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Assessing the Magnetic Mineralogy of the Pre-Variscan Manteigas Granodiorite: An Unexpected Case of a Magnetite-Series Granitoid in Portugal

The Early Ordovician Manteigas granodiorite has been characterized as having a magnetic susceptibility with high heterogeneity and values compatible with the presence of ferromagnetic phases, such as magnetite. However, granitoids with magnetite are rare in the Iberian Massif. Therefore, an in-depth study of mineralogical composition was carried out on the Manteigas granodiorite by evaluating its magnetic susceptibility, analyzing the isothermal remanent magnetization (IRM) and the IRM curve, identifying the magnetic coercivity components, evaluating the magnetic grain size and performing a petrographic study. Low concentrations of very small grains of magnetite and/or titanomagnetite have been found in areas with low magnetic susceptibility and with values not compatible with ferromagnetic phases. The petrographic study has explained this phenomenon as a result of the transformation of magnetite into hematite (martitization), indicating a redox reaction. A high concentration of magnetite and/or titanomagnetite, occurring as multidomain crystals, has been found in areas with high magnetic susceptibility. This study indicates that the Manteigas granodiorite has a deep magma origin, and formed under oxidizing conditions since it has magnetite instead of ilmenite, which is an uncommon phenomenon in the Iberian massif.