Rock magnetic properties of the Arunta Block, Central Australia, and their implication for the interpretation of long‐wavelength magnetic anomalies

Rock magnetic and petrologic studies of a suite of deep crustal rocks from the Arunta Block of Central Australia reveal that the granulite grade rocks are in general much more magnetic than the amphibolite grade samples irrespective of bulk rock composition. The dominant magnetic mineral in all samples is relatively pure magnetite as determined from thermomagnetic and electron microprobe analysis. The bulk magnetic properties are typical of pseudo‐single‐domain to multidomain size material. The samples from our study have very large remanences compared to previous crustal magnetic studies, with the granulites having a median natural remanent magnetization of 4.1 A/m and Koenigsberger ratio of 7.2. These remanences are relatively resistant to thermal demagnetization, with nearly 50% of the magnetization remaining after 400°C demagnetization. Thus remanence may contribute significantly to the observed magnetic anomalies, including long‐wavelength magnetic anomalies, the source of which resides at depth and therefore at elevated temperature, where a thermoviscous remanent magnetization along the present‐day field is likely to dominate. The magnetic susceptibilities of the samples are only capable of producing a magnetization of less than 1 A/m in the 0.05 mT present‐day field of Central Australia. Susceptibility is nearly constant with temperature to within 30°C of the Curie temperature where it decreases rapidly, i.e., there is no significant Hopkinson peak. The granulite samples from this study have magnetizations, both remanent and induced components, that are large enough to account for most long‐wavelength magnetic anomalies if they are juxtaposed with relatively nonmagnetic rocks, similar to the high‐grade rocks in the Canadian Shield.