Studies of density and magnetic properties of rocks from Northern Sweden

Summary Over the shield region of Northern Sweden, the Geological Survey runs a low altitude (30 m) aeromagnetic survey and regional gravity surveys cover almost the same areas. The production of detailed geological maps would be almost impossible without these geophysical measurements. To enhance their interpretation, a study of the appropriate petrophysical properties was started by measurements of density, susceptibility and remanence on all rock specimens collected by field geologists. Up to now more than 30,000 specimens have been measured and almost as many in situ susceptibility determinations have been made. About 10% of the samples are oriented. The information obtained is useful not only for the direct interpretation of geophysical surveys but also reflects the effects of various geological processes acting during and after the formation of the rocks considered.In precambrian rocks, density is obviously closely correlated to mineral composition and thus to chemical composition. This is demonstrated for igneous rocks by the correlation trends between density and SiO₂-content and the CM/AF-index.Susceptibility mainly reflects the magnetite content of rocks. As magnetite is an accessory mineral it is seldom considered by geologists. However, a closer study of the magnetic susceptibility of rocks reveals that its extreme complexity reflects the effects of primary and secondary geological processes. The susceptibility spectrum of a certain rock may prove to be a useful classification tool — at least regarding intermediate and basic igneous rocks.The combination of the physical parameters density and susceptibility in 2-dimensional frequency distributions seems to be a promising approach to the understanding of certain petrological processes and makes possible the delineation of local or regional secondary processes. Magmatic differentiation and serpentinization can be demonstrated in this manner.Remanent magnetization plays a secondary role in the majority of precambrian rocks. Still there are places where the natural remanence dominates and shows pronounced directions deviating from today's magnetic field. So far we have only one case where a follow up by demagnetization has been attempted, but intensified paleomagnetic research should most certainly add to our understanding of precambrian geology.Combination of susceptibility and remanance shows some characteristic correlations for highly remanent rocks, reflecting mainly exosolution phenomena among magnetic opaques and grain size distributions. Even these features might be used as diagnostic or classifying tools. They also explain some of the diversity of susceptibility spectra.