Magnetic Fabric and Natural Remanent Magnetization in the MCU IVe’ Unit of the Bjerkreim Sokndal Layered Intrusion

Abstract:

The Bjerkreim Sokndal intrusion consists of layers of plagioclase-pyroxene cumulates representing recharge and fractional crystallization of a jotunitic magma. The layers form a syncline with a SE plunging fold axis, and strong foliation at the limbs. One of these layers, MCU IVe’, is associated with a strong negative magnetic anomaly on the east of the syncline, reducing the magnetic field intensity at the surface to about 1/3 to 1/2 of the expected value. The anomaly is less prominent where MCU IVe’ outcrops in the hinge and west limb. Such a strong negative anomaly must be caused by remanent magnetization, most likely carried by lamellar hemo-ilmenite, which is known for its strong and stable remanence. A second magnetic phase present is magnetite. The aim of this study is to understand how the magnetic fabric affects the orientation and intensity of the remanence, which in turn controls the observed anomaly. Natural remanent magnetization (NRM) and anisotropy of magnetic susceptibility (AMS) have been measured on 500 cores from sites along MCU IVe’. The NRM direction changes along the layer, possibly related to mineral foliation. Both NRM intensity and mean susceptibility show a general decrease from east to north to west. In most sites, especially those with strong remanence, the NRM is sub-parallel to the maximum susceptibility, or lies within the magnetic foliation plane. Minimum susceptibility is normal to the expected mineral foliation plane in some sites, whereas it can lie within the foliation plane in others. This may be explained with the presence of two magnetic carriers, hemo-ilmenite and magnetite, which can give rise to different magnetic anisotropies. Understanding the relationship between mineral fabric, AMS, the intensity and direction of NRM, and the observed anomaly will help in interpreting other strong magnetic anomalies.