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

Magnetic fabrics were determined by applying two techniques: anisotropy of low-field magnetic susceptibility (AMS); and anisotropy of anhysteretic rem anence (AAR). AMS was performed in 81 dikes from Ilheus, Olivenca and Itaju do Colonia swarms located in SE Bahia. The dikes intrude high-grade metamo rphic terrains as old as 3.2 Ga. Dikes are vertical or subvertical in dip a nd trend mainly E-W. They are tholeiitic in composition and are classified into two groups, characterized by high TiO2 (> 2%) and incompatible element s contents, and low TiO2 and incompatible elements contents, respectively. Thicknesses vary from a few centimeters to ca. 20 m (average of 3 m). Ar-40 /Ar-39 data indicate ages of 1.012 (Ilheus dikes) and 1.078 Ga (Olivenca di kes). Rock magnetism shows that Ti-poor titanomagnetites up to pure magneti tes 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 axe s are nearly perpendicular to it. This fabric is interpreted as due to magm a flow. (ii) 'Intermediate' fabric (seven dikes) characterized by K-1-K-3 a xes clustering close to dike plane and K-2 axes perpendicular to the dike w all. It was interpreted as due to vertical compaction of a static magma col umn with the minimum stress along the dike direction; and (iii) 'Inverse' f abric (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 di kes; two of them with 'inverse' and one with 'normal' AMS fabrics. AAR was coaxial for 'normal' AMS fabric and it resulted in 'intermediate' and 'norm al' for 'inverse' AMS fabrics. A combination of AMS and AAR fabrics suggest s that the magmatic fabric for these dikes that should be flow, was overpri nted by some local event, probably related to Brasiliano Orogeny processes, after dike emplacement. 'Inverse' AMS fabric would record this event. 'Nor mal' fabric is the most important one found for the studied swarms. The ana lysis of K-1 inclination permitted one to infer that the dikes from Ilheus were fed dominantly by horizontal fluxes (K-1 < 30 degrees) whereas those f rom Olivenca and Itaju do Colania were mainly fed by inclined up to vertica l fluxes (K-1 > 60 degrees). Dikes from Itaju do Colania region have the st eepest K-1 suggesting that this region could be closer to a magma source. O n the other hand, a magma source near to Olivenca area could be also inferr ed 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 majorit y of dikes were fed from upward flow coming from west to east and only a fe w of them were fed from upward flow from east to west. (C) 2000 Elsevier Sc ience B.V. All rights reserved.