Low-field magnetic fabrics and mineralogy of subvolcanic dykes from northern Portugal: Magma flow and petrogenetic implications

• Type: Article in International Scientific Journal
• Date: 2025

Authors

Oliveira, AJT (Author) Other; Sant'Ovaia, H (Author) FCUP; H. C. B. Martins (Author) FCUP; Font, EC (Author) Other

Journal

Title: Journal Of Iberian Geology

ISSN: 1698-6180

Publisher: Springer Nature

Indexing

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Other information

Authenticus ID: P-017-YG7

DOI: 10.1007/s41513-024-00276-4

Abstract (EN): The Permo-Carboniferous hypabyssal magmatism is a well-recorded event throughout southwestern Europe, associated with the post-collisional setting that followed the Variscan orogeny. In northern Portugal, this episode is represented by porphyry, lamprophyre, and dolerite dykes, many of which were studied within the scope of this work using methodologies such as isothermal remanent magnetization, frequency-dependence of the magnetic susceptibility, thermomagnetic analyses, and low-field anisotropy of magnetic susceptibility. The main goals are to report the respective magnetic mineralogy and fabrics, to provide insights into magma flow directions and emplacement conditions, and to complement our understanding about the petrogenetic evolution of the aforementioned subvolcanic rocks. The AMS fabrics of most leucocratic lithotypes is normal and undisturbed by tectonic deformation. Such observation is compatible with the average low anisotropy degrees (Ppara(%) = 0.92-4.28%) implying passive emplacement of the melts. Kfd measurements indicate that superparamagnetic particles are likely to exist in rare cases, inducing low bulk susceptibilities. Furthermore, inverse fabrics are probably associated with deuteric and/or post-magmatic alterations. Considering the results of the rock magnetism tests, magnetite is present in all specimens (felsic, intermediate, and mafic). Magnetite is dominantly primary in the mafic lithologies, but mostly secondary in the felsic dykes, as well as generated from exsolutions from iron-bearing silicates. Nonetheless, the presence of primary magnetite in the porphyries and microgranites cannot be completely discarded due to possible magma mixing, as suggested by the coexistence of mantled and non-mantled feldspars, as well as previous geochemical and isotopic evidence. El magmatismo hipabisal del P & eacute;rmico-Carbon & iacute;fero es un evento bien registrado en todo el suroeste de Europa, asociado con el contexto p & oacute;s-colisional que sigui & oacute; a la orogenia Varisca. En el norte de Portugal, este episodio est & aacute; representado por diques de p & oacute;rfido, lampr & oacute;firo y dolerita, muchos de los cuales se estudiaron en esto trabajo utilizando metodolog & iacute;as como la magnetizaci & oacute;n remanente isot & eacute;rmica, la variaci & oacute;n de la susceptibilidad magn & eacute;tica con la frequencia, los an & aacute;lisis termomagn & eacute;ticos y la anisotrop & iacute;a de bajo campo de la susceptibilidad magn & eacute;tica. Los objetivos principales son informar sobre la mineralog & iacute;a y las f & aacute;bricas magn & eacute;ticas respectivas, proporcionar informaci & oacute;n sobre las direcciones del flujo magm & aacute;tico y las condiciones de emplazamiento, y complementar nuestra comprensi & oacute;n sobre la evoluci & oacute;n petrogen & eacute;tica de las rocas subvolc & aacute;nicas mencionadas anteriormente. Las f & aacute;bricas ASM de la mayor & iacute;a de los litotipos leucocr & aacute;ticos son normales y no se ven perturbadas por la deformaci & oacute;n tect & oacute;nica. Tal observaci & oacute;n es compatible con los bajos grados de anisotrop & iacute;a promedio (Ppara(%) = 0,92-4,28%), lo que implica un emplazamiento pasivo de los melts. Las mediciones de Kfd indican que es probable que existan part & iacute;culas superparamagn & eacute;ticas en casos raros, lo que induce bajas susceptibilidades en masa. Adem & aacute;s, las f & aacute;bricas inversas probablemente est & eacute;n asociadas con alteraciones deutericas y/o p & oacute;s-magm & aacute;ticas. Teniendo en cuenta los resultados de los estudios de magnetismo de roca, la magnetita est & aacute; presente en todos los espec & iacute;menes (f & eacute;lsicos, intermedios y m & aacute;ficos). La magnetita es predominantemente primaria en las litolog & iacute;as m & aacute;ficas, pero principalmente secundaria en los diques f & eacute;lsicos, as & iacute; como generada a partir de exsoluciones de silicatos que contienen hierro. No obstante, la presencia de magnetita primaria en los p & oacute;rfidos y microgranitos no se puede descartar por completo debido a la posible mezcla de magma, como lo sugiere la coexistencia de feldespatos mantados y no mantados, as & iacute; como la evidencia geoqu & iacute;mica e isot & oacute;pica previa.

Language: English

Type (Professor's evaluation): Scientific

No. of pages: 40