Magnetic fabric of a basaltic dyke swarm associated with Mesozoic rifting in northeastern Brazil

The anisotropy of magnetic susceptibility (AMS) has been studied in a 120 km long, Early Cretaceous tholeiitic dyke swarm emplaced during the early stages of rifting and opening of the equatorial Atlantic Ocean. The vertical dykes filled a set of E-trending fractures that cut the structural grain of the Precambrian basement of northeastern Brazil at a high angle. These strongly magnetic rocks contain pseudo-single domain, Ti-poor magnetite and secondary maghemite as revealed by thermomagnetic and hysteresis data. The contribution of the paramagnetic and the high coercivity antiferromagnetic fractions to the bulk susceptibility is less than 1.2%. The dykes generally show well-clustered AMS principal directions. The plunge of the magnetic lineation varies from nearly subvertical in the center of the swarm to horizontal in the west. The strike of the magnetic foliation is generally oblique to the dyke wall and exhibits a curved trend at the regional scale. This fabric pattern suggests that the magma source that fed the dykes was situated in the center of the swarm, which is presently below Tertiary sandstones.