Geochemistry and Geochronology of W-Mineralized Fourque Granodiorite Intrusion, Pyrenean Axial Zone, Southern France

Abstract

This study focuses on the Fourque massif, one of the thirty Variscan plutons outcropping along the Axial zone of the Pyrenees. It hosts a significant tungsten deposit that was actively mined until 1986. However, since the closure of the mine, no detailed geochemical or geochronological studies have been conducted until recent investigations in 2019, leaving a significant gap in our understanding of this intrusion. This lack of research, along with the ongoing debate and uncertainties regarding the timing and magmatic processes of Variscan plutonism in the Pyrenees, underscores the importance of further investigations. To address these gaps, we present new zircon U–Pb geochronology, whole-rock and zircon geochemistry (X-ray fluorescence and LA-ICP-MS), and Ti-in-zircon thermometry. Our study compares nine new whole-rock geochemistry samples with the limited previous dataset from 1987, refining the petrogenetic interpretation of the intrusion. These efforts are framed within the ongoing debate surrounding the different Variscan intrusions in the Pyrenees, including the discussions on their emplacement age, magmatic context, type, and origin. Geochronological data indicate an age ranging from 304.6 ± 2.3 to 308.4 ± 2.6 Ma, with crystallization temperatures ranging from 700 to 800 °C. The granodiorite is characterized by differentiated petrogenetic facies, related to successive batches of magma rising from a deeper source. The granodiorite exhibits high ASI ratios (>1.3), classifying it as strongly peraluminous. While I-type granites are typically metaluminous to weakly peraluminous, such elevated ASI values suggest a significant influence of crustal assimilation during magmatic evolution. The geochemical signature of the intrusion is enriched in large ion lithophile elements (LILE) and light rare earth elements (LREEs) while showing depletion in heavy rare earth elements (HREEs), consistent with a high-K calc-alkaline, magnesian, syn-orogenic setting. Whole-rock and zircon trace element data suggest that the magma source involved partial melting of the continental crust, with evidence of interaction with a subduction-modified mantle component. By applying methods previously unapplied to this pluton, this study provides new data on its geochemistry and geochronology, revealing significant differences from previous interpretations. These findings offer deeper insights into the emplacement and evolution of the Fourque granodiorite, refining its role within the broader context of Variscan orogenesis in the Pyrenean Axial Zone and similar plutonic systems worldwide.

1. Introduction

The Pyrenees Mountain range, southern France, stands out as a geologically diverse area with significant metallogenic potential. Stretching across 530 km, this mountain range serves as the natural boundary between Northeast Spain and Southwest France. The Pyrenees are bordered by the Aquitaine and Ebro foreland basins to the north and south, respectively, and by the Mediterranean Sea to the east and the Atlantic Ocean to the west. The Alpine orogeny, caused by the collision of the Iberian and Eurasian plates around 60 to 40 million years ago, uplifted this mountain range. Erosion exposed a much older Paleozoic basement outcropping within the axial zone, deformed during the Variscan orogeny (Middle to Late Paleozoic) [1]. These Paleozoic terranes were intruded by several plutonic bodies, with around 30 outcropping within the axial zone [2]. The Variscan plutons have undergone extensive geological investigation and exploration, providing important information concerning the regional geological context and revealing significant mineralization potential. Efforts over previous centuries have led to the discovery and exploitation of important deposits across the Pyrenean axial zone [3,4].

Located in the central Pyrenees within the Axial zone, the Salau deposit stands as France’s most significant tungsten deposit ever mined until its closure in 1986. A total of 930,000 tons of ore with an average grade of 1.5% WO₃ were mined. This deposit is located along the edge and within the Salau massif, a Variscan granodiorite intrusion, primarily in scheelite lenses and mineralized breccia. The granodioritic massif outcrops over a small area of 1.2 km² in the southern part of Salau village. It intrudes a sedimentary series of Paleozoic carbonate rocks that underwent significant deformation and fracturing during the Variscan orogeny [5]. This granodioritic intrusion is associated with a complex polyphase mineralization, including skarn and vein type deposits [6].

Before its closure in 1986, research on the Salau mine and its associated geological features was conducted, though it was relatively limited. These studies are now dated and lack the benefits of modern analytical techniques. After the mine ceased operations, research on the topic stopped entirely. Recent debates regarding the reopening of former mining sites have revived interest in France’s tungsten deposits [7]. Despite this renewed attention, only a few recent studies have revisited the geological aspects of the Fourque granodiorite, leaving significant gaps in its petrographic and geochemical characterization. Recent research focuses on a particular aspect of the mineralization. The pluton emplacement is associated with a specific syn-tectonic deformation phase identified as D2 phase, one of the four main deformation events documented in the Pyrenean Axial Zone, characterized by high-temperature foliations and lineations in the middle crust (D2-a) and large-scale open folding affecting both the middle and upper crust (D2-b) [8]. While the previous U–Pb analysis indicated a crystallization age of 295 Ma [6], this investigation presents older zircon U–Pb ages, between 304 and 308 Ma, similar to other Variscan intrusions related to the same deformation phase [9,10], highlighting the need for updated geochronological analysis to better constrain and understand the tectonic events that shaped the region.

To address the research gaps surrounding the Fourque granodiorite intrusion, this study aims to clarify its emplacement age, magmatic processes, and tectonic setting. A comprehensive geological investigation is conducted using techniques such as LA-ICP-MS and X-ray fluorescence across different facies and locations of the intrusion. Specifically, this study seeks to: (1) refine the emplacement age through zircon U–Pb dating, (2) characterize the geochemical composition and magmatic evolution using whole-rock and zircon trace element geochemistry, and (3) apply Ti-in-zircon thermometry to constrain crystallization conditions. This research also includes a comparison with previous studies from Raimbault and Kaelin (1987) [5] to assess variations in magma characteristics and composition, though the study is older and may have used different methods; therefore, the analysis emphasizes trends rather than direct numerical comparisons. The findings contribute to refining the tectonic chronology of the Variscan orogeny within the Pyrenean Axial Zone and updating prior geological interpretations of the intrusion.

2. Geological Context

2.1. Regional Geological Context

The Pyrenees region is segmented into five distinctive zones: the Northern Folded Foreland, characterized by prominent folds within a dense Mesozoic to Tertiary sequence; the North Pyrenean zone, featuring northward-thrusted Mesozoic and older rocks; the South Pyrenean zone, comprising a southward-directed thrust sheet of Tertiary and Cretaceous rocks overlying a detachment surface of Triassic evaporites; the Southern Folded Foreland, showcasing typically folded Tertiary and Mesozoic rock formations; and the Axial zone, hosting thrust and imbricated sediments atop a vast basement comprising Precambrian and Paleozoic rocks that underwent folding and high-temperature/low-pressure metamorphism during the Variscan orogeny, further intruded by late-stage Variscan granitoids [11,12].

Geologically, the Pyrenees exhibit a combination of both young and old formations, as evidenced by their presence on the geological time scale. The Pyrenees stand among several relatively small alpine mountain ranges formed across southern Europe by the convergence between the microcontinent of Iberia driven by the African plate (Figure 1), and the European plate, from the Late Cretaceous period to the present day [13,14]. The first stage of the convergence occurred from 100 to 85 Ma ago, when the continental crust underwent stretching and thinning along a massive shear plane, creating a rifting zone that did not evolve into oceanic crust. Around 84 million years ago, the small Iberian plate, driven by the African plate, began to rise to the north, eventually colliding with the European plate. The convergence of the two plates at 67 million years ago caused the Iberian plate to dip beneath the European plate, initiating the Pyrenean orogenic phase. Between 44 and 24.7 Ma, the Pyrenees had already taken shape, with the Variscan basement overlapping and layering successively within the Axial zone [15,16] (Figure 1). This basement was deformed and metamorphosed during the Variscan orogeny around 370 to 290 million years ago [1]. This orogeny resulted from the collision between the continents of Laurussia and Gondwana, giving rise to the supercontinent Pangea and the Variscan orogenic belt, which can still be observed in certain parts of Europe, including the Pyrenean axial zone [17].

Compressional tectonic activity from 340 to 300 Ma included mafic and felsic magmatism, causing lower crust temperatures to approach the wet granite solidus. Around 315–305 Ma, regional extension and asthenospheric upwelling introduced more mafic magma into the lower crust, leading to fluid-absent melting, melt migration, and mid-crustal plutonism [18]. The main deformation phase related to pluton emplacement is divided into three stages: the D1 Phase (323 to 308 Ma) corresponding to folding and south-verging thrusting under medium-temperature and medium-pressure conditions; the D2 Phase (308 to 301 Ma) characterized by N-S compression accommodated by eastward-directed flow under partial melting conditions and high-temperature, low-pressure metamorphism, marked by the formation of domes in the infrastructure and emplacement of syn-tectonic intrusive granites; and the D3 Phase (around 300 Ma) associated with late-orogenic tectonics in a retrograde metamorphism context [19]. The related granitic intrusive bodies within the Pyrenees (Figure 2) [20] reveal three genetically associated magmatic suites: calc-alkaline granitoids forming plutons up to several hundred square kilometers; mafic complexes occurring as small intrusions, ranging from decametric bodies to stocks of tens of square kilometers; and anatectic S-type granites and migmatites [21]. Consequently, the Axial zone of the Pyrenees features Paleozoic rocks shaped by two distinct orogeneses: the Alpine and Variscan orogeneses [8,22,23].

2.2. Local Geological Context and Petrographic Study

The study area is Salau, located in the commune of Couflens within the Ariège department. The granodiorite massif of the Fourque, the Variscan pluton linked to the W deposit, covers an area of 1.2 km² in the south of the village of Salau, at the bottom of the Salat valley, 3 km from the Franco-Spanish border (Figure 3). It intrudes into a sedimentary series with carbonate levels of Lower to Middle Paleozoic age, intensely folded and fractured, and weakly metamorphosed during the Variscan orogeny [5]. This series plays a crucial role in contextualizing the geological evolution of the region. These metasedimentary rocks are oriented east to west, exhibit vertical dip and form a significant monocline with the youngest rocks exposed in the north and the oldest in the south. Among the crosscut metasedimentary series can be distinguished three main formations: the Mont Rouch formation, the Salau limestone formation, and the blue shales formation (Figure 4; [5]).

The Mont Rouch formation, with a local thickness of approximately 120 m that expands to a maximum of 2 km, is exposed to the south of the intrusion. Its origin dates from the Lower to Middle Ordovician period. This formation is characterized by a flysch complex featuring rhythmic sequences of sandstones, shales, and fine-grained sandstone beds, and plays a regional role as a thrusting level. Enveloping the granodiorite intrusion at the outcrop is the Salau Limestones formation, ranging from 100 to 150 m in thickness, dolomitic to the east of the Fourque granite and featuring skarn development. This series comprises the Salau upper marble in its upper portion, dating from the Emsian age [25]. Additionally, it includes sequences of graphitized marble beds with substantial clay beds at the base, forming the “Barrégiennes” formation from the Upper Silurian to the Lower Devonian. The third main formation is the Salau lower marble, a substantial shale unit measuring 200 to 250 m in thickness, consisting of bluish metapelites interspersed with blue or black limestone, exposed to the north of the intrusion (Figure 4; [5]).

Following a period of moderate thickening due to fold-thrust belt formation in the upper crust between 323 and 308 Ma, the Variscan region of the Pyrenees underwent crustal flow around 306 Ma [8]. The late phase of Variscan orogeny witnessed a strong heat anomaly in the upper crust between 305 to 285 Ma with a rapid cooling [26] leading to the formation of magmatic plutons. Those plutons are large bodies of intrusive igneous rocks that were emplaced, uplifted and exposed through erosion, outcropping across the axial zone [27]. The ascent mechanism of those plutons could be closely related, as in other Variscan granitoid examples [28], to the activation of shear zones and successive pulses of magma [29]. This heat anomaly is observed along the Axial zone, outcropping in several places including the Fourque massif, and shows evidence of its existence with the presence of a contact metamorphism halo between the outcropping granitoids across the Pyrenees. The contact metamorphic halo is characterized by minerals such as biotite in distal zones and calc-silicate assemblages, including garnet, vesuvianite, clinopyroxene, and clinozoisite, with fluorine-rich garnet and zoned vesuvianite reflecting proximity to the intrusion and varying metamorphic intensities [30]. Studies of the different intrusions and the east-west elongation of the contact metamorphism zone guided by shear zones [31] suggests that other plutons are from the same intrusion near the surface [32].

The Fourque granodiorite intrusion itself contains two main facies, a porphyritic monzogranite in the massif core, and an apical granodiorite on the periphery. The core facies consist of a medium-grained monzogranite, typically equigranular but occasionally exhibiting porphyritic features. In contrast, the peripheric granodiorite facies is characterized by a fine-grained texture between 0.5 and 1 mm and a thickness of 100 to 250 m surrounding the monzogranitic core, becoming thinner at depth. It is composed of plagioclase and abundant biotite, surrounded by large xenomorphic and interstitial crystalline quartz [5], sparse microcline, zircon and apatite as accessory minerals (Figure 5) [5,33], giving evidence of a multistage intrusion with successive batches of magma rising.

3. Sampling and Analytical Methods

3.1. Sampling

The Fourque granodiorite intrusion represents a challenge for sample collection due to a limited bedrock exposure on the sides of the mountain, and generally steep slopes and cliffs difficult to access. To address this, the eastern side of the peak with trail access was sampled along the intrusion. Nine granodiorite samples were collected on the surface for whole-rock geochemistry and zircon single-mineral separation. Samples were extracted from various locations, encompassing the northern part of the intrusion G11, G21, G51, above the open pit of the Bois d’Anglade, in the southern part (GK, GF), on different outcropping cliffs in the southeast of the intrusion (GI, GH, GE) and in the eastern part of the intrusion (sample S1-1) (Figure 4b). Samples were collected from the edge of the core zone of the Fourque granodiorite. Among these, two samples, GK and G21, exhibit clear petrographic differences: they are notably lighter in color and display distinct textures compared to the other samples, indicating different facies within the granodiorite zone.

3.2. Petrography and Thin Section Analyses

Polished thin sections were prepared by ALS Global Analytical Company in Guangzhou, China. These sections were then imaged in plane-polarized and cross-polarized transmitted light using a Zeiss Axioscope.A1 optical microscope and a ZEISS Axiocam 506 color camera (Zeiss, Oberkochen, Baden-Württemberg, Germany) at the School of Geosciences and Info-Physics, Central-South University.

3.3. Whole-Rock Major and Trace Element Analyses

Each sample was sent to ALS Minerals in Guangzhou for whole-rock geochemical analysis. Major elements were analyzed using X-ray Fluorescence Spectrometry (XRF) following the ME-XRF26 method [34]. Loss on Ignition (LOI) was measured using an electronic balance. Trace and rare earth elements were determined using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and Optical Emission Spectrometry (ICP-OES) following the ME-MS61r and ME-MS81g methods. The instruments used included the PANalytical PW5400 XRF spectrometer (Malvern Panalytical, Almelo, Overijssel, The Netherlands) for major elements and the Agilent 5110 ICP-OES and 7900 ICP-MS (Agilent Technologies, Santa Clara, CA, USA) for trace and rare earth elements.

3.4. LA-ICP-MS Zircon U–Pb and Trace Elements

The mineral separation process began with crushing the sample to remove surface dust, followed by coarse and fine crushing using a jaw crusher and double-roll crusher, respectively. Subsequently, the crushed sample underwent panning using a shaking table. The heavy fraction was then subjected to strong magnetic separation, electromagnetic separation, and fine panning of the non-magnetic portion using alcohol or heavy liquids. The zircons were embedded in epoxy resin for subsequent experiments. Pure zircon grains were handpicked under a binocular microscope. After polishing with a Unipol-1200M machine (Laizhou Weiyi Experimental Machinery Manufacture Co., Ltd., Shandong, China), the zircon grains were imaged using transmitted and reflected light photomicrography methods, along with cathodoluminescence (CL) imaging in the field emission scanning electron microscope.

Zircon samples were observed using a JSM-IT 500 scanning electron microscope (SEM) (JEOL Ltd., Tokyo, Japan) in both transmitted and reflected light modes. Cathodoluminescence (CL) imaging was utilized to reveal the internal structure of the zircon crystals, which appeared transparent, free from cracks, and devoid of fluid or mineral inclusions. These microphotographs were used to identify the internal structure and hydrothermal mineral inclusions within the minerals, as well as to select clean, inclusion-free areas devoid of cracks for U–Pb dating analyses.

U–Pb dating of zircon and trace element content measurement were conducted by LA-ICP-MS at Microtrace Geochemistry Technology (Langfang) Co., Ltd., China. Laser ablation was performed using the UP213 Nd:YAG solid-state laser (New Wave Research, Fremont, CA, USA), while the mass spectrometer used was the Agilent 7900 quadrupole inductively coupled plasma mass spectrometer from Agilent Technologies, USA. During the experiment, helium served as the carrier gas, while argon acted as the compensation gas to adjust sensitivity. They were mixed and connected to the ICP-MS via a T connector.

For Zircon LA-ICP-MS analyses, the laser’s spot size was set to 30 µm, with energy density and frequency set at 10 J/cm² and 10 Hz, respectively. NIST SRM610 was utilized to achieve the highest sensitivity, lowest oxidation, minimal background, and stable signal, ensuring optimal conditions. A background time of 20 s was collected, followed by a 40 s sample analysis and a 30 s sample purge time. Peak skipping methodology was employed for data collection, with different collection times set for specific elements: 10 ms for ²⁰²Hg and ²³²Th, 20 ms for ²⁰⁴Pb and ²⁰⁶Pb, 30 ms for ²⁰⁷Pb, 15 ms for ²⁰⁸Pb and ²³⁸U, and 6 ms for other elements. The external standard zircon 91500 was used for isotope ratio correction [35], while standard zircon plesovice (337 Ma) served as the monitoring blind sample [36]. The international standard sample NIST SRM610 was employed for correcting trace element content [37].

Isotopic ratios and element contents were calculated using GLITTER4.0 software. Concordant ages and images were obtained using Isoplot/Ex (3.0). The errors (standard deviations) of the isotopic ratios and isotopic ages obtained from the experiment were both within the 1σ level. The Ti content of zircon was used as a thermometer to estimate the temperature of magma. To ensure consistency in temperature estimation, zircons with relative extremely high values (over 100 ppm) and extremely low values (below 1 ppm) were excluded due to potential analytical uncertainties. In this research, we employed the zircon Ti thermometer developed by Watson et al. (2006) [38]. $Log\left(T{i}_{zircons}\right)=\left(6.01\pm 0.03\right)-{\displaystyle \frac{5080\pm 30}{T\left(K\right)}}$.

4. Analytical Results

4.1. Whole-Rock Major and Trace Element Composition

The whole-rock geochemistry composition of major elements (wt.%) from the Fourque intrusion is listed in Supplementary Table S1. The loss of ignition for all samples is less than 1.77 wt.%. The composition varies significantly, likely due to differences in sample location and facies. Specifically, SiO₂ content ranges from 60.61 to 71.80 wt.%, with samples G21 and GK at around 61 wt.% while samples GE, GH, GI, GF and G51 are around 71 wt.%. Other major elements include Na₂O (3.25–8.46 wt.%), K₂O (1.83–4.44 wt.%), Al₂O₃ (14.62–19.97 wt.%), and CaO (0.69–1.94 wt.%). MgO values range from 0.44 to 1.39 wt.%. The FeOT/(FeOT + MgO) ratio falls between 0.65 and 0.75, indicating a magnesian granite series (Figure 6a).

The samples exhibit overall enrichment in alkali elements (Na₂O + K₂O: 6.47–8.27 wt.%), with G21 and GK showing elevated values (10.45 and 11.36, respectively). The levels of P₂O₅, TiO₂, Fe₂O₃T, MgO, Al₂O₃, Sr, Ba, Zr, Na₂O, and ∑REE show negative correlations with SiO₂, whereas K₂O exhibits a positive correlation.

On the SiO₂ versus K₂O diagram (Figure 6b), the samples are predominantly located within the High-K alkaline series. However, two samples, GF and G51, deviate from this trend and fall within the calc-alkaline series. Additionally, all samples are classified as strongly peraluminous based on the aluminum saturation index ASI vs. A/NK plot (Figure 6a), with a significant contrast for the G21 and GK samples showing much weaker peraluminous characteristics. The Debon and Lefort P-Q diagram shows that most of the samples range from tonalite to granite, mostly into the granodiorite and adamellite groups, except for the two samples G21 and GK which range into the monzogabbro group (Figure 6c). The TAS diagram (Figure 6d) shows most of the samples ranging into the granite group, nearby both granodiorite and quartz monzonite groups. The two samples G21 and GK again show a different trend, part of the Syenite group. The MALI (MALI = (Na₂O + K₂O) − CaO) values range from 5.11 to 10.2. Mg# (Mg/(Mg + Fe)) values range from 0.344 to 0.526, and Sr/Y ratio ranges from 12.56 to 17.57.

Raimbault & Kaelin (1987) proposed that the TiO₂ content discriminates the different granodiorite facies [5]. They demonstrated that the TiO₂ (%) content of the 14 samples clustered around four values of TiO₂, distributed as follows: 0.17 (one sample), 0.32 ± 0.02 (seven samples identified as granodiorite porphyry outcropping), 0.45 (two samples), and 0.53 ± 0.02 (four samples identified as garnet-bearing granodiorite outcroppings). The values obtained in the nine samples of this study differ from this observation: the values group from 0.20 to 0.23 (six samples) and between 0.34 and 0.39 for samples G11, G21 and S1-1 (Figure 7a). Harker diagrams (Figure 7b–e) highlight the varying trends as SiO₂ increases, concentrations of MgO, Fe₂O₃, and TiO₂ decrease, while K₂O increases.

The rare earth element (REE) content analysis of the La Fourque granodiorite samples reveals a clear trend of heavy REEs (HREEs) depletion and enrichment in light REEs (LREEs), resulting in a right-dipping chondrite-normalized REE pattern (Figure 8a). Strong Ba, Ti, Sr and P negative anomalies also appear in the primitive mantle normalized REE diagram (Figure 8b). Some slight variation in HREE content is observed depending on the samples, which were already grouped into three categories in the previous study. The first group, which includes sample G11, displays the highest HREE content and aligns with the garnet-bearing granodiorite facies observed in previous studies. The second group, including samples G21 and S1-1, exhibits intermediate HREE content similar to the granodiorite porphyry facies. The third group, comprising the remaining samples, presents the lowest HREE content (Figure 8a).

4.2. Zircon U–Pb Geochronology

Zircons were extracted from nine samples and analyzed under cathodoluminescence imaging. A total of 40 targets were selected on zircons devoid of cracks and inclusions for simultaneous U–Pb dating and trace element analyses using LA-ICP-MS. The analytical results are presented in Supplementary Table S2 and the trace element data are presented in Supplementary Table S3. Some typical targeted and numbered zircons for the samples S1-1, G51 and GH are presented (Figure 9) and the nine U–Pb concordia diagrams are presented in Figure 10.

Most zircons have dimensions typically falling within 100–130 μm in length and 70–100 μm in width and are euhedral to subhedral, appearing as equant to elongate prismatic grains, with ratios of 1:1 to 4:1 (Figure 9). They show concentric oscillatory zoning in cathodoluminescence (CL), characteristic of magmatic origin.

Some zircon U–Pb age analyses were manually invalidated due to incoherent ages or low concordance (generally under 96%), likely caused by inherited zircon cores. Samples G11, G21, and G51 yielded 38, 28 and 35 valid analyses, respectively; samples GH, GE and GF yielded 34, 30 and 36 valid analyses, respectively, and samples GI, GK and S1-1 yielded 34, 33 and 37 valid analyses, respectively, ensuring a comprehensive age range and distribution. The zircon Th/U ratio range of all samples also confirms their magmatic origin (Supplementary Table S2 [48]). The results of U–Pb dating on zircon plotted in Figure 10 yield the following weighted average ages: G11: 313.0 ± 2.6 Ma (MSWD = 0.045, n = 29), G21: 308.4 ± 2.6 Ma (MSWD = 0.43, n = 28), G51: 305.2 ± 2.3 Ma (MSWD = 0.11, n = 35), GH: 306.8 ± 2.3 Ma (MSWD = 16, n = 34), GE: 306.5 ± 2.4 Ma (MSWD = 11, n = 31), GF: 305.5 ± 2.3 Ma (MSWD = 5.6, n = 34), GI: 308.4 ± 2.3 Ma (MSWD = 3.6, n = 34), GK: 304.4 ± 2.3 Ma (MSWD = 0.27, n = 33), and S1-1: 305.5 ± 2.3 Ma (MSWD = 0.52, n = 37). Most of the samples, eight out of nine, show U–Pb dating results between 304 and 308 Ma. The sample G11 shows a slightly older age, around 313 Ma.

The extracted zircons show varying U and Th content depending on the sample. Sample S1-1 exhibited relatively low concentrations, with U ranging from 46.03 to 351.47 ppm and Th from 8.69 to 91.61 ppm (Th/U ratio 0.09–0.46). The majority of samples (G11, G21, G51, GK, GI, GH, GE, GF) displayed medium concentrations, with U ranging from 125.12 to 2004 ppm, except for GE, which reached up to 2751 ppm. Th concentrations for these samples varied from 27 to 992 ppm. The samples thus show Th/U ratios between 0.07 and 0.54 (Table S2).

4.3. Zircon Trace Elements

The chondrite-normalized rare earth element (REE) patterns display a general left-dipping trend, indicating a depletion in light REEs and a higher content of heavy REEs compared to the whole-rock REE content (Figure 11a). There is a negative anomaly for Eu and positive anomaly for Ce present in the zircons from the granodiorite samples, indicating typical values for magmatic zircons [49]. The Eu anomaly (Eu/Eu*) values of zircons mostly vary between 0.1 and 0.22. The trace elements of zircons were plotted on various diagrams for rock type discrimination among granitoids, syenites, pegmatites, and mafic rocks (Figure 11b,c) [50]. The Yb versus U plot confirmed the zircons’ origin from the continental crust (Figure 11d) [51,52]. Additional discrimination diagrams, such as Nb/Yb versus U/Yb (Figure 11e), placed the zircons partially within the continental arc group and above [52]. The Th/U versus Nb/Hf diagram distinguishes zircons from “Within-plate” or non-orogenic and Arc-related or orogenic contexts (Figure 11f) [53] with all analyzed zircons falling within the orogenic field.

The Ti content of zircon was used as a thermometer to estimate the temperature of magma [38]. The titanium contents range from 1.33 to 53.78 ppm, and the Ti temperatures deduced range from 600 to 915 °C, most of the values falling within 700 to 800 °C, with an average value of 733 °C. The histogram of calculated temperature (Figure 11i) shows the peak of temperature between 730 and 740 °C. Zircon crystallization temperatures (Ti-in-zircon) show a negative correlation with Hf content (Figure 11h), where higher-temperature zircons are systematically depleted in Hf relative to lower-temperature grains.

5. Discussions

The Fourque granodiorite’s distinct facies reveal significant geological and chronological relationships with other Variscan intrusions within the Pyrenean Axial Zone. These similarities emphasize the importance of studying the intrusion to refine our understanding of the tectonic and magmatic events that led to its emplacement. This study not only enhances knowledge of its geochemical characteristics, magmatic evolution and timing of intrusion but also contributes to a broader understanding of Variscan plutons within the Pyrenean Axial Zone.

5.1. Geochronological Framework

Based on previous research and the new U–Pb dating data obtained in this study, the aim is to interpret the age, geochemistry, and isotopic characteristics of the Fourque granodiorite. This study seeks to establish and constrain the emplacement framework of this intrusion and link it to the magmatic and tectonic events described by previous studies [5,6,8].

The Axial zone of the Pyrenees primarily comprises Precambrian to Paleozoic metasediments intruded by numerous late Variscan granitic plutons, defining its suprastructure [8]. These plutons are typically associated with extensive crustal reworking due to tectonic processes related to the Variscan orogeny which occurred between 330 and 290 million years ago [1]. Previous research has established crucial age constraints for these intrusions, revealing a complex magmatic history. The emplacement of these calc-alkaline plutons occurred between 312 and 295 Ma [8,26,56,57,58]. The Bordères-Louron pluton has been dated at 309 ± 4 Ma, while the Saint Arnac pluton is dated at 308.3 ± 1.2 Ma [59]. The Quérigut massif has a similar age of 307 ± 3 Ma [60]. The Mont-Louis-Andorra intrusion is slightly younger at 305 ± 5 Ma [61]. The most adjacent intrusion, Bassiès, is dated at 312 ± 3 Ma [62]. Most of the U–Pb dating of the plutons fall within a 10-million-year period between 300 and 310 Ma, according to the timeline of emplacement ages for the main plutonic massifs in the Axial Zone of the Pyrenees [8]. Eight out of nine samples from the Fourque granodiorite show zircon U–Pb dating ages ranging from 304.4 ± 2.3 to 308.4 ± 2.6 Ma, depending on the granodiorite sample, and yield older zircon U–Pb ages compared to the previous study [6], which reported ages of 295 ± 2 Ma.

The age discrepancy observed between the current dataset and previous studies, such as Poitrenaud (2018) [6], where an age of 295 ± 2 Ma was reported for the emplacement of the La Fourque intrusion, may be attributed to several factors. One possibility is that the analyzed zircons in this study were sourced from the border regions of the intrusion, where crystallization likely occurred earlier in the magmatic history. Additionally, the geochemical heterogeneity observed across the intrusion, with zircon grains potentially influenced by contamination or inheritance from older crustal material, could have led to some zircons recording older ages. Despite these variations, the mean crystallization age of approximately 305 Ma obtained from the analyzed samples suggests that the Fourque intrusion primarily crystallized around this time, aligning with the D2 deformation phase (308–301 Ma) of the Variscan orogeny.

The zircon U–Pb results and their weighted mean ages (Figure 10) indicate that sample G11, located closest to the intrusion margin, yielded an age of 313.1 ± 2.6 Ma, correlating with its proximity to the border and suggesting it represents the earliest crystallized phase. The age discrepancies between previous studies (~308–313 Ma) and the current dataset, combined with the distinct geochemical and petrographic facies observed across the intrusion, support a model of multi-stage magmatism involving successive magma pulses over millions of years. The mean crystallization age of approximately 305 Ma from the sample analyzed in this study, with individual ages distributed between ~300–310 Ma, further corroborates this interpretation. These results imply that the Fourque intrusion crystallized incrementally over the D2 deformation period (308–301 Ma) of the Variscan orogeny. This period coincided with the intrusion of various plutons within the Pyrenean Axial zone, providing a coherent temporal framework for the observed magmatic events.

5.2. Granite and Magma Type and Classification

Previous research has established that granitic rocks are classified into I-, S-, M-, and A-types based on their distinct geochemical characteristics [63,64]. Understanding the genetic type of magmatic rocks is essential for discerning the magma source region, magmatic processes, and tectonic setting [65]. The Fourque granodiorite is characterized by low (Zr + Nb + Ce + Y) contents and distinct (K₂O + Na₂O)/CaO ratios, indicating a transitional I-type to S-type affinity [64]. The main core facies, peraluminous monzogranite samples, exhibit the typical S-type characteristics, with strongly peraluminous properties, marked by high CaO and Na₂O contents and elevated ASI ratios, around 1.4 (Figure 6a). The alkali granodiorites, characterized by the samples G21 and GK, exhibit I-type affinity, characterized by their weakly peraluminous nature which exceeds the typical range for I-type granites [66]. Additionally, the highest La (46.1–54.1 ppm) and Ce (96.5–106 ppm) contents of the alkali granodiorites align with compositions observed in I-type granites [67], although these elements are not commonly used as primary discriminants for magma genetic classification. Instead, classification is more reliably inferred from high-K calc-alkaline affinity, magnesian composition, and fractional crystallization trends. The coexistence of distinct facies confirms that the intrusion resulted from multiple magma pulses with varying geochemical signatures. However, the observed gradual transition between the core and apical facies suggests that these pulses originated from a progressively evolving magma source. The granite tectonic discrimination diagram SiO₂ versus FeOt/(FeOt + MgO) [39] indicates a magnesian granite series (Figure 6a). Mg# values range from 0.344 to 0.526, which suggests a moderate degree of differentiation. Additionally, Sr/Y ratios (12.56–17.57) indicate a moderately evolved high-K calc-alkaline signature, consistent with fractional crystallization trends.

Based on available data, the Fourque granodiorite can be categorized into three distinct groups, considering their petrographic characteristics and REE patterns. The first group, consisting of samples from the southern part of the intrusion as well as the upper part of the northern sampling area, exhibits significant heavy REE depletion. This pattern aligns with the trend observed in previous studies of microcline-rich granodiorite [5] (Figure 8). The second group corresponds to REE patterns that are slightly depleted in heavy REEs (HREEs) compared to the first group, similar to those observed in porphyritic granodiorite described by Raimbault and Kaelin (1987) [5]. The last group displays REE patterns consistent with garnet-bearing granodiorite. This gradual depletion of heavy REEs across the samples (Figure 8a) supports the hypothesis of two successive crystallization stages in the evolution of the Fourque granodiorite: an initial stage dominated by mafic mineral fractionation (e.g., amphibole, biotite, titanite) and a later stage characterized by felsic differentiation, with K-feldspar and accessory minerals concentrating incompatible elements. These findings indicate a progressive magmatic evolution rather than a single crystallization event.

Fractional crystallization is further highlighted by trends observed in the Harker diagrams (Figure 7), where increasing SiO₂ correlates with decreasing MgO, Fe₂O₃, and TiO₂ decrease, while K₂O concentration increases. The polyphase nature of this intrusion, with distinct facies, aligns with other Variscan plutons, demonstrating progressive differentiation and emplacement through multiple pulses over a few million years [57].

Tectonic classification diagrams provide additional insight. The Peccerillo and Taylor (1976) diagram (Figure 6b) places most samples within the high-K calc-alkaline series [40]. The Debon and Le Fort (1983) multicationic plot (Figure 6c) situates most samples between the granodiorite and granite fields [41]. The Middlemost (1994) SiO₂ vs. (Na₂O + K₂O) classification diagram (Figure 6d) places most samples within the granite field, near the granodiorite boundary [42]. The Co-Th plot (Figure 6e) indicates high-K calc-alkaline and shoshonite series affinities, specifically within the D/R* field, reflecting their evolved magmatic nature [43]. The feldspar triangle classifies the intrusion between granite and trondhjemite groups [45]. Tectonically, the R1-R2 plot (Figure 6g) indicates a syn-collision setting, consistent with the Variscan orogeny [46]. The REE spectra normalized to chondrite/primitive mantle (Figure 8) exhibit an enrichment of large ion lithophile elements (LILE: K, Rb, Ba) relative to high field strength elements (HFSE: Nb, Ta, Ti), suggesting a significant contribution from crustal components. Without isotopic data (e.g., Sr–Nd–Pb systematics), it is difficult to precisely quantify the mantle versus crustal contributions to magma genesis, but the observed geochemical features suggest a hybrid origin with significant crustal assimilation.

Magma differentiation was primarily driven by fractional crystallization, as demonstrated by Harker diagrams (Figure 7) where increasing SiO₂ correlates with decreasing concentrations of MgO, Fe₂O₃, and TiO₂, while K₂O concentrations increase, aligning with a fractionating mineral assemblage. Further evidence of dominant fractional crystallization processes is provided by the high concentration of light REEs, with ΣLREE values ranging from 95.5 to 132.2, and the nearly parallel trends of REE patterns in the average chondrite-normalized diagrams (Figure 8a,b). These trends also indicate that early-stage magma evolution involved garnet fractionation, which may have played a role in shaping the observed REE depletion.

The geochemical classification and tectonic setting of the Fourque granodiorite have several implications. Firstly, whole-rock geochemistry, including Nb-Ta depletion, Sr enrichment, and LILE/HFSE fractionation, suggests that the magma source was influenced by crustal contamination or subduction-related metasomatism, confirming crustal contamination of the magma source, as the influence of a subduction-related metasomatism is inconsistent with the syn- to post-collisional geological setting of the area. However, without isotopic data, the precise mantle versus crustal contribution remains uncertain. Secondly, the high-K calc-alkaline affinity is consistent with magmatism in active continental margins or collisional orogenic settings, reinforcing its emplacement during the Variscan orogeny. Thirdly, geochemical trends in the Harker diagrams indicate that magma differentiation was primarily controlled by fractional crystallization, with early-formed mafic minerals such as amphibole, biotite, and plagioclase being removed, leading to an enrichment of alkali elements in the residual melt.

The petrological and geochemical evidence suggests that the Fourque granitoids represent a moderately fractionated high-K calc-alkaline I-type granite with peraluminous characteristics. These findings align with Soder et al. (2018), who described how subduction of continental crust during the Variscan orogeny led to metasomatism of the lithospheric mantle [68]. This process resulted in the generation of potassic and ultra-potassic magmatism, followed by partial melting of enriched mantle domains to produce high-K calc-alkaline I-type granitoids typical of the region’s post-collisional setting. Furthermore, the Fourque granodiorite exhibits a compositional gradation from more mafic outer zones to felsic cores, a feature commonly observed in late-Variscan plutonic massifs of the Pyrenees [69].

5.3. Tectonic Context

Regional tectonic studies suggest that the activation of shear zones played a crucial role in magma ascent, facilitating pluton emplacement [29]. U–Pb zircon dating indicates that most of these plutons were emplaced from the Upper Carboniferous to the Lower Permian, especially between 300 and 310 Ma. This period is marked by a phase of crustal thickening over a few million years, comprising three subphases [8]. The first subphase involved North-South directed crustal thickening and shortening in the upper and middle crust between 323 and 308 Ma. The second subphase was characterized by a syn-convergence phase, involving crustal-scale lateral flow of the lower crust with East-West stretching, and concurrent high-temperature/low-pressure (HT/LP) conditions reaching partial melting around 306 Ma [70]. This phase concluded with crustal-scale folding, leading to upper crust folds and middle crust open folds, which caused the formation of gneissic domes until 301 Ma. The final subphase involved deformation during the cooling of the Variscan crust, with North-South shortening and the development of shear zones. The Fourque intrusion, like other Variscan plutons, is associated with the D2 syn-convergence phase which began around 308 Ma [71]. U–Pb zircon dating confirms the emplacement event of the Fourque granodiorite between 308 and 304 Ma, aligning it with this significant tectono-metamorphic phase. The R1-R2 diagram [46] further confirms the syn-collision context of the formation of the granodiorites.

Ti-in-zircon serves as a valuable tool for determining the crystallization temperature of zircon [38]. In the case of the Fourque granodiorite, calculated Ti-in-zircon temperatures range from 600 to 915 °C, mostly situated between 800 and 700 °C. The more representative median-T (T_med) value of the series of zircon analyses reaches 734 °C [72]. This calculated temperature aligns with previous analyses that were conducted on micas and amphiboles using microprobe techniques, revealing a temperature exceeding 800 °C at a pressure of approximately 3 kbar (equivalent to a depth of 8 to 10 km) [6], characterizing the unusual high temperature and low-pressure conditions of the tectonic context. The high-temperature emplacement of the Fourque granodiorite, with Ti-in-zircon thermometry values reaching 800 °C, played a role in the prograde skarn development observed in the region. The heat flux from the intrusion drove contact metamorphism in the carbonate host rocks, promoting decarbonation reactions and the formation of calc-silicate minerals. However, no direct geochemical link has been established between the intrusion and the tungsten mineralization, suggesting that later hydrothermal events were responsible for ore deposition. The trace elements of zircons analyzed using LA-ICP-MS were plotted on various diagrams to discriminate rock types among granitoids, syenites, pegmatites, and mafic rocks (Figure 11b,c) [50]. The Yb versus U plot confirmed the zircons’ origin from the continental crust (Figure 11d) [51,52]. Further discrimination diagrams, such as Nb/Yb versus U/Yb (Figure 11e), partially placed the zircons within the continental arc group and above, the higher Nb/Yb and U/Yb ratio suggesting crustal input [52,73]. The Th/U versus Nb/Hf diagram was used to differentiate zircons from “Within-plate” or non-orogenic and arc-related or orogenic contexts, in that case confirming the orogenic context (Figure 11f) [53]. The age range of the zircons, plotted in Figure 11g, shows a span between 295 and 315 Ma. The Ti-in-zircon temperature data, plotted against Hf content (Figure 11h), reveal a magmatic differentiation trend with cooling mostly from 800 to 700 °C. The results confirm that the Fourque granodiorite originated from the Variscan orogenic setting, with its magmatic evolution influenced by both mantle-derived and crustal components. Whole-rock geochemistry, including high-K calc-alkaline to shoshonitic signatures, suggests a primary contribution from a subduction-modified mantle source. However, zircon trace element data, peraluminous characteristics, and LILE enrichment indicate that crustal assimilation also played a role in modifying the magma composition. This suggests that the granodiorite evolved through a combination of fractional crystallization and crustal contamination rather than solely through partial melting of the continental crust. Alternative mechanisms, such as magma mixing between mantle-derived and crustal melts, source heterogeneity (e.g., metasomatized lithospheric mantle), or variable degrees of partial melting, could also contribute to the observed geochemical variability. While isotopic data (e.g., εNd, Sr isotopes) are currently unavailable to definitively quantify the extent of contamination, the elevated LILE/LREE ratios, presence of inherited zircon cores, and geochemical trends (e.g., A/CNK variability) align with assimilation of metasedimentary crust. Future isotopic studies are required to distinguish the relative contributions of crustal contamination, source heterogeneity, and fractional crystallization in shaping these signatures. The Fourque granodiorite aligns with the broader magmatic trends of Variscan plutons in the Pyrenees, displaying a compositional gradation from more mafic outer zones to felsic cores, consistent with an evolving magma system [69]. An example of such magmatic differentiation can be seen in the Saint Laurent—La Jonquera pluton, where zonation between different magmatic bodies reflects a differentiation process [74].

6. Conclusions

The Fourque granodiorite mainly exhibits emplacement ages between 304.4 ± 2.3 and 308.4 ± 2.6 Ma, aligning with the D2 Variscan tectonic event (308–301 Ma) and highlighting its synchronicity with other regional Variscan intrusions.

Zircon trace elements confirm the granodiorite’s continental crust origin, associating it with syn-orogenic, and Ti-in-zircon thermometry values primarily varying from 700 to 800 °C, with T_med reaching 734 °C, in high-temperature, low-pressure conditions during emplacement. These temperatures were sufficient to drive contact metamorphism in the surrounding carbonate rocks, facilitating prograde skarn formation.

The granodiorite exhibits a distinction between an alkalic facies, characterized by high Na₂O and low SiO₂, and an S-type monzogranite core with high-K calc-alkaline composition. This geochemical contrast suggests the involvement of multiple magma pulses derived from distinct sources. The alkalic facies likely originated from a sodium-rich magma, while the monzogranite core reflects a more evolved, high-K calc-alkaline magma. The observed progressive differentiation, fractional crystallization, and tectonic setting indicate that these magmas underwent assimilation and mixing during intrusion, evolving in composition as they emplaced in the host rock.

The classifications overall align with the broader Variscan plutonic pattern in the Pyrenean Axial Zone.