Reduced- or ilmenite-type granites versus oxidized- or magnetite-type granites: occurrence in Iberian Variscan Belt

The magnetic susceptibility (K_m) of granites is an important characteristic and it is mainly controlled by the presence of certain oxide minerals like magnetite and/or ilmenite, as well as ferromagnesian phyllosilicates such as biotite. The abundance of magnetite or ilmenite can be explained by different redox conditions in the magma chamber and distinct magma sources. The presence of magnetite or ilmenite as accessory minerals represents oxidized- or magnetite-type granites and reduced- or ilmenite-type granites, respectively.

This work focuses on the K_m of 20 Variscan granitic massifs from northern and central Portugal and considers the results obtained in about 750 sampling sites, in order to deduce the redox conditions in the magma system. These granites are essentially two mica mesocrustal and biotite-rich basicrustal/infracrustal in origin and their emplacement was related to Variscan orogeny. In the northern and central Portugal, three main ductile deformation Variscan phases were recognized and described: D₁, D₂ and D₃. The studied granites were subdivided in three main groups according to U-Pb dating, field observations and emplacement relative to the D₃ phase. Therefore, the studied granites are subdivided as following: (1) syn-D₃ two-mica (mosc=biot) granites, ca. 311 Ma; (2) late-D₃ monzogranites, biotite-rich and two-mica granites (biot>mosc), ca. 300 Ma; (3) post-D₃ monzogranites and biotite-rich granites, < 299 Ma.

The evaluation of the K_m variation of the different granite groups shows that, as granites become progressively younger, the K_m parameter tends to increase as a result of the increasing in the mantellic contribution to the genesis of the magmas. Syn-D₃ granites display K_m between 17.8 μSI and 186 μSI. This variation is due to the high textural and compositional diversity, including two-mica granites with different relative proportions of muscovite and biotite. Late-D₃ granites are represented by two-mica and biotite-rich granites with calcium plagioclase, with several degrees of post-magmatic alteration, implying iron leaching processes. These processes also promote the crystallization of secondary muscovite, which implied a decrease in the K_m values, and also a wide dispersion of K_m values ranging between 7.3 μSI and 276 μSI. Post-D₃ granites are mostly represented by biotite-rich granites with calcium plagioclase close to I-type granites. This granite group is divided into two subgroups: (i) post-D₃ ilmenite-type granites with K_m values of ca. 113 μSI, typical of biotite-rich granites; and (ii) post-D₃ magnetite-type granites with K_m values between 2078 μSI and 11676 μSI representing magnetite-type granites. In N and Central Portugal, these magnetite-type granites can occur in homogeneous plutons or in composite plutons constituted by ferromagnetic and paramagnetic facies.

As a conclusion, mostly of the granites from northern and central Portugal exhibit average K_m values below 1000 μSI and are characterized by a paramagnetic behavior corresponding to reduced- or ilmenite-type granites. Among the all studied granites, only one pluton showed to be a truly oxidized- or magnetite-type granite, with K_m of the order of 11676 μSI.

Acknowledgements: This work was funded by the Fundação para a Ciência e a Tecnologia (FCT) under UIDB/04683/2020 project.