Rock magnetism and geophysical interpretation of the Black Hill Norite, South Australia

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 Norite gives rise to an unusual magnetic anomaly, a major negative to the north-east with a similar positive to the south-west, which is atypical of the present magnetic inclination of ?65° and declination of +8°. Measurements of natural remanent magnetization on oriented samples revealed a high magnetic susceptibility (2000 to 5000*10?5 SI units) and a very stable remanent magnetization with the following properties: declination 221.2°, inclination 7.6° (a95 = 4°) and a Koenigsberger ratio of approximately 2. The Black Hill Norite is inferred to have formed at equatorial latitudes. Surface measurements are probably representative of the rock at depth since forward modelling incorporating remanent magnetization gave a good match between observed and modelled magnetic anomalies.