Magnetic fabric and petrology of Miocene sub-volcanic sills and dykes emplaced into the SW Flysch Belt of the West Carpathians (S Moravia, Czech Republic) and their volcanological and tectonic implications

Abstract

Magnetic fabrics were investigated in the sub-volcanic bodies occurring within the SW Flysch Belt of the Western Carpathians. Magnetite or low-Ti titanomagnetite is the principal carrier of the anisotropy of magnetic susceptibility (AMS) in these rocks, though various Fe–Ti oxides were also identified in minor amounts. The magnetic fabric in almost all sills and dykes is conformable to the shapes of the bodies and no doubt originated through more or less free magma flow that requires relatively open voids to be injected, that likely originated in an extensive tectonic regime. In an ankaramite sill, where the rock creates typical hexagonal columns, the magnetic fabric is related rather to the column axes than to the overall shape of the sill. The magnetic fabric formation in this sill is controlled by the post-emplacement process of the creation of the columns. At some studied localities, signs of post-magmatic Middle Miocene tectonic deformation are observed in the field, the youngest known in the Flysch Belt. As the magnetic fabric is unrelated to those signs, we conclude that these young tectonic movements did not penetrate ductilely in the volume of the volcanic body. It seems that the strain was focused to thin zones along the faults, since it did not change the preferred orientation of magnetic minerals in the rock volume substantially.