Modalities of anisotropy of magnetic susceptibility in fine-grained sedimentary rocks deformed in a contraction-dominated setting - A case study of the Central Armorican Domain, Brittany, France

An integrated rock-magnetic and mineralogical approach is performed for a case study of low-grade metasedimentary rocks from the Central Armorican Domain (CAD). The objective is twofold. Firstly, gaining a better understanding of the relation between the mineral sources of magnetic susceptibility and the anisotropy of magnetic susceptibility (AMS). Secondly, verifying whether the magnetic fabrics have a potential as a regional strain marker. For this goal, a lithostratigraphical reference unit, which consists of homogeneous siltstone beds (HSB), is sampled in four distinct research areas, i.e. the Crozon south (CS) and Crozon north (CN) research area on the Crozon peninsula and the Monts d Arrée slate belt (MASB) and Montagnes Noires slate belt (MNSB)research area in the inland CAD.It was found that the magnetic fabrics of the HSB samples from the four research areas dominantly reflect two different magnetic fabric types. On the one hand, a type III intermediate magnetic fabric, which is composed of bedding-parallel and a cleavage-parallel component fabric, and on the other hand, a type IV tectonic magnetic, which reflects a single cleavage-parallel fabric. Hence, the magnetic fabrics seems to show ageneral, progressive evolution that is typical for rocks that have been affected by a coaxial, contraction-dominated deformation. There is, however, no systematic regional pattern in the occurrence of these different magnetic fabric types. Both types occur in all four research areas, whereas the four research areas each have their own typical deformation characteristics.Remanence experiments, low temperature and high-field AMS experiments demonstrate that the magnetic signal of the HSB samples is strongly dominated by their paramagnetic phases. A mineralogical analysis revealed that there is a difference in the paramagnetic mineralogy of the HSBs from the Crozon peninsula, i.e. an anchizonal metamorphic mineral assemblage with white mica and chlorite, and the HSBs from the inland slate belts, i.e. an epizonal metamorphic mineral assemblage with white mica and chloritoid. Because of its strong magnetic susceptibility (high FeO content), chlorite is the dominant mineral in the former mineral assemblage. For the same reason, chloritoid is the dominant magnetic mineral in the epizonal mineral assemblage. As we found that the intrinsic magnetic anisotropy of chloritoid is much stronger than that of chlorite, the magnetic anisotropy from the Crozon peninsula and those from the inland slate belts cannot be compared directly and should be treated separately.An independent texture analysis for a single, chloritoid-bearing specimen of the MASB shows that the orientation of the muscovite, chloritoid and chlorite fabric matches the orientation of the magnetic fabric. However, a calculation of the AMS tensor of a 3-phase model from the obtained mineral fabrics and the intrinsic magnetic anisotropy of these three phases, shows that weighted average of the magnetocrystalline anisotropy of the different phases seems not sufficient to explain the measured magnetic anisotropy. The reason for this discrepancy remains unclear to date. Until this issue is solved, we have to conclude that the anisotropy of the magnetic and mineral fabric do not match and hence, the magnetic fabric cannot be used as a reliable proxy for the mineral fabrics.Despite this shortcoming, there are still a number of conclusion that can be drawn from the regional magnetic fabric datasets. Firstly, besides the influence of the metamorphic mineral assemblage, we found that there are two other boundary conditions that are not stable for the entire dataset and hence, that appear to affect the magnetic fabric development. These are (1) composite magnetic fabric variations due to an influence from the angle between the bedding-parallel and the cleavage-parallel component fabric and (2) compositional variations due to an influence from the quartz to total clay ratio. Therefore, the magnetic anisotropy of the HSBs cannot be uniquely related to the tectonic strain in the CAD, even when the issue of the quantitative relationship between the magnetic and mineral fabric can be clarified. Secondly, by comparing the magnetic fabric development for specimens with similar boundary conditions, we can conclude that for the Crozon peninsula there is generally a higher amount of strain for the CN research area than for the CS research area and for the inland slate belts, there is generally a higher strain for the MASB research than for the MNSB research area.