Magnetic Anomalies from Eclogite and Deep-Seated Rocks in Scandinavia: Contrasting Environments and Magnetic Properties
Norwegian University of Science and Technology, NORWAY
The magnetic response of crustal rocks is directly related to the type and abundance of oxides in the rock body. When magnetite is the primary oxide the primary controlling feature is abundance of oxide. This is because in high-grade metamorphic rocks the magnetite is commonly multi-domain in size and is close to end-member with few microstructures. With few features to stabilize the natural remanent magnetization (NRM) the response is dominated by induced magnetization. When exsolved members of the ilmenite-hematite solid solution are present the response is dominated by the NRM. However the intensity of the NRM is more complicated than in magnetite bearing rocks. In addition to the amount of oxide in the rock, the orientation of the oxide grains to the magnetizing field, and amount of exsolution lamellae play an important roles in the total intensity. If there is no coexisting magnetite then the rocks will have very high Q values because the induced magnetization will be very low. In these more oxidized rocks remanent anomalies are more common than the more reduced magnetite bearing rocks at the same pressure and temperature conditions. Here we present data from Western Gneiss Region mafic rocks which have been exhumed from depths as deep as 120 km with abundant eclogite, and the Swedish Granulites which also contains eclogite, but depth estimates are much lower. There is a very strong contrast in magnetic properties between the two regions due to the different oxidation state of the terrains. Aeromagnetic maps from both regions are compared and discussed in relation to the magnetic properties.