Fabric of the Rio Ceará–Mirim mafic dike swarm (northeastern Brazil) determined by anisotropy of magnetic susceptibility and image analysis

Abstract

Anisotropy of magnetic susceptibility (AMS) and petrofabric studies of the Early Cretaceous Rio Ceará–Mirim dike swarm reveal that shape‐preferred orientation of opaque grains controls the magnetic fabric. The 350‐km‐long swarm is characterized by relatively thick dikes, of the order of 20 m up to 150 m in width. The strong magnetic susceptibility of these rocks, around 5 × 10 −2 SI, is attributed to magnetite with low Ti content. The magnetite usually forms euhedral to subhedral, equant to skeletal grains. The resulting AMS has a large proportion of abnormal fabric types, as observed in 58% of the 50 dikes studied. The remaining 42% of the dikes show a “normal” fabric regionally characterized by steep magnetic foliations and subhorizontal lineations. In the central eastern part of the swarm, however, the lineation plunges downdip, suggesting a magmatic feeder zone. The shape alignment of the plagioclase supports the inferred regional flow pattern, indicating a primary origin for the normal AMS. Inverse to highly imbricate magnetic fabrics have lineations perpendicular to, or at a high angle to, the dike wall. A shape lineation of opaque grains lies generally close to the magnetic lineation, while the foliation is provided by the oblate‐shaped plagioclase fabric. Petrofabric studies indicate that the abnormal magnetic fabric is unrelated to flow. Competition of interstitial magnetite grains crystallizing either along, as well as perpendicular to the magma stretching direction seems to account for the weak magnetic anisotropy and the irregular distribution of the AMS types on the swarm.