Composite magmatic/magnetic fabrics evidences late AMS in syn-tectonic dikes in the Monteiro-Sumé plutonic-volcanic complex (NE Brazil)

Abstract Interpretations of flow fabrics in dikes using anisotropy of magnetic susceptibility (AMS) are based on the orientation of the magnetic fabric recorded next to the dike walls. However, uncertainties remain when AMS is sensitive to parameters such as grain size and composition of the magnetic carriers and deformation of the magma. We study subvolcanic andesite to dacite dikes associated with granite plutons that intrude the basement rocks. The dikes have magnetic susceptibilities mostly between 10−3 and 10−2 SI carried by Fe–Ti oxides. AMS shows fabrics that would be consistent with a lateral movement of the magma. However, the anisotropy of anhysteretic remanence (AAR) and silicate shape preferred orientation (SPO) indicate that the melt moved upward based on the steeply plunging lineations detected by AAR and SPO. Furthermore, the obliquity of the AAR foliations in the dike margins indicates a component of transtension resolved in the dike walls. As there is no evidence for internal deformation of titanomagnetite, AMS would record its late growth in the tectonic extension direction. The magnetic subfabric concealed by late AMS therefore documents an earlier tectonic episode and evidence the limits of using just AMS directions to constrain the emplacement of syntectonic magmas.