Magnetic mineral petrogenesis, rock magnetism and aeromagnetic response in the eastern Arunta Inlier, northern territory

Abstract

Measurements of magnetic susceptibility and natural remanent magnetisation of representative rocktypes from the eastern Arunta Inlier reveal the majority of lithologies have distinct magnetic properties. Studies of opaque mineralogy and bulk rock chemistry demonstrate that magnetic rock properties are dependent on total iron content and the manner of its division between Fe, Mg-silicates and the oxides of iron and titanium. The partitioning of iron is controlled primarily by oxygen fugacity and titanium content. All these parameters exhibit a well-defined dependence on weight percent SiO2, and as a result, the magnetic properties of most major rock groups are related to SiO2 content. This relationship appears to be inherited from tectonic environment in the case of igneous rocks and depositional environment in the case of sedimentary rocks. Exceptions are usually due to late-stage alteration involving hydration or dehydration resulting in a change of oxygen fugacity. The aeromagnetic signature of individual lithologies is consistent with their measured magnetic properties and many may be regarded as magnetic litho-stratigraphic marker horizons. Four major rock groups having distinct signatures are recognised. These comprise a predominantly metaigneous basement complex, a metasedimentary, metavolcanic cover sequence, igneous bodies which intrude basement and cover groups and narrow linear features related to mylonitic and retrograde shear zones.