Magnetic Fabric of the Brazilian Dike Swarms: A Review

Abstract

In Brazil, there are many Precambrian and Phanerozoic mafic dike swarms of variable length, chemistry and structural trend. Phanerozoic dike swarms are more abundant and widely distributed than those of Precambrian ages. The longest dikes and the densest swarms are Mesozoic, e.g. the Ponta Grossa swarm in Parana State. It is well accepted that magnetic anisotropies yield the most efficient methods to determine petrofabric orientation in rocks, particularly where standard petrofabric techniques are inadequate or inefficient, and even in rocks that are visually isotropic. Magnetic fabrics, also called magnetic anisotropies, can be determined using either low-field anisotropy of magnetic susceptibility (AMS), which is the most popular method, or anisotropy of magnetic remanence (AMR). These are powerful tools for structural geology and have been applied in many geological situations. Magnetic fabrics were determined in many dike swarms of Precambriam and Mesozoic ages together with extensive rock magnetic studies. The main magnetic fabric for these swarms is related to magma flow, and the relative position between magma sources and emplacement fractures could be inferred. In some swarms AMS and AMR tensors are coaxial, whereas in others the AMS fabric is primary in origin but the AMR fabric is tectonic and acquired after dike emplacement.