The Magnetic Properties and Fabric of Granulite Facies Deep Crustal Rocks in the Mount Hay Region, southeastern Arunta Block, Central Australia

The granulite-facies deformation in the Mount Hay region of the Arunta Inlier in central Australia occurred at 816±27°C and approximately 800 MPa. Mount Hay is comprised primarily of mafic granulite plus lesser amounts of charnockitic, porphyroclastic, and quartzofeldspathic granulite. Magnetic fabric of these variably deformed, deep crustal rocks provide insights into the deformation processes occurring within high grade metamorphic shear zones. Samples were collected along a north-south trending 5.5 km traverse. Magnetic measurements include anisotropy of magnetic susceptibility (AMS), high and low temperature susceptibility measurements, saturation remnant magnetization as a function of low temperature, and hysteresis loops. AMS studies in the Mount Hay region show S>L to S>>L (stronger foliation than lineation) for mafic and porphyroclastic granulites, while S<L for charnockitic and quartzofeldspathic granulite. Hysteresis data suggest the room temperature magnetic grain size of all rock types are pseudo-single-domain to multidomain. The mafic granulites generally have the lowest coercivity of remanence to coercivity ratio, indicating a finer magnetic grain size, typically pseudo-single- domain. Low-temperature saturation isothermal remanent magnetization yielded Verwey transition temperatures ranging from 111-123 K, suggesting the magnetite is in some cases slightly oxidized or contains impurities. A weak correlation between lower Verwey transitions and lower coercivity of remanence to coercivity ratio is consistent with oxidation due to a larger surface area to volume ratio for finer grain magnetite grains. The dominant Curie temperatures are approximately 567°C, typical of magnetite with minor cation substitution. Most samples exhibit a secondary ordering transition of ~330°C and a 35 K low-temperature saturation isothermal remanence transition during warming which are both characteristic of pyrrhotite. AMS ellipsoid principle directions correlate well with the field orientations for lineation and foliation in the quartzofeldspathic granulites. The relationship between AMS and field fabrics is more variable for other rock types. This study suggests that multiple phases may contribute to the AMS of deep crustal rocks and that magnetic techniques may be a powerful tool to quantify variations in their deformation. Differences between AMS and field fabrics may be the result of variable contributions from multiple phases or that different lithologies record different strain events or that different rocks deform by different mechanisms.