Preferred mineral orientation of a chloritoid-bearing slate in relation to its magnetic fabric

Abstract

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.

Elastic anisotropy of Tambo gneiss from Promontogno, Switzerland: a comparison of crystal orientation and microstructure-based modeling and experimental measurements

“…(e.g. Oertel 1983;Haerinck et al 2015). They are followed by gneisses and schists as described in the previous section (Table 6, see also Cholach & Schmitt 2006;Wenk et al 2010b).…”

Kinematics and strain distribution in an orogen-scale shear zone: Insights from structural analyses and magnetic fabrics in the Gavarnie thrust, Pyrenees

“…Nevertheless, clear stretching lineations are difficult to identify in pelitic rocks with very low metamorphic grade, especially where flattening is combined with simple shear. AMS can be helpful to define the dominant set of planes in different domains of the shear zone and also to determine the mineral lineation within the rock (Debacker et al, 2009;Oliva-Urcia et al, 2010;2012a;Haerinck et al, 2015).…”

Multidisciplinary approach to constrain kinematics of fault zones at shallow depths: a case study from the Cameros–Demanda thrust (North Spain)

“…Lineations are scattered within the foliation planes, showing, in individual sites, a trend (1) perpendicular to the slickenside striations (locally parallel to the intersection lineation between C1 and C2 planes, PA1, PA4, PA6, PA7), (2) parallel to the transport direction (considering this as the perpendicular to the average C1/C2 intersection, PA5), or (3) both orientations (PA2, PA3). The different generations of C planes observed in thin sections can also account for deflecting magnetic lineation orientation toward the intersection lineation (see Debacker et al 2009;Haerinck et al 2015) versus the elongation or transport direction within the shear zone. No lithological control seems to exist on this difference because both features are found in argillaceous fault gouge and fault breccia.…”

“…In shear zones, the resultant maximum axis of the magnetic susceptibility ellipsoid is assumed to be parallel to the transport direction on C planes (see Parés and Van der Pluijm 2002), although case studies also indicate (1) intermediate orientations at the bisector between S and C planes (Aranguren et al 1996) and (2) opposite geometrical relationships, with k max perpendicular to the transport direction and, consequently, parallel to the intersection lineation between C and S planes (Oliva-Urcia et al 2009;Ono et al 2010). Because of these ambiguous relationships, it is extremely important to determine in each particular case the type of geometrical relationship between AMS and kinematic indicators both at the outcrop and microscopic scales (Debacker et al 2004(Debacker et al , 2010Haerinck et al 2015).…”