Paramagnetic metamorphic mineral assemblages controlling AMS in low-grade deformed metasediments and the implications with respect to the use of AMS as a strain marker

A regional analysis of the anisotropy of magnetic susceptibility (AMS) has been performed on low-grade 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.