedimentary rocks are volumetrically subordinate; the field area is dominated by intermediate and mafic granulites. Yet shear zones appear to be preferentially located in areas with both significant centimeter- to meter-scale interlayering of different lithologies and a greater volume of quartz-rich metasedimentary granulites.

Within the shear zone, compositional banding in the metasedimentary granulite is thinner, more continuous, and more planar than in less deformed areas, indicating higher strain recorded by rocks inside the shear zone. In the porphyroclastic granulite, abundant plagioclase porphyroclasts are drawn into polycrystalline ribbons. The mafic granulite, however, shows no mesoscopic signs of strain variation across shear-zone margins.

Fabric analyses were completed to explore the strengths and limitations of measures of finite strain in these rocks. We analyzed shape-preferred orientation (SPO), Anisotropy of Magnetic Susceptibility (AMS), and crystallographic preferred orientation (CPO). In the porphyroclastic granulite, the AMS foliation and lineation are parallel to those measured in the field and described by plagioclase SPO. In the mafic granulite, the AMS fabric is not parallel to the field foliation and lineation, although the magnitude of anisotropy is higher in the center than outside shear zones. The AMS ellipsoids of metasedimentary granulites inside the shear zone range from strongly anisotropic to nearly spherical, depending on the bulk mineralogy and distribution of magnetic phases. Field observations and the results of fabric analyses collectively suggest competency contrast is sufficient to localize deformation at granulite facies conditions and, where present, quartz plays an important role in this localization.