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<p>The “Foz do Douro Metamorphic Complex” (FDMC; “Finisterra Terrane”) represents a mid-crustal section affected by intense deformation related to the Porto-Tomar-Ferreira do Alentejo shear zone. Magnetic fabrics of deformed biotite orthogneisses (BG) and garnet leucocratic orthogneisses (GG) of the newly defined sodium-rich series of the FDMC have been studied using the Anisotropy of Magnetic Susceptibility (AMS) technique. AMS and geochemical signatures of these orthogneisses allowed to infer the nature of the magmatic rocks from which they derived and revealed a change in the fugacity conditions of the granitoid magmas. BG derived from “magnetite-type” granitoids, generated from magmas having a contribution of materials formed at greater depth (upper mantle/lower crust), under oxidizing conditions. On the other hand, the protolith of the GG was generated at lower depths than the protolith of the BG, under reducing conditions. AMS data demonstrates the control of the magnetic anisotropy by strain on both lithologies. AMS results combined with structural data suggest that the magnetic foliation is parallel to the XY-plane of the finite strain ellipsoid of each orthogneiss. The parallelism of the magnetic and tectonic foliations and lineations and the variability of the lineations suggest that the sodium-rich orthogneisses were subject to strain controlled by transpressional structures compatible with the presence of a regional dextral NNE-SSW shear zone.</p>

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Deciphering deformation and magmatism along Finisterra Terrane: AMS petrofabric and geochemistry of sodium-rich orthogneisses of the “Foz do Douro Metamorphic Complex”

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  • 发表日期：

    2025年6月1日

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  • Mónica Sousa,
  • Helena Sant’Ovaia,
  • Fernando Noronha

摘要

The “Foz do Douro Metamorphic Complex” (FDMC; “Finisterra Terrane”) represents a mid-crustal section affected by intense deformation related to the Porto-Tomar-Ferreira do Alentejo shear zone. Magnetic fabrics of deformed biotite orthogneisses (BG) and garnet leucocratic orthogneisses (GG) of the newly defined sodium-rich series of the FDMC have been studied using the Anisotropy of Magnetic Susceptibility (AMS) technique. AMS and geochemical signatures of these orthogneisses allowed to infer the nature of the magmatic rocks from which they derived and revealed a change in the fugacity conditions of the granitoid magmas. BG derived from “magnetite-type” granitoids, generated from magmas having a contribution of materials formed at greater depth (upper mantle/lower crust), under oxidizing conditions. On the other hand, the protolith of the GG was generated at lower depths than the protolith of the BG, under reducing conditions. AMS data demonstrates the control of the magnetic anisotropy by strain on both lithologies. AMS results combined with structural data suggest that the magnetic foliation is parallel to the XY-plane of the finite strain ellipsoid of each orthogneiss. The parallelism of the magnetic and tectonic foliations and lineations and the variability of the lineations suggest that the sodium-rich orthogneisses were subject to strain controlled by transpressional structures compatible with the presence of a regional dextral NNE-SSW shear zone.