Abstract
The Lamas de Olo Pluton, in northern Portugal, is a post-tectonic pluton located in the northern part of the Central Iberian Zone of the Iberian Variscan belt. It is a composite massif comprising different granites: Lamas de Olo, Alto dos Cabeços and Barragem. Magnetic susceptibility was measured in 349 oriented cores from 48 sampling sites across the pluton. The measured magnetic susceptibility (Km) values range between 21 µSI (or 21 × 10− 6 SI) and 2769 µSI for all studied granites. The application of the geostatistical methodologies leads to a more detailed mapping of the magnetic susceptibility data and provided a quantification of its spatial dispersion. Kriging interpolation allows the visualization of two different areas in the Lamas de Olo Pluton: (1) areas with higher magnetic susceptibility values, typical of magnetite-bearing granites (Km ≥ 1200 µSI) and (2) areas with lower values, which correspond to ilmenite-type behavior granites (21 µSI < Km < 1200 µSI), showing the heterogeneity of their granites, with a predominance of ilmenite-type granites. Alto dos Cabeços and Barragem are classified as ilmenite-type granites and Lamas de Olo granite presents both behaviors. The mineralogical and geochemical study of the Lamas de Olo granite with different behaviors does not show significant differences between the two types, allowing us to conclude that the Lamas de Olo granite was originated by the same magma and its mineralogical composition varied during the rise and emplacement of the magma implying the alteration of magnetite into hematite.
Resumen
El plutón de Lamas de Olo, en el norte de Portugal, es un plutón postectónico ubicado en la parte septentrional de la Zona Centro Ibérica del macizo Varisco Ibérico. Es un macizo compuesto que engloba diferentes granitos: Lamas de Olo, Alto dos Cabeços y Barragem. La susceptibilidad magnética se midió en un total de 349 muestras orientadas provenientes de 48 estaciones distribuidas por todo el plutón. Los valores de susceptibilidad magnética (Km) oscilan entre 21 μSI (ó 21 × 10-6 SI) y 2769 μSI en los granitos estudiados. La aplicación de herramientas geoestadísticas permite una cartografía más detallada de la susceptibilidad magnética y proporciona una cuantificación de su dispersión espacial. La interpolación por Kriging permite la visualización de dos áreas diferentes en el plutón de Lamas de Olo: (1) áreas con valores de susceptibilidad magnética más altas, típicos de granitos que contienen magnetita (Km ≥ 1200 μSI) y (2) áreas con valores más bajos, que corresponden a granitos de comportamiento tipo ilmenita (21 μSI < Km < 1200 μSI). Por otra parte los granitos de Alto dos Cabeços y Barragem se clasifican como tipo ilmenita. Esta variabilidad muestra la heterogeneidad de los granitos en los que predominan los de tipo ilmenita. Los estudios mineralógicos y geoquímicos del granito de Lamas de Olo no muestran diferencias significativas entre los dos tipos, lo que nos permite concluir que el granito de Lamas de Olo se originó del mismo magma y su composición mineralógica varió durante el ascenso y emplazamiento del magma conllevando la alteración de la magnetita a hematita.
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Acknowledgements
The first author was financially supported by SFRH/BD/109693/2015 (Fundação para Ciência e a Tecnologia-Portugal). The authors acknowledge funding from FCT (Fundação para a Ciência e a Tecnologia) under project UIDB/04683/2020 and from ESMIMET, an INTERREG Spain-Portugal POCTEP project. We thank Helena Brites Martins for the help in interpreting geochemical results. The authors are grateful to the anonymous referee whose comments greatly helped to improve the manuscript.
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Cruz, C., Góis, J., Sant’Ovaia, H. et al. Geostatistical approach to the study of the magnetic susceptibility variation: Lamas de Olo Pluton case study. J Iber Geol 46, 279–289 (2020). https://doi.org/10.1007/s41513-020-00128-x
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DOI: https://doi.org/10.1007/s41513-020-00128-x