New experimental and computational approaches to interpret orientation and intensity of natural remanent magnetization (NRM) carried by lamellar magnetism are applied to historic magnetic measurements on a collection of 82 massive hemo-ilmenite samples from the Allard Lake District in the Grenville Province, Quebec. The anisotropy of magnetic susceptibility (AMS), together with declination and inclination ofNRM, indicate a systematic deflection β of the NRM vector away fromthe unit vector v that represents the Mesoproterozoic magnetizing field direction. The deflection β is caused by a statistical lattice-preferred orientation (LPO) of the individual (0001) basalplanes, to which the NRM is confined in hemo-ilmenite crystals. Here, we study a second deflection Ψ that is the angle the NRM makes with the statistical (0001) basal plane of the crystal assemblage, in relation to the angle α between the statistical (0001) basal plane and v. The relatio between these two angles depends on the scatter of thedistribution of crystal platelets, which also influences the AMS of theassemblage. For a Fisher distribution of basal planes, the distribution parameterK can be determined from Ψ and a. It is then furtherpossible to infer the single-crystal anisotropy of individual platelets. Typical crystals of hemo-ilmenite turn out to have a relatively weak AMS so that samples with a narrow Fisher distribution of plateletsnevertheless can have a weak AMS. This has been confirmed in two samplesby measurement of the (0001) basal plane distribution of crystalsusing electron backscatter diffraction, and in one of these two samples by measuring AMS and NRM of a single hemo-ilmenite crystal. Based on our estimated K values for selected samples, we calculate values of β, NRM intensity and Ψ for any value of α. These data provide striking examples of the influence of the orientation of the crystal LPO on the intensity of lamellar magnetism, and explain the large variation of observedNRM intensities by varying orientation with respect to the magnetizing field, without requiring large variations ofthe paleomagnetic field intensity. This relation between NRM and LPO is also important for anomaly interpretation in areas with strong foliation.
- Magnetic anomalies
- Magnetic fabrics and anisotropy
- Magnetic mineralogy and petrology
- modeling and interpretation
- Palaeomagnetism applied to tectonics