Magnetic mineralogy of the Black Hill Norite and its aeromagnetic and palaeomagnetic implications

Abstract

The Black Hill Norite is a mafic intrusion which formed around 487 Ma ago. It intruded sediments of the Kanmantoo Group and Adelaide Supergroup which were deformed and metamorphosed during the Delamerian Orogeny. The unusual aeromagnetic anomaly caused by the norite gave the first indication of the presence of natural remanent magnetisation (NRM) totally different from the present field direction. The Black Hill Norite exhibits a strong remanent magnetisation (declination = 221.2 degrees, inclination =7.6 degrees, and intensity =4.9 A/m) indicating that it was intruded at equatorial latitudes. The remanence is probably thermal in origin. The known amount of coarse-grained multi-domain magnetite present is sufficient to explain the high magnetic susceptibility. The stable and intense remanence is carried by fine-grained single-domain pure magnetite. Demagnetisation and hysteresis studies suggests the presence of two populations of single-domain magnetite with nearly parallel but consistently different remanent directions. Preliminary electron probe work has confirmed the presence of randomly oriented fine-grained magnetite in the rock matrix as well as magnetite exsolved out of pyroxenes and feldspars. The bimodal NRM direction can be explained as due to secular variation associated with slightly different times of NRM acquisition or due to a shape anisotropy (caused by the orientations of the exsolved single-domain grains being confined to particular crystallographic directions within the host silicates, which in turn, show a preferential orientation, possibly indicating magma flow directions). The 3D analytic signal map and the reduced to the pole map, both made from calculations on the total magnetic intensity data, are similar, thus demonstrating the dual application of the analytic signal in the interpretation of low latitude surveys and the interpretation of magnetic sources suspected of carrying a strong, but unknown, remanent component. The Black Hill Norite is an excellent palaeomagnetic recorder. It provides evidence for the Early Ordovician palaeomagnetic pole position for Australia (interpreted as being near the African Bight in a standard Gondwana reconstruction) which was previously in doubt because of the absence of reliable data. The success of this study illustrates how aeromagnetic maps, by indicating the presence of remanently magnetised rock units, can be constructively used to select sampling sites for NRM studies.