Variscan Plutonism in the Geodynamic Evolution of the Central Iberian Zone of Portugal: Castelo Branco Pluton as Another Piece of the Puzzle

Abstract

A multidisciplinary analysis of the Pennsylvanian Castelo Branco pluton of Central Iberian Zone (Iberian Variscan belt) was made, focusing on its magnetic behavior and fabric, microstructures, microfractures, and radiometric and gravimetric anomalies. The findings reveal that the Castelo Branco pluton is an ilmenite-type granite, characterized by low magnetic susceptibility values. The petrographic observations and high-temperature solid-state deformation indicate that pluton was emplaced during the latest compression phase (D₃) of the Variscan tectonic regime. Magnetic fabric and gravimetric data show that the Castelo Brano pluton has a flat-shaped geometry with a depth of approximately 2–3 km, a feeding zone corresponding to NE-SW-trending regional faults, and that its fabric is oriented parallel to the NW-SE-trending regional foliation of the host rocks. The concentric magnetic foliation in the Alcains granite suggests an earlier ascent and emplacement compared to the Rio de Moinhos and S. Miguel da Acha granites, with Alcains demonstrating a laccolithic shape indicative of significant upward force. The ascent pathways of the different granites seem to have occurred along pre-existing NE-SW faults. The Castelo Branco pluton displays zoned nesting, with fluid inclusion planes indicating NNE-SSW to NE-SW and ENE-WSW trends in biotite-rich granites, and NNE-SSW to NE-SW and ESE-WNW trends in two-mica granites. Structural alignments in the study area show both NE-SW and NW-SE trends. The NE-SW faults and thrust faults are supported by residual gravimetric anomaly data, and NW-SE alignments are evident in magnetic fabric and regional folded structures. These findings enhance our understanding of the geodynamic processes influencing the Variscan plutonism in the Central Iberian Zone, positioning the Castelo Branco pluton as a key component in this geological puzzle.

1. Introduction

The ascent and emplacement of granite plutons are important geological processes fundamental to the understanding of crustal geodynamics (e.g., [1,2,3,4]). Given the importance of granites in understanding the evolution of the crust, many studies have been carried out to establish relationships between magmatic processes, granite geometry, and the relationship between granite and host rocks to understand the role of regional structures in the magma ascent and emplacement, especially the role of shear zones and faults (e.g., [2,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19]).

Granitic bodies can be used as kinematic markers for the detailed description of the tectonic evolution of an orogenic process (e.g., through the analysis of magnetic fabric, microstructures, and microfractures [20,21]). The Central Iberian Zone (CIZ) stands out within the Variscan chain for its extensive outcroppings of granitic bodies and remarkable typological diversity [22]. In this sense it is pertinent to discuss the emplacement kinematics and the chronological position of the plutons relative to the Variscan deformation phases. Also of great importance is the relationship between Variscan granites and critical metals mineralization, which for Iberia is mostly controlled by regional Variscan structures and close spatial relationship with granites (e.g., [23,24,25,26,27]). Several works concluded that the main factor controlling the type of mineralization associated with the nearby granite is the type of iron oxide that this granite contains (e.g., [28,29]); more specifically, the presence of magnetite or ilmenite, which is due to the magma redox conditions.

In the present study, we focus on the Castelo Branco (CB) Variscan pluton, located in the CIZ. The pluton hosts numerous W-Sn occurrences in quartz veins intersecting the pluton [30]. This pluton has been subject to several studies, including geochemical characterization of its granites and geochronological dating (e.g., [31,32]).

To investigate the fundamental processes of the emplacement of this pluton, we drew on studies using anisotropy of magnetic susceptibility (AMS). AMS studies may be used to characterize and measure weak anisotropic fabrics (foliations and lineations), either magmatic or solid-state ones, which can describe the fabric of granite, to identify the constraints leading to the granite intrusion, and finally to link these features to the regional tectonics (e.g., [33,34,35,36]). Also essential for the identification of the iron oxide minerals are the studies of isothermal remanent magnetization (IRM), which provide information about the magnetic carriers. The analyses of IRM acquisition curves allow the estimation of the characteristic coercivity of the ferromagnetic minerals [37].

To complement the studies of petrophysical nature, petrographic studies, particularly focusing on microstructures and their implications for magmatic rheological evolution, are essential for developing a comprehensive emplacement model. Furthermore, to understand microfracture geometry and brittle stress constraints, microscopic observation of fluid inclusion planes (FIP) geometry serves as a powerful analytical tool (e.g., [38,39,40]).

Radiometric imaging offers a frozen image of the last stage of the magmatic building of plutons and proves statistically reliable in distinguishing different rock units [41,42]. Variations in radiation concentration can be due to the presence of different lithologies or different rates of rock alteration.

Gravimetry measures variations in the Earth’s gravitational field due to differences in subsurface rock density. This technique utilizes gravimeters to detect changes in gravity acceleration caused by less dense bodies, such as granite, within denser host rocks, like the metasedimentary formations. It serves as a precise tool for determining the shape of granitic intrusions and inferring emplacement models (e.g., [5,6,8,10,11,12,15,16,17,43,44,45]).

Finally, computed by gravimetric data and complemented by AMS results, a 2D geological profile can constrain the in-depth geometry and sustain the emplacement model (e.g., [46,47,48]). The modeling helps to infer the shape of the pluton and allows us to classify the pluton as a flat- or wedge-shaped pluton. Flat-shaped plutons are, in general, 3–4 km in depth and extend in all horizontal directions with a gently sloping floor towards the root zones. In contrast, wedge-shaped plutons are thicker (>10 km) and more elongated in one direction with fewer root zones than the flat-shaped ones [10,43].

In summary, this paper unveils the outcomes of a comprehensive study involving petrophysical and magnetic analysis (AMS and IRM), microstructural (petrography) and microfractures (FIP) examination, and geophysical studies (radiometric and gravimetric) of the CB pluton. It further integrates these findings into a discussion concerning the late orogenic evolution of the Iberian Varican belt.

2. Geological Setting

2.1. General Framework

The Variscan orogeny resulted from the oblique collision between Laurasia and Gondwana during the Late Paleozoic [49,50,51]. The Iberian Belt is subdivided into several geotectonic zones: the Cantabrian Zone (CZ), the West Asturian Leonese Zone (WALZ), the Galicia Trás-os-Montes Zone (GTMZ), the CIZ, the Ossa Morena Zone (OMZ), and the South Portuguese Zone (SPZ; [52,53,54]).

In the Northwest of the Iberian Variscan belt, three main ductile phases of deformation, D₁, D₂, and D₃, were described with ages ca. 360–337 Ma, 337–320 Ma, and 320–310 Ma, respectively (e.g., [22,55,56,57,58,59]). After D₃, the deformation regime is essentially brittle [60,61,62].

The granites have been divided into two main groups based on geological, petrographic, and geochemical studies [22,55,63,64,65]: (i) two-mica granites (muscovite > biotite or muscovite = biotite)—usually leucocratic granites with primary muscovite and biotite, syn-tectonic with the third phase of deformation (syn-D₃), mesocrustal in origin and are usually emplaced in the nucleus of regional D₃ folds; (ii) biotite granites (biotite > muscovite)—originate at lower crust, and if they contain muscovite, it is mainly from secondary processes; the intrusion and distribution of biotite granites can be either syn-D₃ (321–312 Ma), late-D₃ (312–305 Ma), late- to post-D₃ (ca. 300 Ma), or post-D₃ (<299 Ma).

The study area is in the district of Castelo Branco, in the interior of Portugal, approximately 50 km from the border with Spain. Portugal is a country in southern Europe. The CB pluton cuts the metasedimentary rocks from the Ediacaran Beiras Group, of the Schist-Greywacke Complex, the Ordovician Armorican Quartzite, and the Ordovician Oledo pluton [66,67]. The Beiras Group metagraywackes and schists are unconformably overlain by the Armorican Quartzite, representing the Ordovician, displaying intense folding. The main trend of the Beiras Group metasedimentary rocks is NW-SE, parallel to the Castelo Branco monoclinal (to the west of the CB pluton [66,67]). At the east, the Variscan CB pluton contacts with the Oledo-Idanha-a-Nova Ordovician pluton, emplaced within a short period of time at 480–479 Ma [68]. Furthermore, the Ordovician Fundão pluton dated at ca. 478 Ma [69], and the Penamacor with an age of ca. 311–308 Ma [68], occurs to the NNW and NE of the CB pluton, respectively. The sedimentary cover is composed of Paleogene to Neogene sedimentary rocks from the Beira Baixa group [67] (Figure 1).

The general deformation of the area under study, particularly concerning the structures within the metasedimentary rocks, is essentially the result of the action of Variscan deformation. Variscan deformation produced strong folding of all the pre-Mesozoic formations, with the development of a S₁ cleavage characterized by its pervasive nature throughout the region. These NNW-SSE to NW-SE-trending folds, with sub-horizontal axes and vertical axial planes, define synclines (e.g., Vila Velha de Rodão) or monoclines (e.g., Castelo Branco monocline) [66,70]. In clear association with D₁ folds, NE-SW-oriented overthrusts are also present. To the southeast of the CB pluton, the NE-SW Pônsul fault, reactivated as a reverse fault during the late Tortonian to Zanclean age (Neogene Period; [71]), puts in contact the formations of the Beiras Group with Meso-Cenozoic formations [66,70].

In the area, several W-Sn occurrences are present, hosted by lenticular quartz veins that crosscut the Beiras Group and the granites of the CB pluton, defining the NW-SE-trending Segura-Góis metallogenic alignment [72]. One of these ore deposits, hosted in CB pluton, constituted a mining area, Mata da Rainha Mine, where tungsten was exploited during World War II [30].

2.2. Castelo Branco Granite Pluton

The CB pluton covers an area of 390 km² and consists of peraluminous Variscan Pennsylvanian granitic rocks. The pluton is composed of six granites: (i) Alcains, a fine- to-medium-grained muscovite > biotite granite, that forms the core of the pluton; (ii) Rio de Moinhos, fine-to-medium-grained slightly porphyritic biotite > muscovite granite; (iii) S. Miguel da Acha (or Castelo Branco), medium-to-coarse-grained porphyritic biotite > muscovite granite with cordierite, defined as the external zone of the pluton; (iv) Soalheira, a coarse-grained muscovite > biotite granite with sulfides; (v) Castelo Novo, a medium-grained muscovite = biotite granite in a small outcrop between Soalheira and S. Miguel da Acha granites, and (vi) Póvoa da Palhaça, a fine-to-medium-grained muscovite granite in the N of the pluton [69] (Figure 1 and Figure 2).

Antunes [31] identified different granites on the CB pluton, which are not always identical to the cartographic contours defined on the Geologic Map of Portugal (scale 1: 200,000) [69]. Antunes [31] proposed that the CB pluton was composed of five different granites: (i) G1, a fine-to-medium-grained muscovite > biotite granite (corresponding to the Alcains granite) that forms the core of the pluton and is surrounded by (ii) G2, a fine- to-medium-grained slightly porphyritic biotite > muscovite granite, encircled by, and grading to (iii) G3, a medium-to-coarse-grained porphyritic biotite > muscovite granite (both G2 + G3, roughly corresponded to the Rio de Moinhos granite), grading into (iv) G4, a medium-to-coarse-grained porphyritic muscovite = biotite granite (which is identical to the S. Miguel da Acha granite), which contains equal amounts of biotite and muscovite, and finally, (v) G5, a coarse-grained muscovite > biotite granite (comparable to both Soalheira, or Lardosa, granite, and Castelo Novo granite). The northernmost area of the pluton (where the Póvoa da Palhaça granite outcrops) is not considered in the Antunes [31] work.

The granitic rocks from the CB pluton are peraluminous with a molecular ratio Al₂O₃/(CaO + Na₂O + K₂O) of 1.1 to 1.3. Their normative corundum content ranges from 2.7 to 3.8 and the rocks are mainly magnesian and alkali calcic [32]. The geochemical and isotopic data indicate a crustal origin of the CB pluton, probably from partial melting of heterogeneous pelitic host rock. There is also evidence for derivation from different sources, but also fractional crystallization linking some of the internal plutonic phases [31,32].

Antunes et al. [32] performed U–Pb isotopic analyses of zircon and monazite from representative samples of G1 (or Alcains), G2 (or Rio de Moinhos), and G5 (or Soalheira) granites using the ID-TIMS method. The crystallization of granites G1, G2, and G5 was estimated at 310 ± 1 Ma, 310.5 ± 1 Ma, and 309.7 ± 1 Ma, respectively. Thus, the CB pluton, based on field relations with the host rocks has been classified as late- to post-D₃ pluton [69], but according to geochronology is considered a late-D₃ pluton (Pennsylvanian) [32].

Based on geochemical data, Antunes et al. [32] proposed that the CB pluton shows an uncommon zoning resulting from a pulse of granite magma G1 in the core of the pluton, followed by a pulse of granitic magma G2 surrounding G1 and differentiating to originate granites G3 and G4 outwards. Another distinct pulse of granitic magma originated the G5 outwards.

The intrusion of the CB pluton produced a contact metamorphic aureole up to 2 km wide of altered pelitic hornfels and spotted schists. The pluton is crosscut by quartz veins and mafic NE-SW-trending dykes [32].

3. Sampling and Analytical Methods

3.1. Sampling

Sampling of the CB granites was performed according to the road accesses and the presence of fresh and in situ outcrops. Oriented rock cylinders were sampled with a portable drill machine.

For AMS studies, all samples need to be oriented according to a geographical framework. Accordingly, at each site, four oriented cores (25 mm in diameter and 60–70 mm in length) were collected in situ with a portable drill machine. In the laboratory, the samples were cut perpendicular to the axis, obtaining individual cylinders with dimensions of 22 mm in height. Each location was represented by 8–9 oriented cylindrical samples.

The AMS study was based on 607 rock cylinders from 84 sampling sites roughly distributed in the CB pluton (Figure 3). A total of 16 cylinders were selected to make thin sections for petrographic studies, 11 samples were used for IRM studies, and 7 for the study of fluid inclusion planes (Supplementary Materials).

3.2. Petrography and Microstructural Observations

For petrography and microstructural observations, samples from the different granites of the CB pluton were selected to make oriented thin sections (Supplementary Materials) to find possible changes in microstructure deformation patterns. The petrographic studies were performed using the LEICA-ICC50HD petrographic polarizing microscope. The device has associated image editing software, LAS v4.6, allowing the capture of photomicrographs with a scale bar. The petrographic studies were undertaken at the Department of Geosciences, Environment, and Spatial Planning (DGAOT) of the Faculty of Sciences, University of Porto (FCUP) and Institute of Earth Sciences—Porto Pole (ICT-Porto).

3.3. Petrophysical Studies

3.3.1. Anisotropy of Magnetic Susceptibility

The relationship between the magnetization induced in a material M and the external field H is defined as M = K × H, where K is the magnetic susceptibility. Because M (per unit volume) and H have the same units, K is dimensionless (in the units of the International System, SI). If the material is isotropic, the magnetic susceptibility is scalar; if the material is anisotropic, magnetic susceptibility is represented by a second-order, symmetric tensor, known as the anisotropy of magnetic susceptibility tensor, and is represented as M_i = K_ij × H_j (i, j = 1, 2, 3), where M_i represents the magnetization in the i direction and H_j represents the magnetic field in the j direction [33,36,73,74].

Magnetic susceptibility reflects the mineral composition of the rock, which can have diamagnetic (e.g., quartz, feldspar), paramagnetic (e.g., biotite, muscovite, cordierite), or ferromagnetic behavior (e.g., magnetite, hematite).

The AMS results describe the variation of magnetic susceptibility of direction within the rock sample and represent the contribution of all the rock-forming minerals [75]. AMS data allowed establishing the magnetic susceptibility tensor, which is represented by a triaxial ellipsoid, where K₁ ≥ K₂ ≥ K₃ (or K_max ≥ K_int ≥ K_min). The intensities and orientations of the three axes are the tensorial means of the k₁ ≥ k₂ ≥ k₃ axes of measured specimens.

The magnetic fabric is defined by the magnetic lineation (parallel to the long axis of the mean ellipsoid orientation average, K₁) and the magnetic foliation (plane perpendicular to the orientation of K₃, the short axis). The magnetic anisotropy degree (P%) corresponds to (K₁/K₃ − 1) × 100. To describe the shape of the anisotropy of the magnetic susceptibility ellipsoid, the shape parameter, T, expressed by 2 × ln (K₂/K₃)/ln(K₁/K₃) − 1 [76] is computed.

AMS measurements were performed using a KLY-4S Kappabridge susceptometer Agico model (Czech Republic) from the ICT-Porto. For each site, the ANISOFT 5 program package [77] enabled us to calculate several parameters: magnetic susceptibility (K), magnetic anisotropy (e.g., P%), and shape parameter (T). The mean susceptibility K_m was also calculated, which is the mean of the eight (or more) individual arithmetic means (k1 + k2 + k3)/3.

3.3.2. Isothermal Remanent Magnetization Analysis

The remanence acquired by a sample exposed to a direct magnetic field at atmospheric temperature is called isothermal remanent magnetization (IRM). The IRM acquisition curves are required to estimate the characteristic coercivity of ferromagnetic minerals. The trend of the acquisition curves depends on the relative concentration of low-coercivity minerals, like magnetite, and high-coercivity minerals, like hematite (e.g., [37]).

The IRM values were measured using a Molspin spinner magnetometer and fields were imparted with a Molspin magnetometer from the Paleomagnetism Laboratory from the Department of Earth Sciences, University of Coimbra. Selected samples from the several granites (Supplementary Materials) were magnetized, firstly in the same direction from 12.5 mT up to 1T, and secondly in the opposite direction, also from 12.5 mT up to 1T. The isothermal remanent magnetization at 1T was defined as the saturation isothermal remanent magnetization (SIRM).

Two additional samples were examined (Supplementary Materials), having undergone prior demagnetization through an alternating field demagnetizer coupled with an anhysteretic magnetizer (Molspin), with each step of demagnetization measured until reaching 100 mT. Then, we applied a consistent magnetic field to the samples in the same direction using a high-field magnetizer (IM-10, Manufacturer by ASC Scientific in the Narragansett, RI, USA) to generate the IRM acquisition curve. The resulting remanence was measured at each induction step until saturation, utilizing the JR-6 Spinner magnetometer from AGICO.

3.4. Microfracturing

Fluid inclusion planes (FIPs) result mainly from the healing of former open cracks and are fossilized fluid pathways (e.g., [38,39,40]). FIPs occur in sets and are formed as mode I cracks materializing the σ1–σ2 plane [78]. In mode I cracks, the growth of the fracture is parallel to the tip line, thus displacing the two separate blocks in a direction perpendicular to the fracture surface. However, some authors consider the existence of curved irregular microfractures that are mode II cracks (Lawn and Wilshaw 1975 in [79]). In mode II cracks, the fracture growth is perpendicular to the tip line, and the relative displacement between the blocks is parallel to the fracture surface. In case of mode III cracks, the fracture grows parallel to the tip line and with relative displacement of the fractured blocks. Mode I microfractures do not cut the mechanical continuity of the mineral grains and do not exhibit shear evidence, as is occasionally visible in the mode II FIPs [40].

FIP study offers valuable insights into local rock stress [39] while also establishing connections between tectonic events and fluid percolations. Nevertheless, no straightforward conclusions can be made, and the results have to be considered in conjunction with other methodologies. FIP can be classified according to the terminology proposed by Simmons and Richer (1976) and Kranz (1983) in [80] as transgranular, intergranular, or intragranular.

Selected oriented rock cylinders from different granite types of the CB pluton (Supplementary Materials), used for the AMS studies, were chosen, and 200 μm doubly polished wafers, cut perpendicular to the cylinders, were prepared for the FIP studies. The strike of the FIP was determined on oriented thick sections (related to geographic north), using an image analyzer adapted to a transmitted light microscope. In each thick section ca. 50 to 150 planes were measured. The image acquisition analysis and the mapping of the FIP were performed using the software GeoImagin v.3 ([81], https://home.uevora.pt/~pmn/geoimagin/ (accessed on 12 September 2023)).

FIPs were studied in quartz since this mineral has a rapid rate of healing [82,83], easily trapping fluids as fluid inclusions, which enables a clearer visualization of the microcracks.

3.5. Radiometric Imaging

The radiometric data used in this study were provided by Mineral Resources and Geophysics Unit of the Geology and Mining Laboratory—National Laboratory of Energy and Geology (LGM—LNEG). The total gamma radiation data correspond to an area covering around 20 military sheets of the Carta Militar de Portugal at a scale of 1:25,000. The area includes the CB pluton outcrop and host rocks.

In total, LNEG provided 24,753 radiometric measurements of the total gamma radiation (nGyh⁻¹). These measurements were acquired by a scintillator SAPHYMO SRAT, model SPP2 (Saphymo-PHY, Massy, France). Additionally, regional surveys were carried out using the hand held Exploranium GR-256 (Manufactured by SAIC Exploranium, 6108 Edwards Blvd, Mississauga, ON, Canada) spectrometer, and the Exploranium GR-650 4L coupled to a car [84].

The total gamma radiation, with a broad spectrum, includes contributions from potassium (K), uranium (U), and thorium (Th) [41,42]. The emission of gamma rays through the radioactive decay of natural radioisotopes has proved useful for mapping the boundaries of geological units [85,86]. Thus, gamma-ray surveys have been used to obtain images of the geochemical characteristics of the Earth’s surface and near the surface down to a depth of 30 cm [87]. The variations in the radiation concentration are closely related to the presence of different lithologies or different degrees of alteration [88].

3.6. Gravimetry Architecture and Model of Pluton Emplacement

Gravity anomalies result from the difference in density, or density contrast, between a rock and its surroundings. The sign of the density contrast determines the sign of the gravity anomaly. In the case of granitic plutons, since the densities are generally lower than that of the host rocks (mostly metasedimentary rocks), the gravity anomaly is usually negative (e.g., [5]). Conversely, if the area under investigation displays a high geological heterogeneity (lithologies and structures), this pattern may be slightly different and more complex to interpret.

For the gravimetric study, the gravimetric values from the National Gravity Network were used. In total, gravimetric data from 282 stations located over the pluton and its host rocks were selected for data analysis [89]. The gravimetric data were processed to obtain the Bouguer anomaly values for each gravimetric station. The Bouguer anomaly values were interpolated through minimum curvature with Oasis Montaj 2023.2 software (https://my.seequent.com/products/oasis-montaj/, accessed on 8 May 2024).

To interpret the specific gravity signature related to the granite intrusion, the residual anomaly needs to be calculated [46,47,48,90]. In this study, the residual and regional anomalies were separated using the spectral filtering method, based on polynomial adjustment (according to the first-, second-, and third-order polynomial), through the Oasis Montaj 2023.2 software.

To model the intrusion on the CB pluton, the gravimetric data were combined with available geological mapping, data from fieldwork, petrography, and geochemistry, together with studies of the magnetic fabric and fluid inclusion planes. Considering all the available information, a 2D gravity profile (NW-SE) was produced using the gravity and magnetic modeling software (GM-SYS) incorporated into the Oasis Montaj 2023.2 software.

4. Results

4.1. Petrography, Textures, and Microstructural Features

All the CB granites present a very similar mineralogical composition, and the differences are essentially textural: fine-to-medium-grained muscovite > biotite and muscovite = biotite granites (Alcains, Soalheira, and Castelo Novo granites) have a hypidiomorphic granular texture, while the fine-to-medium and medium-to-coarse-grained biotite > muscovite granites (Rio de Moinhos, and S. Miguel da Acha) present a porphyritic hypidiomorphic granular texture with K-feldspar and occasional plagioclase megacrystals.

The mineral composition of all the CB granitic samples consisted of quartz, perthitic microcline, plagioclase, biotite, muscovite, and tourmaline. Magmatic andalusite, sillimanite, and cordierite were also observed but were restricted to the samples belonging to the biotite-dominant granites. Apatite, zircon, ilmenite, monazite, and rutile were found as accessory phases.

Quartz was anhedral to subhedral typically interstitial, exhibit undulatory extinction, and sometimes showed a slight recrystallization and recovery processes (e.g., subgrain boundaries; Figure 4). The plagioclase was euhedral to subhedral and showed oscillatory zoning. However, more sodic composition was found in intragranular crystals, in rims that surrounded the zoned plagioclase, and in myrmekitic intergrowths located adjacent to the K-feldspar. The formation of albite (under subsolidus conditions) was common near the contacts and within the K-feldspar crystals, which consisted of perthitic microcline. Microcline was present as megacrystals of an earlier growth stage or as interstitial crystals of later formation. They were subhedral or anhedral respectively, crosshatched twinned, and contain inclusions of zoned plagioclase with albite rims, biotite, muscovite, and globular quartz. Biotite was generally subhedral with reddish-brown to pale yellow pleochroism with abundant inclusions of zircon, apatite, some monazite, ilmenite, and rutile. It sometimes formed polycrystalline aggregates and was locally altered to chlorite. Magmatic muscovite (muscovite I) was characterized by subhedral crystals, with some undulatory extinction sometimes overgrowing biotite, K-feldspar, and plagioclase, and showed a random orientation. Muscovite crystals with irregular shapes were also observed and were formed below solidus temperature (secondary muscovite or muscovite II). Andalusite occurred in euhedral crystals included in muscovite. Sillimanite occurred as needles included in muscovite, in andalusite, and in plagioclase. Cordierite, only found in the biotite-rich granites, occurred as anhedral crystals partially included in muscovite I and was altered into pinite. Tourmaline was subhedral to anhedral, occurring mainly in slightly zoned crystals and partially replacing micas.

Microstructures have been examined to determine the degree of deformation and the physical state of the rock that prevailed during fabric development in the CB granites.

The microstructures observed in thin sections of the studied granitoids were similar in all granites: (i) magmatic microstructures, like undulatory extinction in quartz (Figure 4a–c) and (ii) medium-to-high-temperature post-magmatic (or subsolidus) microstructures, such as a chessboard pattern (Figure 4c) and some grain boundary migration in quartz grains (Figure 4d), kinks in muscovite and in biotite (Figure 4e–g), and also the bending of plagioclase twin planes were frequent (Figure 4h).

Irregular grain shapes and bowed grain boundaries were often observed in quartz, and are characteristic of a high mobility of the grain boundary at high temperature. Quartz grains also presented a chessboard-like texture indicating both ‹a› and ‹c› dislocation slip occurring during high-temperature (>600°) deformation (e.g., [14,91]). Some K-feldspars were affected by myrmekites, and most of the biotite and muscovite grains showed kinking or undulatory extinction microstructures, which suggest plastic deformation.

4.2. Anisotropy of Magnetic Susceptibility Results

4.2.1. Bulk Susceptibility, Anisotropy Degree, and Shape Parameter

The K_m values ranged between 7.3 μSI and 186.0 μSI for all rocks (

Table 1. AMS data for the sampling sites of the CB pluton. K_m: mean magnetic susceptibility (µSI); P%: anisotropy degree (%); T: shape parameter; K₁: long axis of the mean ellipsoid orientation; K₃: short axis of the mean ellipsoid orientation; Tr., Pl.: trend (°), plunge (°); N: number of specimens.

Sites	Lithology	Km (μSI)	P (%)	T	K1		K3		Foliation	N
					Tr.	Pl.	Tr.	Pl.
CB1	Biotite-rich granite	64.8	6.4	0.466	133	72	259	12	N169°, 78° NE	13
CB2	Two-mica granite	35.0	5.5	0.386	24	38	283	13	N13°, 77° SE	11
CB3	Biotite-rich granite	69.3	4.8	0.316	68	46	244	46	N154°, 44° NE	14
CB4	Biotite-rich granite	71.8	4.7	0.391	341	77	204	5	N114°, 85° NE	9
CB5	Biotite-rich granite	87.9	3.4	0.069	341	50	234	21	N144°, 69° NE	8
CB6	Biotite-rich granite	125.1	5.6	0.569	51	81	250	11	N160°, 79° NE	11
CB7	Two-mica granite	56.4	8.3	0.595	163	54	254	1	N164°, 89° NE	8
CB8	Biotite-rich granite	65.8	3.2	−0.194	64	7	181	71	N91°, 19° N	9
CB9	Biotite-rich granite	85.6	1.9	0.191	317	57	96	16	N6°, 74° W	8
CB10	Two-mica granite	53.3	13.2	0.382	278	15	159	62	N69°, 38° NW	10
CB11	Two-mica granite	52.6	5.6	0.097	188	31	15	54	N105°, 36° SW	6
CB12	Two-mica granite	48.7	9.8	0.277	245	56	41	39	N131°, 51° SW	7
CB13	Biotite-rich granite	121.7	6.2	0.264	148	76	43	8	N133°, 82° SW	9
CB14	Biotite-rich granite	77.1	5.2	−0.026	154	48	287	26	N17°, 64° SE	8
CB15	Biotite-rich granite	97.6	4.8	0.336	132	19	225	12	N135°, 78° NE	2
CB16	Biotite-rich granite	93.4	8.7	0.482	295	82	113	8	N23°, 72° NW	8
CB17	Biotite-rich granite	86.9	2.2	−0.084	191	58	67	20	N157°, 70° SW	3
CB18	Biotite-rich granite	79.3	5.6	0.575	117	78	292	13	N22°, 77° SE	6
CB19	Two-mica granite	56.1	2.3	0.550	319	54	95	27	N185°, 63° W	10
CB20	Two-mica granite	54.8	2.2	0.110	338	68	106	20	N16°, 70° NW	8
CB21	Two-mica granite	7.3	7.4	−0.152	70	17	175	41	N85°, 49° N	8
CB22	Two-mica granite	30.6	4.2	0.244	288	66	62	17	N152°, 73° SW	11
CB23	Two-mica granite	36.6	6.7	0.259	202	54	94	12	N4°, 78° W	7
CB24	Two-mica granite	63.8	6.5	0.282	323	61	102	21	N12°, 69° NW	9
CB25	Two-mica granite	48.2	3.9	0.249	13	17	265	61	N175°, 29° NE	2
CB26	Two-mica granite	48.9	2.8	0.342	23	76	148	7	N58°, 83° NW	6
CB27	Two-mica granite	59.8	3.9	0.280	328	77	212	12	N122°, 78° NE	10
CB28	Two-mica granite	55.4	2.9	0.500	103	20	219	46	N129°, 44° NE	8
CB29	Two-mica granite	56.0	3.2	−0.045	153	39	356	40	N86°, 50° S	8
CB30	Biotite-rich granite	88.0	3.7	0.153	314	12	212	54	N122°, 36° NE	7
CB31	Two-mica granite	56.0	5.7	0.087	95	22	196	27	N106°, 63° NE	8
CB32	Two-mica granite	48.9	3.6	0.073	303	20	210	9	N120°, 81° NE	11
CB33	Two-mica granite	63.4	4.0	−0.176	308	26	130	63	N40°, 27° NW	10
CB34	Biotite-rich granite	131.2	6.0	0.375	312	26	50	25	N140°, 65° SW	7
CB35	Biotite-rich granite	85.0	5.3	0.324	135	49	241	11	N151°, 79° NE	6
CB36	Biotite-rich granite	107.1	7.8	0.187	251	57	68	27	N158°, 63° SW	8
CB37	Biotite-rich granite	41.5	4.6	0.388	280	63	73	25	N163°, 65° SW	7
CB38	Biotite-rich granite	127.4	2.8	0.373	147	50	239	3	N149°, 87° NE	4
CB39	Biotite-rich granite	119.3	4.7	0.003	118	58	246	24	N156°, 66° NE	8
CB40	Biotite-rich granite	91.3	2.7	0.308	137	20	235	4	N145°, 86° NE	10
CB41	Biotite-rich granite	67.5	2.7	−0.064	322	8	220	37	N130°, 53° NE	8
CB42	Two-mica granite	24.6	4.0	−0.166	313	12	220	22	N130°, 68° NE	8
CB43	Two-mica granite	65.0	12.5	0.338	185	41	90	6	NS, 84° W	7
CB44	Two-mica granite	40.5	8.3	0.437	107	49	19	4	N109°, 86° SW	9
CB45	Two-mica granite	50.4	5.7	0.077	168	34	267	10	N177°, 80° NE	5
CB46	Two-mica granite	52.5	4.4	0.395	147	2	231	1	N141°, 89° NE	6
CB47	Biotite-rich granite	38.2	2.5	−0.096	299	33	160	47	N70°, 43° NW	7
CB48	Biotite-rich granite	84.2	7.6	0.400	156	44	258	2	N168°, 88° NE	6
CB49	Biotite-rich granite	99.1	3.0	0.562	191	48	302	15	N32°, 75° SE	7
CB50	Biotite-rich granite	53.9	3.0	0.067	119	0	61	77	N151°, 13° SW	7
CB51	Two-mica granite	50.0	2.7	−0.020	55	59	183	24	N93°, 66° N	5
CB52	Two-mica granite	40.9	5.4	0.345	285	2	199	4	N109°, 86° NE	8
CB53	Biotite-rich granite	100.6	4.0	0.694	324	37	236	4	N146°, 86° NE	4
CB54	Biotite-rich granite	114.0	2.3	−0.270	187	27	21	62	N111°, 28° SW	4
CB55	Biotite-rich granite	104.7	2.4	0.081	65	20	168	67	N78°, 31° NW	3
CB56	Biotite-rich granite	71.5	3.4	0.445	145	26	31	29	N121°, 61° SW	7
CB57	Two-mica granite	56.0	3.2	0.558	159	70	30	13	N120°, 77° SW	8
CB58	Two-mica granite	45.3	2.9	0.345	145	28	20	46	N110°, 44° SW	5
CB59	Two-mica granite	42.7	2.1	0.200	25	61	221	28	N131°, 62° NE	6
CB60	Two-mica granite	65.4	5.1	0.380	325	12	58	19	N148°, 71° SW	11
CB61	Two-mica granite	56.9	1.9	0.247	328	27	58	0	N146°, 90°	8
CB62	Biotite-rich granite	84.3	3.4	0.051	316	2	50	6	N140°, 84° SW	7
CB63	Biotite-rich granite	95.2	4.7	0.228	327	16	230	19	N140°, 71° NE	4
CB64	Biotite-rich granite	81.2	4.3	0.340	284	24	21	6	N111°, 84° SW	4
CB65	Biotite-rich granite	73.7	4.9	0.496	136	33	33	20	N123°, 70° SW	7
CB66	Biotite-rich granite	116.0	3.1	0.593	158	64	267	11	N177°, 79° NE	3
CB67	Biotite-rich granite	76.8	4.5	0.435	146	20	48	19	N138°, 71° SW	4
CB68	Biotite-rich granite	97.7	3.2	0.422	263	26	25	51	N115°, 39° SW	6
CB69	Biotite-rich granite	66.5	4.7	0.318	132	66	337	26	N67°, 64° SE	9
CB70	Biotite-rich granite	71.4	5.8	0.301	139	74	337	14	N67°, 76° SE	7
CB71	Biotite-rich granite	186.0	3.6	−0.510	256	56	160	12	N70°, 78° NW	5
CB72	Biotite-rich granite	68.2	7.0	0.221	110	51	13	4	N103°, 86° SW	2
CB73	Biotite-rich granite	93.4	2.7	0.620	264	9	360	4	N90°, 86° S	2
CB74	Two-mica granite	70.5	5.7	0.141	145	1	53	50	N143°, 40° SW	11
CB75	Two-mica granite	54.2	4.7	0.271	275	53	99	37	N189°, 53° W	7
CB76	Biotite-rich granite	92.1	3.3	0.220	299	11	87	77	N177°, 13° W	8
CB77	Biotite-rich granite	87.2	2.0	0.089	303	59	201	7	N111°, 83° NE	5
CB78	Biotite-rich granite	74.8	3.4	0.285	316	52	56	8	N146°, 81° SW	10
CB79	Two-mica granite	60.7	3.0	0.039	263	32	6	21	N96°, 69° S	8
CB80	Two-mica granite	44.3	4.9	0.231	126	34	280	53	N10°, 37° E	6
CB81	Two-mica granite	58.7	3.2	0.115	118	22	28	1	N118°, 89° SW	8
CB82	Two-mica granite	29.8	4.6	0.039	64	45	158	4	N68°, 86° NW	7
CB83	Two-mica granite	89.7	4.4	−0.020	305	5	57	77	N147°, 13° SW	5
CB84	Two-mica granite	64.5	7.5	0.731	141	52	44	5	N134°, 85° SW	10

). Considering the biotite content, on petrographic analyses, and the magnetic signal in the AMS studies, two major groups can be established:

(i)

Biotite-rich granites, comprising the Rio de Moinhos, and S. Miguel da Acha (biotite > muscovite granites), with 38.2 µSI < K_m < 186.0 µSI (mean value: 89.2 µSI);

(ii)

Two-mica granites, Alcains and Soalheira (muscovite > biotite) granites, and Castelo Novo (muscovite = biotite) granite, with 7.3 µSI < K_m < 89.7 µSI (mean value: 51.1 µSI) (Figure 5).

The magnetic anisotropy, P%, ranged between 1.9% and 8.7% in the biotite-rich granitic rocks (Rio de Moinhos and S. Miguel da Acha granites) with a mean of 4.3%, and between 1.9% and 13.2% in the two-mica granites (Alcains, Soalheira, and Castelo Novo granites) with a mean of 5.1% (Figure 6). However, the distribution of P% values in the pluton is complex: anisotropy values were higher in the north of the pluton (Soalheira granite), at the western border (S. Miguel da Acha granite), and in a small zone in the center of the pluton (Alcains granite), with values higher than 4.5%.

The ellipsoid shape parameter, T, ranged between −0.510 and 0.731 in all granitic types, with a similar mean value for both groups, 0.253 for Rio de Moinhos, and S. Miguel da Acha granites, and 0.231 for Alcains, Soalheira, and Castelo Novo granites, indicating oblate ellipsoids, about 84.5% (Figure 7). The prolate ellipsoids, which were 15.5%, were present in both groups of granitic rocks.

4.2.2. Magnetic Fabric Patterns

The magnetic fabric, lineation, and foliation, is generally directly correlated to crystallographic fabric of paramagnetic phyllosilicates in magnetite-free granites [36,75,92,93]. However, complex fabrics can arise when Fe-bearing silicates have an inverse magnetic fabric (e.g., cordierite). In the studied granites, displaying magmatic to high-temperature microstructures, the magnetic foliation and lineation served as indicators of the flattening and stretching, respectively, experienced by the magmas during the final stages of their emplacement.

Magnetic fabric analysis was conducted on the entire CB pluton. Furthermore, separate interpretations were performed for each of the two identified groups of granitoids: (i) Rio de Moinhos and S. Miguel da Acha granites (biotite > muscovite granites) and (ii) Alcains, Soalheira, and Castelo Novo granites (two-mica granites).

The mean values of magnetic foliations were very similar across the whole pluton, dominantly NW-SE-striking and with steep dips (the mean magnetic foliation of CB pluton was N141° E, 88° SW).

In the first group, biotite > muscovite granites, the magnetic foliations had inward steep dips and were NW-SE-striking, with a mean foliation orientation of N143° E; 90° (Figure 8). For the second group, two-mica granites (muscovite > biotite, and muscovite = biotite granites), the mean foliation struck N137° E and dipped 84° SW. In the central part of the CB pluton, in Alcains granite, the magnetic foliations were concentrically distributed with outward steep dips (Figure 8).

Across the whole pluton, the magnetic lineations had a NW-SE strike and variable plunges, meaning the lineations plunged independently of the type of granite. The mean magnetic lineation dipped 56° to N137° E. In the north, center, and south of the CB pluton, the magnetic lineations had a relatively shallow plunge (<30°), and they tended to be steeper in the other areas (between 30° and 60°), reaching high values in the western area (≥60°; Figure 9).

4.3. Isothermal Remanent Magnetization Results

The saturation isothermal remanent magnetization (SIRM) values ranged from 6.15 to 45.22 mA/m (mean: 22.78 mA/m, N = 5) in biotite-rich granites, and between 5.87 and 21.60 mA/m (mean: 12.31 mA/m, N = 6) in two-mica set granites. In both groups of granites, the acquisition curves showed that saturation was not obtained at the applied fields (Figure 10).

The S-ratio, defined as the mean ratio between the isothermal remanent magnetization at 300 mT and saturation isothermal remanent magnetization (1T), when close to the unity, served as a measure of the relative amounts of high-coercivity remanence and low-coercivity remanence [94]. For the biotite-rich group, this ratio was 0.860, and 0.801 for the two-mica granites.

4.4. Fluid Inclusion Planes Studies

In the CB pluton, fluid inclusion planes (FIPs) occurred in sets and were formed mostly as mode I cracks. Some FIPs from mode II cracks were also visible, but there was no evidence of mode III crack occurrences. The most frequent type of FIPs in the samples studied was the intragranular type, which extends from the grain boundary into the interior of the grain. Conversely, transgranular FIPs were observed to be the least common.

Figure 11 illustrates rose diagrams showing the FIP directions for each granite type. The analysis of rose diagrams showed that two main orientations were present: (i) NNE-SSW to NE-SW in both sets of granite, this last FIP family being more representative in two-mica granites; (ii) E-W, with an ESE-WNW trend in two-mica granites and an ENE-WSW trend in biotite-rich granites.

4.5. Radiometry Data

The descriptive statistics were handled utilizing two distinct methodologies: by encompassing all values over the study area (including the Castelo Branco and Penamacor Variscan plutons, and the Fundão, Oledo-Idanha-a-Nova and Zebrera Ordovician plutons), and by exclusively considering values situated within the CB pluton (

Table 2. Statistics data from total gamma radiometric data (total gamma radiometric data in nGyh⁻¹).

Statistical Parameters	In the Studied Area	Within the CB Pluton	CB—Two Mica Granites	CB—Biotite-Rich Granites
Mean value	86.48	133.87	126.74	135.98
Standard deviation	52.60	71.93	51.83	76.77
Range	4994.43	4944.67	1352.28	4944.67
Minimum	12.22	61.98	78.57	61.98
Maximum	5006.66	5006.66	1430.85	5006.66
Count	24753	6280	1435	4845

).

Radiation levels within the study area ranged from 12.22 to 5006.66 nGyh⁻¹, with a mean value of 86.48 nGyh⁻¹. However, when specifically considering values within the CB pluton, the mean increased to 133.87 nGyh⁻¹, highlighting those granites typically manifest higher radiation levels compared to their host rocks (Table 2). The interpolation of the total gamma radiometric data was made using kriging through ArcGIS Pro 3.2.2. Figure 12 shows a higher concentration of the total gamma radiation (brown colors that correspond to intervals between 120 nGyh⁻¹ and 250 nGyh⁻¹, and values higher than 250 nGyh⁻¹). These elevated radiation levels were observed predominantly in areas occupied by granites from CB and Penamacor plutons. In contrast, the Ordovician plutons (Fundão, Oledo-Idanha-a-Nova, and Zebreira) showed intermediate radiometric values (orange color between 75 nGyh⁻¹ and 120 nGyh⁻¹). Notably, these values correlated spatially within a NE-SW band, while lower values (<75 nGyh⁻¹) were mostly located in the SE area, seemingly delimited by the thrust fault (Pônsul fault) to the north.

The current contours of the granites of CB pluton did not match perfectly with the contours of radiometric map, but there was a notable relative depletion of radiation in the two-mica granites—Alcains and Soalheira granites—compared to the biotite-rich granites (Figure 12). The depletion in total radioactivity in the two-mica set granites was also evident in the statistical data (Table 2), where the mean value was 135.98 nGyh⁻¹ in the biotite-rich granites and 126.74 nGyh⁻¹ in the two-mica granites.

Figure 12. Radiometric interpolation map displaying the presence of a high gamma radiation values within the Castelo Branco and Penamacor plutons (map interpolated by kriging using ArcGIS Pro 3.2.2 software).

Figure 12. Radiometric interpolation map displaying the presence of a high gamma radiation values within the Castelo Branco and Penamacor plutons (map interpolated by kriging using ArcGIS Pro 3.2.2 software).

4.6. Gravimetric Data

The Bouguer anomaly (Figure 13) was calculated using a reduction density of 2.67 g/cm³ [95]. The Bouguer anomaly ranged from −26.60 mGal to −14.76 mGal, having a mean value of −25.79 mGal, and a standard deviation of 1.64 mGal.

The regional and residual anomaly maps obtained through a first-order polynomial surface (Figure 14 and Figure 15) are the ones that best combine the surface of the Bouguer anomaly with the known geology. The regional anomaly map of the CB pluton and host rocks displays a mainly NW-SE trend, increasing from SE to NW (Figure 14).

The selection of the residual map derived from a first-order polynomial surface primarily depended on the shape of the residual anomaly within the CB pluton. This anomaly exhibited a similar trend to the existing structures in the region, particularly those oriented NE-SW (Figure 1 and Figure 15).

To better understand the gravity anomaly, the residual anomaly and the geological maps were superimposed (Figure 16). Accordingly, some assumptions can be made: (i) the Ordovician plutons (Fundão and Oledo-Idanha-a-Nova plutons) exhibited a predominant positive gravity anomaly (red colors); (ii) the low gravity anomaly (blue colors) visible within the CB pluton formed a Y-shape, combining a NE-SW trend, with a branch extending westward characterized by a NW-SE to NNW-SSE trend; (iii) three other low gravity anomalies were observed in the vicinity of the CB pluton, namely one located at the center of Fundão pluton, another in the NW zone of Penamacor pluton, and a third situated between the Oledo-Idanha-a-Nova, and Penamacor plutons; (iv) the lowest gravity anomaly values (blue and green colors) showed a main trend NE-SW, and were parallel to the family of NE-SW structures, and constrained by thrust faults.

5. Discussion

5.1. Integration of Magnetic Parameters with Geophysical Characteristics

The magnetic susceptibility values are essentially due to the paramagnetic minerals that constitute the CB pluton, showing that these granites are ilmenite-type granites according to the Ishihara [28] classification. The muscovite > biotite and muscovite = biotite granites at the CB pluton center, and at the north, respectively, had the lowest values of K_m. The peripheral CB granites, on the other hand, presented high values of K_m and high anisotropies. The K_m values clearly characterized the difference between the two groups of granites and are related to the high abundance of biotite and the presence of other ferromagnesian minerals, such as cordierite in the peripheral granite. The presence of cordierite did not seem to affect the magnetic fabric, since the NW-SE trend was visible in both biotite-rich and two-mica granites.

The P% mean value was higher in the CB two-mica granite suite when compared to the biotite-rich granitic rocks, with mean values of 5.1%, and 4.3%, respectively. Considering the classification model presented in, e.g., Sant’Ovaia et al. [19], which evaluates the AMS of CIZ granites, P% values exceeding 4% categorize the CB pluton as among the plutons emplaced during high compression. These values of P% are supported by numerous microstructures observed within the CB pluton, including quartz exhibiting a chessboard pattern and polycrystalline quartz with grain boundary migration, as well as the bending of plagioclase twin planes, suggesting significant solid-state deformation at high temperatures.

The degree of the oblateness of the AMS ellipsoid was homogeneous inside the CB pluton and is related to the magnetic anisotropy within the basal plane of the biotite [76]. The usual occurrence of oblate AMS ellipsoids is very common in granites where the main carrier of the magnetic susceptibility is defined by phyllosilicates [36].

The SIRM curves analyses pointed to the residual presence of ferrimagnetic minerals in the samples. Otherwise, the S-ratio values corroborated the scarcity of ferromagnetic minerals, as they were consistently below 1.

Radiometric data emphasized that in the studied area the high values were spatial and associated with the CB and Penamacor plutons. Conversely, low radiation values were associated with host metasedimentary rocks.

5.2. Pluton Geometry Modeling

The shape and depth of the CB pluton was inferred through a NW-SE gravimetric profile. The profile is drawn in Figure 16 and represented in Figure 17.

The gravimetric profile represents an observed anomaly based on gravimetric data and all available geologic information. The lithologies and geologic contacts both in outcrops and at depth, as well as the known and probable faults, are represented in 2D model.

The following densities were used for modeling: 2.50 g/cm³ for Castelo Novo granite, 2.54 g/cm³ for Alcains and Soalheira granites, 2.67 g/cm³ for Rio de Moinhos and S. Miguel da Acha granites, and 2.7 g/cm³ for the metasedimentary rocks.

Profile P1 (Figure 17) was drawn along a NW-SE orientation to acquire information about the negative residual anomaly zone, interpreted as a feeder zone. This profile cut all the CB granites, revealing the presence of an individual feeder root within this zone. The feeder zone’s location was inferred based on the lowest residual anomaly values (<−1.37 mGal).

Moreover, the profile P1 highlights the impact of denser rocks to the northwest, notably the Fundão pluton and Beiras Group metasedimentary host rocks. Upon examining the residual anomaly map, another feeder root can be inferred to the north of the CB pluton.

The feeding zone (Figure 17), coupled with available geological data, allowed the formulation of a kinematic model for the CB pluton. A pulse of granitic magma ascended through the feeding zone, using existing fractures, which led to the formation of the Alcains granite. Subsequently, other pulses of magma used the same feeding zone, resulting in the emplacement of the Rio Moinhos granite, which through differentiation gave rise to a border granite, the S. Miguel da Acha granite. Another different magma pulse led to the emplacement of the Castelo Novo and Soalheira granites, which benefited from pre-existing fractures located further northwest of the CB pluton.

In summary, different pulses used the same feeding zone, benefiting pre-existing subvertical structures that allowed the magma’s ascent. The northwest and southeast boundaries of the pluton are delineated by thrust faults, thereby influencing the pluton’s overall shape.

Furthermore, the modeling allowed us to classify the CB pluton as an asymmetric, flat-shaped pluton, with a thickness slightly more than 2 km and a feeder zone with moderate to gently sloping walls. These features align with the classification proposed by Vigneresse et al. [89], which defines the flat-shaped pluton family as bodies with ca. 2–3 km and extending approximately 30–40 km in each horizontal direction. This kind of pluton typically exhibits a predominantly sub-horizontal floor, with gently dipping areas interpreted as feeder zones. The modeling also displays the laccolithic form of the Alcains granite, explained by the subsequent magmatic pulse of the Rio de Moinhos and S. Miguel da Acha granites, which pushed the Alcains magma to the top of the intrusion.

Furthermore, the flat-shaped geometry for the CB pluton aligns with the findings of Cruz et al. [16] regarding the relation between pluton shape and host rock lithology. Plutons encircled by other granites typically exhibit wedge shapes, featuring irregular outlines influenced by faulting. In contrast, when situated within metasedimentary rocks, plutons tend to adopt flat shapes [16].

Considering the ascent, emplacement, and structural control of the various granites within the Castelo Branco pluton, a nested incremental growth model (e.g., [96,97]) can be applied to the CB pluton. The younger pulses of CB tended to nest within the earlier pulses, primarily due to rheological control. These magmas ascended where the prior intrusive material remained hot and, or, still contained melt.

In addition to the negative residual anomaly areas detected within the CB pluton, interpreted as a feeder zones, other areas exhibiting low anomaly values have been identified, prompting further interpretation: (i) A low anomaly was found at the center of the Fundão pluton, suggesting a potentially thick zone within this pluton; (ii) In the northwest zone of the Penamacor pluton, another low anomaly was identified, possibly indicating its feeder root; (iii) Finally, between the Oledo-Idanha-a-Nova and Penamacor plutons, a low anomaly was detected, hinting at the potential presence of granite at depth.

5.3. Emplacement Model and Structural Control

The NW-SE-trending subvertical magnetic foliations of the CB granites suggest that the pluton used deep Variscan structures aligned in this orientation for its emplacement. While most mapped faults and thrust faults on the surface displayed an approximate NE-SW direction, there were also notable NW-SE alignments observed in the study area. This was evident in the general foliation of the host rocks, and was also the predominant orientation of D₃ structures such as synclines (e.g., Vila Velha do Rodão) and monoclines (e.g., Castelo Branco monocline) [66,70].

Magnetic lineations have a distinct behavior within the CB pluton, but they are mostly subvertical, and with a prevailing NW-SE trend. These lineations may be interpreted as stretching directions of the granitic magmas developed during late emplacement [36].

The observed NW-SE-trending magmatic flow, as indicated by the magnetic fabric, underscores the significance of the intersection between NE-SW-trending faults and thrust faults, and NW-SE-trending deep structures during the ascent and emplacement of CB granites.

Recent research has shown that Variscan plutons with both NW-SE-trending subvertical magnetic foliations and magnetic lineations, like the CB pluton, are temporally related to the Variscan D₃, a horizontal NE-SW-trending compressional event.

Regarding the feeding zone, gravimetric data (i.e., a residual gravity anomaly map) suggest the presence of a deep region oriented along a NE-SW direction. Furthermore, the gravimetric anomaly extends towards the west, following a NW-SE to NNW-SSE trend. This NW-SE to NNW-SSE trend, as mentioned above, records the magmatic flow within the CB pluton. Magnetic lineations with moderate plunges, and primarily oriented NW-SE, reflect the enlargement of this Variscan pluton in the same direction and signify either magmatic flow or stretching during the final stages of emplacement.

The NNW-SSE trending magnetic fabric was also noted by Sant’Ovaia et al. [98] in other Variscan granites from the CIZ, particularly in Serra da Estrela and Castro Daire granites. However, this orientation in Serra da Estrela and Castro Daire granites is characterized by magnetic lineations with shallow plunges.

Considering these findings, we propose that the ascent and emplacement of the CB pluton were significantly influenced by the intersection of NE-SW-trending Variscan faults with NW-SE to NNW-SSE-trending Variscan deep structures. It is likely that the ascent of CB granitic magmas occurred concurrently with the initiation of emplacement for other Pennsylvanian granites (e.g., Serra da Estrela and Castro Daire plutons).

After emplacement, the CB pluton recorded increments of the regional tectonic strain as documented by fluid inclusion planes (FIPs) in quartz. All the FIPs are post-magmatic and therefore no relationship can be established with the magnetic fabric. The FIP analysis suggested that the observed directions NE-SW to NNE-SSW and approximately E-W can be interpreted as being mostly related to a maximum stress (σ1) oriented NE-SW.

The results suggest that the magmatic cooling was fast (the pluton emplacement probably occurred in the upper crust), since the stress field was the same during the transition from ductile to ductile–brittle behavior. Additionally, the FIP indicates that the stress field (NE-SW to N-S maximum compressive stress) was the same in the two sets of granites (two-mica and biotite-rich granites) and is compatible with the stress field prevailing in the final stages of Variscan orogeny (D₃) during the cooling of this granite.

Hence, the kinematic characteristics identified in this study appear to mirror the uninterrupted sequence of events during the emplacement and subsequent cooling of the granitic magmas, occurring within a relatively brief timeframe.

6. Conclusions

This multidisciplinary study allowed us to characterize the magnetic behavior, geometry, and emplacement model of the CB pluton considering the following findings:

• The values of magnetic susceptibility of the CB pluton are typical of ilmenite-type granites.
• The P% mean value higher than 4%, and the evidence of solid-state deformation at high temperatures, suggest that the CB pluton was emplaced during deformation.
• The magnetic fabric and gravimetric data analyzed together yield the following insights into the emplacement model:

  (i)

  The 2D model allows us to infer that CB pluton is a flat-shaped pluton and seems to extend to ca. 2–3 km in depth.

  (ii)

  The feeding zone of the CB pluton exhibits a predominant NE-SW alignment, which aligns with the NE-SW-trending regional faults and thrust faults. Additionally, there is evidence of a NW-SE trend, consistent with the orientation of the main regional foliation of the host rocks.

  (iii)

  The concentric magnetic foliation with outward steep dips observed in the Alcains granite suggests that the ascent and emplacement of this granite probably predates that of the Rio de Moinhos and S. Miguel da Acha granites. The magnetic fabric indicates that Alcains had more room to spread, and its laccolithic shape suggests that this granite was pushed to the top of the CB pluton.

  (iv)

  The ascent of Alcains, Rio de Moinhos, and S. Miguel da Acha granites most likely took place along a pre-existing NE-SW-trending fault; the Soalheira, and Castelo Novo granites probably took advantage of other pre-existing NE-SW faults, situated further northwest.

  (v)

  CB is a nested pluton.

• FIPs have two main trends, both related to a maximum stress (σ1) oriented NE-SW:

  (i)

  NNE-SSW to NE-SW-trending visible in all CB granites.

  (ii)

  ENE-WSW trend in biotite-rich granites, and ESE-WNW trend in two-mica granites.

• Two main directions of structures/ alignments are clearly noticeable in the study area: NE-SW and NW-SE.

  (i)

  The NE-SW trend characterizes the primary direction of the faults and thrust faults within the study area. Moreover, the residual anomaly suggests a feeding zone extending along the NE-SW to NNE-SSW direction.

  (ii)

  The NW-SE alignments are also evident both within CB pluton and also in the surrounding area. The magnetic fabric exhibits a predominant NW-SE trend, aligning parallel to regional structures such as the kilometer-scale D₃ regional folds.