A regional analysis of the anisotropy of magnetic susceptibility (AMS) has been performed on lowgrade metamorphic, deformed homogeneous siltstone beds (HSBs) of the Plougastel Formation in the Central Armorican Domain together with exhaustive compositional analyses of the studied specimens. Despite sampling a single horizon, different paramagnetic minerals are controlling the AMS in the Crozon peninsula sites (white mica and chlorite) and in the inland sites of the Central Armorican Domain (white mica, chloritoid and some chlorite). Both the Crozon peninsula and inland datasets show a hockey-stick shaped pattern on a plot of the shape parameter T versus the corrected degree of anisotropy P J , although the whole pattern is shifted to higher P J values for the inland dataset. High-field AMS indicates that the low-field P J and T values from two inland sites are slightly enhanced by a small ferromagnetic (sensu lato) contribution. Furthermore, variation in P J and T values within single sites can be attributed to an effect of the observed quartz/white mica ratio, as quartz grains disrupt the petrofabric intensity. Our findings clearly demonstrate that the AMS of the HSBs in the Central Armorican Domain is strongly influenced by compositional variations and does not merely indicate tectonic strain, even in very similar tectonostratigraphic settings. Supplementary material:Outcrop coordinates with information on the HSBs and samples, and the summarized low-field, room-temperature AMS data are available at www.geolsoc.org.uk/SUP18578. research-articleResearch Article170X10.1144/jgs2012-062T.

[1] The magnetocrystalline anisotropy of monoclinic chloritoid, a relatively common mineral in aluminum-rich, metapelitic rocks, has been determined for the first time by measuring the high-field anisotropy of magnetic susceptibility (HF-AMS), using two independent approaches, i.e., (a) directional magnetic hysteresis measurements and (b) torque magnetometry, on a collection of single crystals collected from different tectonometamorphic settings worldwide. Magnetic remanence experiments show that all specimens contain ferromagnetic (s.l.) impurities, being mainly magnetite. The determined HF-AMS ellipsoids have a highly oblate shape with the minimum susceptibility direction subparallel to the crystallographic c-axis of chloritoid. In the basal plane of chloritoid, though the HF-AMS can be considered isotropic. The degree of anisotropy is found to be 1.47, which is significantly higher than the anisotropy of most paramagnetic silicates and even well above the frequently used upper limit (i.e., 1.35) for the paramagnetic contribution to the AMS of siliciclastic rocks. The obtained values for the paramagnetic Curie temperature parallel (θ ∥ ) and perpendicular (θ ⊥ ) to the basal plane indicate that this pronounced magnetocrystalline anisotropy is related to strong antiferromagnetic exchange interactions in the direction of the crystallographic c-axis (θ ⊥ < 0) and rather weak ferromagnetic exchange interactions within the basal plane (θ ∥ > 0). As a consequence, chloritoid-bearing metapelites with a pronounced mineral alignment can have a high degree of anisotropy without the need of invoking a significant contribution of strongly anisotropic, ferromagnetic (s.l.) minerals. The newly discovered magnetocrystalline anisotropy of chloritoid thus calls for a revised approach of magnetic fabric interpretations in chloritoid-bearing rocks.

a b s t r a c tA regional analysis of the anisotropy of the magnetic susceptibility on low-grade metamorphic, chloritoid-bearing slates of the Paleozoic in Central Armorica (Brittany, France) revealed very high values for the degree of anisotropy (up to 1.43). Nonetheless, high-field torque magnetometry indicates that the magnetic fabric is dominantly paramagnetic. Chloritoid's intrinsic degree of anisotropy of 1.47 ± 0.06, suggests that chloritoid-bearing slates can have a high degree of anisotropy without the need of invoking a significant contribution of strongly anisotropic ferromagnetic (s.l.) minerals. To validate this assumption we performed a texture analysis on a representative sample of the chloritoid-bearing slates using hard Xray synchrotron diffraction. The preferred orientation patterns of both muscovite and chloritoid are extremely strong (~38.6 m.r.d. for muscovite, 20.9 m.r.d. for chloritoid) and display roughly axial symmetry about the minimum magnetic susceptibility axis, indeed suggesting that chloritoid may have a profound impact on the magnetic fabric of chloritoid-bearing rocks. However, modeling the anisotropy of magnetic susceptibility by averaging single crystal properties indicates that the CPO of chloritoid only partially explains the slate's anisotropy.