Unveiling the magnetic mineralogy and magma flow dynamics within subvolcanic dykes from northern Portugal, Central Iberian Zone

Several hypabyssal dykes and masses, generated during the Permo-Carboniferous, outcrop throughout southwest Europe. In Portugal, this magmatic event is related to the transition from a post-collisional to extensional setting that followed the Variscan orogeny and represented by porphyries, lamprophyres, and dolerites which intruded into older metasediments and granites. The magnetic mineralogy, susceptibility, and fabrics of selected dykes from northern Portugal were studied using methodologies such as IRM curve acquisition and treatment, frequency-dependence of the magnetic susceptibility (Kfd), and low-field AMS.

Based on the bulk magnetic susceptibility, the felsic lithotypes are paramagnetic (K_m = 0.9-148.2 µSI) but mostly composed of diamagnetic minerals (i.e., quartz and feldspars). AMS in these rocks is mainly carried by iron-bearing silicates, such as biotite and cordierite, as well as ilmenite. However, other iron oxides, namely hematite, goethite, or fine-grained magnetite, also play an important role in the magnetic anisotropy. By contrast, in most of the mafic lithologies (K_m = 238.2-15,640.7 µSI), magnetite is an essential component. Other iron-rich minerals such as biotite, amphibole, and pyroxene also influence the anisotropy of these rocks.

Following the IRM data treatment (Kruiver et al., 2001; Maxbauer et al., 2016), all samples reveal at least one magnetite grain population whose mean coercivities (B_1/2) and dispersion parameter (DP) range from 18.2 to 70.8 mT and 0.26 to 0.40, respectively. Petrographic observations suggest that most magnetite composing the mafic rocks is multidomain-type. However, Kfd measurements (4.66-18.18%) indicate that superparamagnetic particles are likely to exist in some lithologies, inducing low bulk susceptibilities and anomalous AMS fabrics. On the other hand, inverse fabrics are probably associated with hydrothermal and/or post-magmatic alterations.

Many felsic specimens display a normal fabric, where the magnetic minerals were oriented along the magma flow direction within the dyke and undisturbed by tectonic strains. Such observation is compatible with the average low anisotropy degree (P_para% = 0.92-4.28%), implying passive emplacement of the melts, and possibly reflects a weak contribution from single-domain-magnetite. Rare cases where the magnetic fabric is intermediate presumably point to more intense deformations.

Magnetite on the mafic rocks is mainly primary. By contrast, since the felsic dykes derived from anatexis of pelitic sources, their generation occurred under reducing conditions, being similar to Ilmenite-type granites. As such, most magnetite in the felsic samples is probably secondary, having resulted from exsolution from Fe-bearing minerals. However, the presence of primary magnetite cannot be ruled out due to possible magma mixing, as suggested by the coexistence of mantled and non-mantled feldspars. The less evolved member involved in the mixing process is likely to carry a more oxidized composition, bearing the potential to crystallize primary magnetite.

This work was supported by the Portuguese Foundation for Science and Technology (FCT), through the project reference UIDB/UIDP/04683/2020 and ICT (Institute of Earth Sciences). The main author is also financially supported by FCT through an individual Ph.D. grant (reference SFRH/BD/138818/2018). References: Kruiver, P.P., Dekkers, M.J., Heslop, D., 2001. Earth Planet. Sci. Lett. 189, 269–276. Maxbauer, D.P., Feinberg, J. M., Fox, D.L., 2016. Computers & Geosciences 95, 140–145.