Magnetic fabrics in the Bjerkreim Sokndal Layered Intrusion, Rogaland, southern Norway: Mineral sources and geological significance

Abstract

Magnetic anisotropy can provide important information about mineral fabrics, and thus magmatic processes, particularly when it is known how multiple mineral species contribute to the anisotropy. It may also affect the direction of induced or remanent magnetization, with important consequences for paleomagnetic studies or the interpretation of magnetic anomalies. Here, we aim at describing the magnetic fabrics in the Bjerkreim Sokndal Layered Intrusion and identifying their carriers.Anisotropies of magnetic susceptibility and remanence were measured on samples covering different geographic locations and stratigraphic units within the Bjerkreim Sokndal Layered Intrusion. The intrusion is characterized by magmatic layering and has a synform structure, with strong foliation on the limbs. Detailed comparison between magnetic and mineral fabric shows that they are not necessarily coaxial, but the minimum susceptibility, and minimum anhysteretic remanence are generally normal to the foliation or the magmatic layering. The minimum susceptibility and anhysteretic remanence are associated with pyroxene (100) axes, and the maximum susceptibility and anhysteretic remanence are sub-parallel to the pyroxene [001] axes in layers MCU IVc and MCU IVe for which electron backscatter data are available. However, the paramagnetic anisotropy of pyroxene is too weak to explain the observed anisotropy. We propose that the magnetic anisotropy of magnetite-free specimens is carried by hemo-ilmenite exsolutions within pyroxene, in addition to pyroxene itself. When present, multi-domain magnetite dominates both the anisotropy of magnetic susceptibility and anhysteretic remanence, due to shape-preferred orientation and distribution anisotropy. The orientation of the magnetic fabric appears independent of carrier, due to their common deformation history, but the degree of anisotropy is stronger for magnetite-bearing specimens. The results of this study will facilitate future structural interpretations and may be used to correct for magnetization deflection.

Effect of magnetic anisotropy on the natural remanent magnetization in the MCU IVe' layer of the Bjerkreim Sokndal Layered Intrusion, Rogaland, Southern Norway

“…The orientation of the site mean principal susceptibility axes is generally similar in sites that are located close together but changes in accordance with the trend of the layers. Overall, the minimum susceptibility is approximately perpendicular to the foliation and magmatic layering, as has been found on a subset of specimens previously studied [ Biedermann et al ., ; Paludan et al ., ].…”

“…Newly collected specimens are labeled BK2015_xx, and specimens available from earlier studies are named BKxx. The magnetic fabrics of some of these specimens have been described previously [ Biedermann et al ., ]. In total, 371 specimens from 46 sites, including 265 new specimens from 95 drill cores in 29 sites, and 106 existing specimens from 16 drill cores and 13 oriented blocks in 17 sites, were selected for this study (Figure ).…”

“…Strong foliation has been reported on the limbs of the syncline and a lineation‐dominated fabric caused by the superposition of an initial magmatic layering and a tectonic overprint in the hinge zone. Magnetic fabrics, as defined by anisotropy of susceptibility and anisotropy of remanence, are strong in the entire intrusion and broadly reflect the orientation of the layering [ Biedermann et al ., ; Bolle et al ., ; Paludan et al ., ]. The folding has been interpreted as a synmagmatic to postmagmatic event caused by gravitational instability related to the emplacement of the surrounding anorthosite bodies at 930 Ma [ Bolle et al ., ].…”

“…Anisotropy of remanence has been reported in the Stillwater Complex [ Selkin et al ., ]. Anisotropy of susceptibility and anisotropy of remanent magnetization have recently been characterized for the cumulate series of the BKS intrusion [ Biedermann et al ., ].…”

Effects of Magnetic Anisotropy on Total Magnetic Field Anomalies

“…Surface samples have been collected from various locations in the MCU IVe′ layer, and additional samples from the MCU IVe′ layer and other layers of the intrusion were available from previous studies (Figure ). A detailed description of sampling and sample preparation is given in Brown and McEnroe () and Biedermann et al (). Throughout this study, magnetic property results will be shown for two sets of samples: in total, 570 samples from 101 sites have been used, including samples from layers IAa, IBa, IBc, IBe, IIc, IIIa, IIIc, IIId, IIIe, IIIf, IVa, IVb, IVc, IVd, IVe, IVe′, IVf, and mangerite, and 310 of these from 39 sites originate within the MCU IVe′ layer.…”