TSK 11 Göttingen 2006 Hirt et al. Magnetic fabric in ilmenite- rich norites of the Bjer- kreimer-Sokndal Layered In- trusion, Norway Poster Ann M. Hirt1 Volkmar Schmidt1 Suzanne McEnroe2 Florian Heidelbach3 Peter Robinson2 Introduction The Bjerkreim-Sokndal (BKS) is a lay- ered intrusion, located in the Mid- Proterozoic Egersund anorthosite-norite province within the Sveconorwegian province of the Baltic Shield, south Norway. The layered intrusion formed by influxes of more primitive magma into more evolved magma to produce six Megacyclic units (MCU), each of which can be divided into up to six subunits. From bottom to top in each megacycle the rocks consist of early plagioclase-rich norites, intermedi- ate hemo-ilmenite-rich norites and later magnetite-rich norites. Aeromagnetic maps over the intrusion show large negative and positive anomalies. A negative anomaly with amplitude to - 13000 nT at 60m above ground is as- sociated with hemo-ilmenite-rich norite layer MCU Ive. This layer IVe con- tains plagioclase, orthopyroxene, hemo- ilmenite, magnetite, and minor clinopy- roxene, biotite, apatite and sulfides. Multi-domain (MD) magnetite makes up 2–3% of the rock. The negative magnetic anomaly associ- ated with MCU IVe reaches its most negative value on the east limb of the Bjerkreim Lobe near Heskestad. The 1 Institute of Geophysics, ETH Hoengger- berg, CH-8093 Zurich, Switzerland 2 Geo- logical Survey of Norway, N-7040, Trondheim, Norway 3 Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany anomaly at Heskestad is part of a longer negative anomaly, which follows MCU IVe for more than 20 km around a large syncline. The average NRM intensity decreases from 25AM−1 along the east fold limb to 10AM−1 towards the hinge area to 7AM−1 at the hinge. The BKS has a penetrative deformation fab- ric within the syncline with the weak- est deformation found in the hinge area and the strongest on the east limb. Elec- tron backscatter diffraction (EBSD) was used to determine the lattice-preferred orientation (LPO) of orthopyroxene and ilmenite. The (100)-planes of the or- thopyroxenes are found to lie parallel to a foliation in the rock, which is sub- parallel to the cumulate layering. Or- thopyroxene c-axes form the steep lin- eation within the foliation plane. The anisotropy of magnetic susceptibil- ity (AMS) was measured for samples that were taken at five locations from the eastern limb to the hinge area of the syncline to investigate if the change in NRM intensity could be related to mag- netic fabric. Magnetic Fabric The AMS of the norite samples was measured in low fields with an AGICO KLY-2 susceptibility bridge at room temperature (293K) and liquid nitro- gen temperature (77 K). At room tem- perature the AMS ellipsoid is triaxial with a grouping of maximum axes down- dip in the foliation plane and minimum axes sub-parallel to the pole to folia- tion. The degree of anisotropy is lower in the hinge area compared to localities on the limb of the syncline. The aver- age, low-field susceptibility increases by a factor of 2.2 to 3.2 at low temperature, which indicates that both ferromagnetic and paramagnetic minerals are responsi- 1 Hirt et al. TSK 11 Göttingen 2006 ble for the low-field susceptibility. This grouping becomes less distinct at low temperature; the shape remains triax- ial although the degree of anisotropy in- creases. There is also a slight change in the orientation of the ellipsoid at 77K, which may be related to the increased contribution of the paramagnetic phases to the magnetic susceptibility at low temperature, and this will be discussed in conjunction with the mineral fabric. To understand better which mineral fraction is controlling the AMS at the two temperatures, i.e., the ferromag- netic or paramagnetic, the AMS was measured subsequently in high-fields with a torque magnetometer at 293K and 77K on selected samples. The torque is dominated by the ferromag- netic phases at both temperatures. The orientation of the ferromagnetic ellip- soid is at 293K is in agreement with the orientation of the low-field AMS. The paramagnetic AMS is similar is tilted slightly with respect to the ferro- magnetic AMS, but both are triaxial in shape. At 77K the orientation of the ferromagnetic subfabric rotated clock- wise with respect to its orientation at 293K. Magnetite is the dominant car- rier of the susceptibility anisotropy. It must be noted that magnetite under- goes a transition in its crystallographic structure from cubic to lower symme- try below the Verwey transition at ap- proximately 120K. Above the Verwey transition magnetite is dominated by shape anisotropy, i.e., shape of the mag- netite grains, whereas below the Ver- wey transition, magnetite is dominated by a strong crystalline anisotropy. The change in magnetic fabric will be dis- cussed in the context of the mineral fab- ric of orthopyroxene and ilmenite. 2