Disclosing the redox conditions in Central Portugal magmatism: the Manteigas granodiorite case study

The presence of magnetite in granitoids is usually rare in the Iberia Peninsula, but can occur depending on the crystal fractionation processes and redox conditions. Here we studied the Manteigas granodiorite in order to characterize and quantify the nature and abundance of ferromagnetic minerals by using petrographic, Isothermal Remanent Magnetization (IRM) curves, and Frequency-dependent Susceptibility (KfD%) experiments. Igneous rocks of the Variscan age are particularly abundant in Central Portugal, including the Manteigas granodiorite that crops out in the Serra da Estrela region (Central Iberian Zone), Portugal. The Manteigas granodiorite is classified as a medium- to coarse-grained slightly porphyritic biotite rock and was dated by U-Th-Pb methods on zircon at 481.1 ± 5.9 Ma. Petrographic studies show that Manteigas is mainly composed of quartz, Ca-plagioclase, K-feldspar, and biotite. As accessory minerals, apatite, chlorite, magnetite ± hematite, and zircon are identified. Muscovite is rare and most of secondary origin. Monazite, sphene-leucoxene, and brookite-anatase are also present but in minor amounts. Microstructures indicate a slight deformation that is reflected in the undulatory extinction in quartz, microfractures in K- feldspar, and curved biotites. Sometimes microfractures in feldspar are filled by later Fe-oxide/hydroxide. Values of magnetic susceptibility (K_m) indicate that the Manteigas granodiorite belongs to the magnetite-type rocks, with K_m values higher than 1.9 x 10^-3 SI (1.9 x 10^-3 SI < K_m < 188.37 x 10^-3 SI). This is consistent with oxidizing conditions in the magma genesis [1]. Furthermore, the oxygen isotope composition (δ¹⁸O) measured on whole-rock samples ranges between 8.8 ‰ and 8.9 ‰ [1,2], which suggests a mantle contribution. The analysis of IRM data through the Cumulative Log-Gaussian (CLG) function with the software developed by Kruiver et al. [3] indicates the presence of a single ferromagnetic s.l. component, with values of mean coercivity (Log B1/2) of 1.61 mT and dispersion parameter (DP) of 0.36, typical of magnetite [4]. The value of the IRM at saturation (SIRM) of 18 A/m indicates a significant contribution of magnetite. We also fitted the IRM curve by using a Skewed Generalized Gaussian function with the MaxUnmix software [5] and obtained similar results. Kfd% analyses were conducted in four samples. Values of the Kfd% are inferior to 6%, suggesting a very weak contribution of superparamagnetic particles. Acknowledgments: The first author is financially supported by UIDP/04683/2020 project (FCT-Portugal). This work is also supported by national funding awarded by FCT under UIDB/04683/2020 project. References: [1] Sant’Ovaia, H., Olivier, P., Ferreira, N., Noronha, F., Leblanc, D. 2010. J. Struct. Geol. 32, 1450-1465. [2] Neiva, A., Williams, I.S., Ramos, J.M.F., Gomes, M.E.P. Silva, M.M.V.G., Antunes, I.M.H.R. 2019. Lithos 111, 186-202. [3] Kruiver, P.P., Dekkers, M.J., Heslop, D. 2001. Earth Planet. Sci. Lett. 189, 269–276. [4] Egli, R., 2003. J. Geophys. Res. 108, 2081. [5] Maxbauer, D.P., Feinberg, J. M., Fox, D.L. 2016.  Comput. Geosci. 95, 140–145.