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 k m long, Early Cretaceous tholeiitic dyke swarm emplaced during the early st ages 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 str ongly magnetic rucks contain pseudo-single domain, Ti-poor magnetite and se condary maghemite as revealed by thermomagnetic and hysteresis data. The co ntribution of the paramagnetic and the high coercivity antiferromagnetic fr actions to the bulk susceptibility is less than 1.2%. The dykes generally s how well-clustered AMS principal directions. The plunge of the magnetic lin eation varies from nearly subvertical in the center of the swarm to horizon tal 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 fa bric pattern suggests that the magma source that fed the dykes was situated in the center of the swarm, which is presently below Tertiary sandstones. (C) 2000 Elsevier Science Ltd. All rights reserved.