Magnetic fabrics of dike swarms from SE Bahia State, Brazil: their significance and implications for Mesoproterozoic basic magmatism in the São Francisco Craton

Abstract

Magnetic fabrics were determined by applying two techniques: anisotropy of low-field magnetic susceptibility (AMS); and anisotropy of anhysteretic remanence (AAR). AMS was performed in 81 dikes from Ilhéus, Olivença and Itaju do Colônia swarms located in SE Bahia. The dikes intrude high-grade metamorphic terrains as old as 3.2 Ga. Dikes are vertical or subvertical in dip and trend mainly E–W. They are tholeiitic in composition and are classified into two groups, characterized by high TiO 2 (>2%) and incompatible elements contents, and low TiO 2 and incompatible elements contents, respectively. Thicknesses vary from a few centimeters to ca. 20 m (average of 3 m). 40Ar/ 39Ar data indicate ages of 1.012 (Ilhéus dikes) and 1.078 Ga (Olivença dikes). Rock magnetism shows that Ti-poor titanomagnetites up to pure magnetites pseudo-single-domain/multidomain grain sizes carry the magnetic fabrics. Three types of AMS fabric are recognized: (i) ‘Normal’ fabric (64 dikes) defined by the clustering of K 1– K 2 axes on the dike plane whereas K 3 axes are nearly perpendicular to it. This fabric is interpreted as due to magma flow. (ii) ‘Intermediate’ fabric (seven dikes) characterized by K 1– K 3 axes clustering close to dike plane and K 2 axes perpendicular to the dike wall. It was interpreted as due to vertical compaction of a static magma column with the minimum stress along the dike direction; and (iii) ‘Inverse’ fabric (nine dikes) defined by K 2– K 3 plane parallel to the dike plane and K 1 perpendicular to dike wall. AAR measurements were performed in three dikes; two of them with ‘inverse’ and one with ‘normal’ AMS fabrics. AAR was coaxial for ‘normal’ AMS fabric and it resulted in ‘intermediate’ and ‘normal’ for ‘inverse’ AMS fabrics. A combination of AMS and AAR fabrics suggests that the magmatic fabric for these dikes that should be flow, was overprinted by some local event, probably related to Brasiliano Orogeny processes, after dike emplacement. ‘Inverse’ AMS fabric would record this event.‘Normal’ fabric is the most important one found for the studied swarms. The analysis of K 1 inclination permitted one to infer that the dikes from Ilhéus were fed dominantly by horizontal fluxes ( K 1<30°) whereas those from Olivença and Itaju do Colônia were mainly fed by inclined up to vertical fluxes ( K 1>60°). Dikes from Itaju do Colônia region have the steepest K 1 suggesting that this region could be closer to a magma source. On the other hand, a magma source near to Olivença area could be also inferred since the majority of the dikes were fed by inclined flow. The existence of more than one source is supported by geochemical, geochronological and paleomagnetic data which have shown that contemporaneous dikes were fed by compositionally distinct magmas. The AMS data also suggest that the majority of dikes were fed from upward flow coming from west to east and only a few of them were fed from upward flow from east to west.