Petrogenesis and U–Pb Dating of Variscan S-Type Granites from the Junqueira Batholith (Central Iberian Zone)

Abstract

The Junqueira massif is a syn- to late-kinematic Variscan batholith intruded into Ediacaran-Cambrian metasedimentary rocks of the Douro-Beiras Supergroup (DBSG) in the Central Iberian Zone. The batholith occupies the axial zone of the Porto-Viseu antiform, a large NW-SE trending megascopic domal structure formed during the last Variscan ductile deformation event. Field and petrographic evidence reveals that the Junqueira batholith comprises several units of leucocratic granites distinguished by variations in grain size and relative proportions of the main rock-forming minerals. This work provides new petrographical, geochemical, Sr–Nd isotope data and ID-TIMS U–Pb ages for the Junqueira batholith. U–Pb dating of zircon and monazite by ID-TIMS gives a crystallization age of ca. 312–309 Ma for this batholith. Combined geochemical and Sr–Nd isotopic data for the different granite units (ASI > 1.1; high SiO₂ and K₂O contents, low CaO, MgO, Ba, Sr, moderately fractionated REE patterns, Eu negative anomalies, ⁸⁷Sr/⁸⁶Sr_i > 0.713, εNd₃₁₀ = −3.5 to −5.9; T_DM = 1.1–1.4 Ga) support a provenance by fluid-absent melting processes of exclusively supracrustal sources (mainly metapelites), similar to the adjoining country rocks of the Beiras Group of the DBSG.

1. Introduction

Granitoids are essential components of Earth’s continental crust and play a key role in its evolution [1,2,3,4]. In continent–continent collisional orogens, strongly peraluminous two-mica granites formed through partial melting of aluminous metasedimentary rocks, such as metapelites and metapsammites, are classically referred to as “S-type” granites [5,6,7]. The anatexis of fertile supracrustal materials can occur under different melting conditions, including influx of external fluids (fluid-present melting) or incongruent breakdown of hydrated mineral phases such as muscovite and biotite (fluid-absent melting), yielding magmas with distinct geochemical signatures (e.g., [8,9,10,11] and references therein).

S-type granites are typically strongly peraluminous (muscovite- and/or cordierite-bearing) (e.g., [11] and references therein). Additionally, their whole-rock isotopic compositions (initial ⁸⁷Sr/⁸⁶Sr ratios > 0.708; negative initial εNd values and δ¹⁸O values > 10 ‰) provide further distinction from other granite types [5]. The study of S-type granitic systems (including field observations/relations, petrology, mineralogy, major- and trace-element geochemistry, and isotopic composition) is fundamental to understanding the nature of source rocks and evolving melting conditions in the continental crust and providing critical insights into crustal reworking processes during the final stages of orogenic evolution (e.g., [3,11,12] and references therein).

The Iberian Massif of the Variscan orogenic belt in Europe hosts abundant post-collisional plutonic bodies ranging from strongly peraluminous granites (S-type) to metaluminous or weakly peraluminous, calc-alkaline, medium- to (rarely) high-K granites (with I-type or transitional I- and S-type affinities). Most of these granitoids were intruded into metasedimentary rocks of low to high metamorphic grade in the Late Carboniferous, at the end of the Variscan collision orogeny (e.g., [13,14,15,16,17,18,19]).

Previous studies in the Iberian Massif have examined the complex interplay between tectonics, metamorphism and magmatism, revealing that the strongly peraluminous granitoids correspond to purely or dominantly crustal melts (e.g., [17,19,20,21,22,23,24]), while the calc-alkaline, medium- to (rarely) high-K, metaluminous to weakly peraluminous granites are diversely interpreted as products of partial melting of infracrustal materials (e.g., [16,18,25,26,27]) or as the result of hybridization between mantle-derived magmas and felsic crustal melts (e.g., [13,14,15,20,28,29,30,31]).

In this paper, we use new petrographic, geochemical, Sr–Nd isotope and ID-TIMS U–Pb age data for the late-Variscan Junqueira batholith (occurring in the central sector of the Central Iberian Zone), in combination with pertinent data from the literature, to further investigate the genesis of post-collisional S-type granitoids in Iberia, constrain their putative sources and melting conditions, deduce the timing of granite emplacement and explore the links between magmatic and tectonic activity during the evolution of the orogen.

2. Geological Setting

The Iberian Massif represents the southwestern extension of the European Variscan Belt (Figure 1a) formed at the end of Paleozoic as the result of oblique continent-continent collision and merging of Gondwana, peri-Gondwanan terranes and Laurussia (e.g., [32,33,34]). As shown in Figure 1b, the Iberian Massif comprises six major tectonostratigraphic units: the Cantabrian (CZ), West Asturian-Leonese (WALZ), Galicia-Trás-os-Montes (GTMZ), Central Iberian (CIZ), Ossa-Morena (OMZ) and South Portuguese (SPZ) zones [35,36,37].

The Cantabrian (CZ) and South-Portuguese (SPZ) zones represent foreland (external) thin-skinned fold-thrust belts located at the Gondwanan and Laurussian (Avalonia-Meguma) flanks of the orogen, respectively, whereas the West Asturian-Leonese (WALZ), Central Iberian (CIZ) and Galicia-Trás-os-Montes (GTMZ) zones, together with the Ossa-Morena Zone (ZOM), constitute the hinterland (internal) orogenic domain of the Iberian Massif (e.g., [39,40,41,42] and references therein).

On a broad scale, the Iberian Variscan belt is part of a major arc-shaped macrostructure extending into the Armorican Massif, known as the Ibero-Armorican Arc (Figure 1a). Current models for the origin of the Ibero-Armorican Arc are still debated (e.g., [39,41,43,44,45,46,47,48,49,50,51,52,53] and references therein).

The studied area, extending from Porto to Viseu (Figure 2), is located in the Portuguese sector of the CIZ (the innermost zone of the Iberian Massif). In the CIZ, the oldest exposed rocks belong to the Schist-Graywacke Complex, also called the Douro-Beira Supergroup (DBSG; Figure 2), a thick, monotonous turbidite-like megasequence of Ediacaran-Cambrian age, consisting of interlayered metapelites and metagraywackes and minor calc-silicate and metaconglomerate horizons [54,55,56,57,58]. In Portugal, the DBSG megasequence comprises two lithostratigraphic units: the Beiras Group in the southern CIZ and the Douro Group in the northern CIZ, showing distinctive geochemical and isotope signatures (e.g., [59,60]).

Across the CIZ, the Ediacaran-Cambrian metasediments of the DBSG are unconformably overlain by Early to Middle Paleozoic shallow marine siliciclastic deposits and volcanic rocks formed at the passive margin of northern Gondwana (e.g., [16,39,61,62] and references therein). These sequences are exposed in the core of narrow, NW–SE trending synclines crossing the CIZ, as the Porto-Sátão syncline shown in Figure 2. The stratigraphic record of the CIZ ends with the deposition of Upper Carboniferous molasse sediments in elongated fault-bounded intracontinental basins (Figure 2) (e.g., [15]).

The dominant Variscan regional structural pattern in the CIZ results from three main ductile deformation events (D₁, D₂ and D₃). The D₁ deformation event started at around 370–350 Ma and was marked by crustal thickening and prograde intermediate-P, Barrovian-type regional metamorphism (e.g., [15,63,64]). In the Porto-Viseu area, the D₁ contractional structures include tight, regional-scale, subvertical folds and a penetrative axial plane S₁ schistosity imprinted in all pre-Carboniferous formations [15].

The D₂ deformation event occurred during Lower-Middle Carboniferous (at ca. 337–320 Ma) and corresponds to an extensional episode responsible for the onset of pervasive migmatization at deep crustal levels and exhumation of middle crust high-grade rocks (e.g., [15,65,66,67,68,69,70,71] and references therein). Minor folds and a flat-lying S₂ schistosity are occasionally recognized in medium- to high-grade metamorphic rocks from the studied area [15]. The peak metamorphic conditions were attained during the D₂ deformation event (e.g., [15,65,67,70,71,72,73] and references therein). P–T estimates for non-migmatitic rocks of some anatectic complexes from the Portuguese sector of the CIZ yielded P = 5.7 kbar and T = 635 °C (Figueira de Castelo Rodrigo–Lumbrales Anatectic Complex) and P = 5.0 ± 1.0 kbar and T = 761 ± 50 °C (Porto-Viseu Metamorphic Belt), while migmatization would have occurred at consistently higher P–T conditions (P = 7.6–7.9 kbar and T = 770–810 °C) [70,72].

At ca. 320–305 Ma, the CIZ was affected by a transpressional deformation event (D₃) accompanied by LP/HT metamorphism and further retrogression (e.g., [15,41,67] and references therein). According to some authors, the D₃ event is linked to the oroclinal bending of the initially linear Variscan belt, generating the Ibero-Armorican Arc (Figure 1a) (e.g., [61]), although this view is disputed by other authors (e.g., [41,74]). In the Porto-Viseu area, the D₃ tectonic event was mostly coaxial with D₁ and involved the development of large wavelength upright folds and crustal-scale transcurrent shear zones with sinistral or dextral movement, such as the dextral Porto–Tomar shear zone (PTSZ), the sinistral Douro–Beira shear zone (DBSZ) and the sinistral Juzbado–Penalva do Castelo shear zone (JPCSZ) (Figure 2), that have accommodated part of the shortening associated with the final stages of continental collision (e.g., [15,31,41,75] and references therein). Overall, the D₃ transcurrent regime was responsible for the formation of large antiforms and synforms with NW-SE to NNW-SSE strikes, refolding and steepening of D₁ + D₂ structures and development of mylonitic shear bands (e.g., [15,31]).

Lastly, from Late Carboniferous to Permian times, the basement of the CIZ experienced brittle-ductile deformation and subvertical faulting under a dominant extensional tectonic regime associated with the final gravitational collapse of the Variscan orogen.

During the post-collisional stage of the Variscan evolution, the CIZ was intruded by abundant volumes of granitoids spanning a wide spectrum of granite types. The vast majority of the post-collisional granitoids from the Portuguese sector of the CIZ were emplaced during and/or immediately after the last ductile deformation event (D₃), leading to their subdivision into syn-, late- and post-D₃ plutons (Figure 2) [76].

The syn-D₃ granitoids (ca. 320–305 Ma) of central-northern Portugal tend to form batholithic complexes, concordant with regional D₃ structures and comprise two main plutonic suites: (a) an early suite of calc-alkaline granodiorites and biotite monzogranites (ca. 322–311 Ma) and (b) a dominant suite of two-mica granites and leucogranites with strongly peraluminous signatures (ca. 320–308 Ma). In contrast, the late-post-D₃ granitoids (ca. 305–290 Ma) occur as discordant intrusions of calc-alkaline granodiorites and coarse porphyritic biotite granites associated with minor bodies of basic and intermediate igneous rocks and porphyritic and non-porphyritic biotite-muscovite granites (e.g., [13,14,15,17,19,21,24,25,27,30,31,59,68,77,78,79,80,81,82,83]).

3. The Junqueira Syn- to Late-D₃ Batholith

The syn- to late-D₃ suite of two-mica granites and leucogranites from the Junqueira massif constitute a large NW-SE trending elongated batholith (55 km long and 15 km wide), located in central-northern Portugal and intruded into Ediacaran-Cambrian metasedimentary rocks of the Douro-Beiras Supergroup (DBSG) of high metamorphic grade (Figure 2) (e.g., [15,20,70,73,84,85]). As shown in Figure 2, the Junqueira batholith occupies the core of a wide D₃ antiformal structure, the Porto-Viseu Metamorphic Belt (PVMB), consisting of metasediment-derived migmatites flanked to the NE and SW by metamorphic rocks of progressively lower metamorphic grade (amphibolite- to greenschist facies) (e.g., [15,20,70,73,84,85,86,87]). Within the PVMB, the metamorphic isograds are symmetrically arranged relative to the axial zone, defining a concentric zonal pattern, marked by the occurrence of migmatites (both metatexites and diatexites) in the core of the antiform, followed by sillimanite-, staurolite ± kyanite—and staurolite- bearing schists, biotite phyllites and biotite- and chlorite-slates towards both the northern and southern limbs (e.g., [15,20,70,73]).

Based on mineral assemblages and reaction textures observed in pelitic rocks of different crustal levels from the PVMB, Valle Aguado et al. [15] proposed that the tectonometamorphic evolution of the Porto-Viseu area involved a prograde regional metamorphic event of Barrovian-type, coeval with crustal thickening (D₁), followed by a decompression event at nearly isothermal conditions during D₂ extensional tectonics and ended with LP/HT metamorphism and further retrogression in an essentially transcurrent regime related to D₃ shear zones.

D₃ ductile deformation, continued tectonic exhumation of high-grade metamorphic rocks, and the emplacement of the Junqueira batholith appear to have occurred almost at the same time under the influence of the NW-SE Douro–Beira sinistral shear zone (DBSZ) and the NNW-SSE Porto–Tomar dextral shear zone (PTSZ) that bound the PVMB to the north and west, respectively (Figure 2).

However, structural relationships reveal that the Junqueira peraluminous two-mica granites and leucogranites are probably syn- to late-kinematic relative to D₃ as the different plutonic units of the batholith crosscut the high-grade metamorphic isograds and show only local evidence of solid-state deformation (shear bands, mylonitic fabrics, gneissic foliation). In general, the Junqueira granitoids exhibit a NW-SE trending magmatic flow fabric (defined by the alignment of micas and/or K-feldspar megacrysts), parallel to the D₃ regional structures from the adjacent host rocks, pointing to a relatively late intrusion for these magmas.

The Junqueira syn- to late-D₃ batholith comprises several facies (Silvares, Vouzela, Junqueira-Serra da Freita, Campia, Fajões and Routar) ranging from fine- to medium- and coarse-grained porphyritic and non-porphyritic two-mica granites (Figure 3 and Figure 4). Biotite schlieren and centimeter-sized metasedimentary xenoliths are rare and unevenly distributed throughout all facies. The contacts between the granite units and the country rocks are sharp and frequently marked by the occurrence of narrow bands of leucogranites [85]. However, occasional interpenetrations between granitic and metasedimentary material are observed. Internal contacts between granite facies vary from sharp to transitional, lobate and interdigitated.

Early attempts to date the intrusion of the different plutonic units of the Junqueira batholith using muscovite K–Ar ages and Rb–Sr whole-rock isochrons are compiled in Table 1 [85,88,89].

Table 1. Geochronological summary of the Junqueira batholith. Ms: muscovite; WR: whole-rock.

Granite Facies	Massif	Age	Method	Reference
Medium- to coarse-grained porphyritic two-mica granite (Silvares)	Silvares	316 ± 8 Ma	K–Ar in Ms	[89]
	Abraveses	315 ± 7 Ma	K–Ar in Ms	[89]
Medium- to coarse-grained two-mica granite (Vouzela)	Vouzela	314 ± 6 Ma	K–Ar in Ms	[89]
Fine- to medium-grained two-mica granite (Junqueira–Serra da Freita)	Boa Aldeia	305 ± 6 Ma	K–Ar in Ms	[89]
	Sra. do Crasto	303 ± 6 Ma	K–Ar in Ms	[89]
	Serra da Freita	329 ± 4 Ma *	WR Rb–Sr	[85]
Medium-grained porphyritic two-mica granite (Campia)	Sra. do Crasto	301 ± 6 Ma to 283 ± 5 Ma	K–Ar in Ms	[89]
	Serra da Freita	320 ± 3 Ma *	WR Rb–Sr	[88]

* Values were recalculated to the new ⁸⁷Rb decay constant [90].

Table 1. Geochronological summary of the Junqueira batholith. Ms: muscovite; WR: whole-rock.

Granite Facies	Massif	Age	Method	Reference
Medium- to coarse-grained porphyritic two-mica granite (Silvares)	Silvares	316 ± 8 Ma	K–Ar in Ms	[89]
	Abraveses	315 ± 7 Ma	K–Ar in Ms	[89]
Medium- to coarse-grained two-mica granite (Vouzela)	Vouzela	314 ± 6 Ma	K–Ar in Ms	[89]
Fine- to medium-grained two-mica granite (Junqueira–Serra da Freita)	Boa Aldeia	305 ± 6 Ma	K–Ar in Ms	[89]
	Sra. do Crasto	303 ± 6 Ma	K–Ar in Ms	[89]
	Serra da Freita	329 ± 4 Ma *	WR Rb–Sr	[85]
Medium-grained porphyritic two-mica granite (Campia)	Sra. do Crasto	301 ± 6 Ma to 283 ± 5 Ma	K–Ar in Ms	[89]
	Serra da Freita	320 ± 3 Ma *	WR Rb–Sr	[88]

* Values were recalculated to the new ⁸⁷Rb decay constant [90].

Figure 3. Simplified geological sketch map of the Porto-Viseu antiform (modified from [86,91,92]). Red stars represent the location of samples selected for U–Pb geochronology. Inset on the right shows the distribution of Variscan plutonic rocks and metasediments of the Douro-Beiras Supergroup (DBSG) in the CIZ.

Figure 3. Simplified geological sketch map of the Porto-Viseu antiform (modified from [86,91,92]). Red stars represent the location of samples selected for U–Pb geochronology. Inset on the right shows the distribution of Variscan plutonic rocks and metasediments of the Douro-Beiras Supergroup (DBSG) in the CIZ.

Figure 4. Field aspects of the different facies of the Junqueira syn- to late-kinematic batholith. (a) Vouzela medium- to coarse-grained two-mica granite; (b) magmatic flow alignment of micas in the Vouzela granite; (c) Junqueira fine- to medium-grained two-mica granite; and (d) Campia medium-grained porphyritic two-mica granite.

Figure 4. Field aspects of the different facies of the Junqueira syn- to late-kinematic batholith. (a) Vouzela medium- to coarse-grained two-mica granite; (b) magmatic flow alignment of micas in the Vouzela granite; (c) Junqueira fine- to medium-grained two-mica granite; and (d) Campia medium-grained porphyritic two-mica granite.

The Silvares medium- to coarse-grained K-feldspar porphyritic two-mica granite occurs in the southeastern sector of the Junqueira batholith (Figure 3) and yielded a K–Ar muscovite age of 316 ± 8 Ma at Silvares (Table 1; [89]) and a similar K–Ar muscovite age of 315 ± 7 Ma at Abraveses (Table 1; [89]).

The Vouzela medium- to coarse-grained two-mica granite containing small and sparsely distributed K-feldspar megacrysts is exposed both in the northwestern and southeastern termination of the batholith (Figure 3) and provided a K–Ar muscovite age of 314 ± 6 Ma (Table 1; [89]).

The Junqueira-Serra da Freita fine-to-medium-grained two-mica granite constitutes the main facies of the batholith (Figure 3). A transition from magmatic to solid-state flow is frequently seen in this granite. Metasedimentary xenoliths (micaceous restites with sillimanite) and biotite schlieren may occur in the Junqueira-Serra da Freita granite, particularly in the vicinity of the contacts with the host metamorphic rocks. K–Ar muscovite ages of 305 ± 6 Ma and 303 ± 6 Ma were reported by Neves [89] for samples from the Junqueira-Serra da Freita facies collected at Boa Aldeia and Sra. do Crasto, respectively (Figure 3; Table 1). A significantly older whole-rock Rb–Sr age was obtained by Reavy et al. [85] for the Serra da Freita massif (329 ± 4 Ma, Table 1). However, more precise ID-TIMS U–Pb monazite geochronological data for the Junqueira granite gave an emplacement age of 307.8 ± 0.7 Ma for this intrusive unit [15], suggesting that the “older” Rb–Sr age could reflect disturbance of the Rb–Sr isotope system and should not be used to infer the time of emplacement.

Due to the transitional contact relationships between the Campia and Junqueira granites, the Campia medium-grained porphyritic two-mica granite is interpreted as a textural variant of the Junqueira-Serra da Freita facies [86]. K–Ar muscovite ages for the Campia facies from Sra. do Crasto (Figure 3) range from 301 ± 6 Ma to 283 ± 5 Ma (Table 1; [89]). The whole-rock Rb–Sr age of 320 ± 3 Ma obtained for this granite [88] is also probably fictitious and does not correspond to the crystallization age.

The Fajões fine-grained two-mica granite forms small, dispersed masses within the Junqueira batholith (Figure 3). According to Pereira et al. [91], this granite unit is intrusive in the Vouzela and Junqueira-Serra da Freita granites and may therefore represent one of the youngest facies of the batholith.

Finally, the highly differentiated Routar fine- to medium-grained muscovite granite tends to constitute very small bodies with little or no evidence solid-state deformation in the Junqueira batholith (Figure 3). The presence of disrupted fragments of the other granite units within Routar granite favors a late-stage emplacement for this unit.

As shown in Figure 2, the Junqueira batholith is truncated in the east by the late-post-D₃ Viseu batholithic complex. ID-TIMS U–Pb zircon ages for the Viseu granite suite indicate that pluton assembly took place from 299.4 ± 0.4 Ma to 296.0 ± 0.6 Ma [31], providing further constraints for the emplacement age of the Junqueira granites, which are clearly older than the Viseu late- to post-D₃ granites.

4. Analytical Methods

Representative whole-rock samples from the different plutonic units of the Junqueira batholith were analyzed for major and trace elements (Ba, Cr, Ni, Sr, V, Zr) by inductively coupled plasma atomic emission (ICP-AES) at the University of London Imperial College (UK). Rb was analyzed by flame emission spectrometry. Rare earth elements (REE), Y and Sc in subsets of these samples were determined at the University of London Royal Holloway and Bedford New College (RHBNC) using cation exchange separation and ICP-AES [93,94]. Analytical errors are less than 5% for most elements [95,96].

Additional major- and trace element data (including REE) were obtained at Activation Laboratories (Ontario, Canada) using the 4Lithoresearch analytical package. Major element compositions and Sc, Be, V, Sr, Zr and Ba contents were determined by ICP-AES, while the remaining trace elements were measured by inductively coupled plasma mass spectrometry (ICP-MS). Further details on analytical techniques, calibration and detection limits can be found at http://www.actlabs.com/.

Strontium and neodymium isotope analyses were performed at the Laboratory of Isotopic Geology of the University of Aveiro (LGI-UA, Portugal). Aliquots of whole-rock powders were dissolved in a HF-HNO₃ mixture in pressurized Parr vessels for 3 days. After dissolution, sample solutions were dried and retaken in HCl for chemical separation. Cation exchange chromatography was used to extract Sr and REE and separate Nd from the total REE fraction. Sr and REE were extracted in quartz glass columns filled with Biorad AG50W-X8 resin, while Ln Resin ion-exchange resins were used to separate Nd from the total REE fraction.

⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd isotope ratios were measured at LGI-UA on a VG Sector 54 multicollector thermal ionization mass spectrometer (TIMS) operating in dynamic mode. The measured Sr and Nd isotope ratios were corrected for mass fractionation using normalization values of ⁸⁶Sr/⁸⁸Sr = 0.1194 and ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219 [93]. Repeated analysis of international standards SRM-987 and JNdi-1 yielded average values of ⁸⁷Sr/⁸⁶Sr = 0.710258 ± 18 (2σ; n = 12) and ¹⁴³Nd/¹⁴⁴Nd = 0.5121006 ± 69 (2σ; n = 12), respectively. Concentrations of Rb, Sr, Sm and Nd were obtained by ICP-MS at Activation Laboratories. The decay constants used for the Rb–Sr and Sm–Nd isotope systems are 1.3972 × 10⁻¹¹ a⁻¹ [90] and 6.524 × 10⁻¹² a⁻¹ [94], respectively. The calculation of initial ⁸⁷Sr/⁸⁶Sr ratios and εNd values was based on the isotopic ratios for the Uniform Reservoir (UR) and Chondritic Uniform Reservoir (CHUR) of DePaolo and Wasserburg [97]. Depleted mantle model ages (T_DM) were calculated according to DePaolo and Wasserburg [97], using ¹⁴³Nd/¹⁴⁴Nd = 0.513151 and ¹⁴⁷Sm/¹⁴⁴Nd = 0.219 for the depleted reservoir [98]. For samples with highly fractionated ¹⁴⁷Sm/¹⁴⁴Nd ratios (¹⁴⁷Sm/¹⁴⁴Nd > 0.12), a correction to an average crustal value of 0.12 [98] was applied.

Zircon and monazite crystals from two representative samples of the Vouzela and Junqueira-Serra da Freita granites were selected for ID-TIMS U–Pb geochronology (Figure 3). About 5 kg of each sample was crushed with a steel jaw-crusher and sieved at the Department of Geosciences of the University of Aveiro. Zircon and monazite were then concentrated by a combination of Frantz magnetic and heavy liquid mineral separation techniques and further handpicked under a binocular microscope. In order to avoid possible inheritance, the most idiomorphic and transparent crystals were selected for U–Pb geochronology.

The U–Pb isotopic analyses were performed at the Department of Geosciences of the University of Oslo, Norway. The zircon fractions selected for ID-TIMS were pre-treated with chemical abrasion [99]. Zircon annealing was carried out at 900 °C for three days and followed by partial dissolution in HF at 180 °C overnight. Monazite crystals were not abraded. Both zircon and monazite fractions were washed with HNO₃, acetone and ultra-pure H₂O and then transferred into Teflon minibombs, spiked with a mixed ²⁰²Pb-²⁰⁵Pb-²³⁵U tracer solution, and dissolved at ca. 190 °C on a hotplate, according to the procedure outlined by Krogh [100] and modified by Corfu [101].

Uranium and lead isotopic compositions were measured on a Finnigan MAT 262 multicollector thermal ionization mass spectrometer at the Isotope Laboratory of the Department of Geosciences of the University of Oslo (Norway). Total procedure blanks were ≤2 pg for Pb and 0.1 pg for U. All isotopic ratios were corrected for mass fractionation, blank and initial common-Pb after the model of Stacey and Kramers [102]. The decay constants for the U–Pb isotopic system are those of Jaffey et al. [103]. Data plotting and calculations were performed using IsoplotR software (version 6.6) [104].

For the following discussion, we have also used mineral compositions and whole-rock geochemical and Sr–Nd isotopic data from Neves [89], Beetsma [13] and Valle Aguado et al. [15], included in Supplementary Tables S1–S3.

5. Results

5.1. Petrography and Mineral Chemistry

The main petrographic features of the different plutonic units of the Junqueira batholith were described in previous works [13,15,85,89]. All granite facies contain quartz, plagioclase, K-feldspar, muscovite and biotite as major rock-forming minerals. Apatite, zircon, monazite and ilmenite are ubiquitous accessory phases in these granitoids, while sillimanite, andalusite, tourmaline and rutile show a more random distribution. Secondary alteration minerals include fine-grained secondary muscovite replacing feldspars and late-stage chlorite pseudomorphs after biotite.

Equigranular hypidiomorphic textures are dominant in the fine-grained, fine-to-medium-grained and medium-to-coarse-grained granites of Fajões, Junqueira-Serra da Freita, Routar and Vouzela (Figure 3), whereas the Campia medium-grained and the Silvares medium-to-coarse-grained granites exhibit distinctive porphyritic textures due to the occurrence of large K-feldspar megacrysts (Figure 3). The distinction between individual granite facies is based on variations in modal abundances, grain size and texture (porphyritic/non-porphyritic). However, irrespective of these differences, the Junqueira granites share significant petrographic similarities.

Quartz is characteristically anhedral and interstitial and shows undulose extinction and incipient subgranulation. Small, rounded quartz inclusions within K-feldspar megacrysts are common in the porphyritic varieties. Vermicular intergrowths between quartz and plagioclase (myrmekites) may occasionally develop at the contacts between plagioclase and K-feldspar.

K-feldspar, ranging from orthoclase to microcline, is present in the groundmass of all the studied granites as anhedral to subhedral crystals (Figure 5a). In the porphyritic facies, K-feldspar can also occur as subhedral megacrysts with well-developed cross-hatch twinning, perthitic textures (film, vein and patch albite exsolution lamellae) and inclusions of biotite and plagioclase (Figure 5b). Chemical analyses for both megacrystic and groundmass K-feldspar overlap and fall within a rather restricted spectrum of orthoclase contents (Or₈₅ to Or₉₂).

Plagioclase is mostly subhedral, weakly zoned and twinned on the albite polysynthetic law (Figure 5c). White mica alteration, particularly towards the center of the crystals is frequently seen. Plagioclase compositions vary from oligoclase to almost pure albite-(An₂-An₁₁) and show limited variation across the different granite facies.

Primary muscovite is often the prevalent mica in the Junqueira granites, occurring either as single isolated flakes or intergrown with biotite. Major element compositions for the primary muscovite trend towards pure muscovite [105]. Secondary muscovite is found as sheaf-like aggregates replacing feldspars, or as tiny laths after biotite and primary muscovite.

Biotite is anhedral to subhedral, strongly pleochroic (from dark red brown in γ = β to pale brown in α) and contains numerous inclusions of zircon and monazite. Individual crystals and biotite “patches” are common in all facies (Figure 5c,d). Biotite compositions (Al_total = 3.37–4.50 apfu; Fe_T/[Fe_T+Mg] = 0.64–0.87) are typical of biotites from aluminopotassic associations [106]. Chlorite alteration of biotite is widespread and of variable extent, particularly along cleavages.

Apatite, zircon, monazite and opaque minerals occur as subidiomorphic to idiomorphic grains, preferentially associated with micas. Sillimanite (mainly fibrolite) of probable xenocrystic origin is present in some samples, as fine-oriented needles included in primary muscovite (Figure 5e) and biotite. More rarely, the studied granites may also contain andalusite subhedral grains with pink pleochroic cores or small subhedral tourmaline crystals (with limited zoning from pale to dark yellow) disseminated in the groundmass.

As a result of heterogeneous ductile deformation, the different facies of the Junqueira batholith exhibit, in places, a weak to strongly developed tectonic fabric and shear-related microstructures. In the more intensively deformed domains, an incipient gneissic foliation, defined by the alignment of micas (Figure 5e) and/or elongation of shape-oriented K-feldspar megacrysts can be observed. Further evidence of deformation at sub-magmatic conditions is provided by local recrystallization of some quartz and feldspar grains (Figure 5e) and the presence of plagioclase crystals with bent twin lamellae and kinked muscovite laths with undulose extinction (Figure 5f) [107].

5.2. ID-TIMS U–Pb Geochronology

Two representative samples of the main granitoid facies of the Junqueira batholith were selected for U–Pb geochronology. Sample 154-7 from the Junqueira-Serra da Freita facies was collected in the northwestern sector of the batholith, while sample 178-108 is located in the southernmost exposure of the Vouzela granite (see Figure 3 for sample locations). The results of ID-TIMS U–Pb isotope analyses of zircon and monazite are presented in Table S4 and plotted on Wetherill Concordia diagrams of Figure 6.

Zircons extracted from sample 154-7 from the Junqueira-Serra da Freita granite are clear and colorless and tend to occur as relatively short prismatic crystals. Some zircon grains are fractured and contain inclusions. Four zircon fractions, including tips, fragments and prismatic crystals were analyzed for U/Pb isotopes. The four zircon fractions yielded sub-concordant ages ranging between 309 Ma and 307 Ma, reflecting variable proportions of Pb-loss (Figure 6a). Tracing a discordia line anchored at 0 Ma through all zircon fractions provided an upper intercept age of 308.9 ± 3.3 Ma (MSWD = 1.4) (Figure 6a). Monazite grains from sample 154-7 are euhedral, rounded, clear to pale yellow and inclusion-free. The analyses obtained in two monazite fractions are concordant and yield a weighted mean ²⁰⁷Pb/²³⁵U age of 312.4 ± 2.7 Ma (Figure 6a). As the monazite age overlaps, within error, with the zircon upper intercept age, the 312.4 ± 2.7 Ma monazite age is interpreted here as the best estimate for the crystallization age of the Junqueira-Serra da Freita granite.

In sample 178-108 from the Vouzela granite, zircon crystals are transparent, colorless to pale yellow and short- to long-prismatic. The isotope data for four zircon fractions are concordant to sub-concordant and give a weighted mean ²⁰⁶Pb/²³⁸U age of 310.1 ± 0.4 Ma (MSWD = 1.3) (Figure 6b). Monazite crystals extracted from this sample are euhedral to subhedral and yellow to brownish. The two analyzed single-grain monazite fractions yielded concordant ²⁰⁷Pb/²³⁵U ages of 311.7 ± 1.9 Ma and 313.3 ± 1.4 Ma (Figure 6b). The youngest monazite age agrees within error with the zircon age, suggesting that the 310.1 ± 0.4 Ma represents the emplacement age of the Vouzela granite.

5.3. Geochemistry

Major- and trace-element compositions for representative samples of the main petrographic types of the Junqueira batholith, including the geochemical data available in the literature, are listed in Table S1.

The analyzed samples are classified as adamellites and granites in the P-Q classification diagram of Debon and Le Fort [108] (Figure 7a) and plot in the field of leucogranites in the A-B diagram proposed by the same authors (Figure 7b). Their high SiO₂ contents (70–75 wt.%), strongly peraluminous signatures (ASI = 1.17–1.63; Figure 7c), relatively low MgO and CaO contents (MgO < 0.68 wt.%; CaO < 1.03 wt.%) and K₂O/Na₂O ratios of 1.08 to 2.24 are typical of S-type granites (e.g., [109,110,111,112]). According to the classification scheme of Frost et al. [113], the granitic rocks from the Junqueira batholith range from magnesian to ferroan (Figure 7d) and are dominantly alkali-calcic (Figure 7e). It should be stressed that the studied granites, classified as alkali-calcic using the modified alkali-lime index (MALI) defined by Frost et al. [113], are subalkaline in the sense of Peacock [114] and Irvine and Baragar [115] and display Rittmann Serial Index values (σ = 1.6–2.9; [116,117]) indicating calc-alkaline affinities.

The B parameter (B = Fe_T + Ti + Mg calculated in millications; [108]), expressing the total mafic mineral content, was used as a differentiation index to better resolve the limited geochemical variation observed across the Junqueira batholith. As shown in Figure 8, the samples from the different petrographic units of the batholith tend to define relatively scattered overlapping trends on most variation diagrams. For decreasing B values, CaO, K₂O, Sr, Ba, Zr, Th and ΣREE contents decrease whereas SiO₂ and Rb concentrations increase. Al₂O₃ contents display an almost flat trend (Figure 8), while the distribution of Na₂O and P₂O₅ contents is more diffuse.

The samples from the Junqueira-Serra da Freita facies exhibit the widest compositional variation (Figure 8). The Campia and Fajões granites have a comparatively more restricted spectrum of compositions and the samples from the highly fractionated Routar granite tend to cluster at the extreme acid end of the trends described above (Figure 8).

Overall, the chemical variation patterns observed in the Junqueira two-mica granitoids are consistent with fractionation of a mineral assemblage consisting of plagioclase, K-feldspar, biotite (±ilmenite) and zircon. However, the significant degree of overlap between individual plutonic units suggests that they may represent independent magma batches that evolved along similar liquid lines of descent.

Due to the lack of a complete REE dataset for some granite units, we only present the distribution of rare earth elements in the Vouzela and Junqueira-Serra da Freita facies. Chondrite-normalized REE patterns for these facies are subparallel and show a moderate to strong enrichment of LREE over HREE ([La/Yb]_N = 7.9–53.2), moderate fractionation of HREE ([Gd/Yb]_N = 2.7–12.8) and negative Eu anomalies (Eu/Eu* = 0.22–0.65) (Figure 9a,b). The total REE abundances (ΣREE) range from 59 to 134 ppm in the Vouzela granite and from 37 to 224 ppm in the Junqueira-Serra da Freita granite. In general, the REE contents decrease from the least to the most evolved samples of the suite. Compared to the metapelite of the DBSG sequence plotted in the diagrams of Figure 9a,b, the granitoids from the Junqueira batholith tend to display a marked depletion in all REE, particularly in HREE and stronger negative Eu anomalies (Figure 9a,b).

The multielement diagrams normalized to the Neoproterozoic Iberian Average Shale (NIBAS) [118] for the granitic rocks from the Junqueira batholith are characterized by significant enrichments in Rb, K, and P and marked depletions in Ba, Sr, REE, Zr, Hf, Ti and Y in all granite facies (Figure 9c–g). Nb, Ta and Th exhibit more erratic behavior, with variable highs and troughs (Figure 9c–g). The two metasedimentary enclaves analyzed in this study are hosted in the Junqueira-Serra da Freita granite and display strong compositional similarities with the metapelite sample from the DBSG sequence (Figure 9h), indicating that the enclaves may correspond to disaggregated metapelitic xenoliths of the wall rocks.

Figure 9. Chondrite-normalized REE patterns and multielement-normalized diagrams for the Junqueira suite. (a,b) Chondrite-normalized REE patterns for the Vouzela, Junqueira-Serra da Freita and Campia granitoids, (c–h) multielement normalized diagrams for the different units of the Junqueira batholith and two metasedimentary xenoliths. The composition of one metapelite sample from the DBSG sequence (green squares) is also plotted in all diagrams. REE data were normalized for the C1 chondrite values of McDonough and Sun [119] and multielement data were normalized for the composition of Neoproterozoic Iberian Average Shale (NIBAS) from Ugidos et al. [118].

Figure 9. Chondrite-normalized REE patterns and multielement-normalized diagrams for the Junqueira suite. (a,b) Chondrite-normalized REE patterns for the Vouzela, Junqueira-Serra da Freita and Campia granitoids, (c–h) multielement normalized diagrams for the different units of the Junqueira batholith and two metasedimentary xenoliths. The composition of one metapelite sample from the DBSG sequence (green squares) is also plotted in all diagrams. REE data were normalized for the C1 chondrite values of McDonough and Sun [119] and multielement data were normalized for the composition of Neoproterozoic Iberian Average Shale (NIBAS) from Ugidos et al. [118].

5.4. Whole-Rock Sr–Nd Isotopic Data

Whole-rock Sr–Nd isotopic compositions and Nd model ages for selected samples of the Junqueira syn- to late-D₃ granitoids are listed in Table S2. The initial ⁸⁷Sr/⁸⁶Sr_i ratios and εNd_i values for the analyzed samples were age-corrected back to 310 Ma based on the geochronological data obtained in the present study (Section 5.2).

All the plutonic units of the Junqueira batholith exhibit strongly radiogenic initial ⁸⁷Sr/⁸⁶Sr ratios (≥0.713) (Table S2): Vouzela granite (⁸⁷Sr/⁸⁶Sr_i = 0.7130–0.7147), Junqueira-Serra da Freita granite (⁸⁷Sr/⁸⁶Sr_i = 0.7146–0.7163) and Fajões granite (⁸⁷Sr/⁸⁶Sr_i = 0.7179–0.7194), in close agreement with the Sr isotopic data published in the literature for the granitic rocks of this massif [13,15,85] (see compilation in Table S2). The initial ⁸⁷Sr/⁸⁶Sr ratio for one sample from the metasedimentary xenoliths (⁸⁷Sr/⁸⁶Sr_i = 0.7161) is also similar (Table S2).

With the exception of one sample from the Fajões granite yielding a comparatively less negative εNd_i value (εNd_i = −2.41), the great majority of the analyzed rocks have εNd_i values ranging from −5.86 to −4.03 (Table S2). The results obtained in this study are fully consistent with the Nd data reported by Beetsma [13] for the Vouzela, Junqueira-Serra da Freita and Campia granites (εNd_i = −5.72; εNd_i = −4.63 to −3.51; εNd_i = −4.49, respectively).

As a result of the relatively wide range of ⁸⁷Sr/⁸⁶Sr_i ratios and narrow spectrum of εNd_i values, the Junqueira granitoids tend to define a broad horizontal trend in the ⁸⁷Sr/⁸⁶Sr_i–εNd_i diagram (Figure 10). The observed trend partially overlaps the field of the metasediments of the lower stratigraphic unit of the Douro-Beira Supergroup (the so-called Beiras Group) and plots clearly above the metasedimentary rocks of the uppermost stratigraphic unit of the DBSG complex (Douro Group) (Figure 10), pointing to a major role for source materials with the composition of the Beiras Group in granite petrogenesis.

The depleted-mantle Nd model ages (T_DM) [97] for the different facies of the Junqueira batholith cluster at about 1.1 to 1.4 Ga (Vouzela granite: T_DM = 1.2–1.4 Ga; Junqueira-Serra da Freita granite: T_DM = 1.1–1.2 Ga; Campia granite: T_DM = 1.3 Ga; Table S2). The close match between the Nd model ages of the Junqueira granitoids and those of the host metasediments (Beiras Group; T_DM = 1.1–1.5 Ga) provides further evidence for a genetic linkage between the Junqueira plutonic units and the Ediacaran-Cambrian metasediments of the Beiras Group.

Although the emplacement age of the Junqueira batholith is well constrained by U–Pb dating (Section 5.2) and supported by regional structural relationships, we have also tried to date these granites with the Rb–Sr method as a complementary tool for petrogenetic considerations.

The whole-rock Rb–Sr isochron obtained for six samples of the Vouzela granite (five from this investigation and one from Beetsma [13]) gave an age of 306 ± 13 Ma with an ⁸⁷Sr/⁸⁶Sr_i ratio of 0.7153 ± 32 (MSWD = 0.21) [106], which is concordant within error with the U–Pb zircon age (310.1 ± 0.4 Ma) for this granite (Section 5.2). The good agreement between the Rb–Sr age and the inferred intrusion age indicates that the Sr initial ratios of these samples were not highly disturbed.

In contrast, the Rb–Sr isotope data for four samples of the Junqueira-Serra da Freita granite (two from this work and two from Beetsma [13]) yield an unrealistic old Rb–Sr age of 343 ± 5 Ma (⁸⁷Sr/⁸⁶Sr_i = 0.7094 ± 9; MSWD = 2.2). Similarly, a too-old whole-rock isochron age was determined for four samples of the Fajões granite (365 ± 25 Ma; ⁸⁷Sr/⁸⁶Sr_i = 0.7062 ± 57; MSWD = 0.87). These apparent Rb–Sr ages point to incomplete homogenization of the Sr isotopic compositions. The spread of ⁸⁷Sr/⁸⁶Sr ratios observed in many granitic intrusions can be ascribed to several causes: (a) isotopic heterogeneity of source materials; (b) isotopic disequilibrium during melting; (c) mixing of magmas with contrasting isotope signatures; (d) assimilation/fractional crystallization (AFC) processes; (e) contamination of magmas during emplacement and (f) post-magmatic hydrothermal overprint (e.g., [22,81,93] and references therein).

In the present case, field and petrographic evidence reveals that the studied granitoids may contain variable proportions of disseminated restitic material. Partial to complete isotopic re-equilibration of basement-derived mineral phases (xenocrysts) entrained in the melts may have been responsible for the discrepancy between Rb–Sr “ages” and the more precise U–Pb ages (e.g., [120]).

6. Discussion

6.1. Emplacement Age of the Junqueira Batholith

The new ID-TIMS U–Pb zircon and monazite ages presented in Section 5.2 for the Junqueira-Serra da Freita granite (312.4 ± 2.7 Ma) and the Vouzela granite (310.1 ± 0.4 Ma) provide solid constraints for their emplacement ages and show that the intrusion of these magmas was roughly synchronous. A younger U–Pb age (307.8 ± 0.7 Ma) was previously reported for the Junqueira-Serra da Freita granite [15]. The monazite grains used to determine this age were extracted from one sample located at the same outcrop as the sample collected for the present investigation. Given the similarity between the 307.8 ± 0.7 Ma monazite age determined by Valle Aguado et al. [15] and the zircon upper intercept age of 308.9 ± 3.3 Ma (this work), it seems plausible to assume the 308–309 Ma age as the timing of crystallization of the Junqueira-Serra da Freita granite. However, the older monazite age obtained in this work (312.4 ± 2.7 Ma) in two concordant fractions may also correspond to the granite emplacement age (as there is no evidence of inherited components in the analyzed grains). For these reasons, we prefer to consider that the crystallization age of the Junqueira-Serra da Freita granite is bracketed between 312 Ma and 309 Ma.

The close spatial relationships between individual plutonic units within the Junqueira massif and the occurrence of transitional contacts between facies further suggest that the whole batholith could have been emplaced at the time interval of ca. 312–309 Ma. The proposed time interval is in full agreement with the ages reported for other syn- to late-D₃ intrusions from both the Portuguese and the Spanish sectors of the CIZ (e.g., [14,15,19,21,24,27,68,78,121,122,123]) and documents an important event of felsic magmatism during the post-collisional stage of the Variscan orogeny. As stated by many authors, the intrusion of the Junqueira batholith is strongly linked to the D₃ strike-slip shear zones that cross the Porto-Viseu area (Figure 2; [15,124]). Evidence of solid-state deformation fabrics in the Junqueira granitoids is not very pronounced (Section 5.1), suggesting that their emplacement occurred during the waning stages of the shear zone activity.

Inconsistencies between the U–Pb and Rb–Sr “ages” for some of the studied granitoids are attributed to disturbance of the Rb–Sr isotopic system induced by the entrainment of restitic mineral phases in the host melts (see Section 5.4). On the other hand, the spread of muscovite K–Ar ages obtained for the Junqueira granitoids (Table 1; [89]) suggests that the K–Ar isotope system was variably affected by post-magmatic alteration leading to either younger or older ages (e.g., [22,81,93] and references therein).

6.2. Petrogenetic Constraints

6.2.1. Petrographic, Geochemical and Sr–Nd Isotope Inferences

Despite the internal petrographic and compositional variability observed within the Junqueira massif, the different granite units of the batholith share a common set of mineralogical, geochemical and isotopic characteristics.

All facies contain abundant muscovite, biotite as the only mafic phase and monazite + zircon + ilmenite ± Al-rich minerals (restitic sillimanite and andalusite) as accessory phases, reflecting their strongly peraluminous character (Section 5.1). As described in Section 5.3 and Section 5.4, the elemental and isotopic compositions of the Junqueira two-mica granites (ASI > 1.1; high SiO₂ contents, low MgO, TiO₂, CaO, Ba, Sr, ΣREE contents, elevated K₂O/Na₂O ratios, highly radiogenic ⁸⁷Sr/⁸⁶Sr_i ratios > 0.713 and negative εNd_i values = −4.03 to −5.86) are typical of S-type granites supporting a major involvement of metasedimentary sources in the genesis of these magmas [5].

The samples from the different granite units of the batholith define overlapping trends or clusters in major- and trace-element variation diagrams (Figure 8), which excludes an origin by simple fractional crystallization from a single parent magma and indicates, instead, that the batholith could have resulted from the assemblage of discrete magma batches evolving along their own liquid lines of descent.

6.2.2. Source Rocks

The basement in the Junqueira region is largely composed of Neoproterozoic metasedimentary rocks of the Beiras Group of the DBSG, consisting of alternating layers of metapelites and metagraywackes (Figure 2 and Figure 3). Previous studies have shown that the DBSG megasequence is a plausible protolith for the Junqueira granites [13,15,20,85].

According to Sylvester [125], the CaO/Na₂O ratios can provide valuable information on the source composition of strongly peraluminous granite magmas. This ratio is dominantly controlled by the proportions of plagioclase and clay in the source. Therefore, pelite-derived melts (plagioclase-poor and clay-rich sources) tend to exhibit low CaO/Na₂O ratios (< 0.3), whilst the melts produced from plagioclase-rich and clay-poor sources, such as metagraywackes or metaigneous rocks of intermediate/acid composition, are characterized by high CaO/Na₂O ratios (>0.3). All the plutonic units of the Junqueira batholith exhibit low CaO/Na₂O ratios (0.03–0.23), suggesting that these magmas formed through partial melting of predominantly pelitic sources. However, some contributions of more immature metasediments included in the DBSG megasequence cannot be entirely ruled out.

In multielement diagrams normalized to the Neoproterozoic Iberian Average Shale (NIBAS) [118], all granite facies of the Junqueira batholith show significant enrichments in Rb, K and P and marked depletions in Ba, Sr, REE, Zr, Hf, Ti and Y (Figure 9c–g). The observed patterns are consistent with a derivation from a metapelitic source compositionally similar to NIBAS (and to the metapelites of the Beiras Group of the DBSG). The depletions in Ba, REE, Zr, Hf, Ti and Y recorded in the studied granitoids could then be attributed to the presence of biotite and minor amounts of garnet and zircon in the residual mineral assemblage (retaining the HFSE and the HREE), while the more elevated Rb, K and P contents suggest that K-feldspar + muscovite ± biotite + apatite ± monazite were consumed during partial melting reactions. Sr depletion could have resulted from fractional crystallization of plagioclase during magma evolution.

The close agreement between the ⁸⁷Sr/⁸⁶Sr and εNd values for the presumed Beiras metasedimentary protoliths at the time of partial melting (⁸⁷Sr/⁸⁶Sr_i = 0.7099–0.7291; εNd_i = −7.67 to −3.23; unpublished data) and the initial isotope compositions of the Junqueira syn- to late-kinematic granites (⁸⁷Sr/⁸⁶Sr_i = 0.7130–0.7194; εNd_i = −5.86 to −3.51) supports the possibility that these rock units represent a suitable source for the granitic melts. The considerable scatter in the initial Sr isotope ratios of the granites and the apparent whole-rock Rb–Sr ages, which are older than the real intrusion ages (see Section 5.4), are probably due to the entrainment of restitic materials to varying degrees.

6.2.3. Partial Melting Processes

Partial melting of pelitic sources may be triggered by dehydration melting reactions of muscovite and/or biotite (dry melting) or by water-saturated or water-fluxed conditions (wet melting). Muscovite dehydration melting begins at relatively low temperatures (<750 °C), produces modest amounts of melt, and is generally complete by ca. 825 °C. This reaction primarily consumes muscovite, along with subordinate amounts of plagioclase and quartz, leaving behind a residuum of K-feldspar, sillimanite and biotite [126]. The resulting granitic melts are characterized by strongly radiogenic ⁸⁷Sr/⁸⁶Sr signatures, high Rb/Sr (3–6), low Sr/Ba (0.2–0.7) ratios, and negative Eu anomalies [8,127]. In contrast, biotite breakdown occurs at higher temperatures (ca. 825–900 °C), where it reacts with plagioclase, aluminosilicates and quartz, forming garnet, (±) K-feldspar, and melt [128,129]. This process generates larger amounts of melts, which tend to exhibit higher Rb concentrations compared to those derived from incongruent decomposition of muscovite [8]. On the other hand, vapor-present melting of muscovite-bearing pelites depletes the source in plagioclase, due to the high stoichiometric plagioclase/muscovite ratio [126]. The resulting melts are trondhjemitic to granitic with high CaO, Sr, Ba, Sr/Ba (0.5–1.6), but low Rb and Rb/Sr (0.7–1.6), along with positive Eu anomalies and relatively unradiogenic ⁸⁷Sr/⁸⁶Sr signatures [8,126].

The low concentrations of CaO, Ba, Sr, coupled with low Sr/Ba ratios (0.15–0.40), high Rb/Sr ratios (Rb/Sr = 3–24), negative Eu anomalies (Eu/Eu* = 0.22–0.65) and strongly radiogenic ⁸⁷Sr/⁸⁶Sr signatures (⁸⁷Sr/⁸⁶Sr > 0.710) observed in the studied granites preclude a derivation from water-fluxed reactions. Instead, dehydration melting of muscovite and/or biotite seems to be implied, with several lines of evidence supporting the latter mechanism. First, strongly peraluminous Variscan granitic magmas were produced in very large volumes, constituting massive batholiths in the CIZ (Figure 2). Second, the studied granites exhibit variable depletion in Ba and HFSE (e.g., Zr, Hf, Nb, Ti, REE; Figure 9c–g), consistent with the retention of variable amounts of biotite ± Fe–Ti oxides + zircon in the residual mineral assemblage. The presence of biotite crystals with inclusions of sillimanite of restitic origin further suggests that this mineral was not fully consumed during partial melting reactions. Third, recent estimates for the core of the Porto-Viseu Metamorphic Belt indicate temperatures exceeding 800 °C and pressures of 7.6–7.9 kbar [70], which are consistent with the onset of incongruent decomposition of biotite. Fourth, the slightly to strongly fractionated HREE profiles ([Gd/Yb]_N = 2.7–12.8; Figure 9a,b) and the strong depletions in Y, and HREE observed in all granites (Figure 9c–g) suggest that garnet played a significant role as a residual phase in the source and, therefore, partial melting occurred at mid- to lower-crustal levels. Additionally, the enrichments in Rb, K, P ± Th contents relative to the Beiras metapelites suggest that feldspar + muscovite + biotite + apatite ± monazite were consumed during partial melting reactions.

In the binary K₂O–(FeO_T+MgO) [130] and ternary K–(Fe*+Mg+Ti)–(Ca+Na) [131] diagrams (Figure 11), which contrast residual ferromagnesian phases with feldspar-rich anatectic melts, there is a close match in the compositions of the Beiras metasediments and those of muscovite-biotite metapelite [126] and biotite metapelite [128] used as starting materials in experimental studies. In these diagrams, the least silicic samples from each of the granite units, with no apparent evidence of restitic phases, plot closer to the feldspar join K–(Ca+Na) and the K₂O axis, in agreement with a melt-dominated (quartzo-feldspathic) component. These samples also cluster closely to the compositions of the glasses produced by dehydration melting of a biotite metapelite at 825 °C and 7 kbar [128], suggesting that they may have formed through biotite (±some muscovite) dehydration melting of similar protoliths, in agreement with previous interpretations.

In the major and trace element variation diagrams, chondrite-normalized REE and multielement diagrams (Figure 8 and Figure 9), the samples from the different granite units exhibit overlapping compositions and variable levels of elemental enrichments for the same B parameter value. This suggests that each granite unit may represent a discrete magma batch, generated through moderate, but varying degrees of partial melting of a common source (Beiras Group metasediments). The Junqueira-Serra da Freita granite probably results from higher degrees of partial melting, as it tends to have higher Al₂O₃, Sr, Ba, and Zr, along with lower SiO₂, P₂O₅, Rb and Rb/Sr values compared to the other granite facies at the same B parameter value.

6.2.4. Heat Sources

Heating of crustal rocks during prograde thickening produces only small volumes of melts through water-fluxed melting of fertile metapelites and metagraywackes, and most of these melts crystallize as in situ trondhjemitic leucosomes or small plutons [126,134]. Larger melt fractions can be generated during decompression by hydrate breakdown reactions. However, extensive metasediment melting can be hardly achieved without additional heat sources such as those provided by underplating of mantle-derived magmas, as predicted by thermo-mechanical and petrological models (e.g., MASH model of Hildreth and Moorbath [135]; deep crustal “hot zone” model of Annen et al. [136]). Other mechanisms proposed to trigger extensive melting in the crust include high internal radiogenic heat production in highly thickened collisional belts (e.g., [18,137]) and focused shear heating related to lithospheric faults (e.g., [134,138]).

In the Central Iberian Zone of the Iberian Variscan Belt, the late-post-collisional stage of orogenic evolution was probably associated with lithospheric thinning and basalt underplating promoting protracted anatexis at HT-LP conditions and the generation of abundant volumes of granitic magmas, large enough to leave the source region and accrete the batholiths (e.g., [15,77,139]). Therefore, basaltic underplating may have represented the main engine to foster extensive intracrustal differentiation. However, it is generally accepted that post-collisional granitoid magmatism in the CIZ was developed within a complex pattern of strike-slip shear zones in a transpressional regime, suggesting that, in addition to the mantle heat input, shear-heating within these crustal-scale shear zones could have enhanced melt production in the lower and middle crust.

6.2.5. Fractional Crystallization

Chemical variation diagrams and REE patterns suggest that modest fractional crystallization of a mineral assemblage consisting of K-feldspar + plagioclase ± biotite + Fe–Ti oxides + zircon + monazite could have played a role during the evolution of the Junqueira magmas.

Major and trace element data were used to test whether samples from the same granite unit are related through fractional crystallization, using mass balance calculations. The least silicic samples were mainly selected as starting magmas, unless chemical data suggested the presence of restitic mineral. The compositions of the most silicic samples, free from alteration effects, were chosen as residual liquids. We used the compositions of pure quartz, ilmenite and apatite, alongside the compositions of biotite, plagioclase and K-feldspar determined by electron microprobe. Examples of the outputs are provided in

Table 2. Fractional crystallization modeling results for the Silvares, Vouzela, Junqueira and Campia granites. PL: parent liquid; RM: residual melt; Calc.: calculated composition. Partition coefficients for Sr, Ba and Rb in biotite, K-feldspar and plagioclase are from ^(a) Nash and Crecraft [140] and ^(b) Arth [141]. Mineral abbreviations according to Whitney and Evans [133]. % Cryst.: percentage of fractional crystallization; ΣR²: sum of the squares of the residuals.

	Silvares			Vouzela			Junqueira			Campia
	PL	RM	Calc.	PL	RM	Calc.	PL	RM	Calc.	PL	RM	Calc.
Sample	VT133	VT168		VCM069	VT158		154-7	VT181		VT131	VT176
SiO2	72.83	75.02	75.13	72.83	74.81	74.89	72.66	74.37	74.46	72.65	74.09	74.26
TiO2	0.25	0.15	0.16	0.33	0.15	0.28	0.36	0.11	0.29	0.23	0.18	0.17
Al2O3	15.68	14.63	15.14	15.05	14.53	14.76	15.41	15.10	15.35	15.74	15.09	15.36
FeOT	1.82	1.07	1.21	1.55	1.08	1.10	1.62	1.03	1.04	1.92	1.39	1.33
MnO	0.04	0.04	0.04	0.02	0.03	0.02	0.01	0.04	0.01	0.03	0.03	0.03
MgO	0.33	0.20	0.10	0.43	0.17	0.28	0.53	0.18	0.35	0.36	0.26	0.21
CaO	0.47	0.51	0.43	0.65	0.53	0.65	0.54	0.38	0.58	0.43	0.47	0.36
Na2O	3.30	3.48	3.18	3.17	3.87	3.53	2.82	3.61	3.14	3.36	3.23	3.12
K2O	4.84	4.48	4.40	5.54	4.46	4.42	5.65	4.72	4.69	4.84	4.85	4.83
P2O5	0.43	0.43	0.47	0.43	0.36	0.40	0.40	0.46	0.42	0.45	0.41	0.48
Total	100.0	100.0	100.3	100.0	100.0	100.3	100.0	100.0	100.3	100.0	100.0	100.2
% Cryst.			12			15			15			8
ΣR2			0.41			0.21			0.38			0.11
Sr (a)	42	37	28	58	26	39	60	31	43	40	40	29
Ba (a)	173	139	111	239	112	105	280	107	129	146	150	122
Rb (b)	434	476	446	345	368	365	348	412	366	469	429	481
Bt (%)			28.4			18.2			21.3			27.9
Pl (%)			33.9			-			-			55.0
Kfs (%)			37.7			71.4			62.7			17.2
Qz (%)			-			9.3			15.3			-
Ilm (%)			-			-			0.1			-
Ap (%)			-			1.2			0.5			-

. In agreement with previous interpretations, major element-based least-squares modeling reveals that the geochemical variation observed within the different units of the Junqueira batholith can be explained by small degrees of fractional crystallization (8%–15%) of a mineral assemblage consisting of K-feldspar (17%–71%), plagioclase (0%–55%), biotite (18%–28%), quartz (0%–15%), apatite (0%–1%) and ilmenite (0%–0.1%). The statistical fit is excellent in all calculations (ΣR² = 0.11–0.41).

As the behavior of Sr, Ba and Rb is predominantly controlled by the major mineral phases of granitic rocks (feldspars and micas), these elements were used to validate the major element model results. To predict their behavior during fractional crystallization, the Rayleigh fractionation equation, along with the partition coefficients of Nash and Crecraft [140] and Arth [141] for these elements, was applied. The Rayleigh equation can be expressed as (Equation (1)):

$$C_L=C_0\times F^{(D-1)}$$

(1)

where C_L and C₀ stand for the trace element composition in the residual melt and in the parental magma, respectively, F is the residual melt fraction and D is the global partition coefficient of the minerals settling out of the melt. As shown in Table 2, the calculated values for Sr, Ba and Rb are generally consistent with analytical data, thereby supporting the major element models and confirming that the evolution of the individual granite units of the Junqueira batholith involved small degrees of crystal fractionation.

7. Geodynamic Considerations

The results obtained in this work document an important peak of Variscan plutonic activity in the CIZ at ca. 312–309 Ma. This age interval falls within the period (ca. 315–290 Ma) proposed for the emplacement of large post-collisional granitic bodies in the Southern Variscides (Figure 1a), such as the French Massif Central (305–290 Ma in the Velay dome; e.g., [142,143]), Maures Massif (305–298 Ma; e.g., [144,145]), Pyrenees (312–305 Ma; e.g., [146,147,148]), Calabria-Peloritani Massif (up to 295 Ma; e.g., [134,149,150]), Alps (305–299 Ma, up to 275 Ma; e.g., [151]) and Corsica (ca. 305 Ma; e.g., [152]).

Indeed, the intrusion of huge granitic batholiths is one of the striking features of the late-collisional evolution of the Southern Branch of the European Variscan Belt, where most of these late Variscan granitoids (ranging from strongly peraluminous S-type granitoids to I-type or transitional I-S-type metaluminous or weakly peraluminous, calc-alkaline granites) are considered to reflect intracrustal recycling of metasedimentary and/or metaigneous rocks of thickened peri-Gondwana crust with minor mantle contributions (e.g., [18,26,134,150,153,154]). Although there is little doubt that the strongly peraluminous S-type granite suites represent purely or dominantly metasediment-derived crustal melts, the origin of the I-type or transitional I-S-type granitoids and associated mafic magmas is more controversial, with some authors regarding them as products of mantle-crust hybridization processes, rather than the result of partial melting of exclusive intracrustal sources (e.g., [11,20,30,155,156,157,158,159]).

Irrespective of the nature of the crustal sources and the degree of mantle input in-volved in granite petrogenesis during the late- to post-collision evolution of the Variscan orogen, it is generally accepted that the heat source necessary to generate the large volumes of granite magmas at this stage is related to lithospheric delamination and asthenospheric upwelling [61,158]. In this regime of lithospheric mantle delamination, enhanced by shearing activity, the intrusion of mantle-derived magmas could have provided the heat supply for extensive melting of metasedimentary crustal levels and produce peraluminous granite magmas, as is the case of the Junqueira batholith.

8. Conclusions

The Junqueira syn-to-late-kinematic batholith was emplaced in the axial zone of the Porto-Viseu Metamorphic Belt at ca. 312–309 Ma. It comprises six facies of strongly peraluminous two-mica granites, which were formed through moderate, but varying degrees of partial melting of metasedimentary sources at mid- to lower-crustal depths. The metasediments of the Beiras group of the Douro-Beiras Supergroup constitute a likely source for the studied granitoids. The breakdown of biotite is considered here as the main mechanism for magma generation. Major and trace element compositions, supported by mass balance and batch fractionation modeling, suggest that each granite unit represents a discrete magma batch that evolved along its own liquid line of descent, through small degrees of fractional crystallization.