INTRODUCTION

The Iberian Massif experienced a polymetamorphic evolution during the Carboniferous as a result of the collision between Laurussia and Gondwana (e.g. Variscan Orogeny; Martínez Catalán et al., 2009; Díez Fernández et al., 2016). The Laurussia-Gondwana convergence caused crustal thickening by the stacking of tectonic nappes and the rejuvenation of reliefs. The regional switch from contractional to extensional deformation was followed by an orogenic collapse and the generation of abundant crustal-derived magmatism (Escuder Viruete et al., 1994; Pereira et al., 2009; 2017a; Martínez Catalán et al., 2014; Alcock et al., 2015; Cambeses et al., 2015; Díez Fernández and Pereira, 2016). Mantle contributions were more abundant towards the SW Iberian Massif, as syn-kinematic magmatism related to intra-orogenic extension was slightly older in the Ossa-Morena and South Portuguese zones (Simancas et al., 2005; Pereira et al., 2009; 2015a) than in the Central Iberian Zone.

The age and compositional ranges of syn-kinematic (Carboniferous) and post-kinematic (Late CarboniferousEarly Permian, LC-EP) intrusions are wide in the NW (e.g. Central Iberian Zone; Castro et al., 2002 and references therein) and SW (Ossa-Morena Zone; Pereira et al., 2015a and references therein) Iberian Massif. Most syn- and postkinematic granitic rocks are S-type peraluminous, but there are also granitic rocks with transitional features between S- and I-type, I-type granites and hybrid granites, including metaluminous varieties (Dias et al., 1998, 2002; Castro et al., 1999, 2002; Bea et al., 1999, 2003; Fernández-Suárez et al., 2000, 2011; Neiva and Gomes 2001; González Menéndez et al., 2006; Moita et al., 2009; Neiva et al., 2009; Solá et al., 2009; Lima et al., 2011, 2012; Teixeira et al., 2011; Villaseca et al., 2012; Merino Martínez et al., 2014; Pereira et al., 2015a, 2017a; López-Moro et al., 2017). These Carboniferous and Permian calc-alkaline granitic rocks are often closely and spatially associated with calc-alkaline mafic-intermediate rocks (e.g.Castro et al., 2003; Bea, 2004; Larrea et al., 2004; González Menéndez et al., 2006; Jesus et al., 2007; Pin et al., 2008; Solá et al., 2009; Moita et al., 2015) and are characterized by many structures and textures formed through the complex interplay of distinct petrogenetic processes. Such composite plutons typically preserve an integrated record of prolonged magmatic evolution, including pluton accumulation, crystallization and emplacement within the crust that evidence the coeval interaction between two or more magmas of contrasting composition and physical properties (Turnbull et al., 2010). Mixing of magmas within a magma chamber to produce a homogeneous hybrid, followed by mingling and partial mixing of the hybrid magma with one of the remaining endmembers is commonly identified in the successive stages of the magmatic history of composite plutons (Barbarin, 2005 and references therein). By studying the temporal and spatial interaction of magmas of contrasting compositions that remained in the contact in liquid state, insights can be gained into the duration of physical and thermal processes responsible for the formation of plutons.

Our study presents original field data and SHRIMP zircon U-Pb dating for a post-kinematic intrusion in the Ossa-Morena Zone with the aim of studying the temporal and spatial interaction of a felsic and a mafic-intermediate magma forming the external ring of the Santa EuláliaMonforte massif. The obtained results will shed light on the timing of crystallization and emplacement of the Santa Eulália-Monforte intrusion, and on the global geodynamic context of the post-kinematic Iberian calc-alkaline magmatism. The results achieved broaden the knowledge on the spatial and temporal relationships between distinct LC-EP post-kinematic intrusions of the Iberian Massif. Our data also show that the crystallization ages of the Santa Eulália-Monforte I-type granite and associated mafic rocks are younger than the Nisa-Albuquerque batholith. Both intrusions cut across the Variscan structures, establishing a maximum range of about 13Ma, from the Late Moscovian to Asselian, for the LC-EP post-kinematic magmatism in the SW Iberian Massif. The subject of this study was chosen because L.G. Corretgé, who receives an honorable mention in this volume, was one of the petrologists who mostly contributed to gather data on the magmatism of the Iberian Massif, thus, fostering scientific cooperation between Portuguese and Spanish geologists.

LP-EC POST-KINEMATIC INTRUSIONS OF THE IBERIAN MASSIF

The LC-EP granitic rocks of the Iberian Massif have been traditionally classified as Late-Variscan because they crosscut the main Variscan structures and syn-kinematic intrusive rocks (ca. 306-287Ma; Ferreira et al., 1987; Dias et al., 1998; Fernández-Suárez et al., 2011; Neiva et al., 2012; Orejana et al., 2012; Villaseca et al., 2012). However, these post-kinematic intrusions are also recently considered as unrelated to the Gondwana-Laurussia collision sensu stricto .e.g.Gutiérrez-Alonso et al., 2011; Pereira et al., 2015b).

The LC-EP post-kinematic intrusions are abundant in the Central Iberian Zone, but are relatively less represented in the Ossa-Morena Zone. The Nisa-Albuquerque massif is a composite elongated body oriented parallel to, and crossing, the boundary between the Central Iberian and Ossa-Morena zones (González Menéndez, 2002) (Fig. 1). This post-kinematic intrusion consists of a dominant S-type monzogranite-syenogranite (ca. 309-307Ma) that surrounds a discontinuous core of I-type tonalitegranodiorite (ca. 306Ma) (SHRIMP zircon U-Pb dating; Solá et al., 2009). The Santa Eulália-Monforte massif, located in the northern domains of the Ossa-Morena Zone, is a composite intrusion (Gonçalves and Coelho, 1969-1970; Gonçalves, 1971; Gonçalves et al., 1972, 1975; Carrilho Lopes et al., 1998; Sant’Ovaia et al., 2015) consisting of S- and I-type granites and mafic-intermediate bodies (González Menéndez et al., 2006). Rb-Sr wholerock ages of ca. 281Ma (Mendes, 1967-1968) and of ca. 290Ma (Pinto et al., 1987) have been, for decades, the only available isotopic ages performed on granites from the Santa Eulália-Monforte massif.

THE SANTA EULÁLIA-MONFORTE MASSIF

Host rocks of the Santa Eulália-Monforte massif

The Santa Eulália-Monforte massif intrudes two important domains of the Ossa-Morena Zone (Figs. 1 and 2): i) the strongly deformed and variable metamorphosed Ediacaran to Ordovician sedimentary and igneous rocks of the Coimbra-Córdoba shear zone (Burg et al., 1981; Pereira et al., 2008, 2010); and ii) the Cambrian-Ordovician sedimentary and igneous rocks of the Alter do Chão-Elvas sector (Apalategui et al., 1990; Oliveira et al., 1991).

The original contacts and textures of the EdiacaranCambrian sedimentary and volcanic rocks (Pereira et al., 2006; Linnemann et al., 2008) and Cambrian-Ordovician plutonic rocks (Pereira et al., 2011; Sánchez-García et al., 2013; Díez Fernández et al., 2015) are best exposed in the northern and southern margins (Fig. 2). In the northern and southern margins of the Coimbra-Córdoba shear zone, as well as in the Alter do Chão-Elvas sector, despite the Variscan ductile deformation and greenschist facies metamorphism, the Ediacaran succession-consisting of metapelite, metagreywacke, black metachert, metamafic rocks, and rare carbonate rocks is unconformably overlain by a Cambrian sequence (Gonçalves, 1971; Gonçalves et al., 1975; Oliveira et al., 1991; Pereira et al., 2006). From the base to the top, the Cambrian sequence consists of: i) felsic metavolcanic rocks, quartzite and marble, and ii) schists, metagreywackes, quartzites, metaconglomerates and metamafics rocks. Cambrian magmatism includes calc-alkaline and MORB compositions (Pereira and Quesada, 2006; Sánchez-García et al., 2010). The Ediacaran-Cambrian succession is intruded by Cambrian-Ordovician peralkaline and alkaline igneous rocks (Gonçalves and Assunção, 1970; Gonçalves et al., 1975: Díez Fernández et al., 2015).

In the Central Unit of the Coimbra-Córdoba shear zone, the Carboniferous (ca. 340-333Ma) ductile deformation and associated metamorphism, reaching amphibolite to granulite and ecologite facies conditions, transformed the original contact and textures of the Ediacaran-Ordovician rocks (Pereira et al., 2008, 2010, 2012) (Fig. 2). CambrianOrdovician plutons occur as elongated bodies of weakly- to moderately-foliated rocks, or as narrow bands of orthogneiss alternating with schists and paragneisses affected by strong Variscan deformation and metamorphism (Gonçalves et al., 1972; Pereira et al., 2008, 2010; Díez Fernández et al., 2015) (Fig. 1).

Field relationships, petrography and geochemistry

The Santa Eulália-Monforte massif (Fig. 2) is a ringshaped composite intrusion 18km wide and 32km long elongated body in E-W direction (Gonçalves and Coelho, 1969-1970; Gonçalves, 1971; Gonçalves et al., 1972, 1975; Oliveira, 1975; Carrilho Lopes et al., 1998; González Menéndez et al., 2002, 2006). It includes a 10km wide and 16km long core of greyish fine- to medium-grained biotite granite (Fig. 3A) surrounded by a ring consisting of a complex association between pinkish coarse-grained biotite granite and mafic-intermediate rocks (Figs. 3, 4, 5 and 6).

The core of this ring-shaped intrusion consists of S-(prevailing) and I-type high-K calc-alkaline peraluminous granite ranging in composition from monzogranite to granite (Fig. 7A; B). Quartz, plagioclase, microcline, biotite and muscovite are the main minerals, being also present cordierite (Fig. 3A). The geochemistry of the greyish granite suggests the derivation from a mixed source of metasedimentary (dominant) with intermediate to acid igneous rocks extracted from the mid-lower crust (González Menéndez et al., 2006). The external pinkish granite is composed of quartz, biotite, and K-feldspar prevails in respect to plagioclase. It also contains chlorite, amphibole, allanite and zircon. The available geochemical data (Carrilho Lopes et al., 1998; González Menéndez et al., 2006) indicate that it is an I-type high-K calc-alkaline peraluminous granite (Fig. 7A; C). The pinkish granite is interpreted to derive from partial melting of intermediate composition gneissic rocks from the mid-lower crust (González Menéndez et al., 2006).

The pinkish granite of the external ring (Fig. 6A) includes variable size and irregular, 0.5-1.5km wide and 0.8-6km long gabbro-diorite to granodioritequartzdiorite-tonalite enclaves, and kilometer-scale xenoliths consisting of Ediacaran and Cambrian host rocks (Fig. 6B). Mafic-intermediate rocks range compositionally (Fig. 7) from gabbro-diorite, which are mainly composed by plagioclase, amphibole, pyroxene, biotite, apatite and zircon, to granodiorite-quartzdioritetonalite consisting of plagioclase, K-feldspar, quartz, biotite, amphibole, apatite and zircon. The maficintermediate associations have been interpreted to derive from distinct sources (González Menéndez et al., 2006): an enriched mantle (gabbro-diorite) and from partial melting of a lower mafic crust (granodioritequartzdiorite-tonalite).

Field relationships show that the greyish granite from the core (Fig. 3A) intrudes both the pinkish granite (Fig. 6A) and the associated mafic-intermediate rocks (Fig. 4A). Maficintermediate bodies at the eastern margin of the Santa Eulália-Monforte massif are intruded by the pinkish granite (Figs. 3B and 6C, D, E). Angular mafic enclaves (Fig. 5B) seem to represent early cooled pieces of gabbro caught up in the granodiorite-quartzdiorite-tonalite magma, but the variation from sharp though irregular contacts to more diffuse contacts suggests that they are contemporaneous (Fig. 4E). It is notable the presence of composite enclaves (Fig. 3E) and irregular patches of granodioritequartzdiorite-tonalite (Fig. 3D) hosted by gabbro-diorite, together with a channel-like enclave swarm of maficintermediate rocks (Fig. 4C; D) suggest intermingling of two magmas. Gabbro-dioritic rocks include leucocratic veins and lenses of quartzdiorite and/or tonalite (Fig. 3C; D). The occurrence of K-feldspar within the gabbro-dioritic enclaves, which may be the same as that of the host granitic rock (Fig. 5), suggests that granitic magma could have interacted with the mafic-intermediate magmas, probably at a time when both were not completely crystallized. The existence of mafic-intermediate enclaves with irregular, rounded and elliptical shapes, suggests mingling with the host granitic magma (Fig. 5).

U-PB GEOCHRONOLOGY

Sample preparation and analytical methods

Zircon separation from a sample of granite (MFT-1) and a sample of gabbro-diorite (BCA-2) of the external ring of the Santa Eulália-Monforte massif (sampling sites on Fig. 2) was performed by means of traditional techniques using dense liquids and magnetic separation at the University of Évora (Portugal). Selected crystals were hand-picked using a binocular lens. Zircon concentrates were cast on a 3.5cm diameter epoxy mount, together with zircon standards (TEMORA zircon, SL13 zircon and GAL zircon), then polished and documented using SEM-CL (Fig. 8), at the IBERSIMS (University of Granada, Spain). Mounts were coated with gold (80-microns thick) and inserted into the SHRIMP for analysis.

Each selected spot was rastered with the primary beam during 120s prior to analysis, and then analyzed over 6 scans following the isotope peak sequence ¹⁹⁶Zr₂O, ²⁰⁴Pb, 204.1 background, ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb, ²³⁸U, ²⁴⁸ThO, ²⁵⁴UO. Every peak of each scan was measured sequentially 10 times with the following total counts per scan: 2s for mass 196; 5s for masses 238, 248, and 254; 15s for masses 204, 206, and 208; and 20s for mass 207. The primary beam, composed of ^16.16.2+, was set to an intensity of 4 to 5nA, using a 120-micron Kohler aperture, which generates 17 x 20micron elliptical spots on the target. The secondary beam exit slit was set at 80microns, achieving a resolution of about 5,000 at 1% peak height. All calibration procedures were performed on the standards included on the same mount. Mass calibration was performed on the GAL zircon (ca. 480Ma, very high U, Th and common lead content; Montero et al., 2008). Analytical sessions initially involved the measurement of SL13 zircon (Claoué-Long et al., 1995), which was used as a concentration standard (238ppm U). TEMORA zircon (ca. 417Ma, Black et al., 2003), used as isotope ratios standard, was then measured every four unknowns.

Data reduction was performed using the SHRIMPTOOLS software specifically developed for IBERSIMS (available at www.ugr.es/~fbea). The intensity of each measured isotope was calculated in two steps using the software: first, the STATA lettervalue display algorithm was used to find outliers in the ten replicates measured at each peak during each scan, discarding them and averaging the rest once they had been normalized to SBM measurements; then, for each isotope, a robust regression of each scan average versus time, if measured, was performed. The final result for each isotope was calculated as the value at the midtime of the analysis resulting from a robust regression line. Errors (95% confidence level) were calculated as the standard error of the linear prediction at the midpoint of the analysis. ²⁰⁶Pb/²³⁸U was calculated from the measured ²⁰⁶Pb⁺/²³⁸U⁺ and UO⁺/U⁺ following the method described by Williams (1998). For high-U zircons (U>2,500ppm) ²⁰⁶Pb/²³⁸U was further corrected using the algorithm of Williams and Hergt (2000). Plotted and tabulated analytical uncertainties are 1σ precision estimates. Uncertainties in calculated mean ages are 95% confidence limits (tσ, where t is the Student’s t multiplier) and, for mean ²⁰⁶Pb/²³⁸U ages, include the uncertainty in Pb/U calibration (0.3-0.5%). Common Pb corrections assumed a model common Pb composition appropriate to the age of each spot (Cumming and Richards, 1975). U-Th-Pb data are presented in Table 1 (electronic appendix, available at www.geologica-acta.com) and plotted in Figure 9, using Isoplot 4 (Ludwig, 2009).

RESULTS

Sample MFt-1 (granite)

Most zircons are stubby and elongated euhedral prisms (150-300μm diameter in the long-axis) (Fig. 8). Morphologically complex zircons analyzed show cores of variable size (dark-cathodoluminescence (CL) subeuhedral cores with concentric zoning and wavy extinction, subeuhedral cores of a patchy to irregular texture with linear or wavy dark and bright-CL bands, sub-rounded dark-CL and unzoned cores). Cores are surrounded by concentric zoned to unzoned rims (Fig. 8). Few grains are simple, showing banded or concentric zoning and some show discordant zircon overgrowths or recrystallization textures. Twentynine U-Th-Pb isotopic analyses of 25 representative zircon grains are listed in Table 1. Twenty-five ²⁰⁶Pb/²³⁸U ages in the range ca. 536 to 273Ma, were obtained using 204-lead correction and show a discordance ≤5%. The oldest age of ca. 537Ma (Cambrian), obtained in analysis 8.2, is interpreted as an inherited old core and, indeed, close inspection showed that it is surrounded by a young overgrowth (8.1). The remaining 19 ²⁰⁶Pb/²³⁸U ages were obtained in zircons with a wide range of U (295-2,277ppm) and Th (118-1,763ppm) contents, and an average Th/U ratio of 0.37 (error standard deviation=0.03; Fig. 9A). These 19 U-Th-Pb isotopic compositions yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 296±4Ma (MSWD=0.62). However, the omission of three analyses with high-U (10.1, 14.1 and 16.1) gave a weighted mean ²⁰⁶Pb/²³⁸U age of 297±4Ma (MSWD=0.43; Fig. 9B), which is probably the best estimate of the crystallization age of the granite.

Sample BcA-2 (gabbro-diorite)

Zircons of this sample present morphological differences with respect to those of the granite described above. CL images of stubby and elongated euhedral prisms (150-350μm diameter in long-axis) show simpler internal textures. Most zircon grains have banded zoning (Fig. 8), but a few show oscillatory concentric zoning or are unzoned.

Twenty-four U-Th-Pb isotopic analyses of representative zircon grains are listed in Table 1. Common lead 204-lead corrected isotope ratios gave ²⁰⁶Pb/²³⁸U ages of ca. 319248Ma, having a cluster between ca. 310Ma and 290Ma. Sixteen U-Th-Pb analyses with discordance of ≤5% were performed on zircons characterized by a wide range of U (89-629ppm) and Th (54-343ppm) content and average Th/U ratio of 0.64 (error standard deviation=0.02; Fig. 9A). This group of analyses yielded a concordia age of 303±3Ma (MSWD=0.079; Fig. 9D) overlapping the weighted mean ²⁰⁶Pb/²³⁸U age of 303±4Ma (MSWD=1.01; Fig. 9E), which is probably the best estimate for the crystallization age of the gabbro-diorite.

DISCUSSION

Significance of the obtained U-Th-Pb data

The new obtained U-Pb ages of the Santa EuláliaMonforte massif suggest that the gabbro-diorite (ca. 303Ma) crystallized somewhat 6Ma earlier than the external granite (ca. 297Ma) (Figs. 9 and 10). However, the overlapping ages of zircons shown in Figure 10 suggest that the maficintermediate rocks and the pinkish granite can also be roughly contemporaneous, as documented by field data. In some cases, it has been demonstrated that zircon populations in a magma chamber are a mix of autocrysts, antecrysts, and xenocrysts, and that the resultant plutonic rocks are, thus, a mechanical mixture of crystals (e.g.Charlier et al., 2005; Miller et al., 2007) that may have different individual ages. The obtained data suggest that the external ring of the Santa Eulália-Monforte massif formed by the crystallization of injections of compositionally distinct magmas. These magmas were close enough in time so that the earlier gabbro-dioritic magma had not cooled yet and had differentiated significantly before the addition of the later granitic magma.

The Th/U ratios obtained from analyzed zircons of both samples (Fig. 10) prove the coexistence of two compositionally distinct magmas from which zircon crystallized during the evolutionary history of the bimodal magmatic chamber. The average Th/U ratio of the granite (0.40) is significantly lower than that of the gabbro-diorite (0.64), indicating felsic-intermediate metaluminous sources for the two magmas (Heaman et al., 1990; Hanchar and Miller, 1993, Hoskin and Schaltegger, 2003). In Figure 6, most zircon analyses of granite (sample MFT-1) are plotted in the range of 0.3<Th/U ratio<0.5, closer to the field of felsic peraluminous sources, in which probably there is abundant monazite (Th>U), and lower than those from the gabbro-diorite (sample BCA-2), in the range of 0.5<Th/U ratio<0.7, in which there is no other U-Th accessory.

Age relationship of the Santa Eulália-Monforte massif with other LC-EP Iberian calc-alkaline plutonic rocks

The reported weighted mean ²⁰⁶Pb/²³⁸U ages (this study) for the plutonic rocks of the external ring of the Santa Eulália-Monforte massif is of 303±3Ma (Late Carboniferous) for the gabbro-diorite and of 297±4Ma (Early Permian) for the granite. T-test indicates that the difference between these two zircon age populations is statistically significant (p-value is 0.02, i.e. lower than 0.05). Field observations of the consistent intrusive relationships between the greyish granite and the external ring (Gonçalves and Coelho, 1969-1970; Gonçalves, 1971; Oliveira, 1975; Gonçalves et al., 1972, 1975; Carrillho Lopes et al., 1998; González Ménendez et al., 2006; Sant’Ovaia et al., 2015) also suggest that these intrusions are coeval. Crystallization ages of the calc-alkaline magmatic rocks of the Santa Eulália-Monforte massif are within the wider range of ages obtained for other post-kinematic magmatic rocks of the Iberian Massif: i) the neighboring calcalkaline Nisa-Alpalhão and Los Pedroches batholiths (U-Pb zircon ages of ca. 314-304Ma; Carracedo et al., 2008; Solá et al., 2009) (Fig. 1); ii) the calc-alkaline plutonic rocks of the Central Iberian and West-Asturian Leonese zones (U-Pb zircon ages of ca. 312-286Ma; Valverde-Vaquero, 1997; Dias et al., 1998; Montero et al., 2004; Valle Aguado et al., 2005; Bea et al., 2006; Neiva et al., 2009; Fernández-Suárez et al., 2011; Gutiérrez-Alonso et al., 2011; Valverde-Vaquero et al., 2011; Díaz Alvarado et al., 2013; Díez Fernández and Pereira, 2017); and iii) the volcanic and plutonic rocks of the Central-eastern Pyrenees (U-Pb zircon ages of ca. 314-283Ma; Roberts et al. 2000; Aguilar et al., 2014; Druguet et al., 2014; Pereira et al., 2014; and ⁴⁰Ar/³⁹Ar ages in the range of ca. 291-285Ma; Solé et al., 2002). Our new U-Pb ages are ca. 20Ma younger with regard to the published age estimates for the Variscan plutonism and high-medium grade metamorphism recorded in the Ossa-Morena Zone (ca. 353-324Ma; Fig. 1). This age relationship indicates that the undeformed calc-alkaline magmatic rocks of the Santa Eulália-Monforte massif are consistent with a post-Variscan intrusion (i.e. postkinematic), representing so far the youngest LC-EP calc-alkaline magmatic event of the Ossa-Morena Zone. The emplacement of the Santa Eulália-Monforte massif immediately followed the neighboring calc-alkaline granite intrusion of the Nisa-Albuquerque massif (ca. 309-306Ma; Solá et al., 2009) and, in part, was almost synchronous to the latest magmatic pulses of the Los Pedroches massif (ca. 309-304Ma; Carracedo et al., 2008) (Fig. 1).

Geodynamic model

There is a general acceptance of a single, consensual interpretation that supports the growth of the Iberian LC-EP batholiths within the framework of the Variscan orogenic cycle (“late-Variscan batholiths”). This interpretation is considered valid even if the origin of these calc-alkaline magmas is about 60-80Ma younger than the development of the Rheic Ocean active margin (Martínez Catalán et al., 2009; Díez Fernández et al., 2016; Pereira et al., 2017b). This paradigm disregards the wider context of the amalgamation of the Pangaea and the spatial proximity of Iberia relative to the Eurasian active margin during the LC-EP closing of the Paleotethys Ocean (Pereira et al., 2015b).

The generation of LC-EP post-kinematic I-type calcalkaline Iberian batholiths in a tectonic setting other than subduction conflicts with essential thermal requirements and phase equilibria constraints on metaluminous magma generation and fractionation (Pereira et al., 2015b). The emplacement and thermal effects of the LC-EP calcalkaline batholiths into a fertile middle crustal level, dominated by several kilometer-thick fertile EdiacaranCambrian metasedimentary successions, explains the late generation of S-type peraluminous granitic rocks and local hybridization of the intrusive magma, which became more peraluminous and potassic (Díaz Alvarado et al., 2011; 2013). The lower crust is usually mentioned as the source of the Iberian calc-alkaline batholiths based on the assumptions that they are intrusive into the middle crust and that melts produced from the mantle underlying the continental crust must be basaltic (Dias et al., 1998; Neiva et al., 2009, 2012; Villaseca et al., 2009, 2012). The new conception of subduction-related magmatism ownig to plume-assisted relamination (Gerya et al., 2004; Castro et al., 2010; Gerya and Meilick, 2011; Vogt et al., 2012) assumes that the generation of magmas of intermediate composition (andesite-diorite) occurs by means of the melting of subducted materials in silicic composite plumes, i.e. composed of oceanic crust and sediments. These materials are later relaminated to a level below the lower crust, where they split by melt segregation into liquids that are emplaced in the middle and upper crust (batholiths) and residues that remain in the lower crust as mafic granulites (Castro, 2013, 2014). In the case of the LC-EP calc-alkaline Iberian magmatism, several factors indicate an extra-crustal origin (Castro et al., 2010, 2013; Castro, 2013, 2014; Pereira et al., 2015b). The LC-EP Iberian batholiths have close compositional similarities with the calc-alkaline silicic magmatism of active continental margins as the Andes (Patagonian batholith) and the North American Cordillera (Sierra Nevada batholith). They also include arc-related appinitic rocks, suggesting a link to a subduction setting (Murphy, 2013) similar to that of the post-Paleozoic Cordilleran batholiths formed at the Pacific active margin of the Americas (Castro et al., 2011). The period during which LC-EP calc-alkaline magmatism was generated (ca. 315-280Ma) coincides with the development of the Iberian-Armorican Arc (Fig. 11) (i.e. Cantabrian orocline; Gutiérrez-Alonso et al., 2008, 2011; Fernández-Suárez et al., 2011; Weil et al., 2012; Johnston et al., 2013), which was probably related to the Pangaea self-subduction (Gutiérrez-Alonso et al., 2008, 2011). In a global context, during the Late Carboniferous the western zones of the Iberian Massif were affected by the late stages of the Laurussia-Gondwana convergence, whereas the subduction of the Paleotethyan Ocean prevailed during and after LC-EP times (Cimmerian cycle) along the Eurasian eastern domains (Stampfli and Borel, 2002; Stampfli et al., 2013) (Fig. 11). The progression of the Paleotethys subduction led to the collision between Cimmerian terranes (detached from the non-collisional northern margin of Gondwana) and the Eurasian active margin (Cocks and Torsvik, 2006; Stampfli and Kozur, 2006), which can explain the inception of a LC-EP Paleotethyan magmatic arc in Iberia (Fig. 11; Pereira et al., 2015b).

CONCLUSIONS

This paper presents the first SHRIMP U-Pb zircon dating of the Santa Eulália-Monforte massif. Concordia and weighted average zircon ages from granite and gabbrodiorite samples indicate magmatic crystallization at ca. 297Ma and 303Ma, respectively. However, field evidence and full analysis of zircon ages do not definitively rule out that they may have been contemporaneous during a certain stage of magma emplacement.

The Santa Eulália-Monforte massif represents so far the youngest LC-EP calc-alkaline magmatic event of the Ossa-Morena Zone (ca. 303-297Ma), which immediately followed the neighboring calc-alkaline granite production of the Nisa-Albuquerque massif (ca. 309-306Ma) and in part, was almost synchronous to the latest magmatic pulses of the Los Pedroches massif (ca. 309-304Ma). The LC-EP Iberian magmatism was coeval with the formation of the Iberian-Armorican arc (Cantabrian orocline). The LC-EP Iberian magmatism can be framed into a plausible model of a transient continental magmatic arc developed in the Eurasian convergent Plate margin above the subducted Paleotethys oceanic Plate.

Acknowledgments

Financial support has been provided by Fundação para a Ciência e Tecnologia (Portugal) through research projects “GOLD” (PTDC/GEO-GEO/2446/2012: COMPETE: FCOMP01-0124-FEDER-029192) and “LITHOS” (CGL2013-48408C3-1-P). C. Rodríguez appreciates the financial support from Universidad de Huelva (Spain) through its postdoctoral program (under the “Estrategia Politíca Científica de la UHU 2016/2017 in collaboration with the Universidade de Évora (Portugal)). This is the IBERSIMS publication Nº 44. The authors are grateful for the critical spirit of the comments made by the editors of this volume, by J.B. Silva, R. Díez Fernández and an anonymous reviewer, who helped to greatly improve this work.

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