On the origin of the magnetic fabric in purple Cambrian slates of North Wales

Anisotropy of magnetic susceptibility (AMS) in samples of Cambrian slate from Penrhynn, Wales, is surprisingly weak (maximum/minimum ≃- 1.1 to 1.2) in view of their large finite strain (X/Z ≃- 3.5 to 4.5), and their hematite-dominated magnetic mineralogy. Several lines of evidence suggest that this is a result of the presence of two or more generations of hematite. SEM observations show a somewhat bimodal size distribution of hematite particles, with modal diameters of approximately 1-3 μm and 10-20 μm. The larger grains tend to have ragged, subequant shapes, and contain titaniferous laminae; they are probably of igneous/detrital origin. The finer grains have crystallographically-controlled shapes, are Ti-free, and are probably authigenic or metamorphic. Initially, chemical leaching of deeply incised solid specimens preferentially removes the more soluble particles, and results in significant changes in AMS. Further leaching attacks the remaining grains on exposed surfaces, and the AMS ellipsoid reverts to its initial shape. Isothermal remanent magnetization (IRM) is significantly more anisotropic than low-field susceptibility; the variation of IRM anisotropy with applied field indicates relatively strong fabric development in the fine-grained hematite, but weak fabric in the coarser fraction. Grain boundaries were digitized from SEM photos in order to calculate Flinn fabric ratios by the R_f-φ method. The X/Z ratios thus obtained for quartz and fine hematite grains are 3.14 and 1.70, respectively. It is therefore clear that the hematite fabric does not reflect the total finite strain.