Magnetic Fabric Studies on Mafic Dykes at a Volcanic Rifted Margin in the Henties Bay – Outjo Dyke Swarm, NW Namibia

Abstract

Mafic dyke swarms are major components of the South Atlantic Large Igneous Province, which originated during the Cretaceous break up of western Gondwana. This contribution presents data on magnetic fabrics and its implications for magma flow from the major Henties Bay-Outjo dyke swarm (HOD) in coastal and inland NW Namibia. The dykes were emplaced in the NE-SW trending, Neoproterozoic Damara mobile belt. The dominant NE-SW strike of the HOD indicates the influence of the Damara Belt structural grain at a regional scale, but locally the dykes crosscut basement foliations and lithologic contacts. Subalkaline tholeiitic basalt is the most common composition. Depending on dyke thickness (c. 0.1 to >100 m), the rocks are variably fine- to medium grained with chilled margins. Vesicles are uncommon, in keeping with an estimated 3–4 km post-Cretaceous denudation. Typical textures are intersertal to subophitic, main mineral phases are plagioclase, clinopyroxene, olivine, Fe-Ti oxides, and accessory apatite and sulphides. Detailed analyses of the anisotropy of magnetic susceptibility (AMS) cover 16 stations in the area between Henties Bay at the coast and ca. 300 km inland. 4–22 standard cylinders were drilled at each sampled point. Magnetic susceptibility varies between 3 and 50×10–3 SI units. Microscopic studies and kappa-T measurements show magnetite and titanomagnetite as the dominant magnetic minerals. AMS is mostly low, with Pj values around 1.03, rarely up to 1.18, indicating a primary fabric. The shape of the AMS ellipsoid ranges from prolate to neutral and oblate. Four fabric types can be recognized, which are comparable to those reported earlier from mafic dykes elsewhere. At least the normal fabric type provides information on flow fabrics. Steep long axes of such fabrics are related here to vertical magma flow and shallow long axes to horizontal flow along the dyke fractures. In places, oblate fabrics with flat-lying magnetic foliation are observed. This flat foliation is attributed to subvertical stresses at the upper end of dykes, where melt was unable to continue flowing upwards. In the HOD as a whole, both flat and steep magma flow directions are observed in every sampled area. Therefore, and because of different dyke chemistries we conclude that the dyke emplacement was a complex process with magma ascending and spreading laterally at several distinct locations.