# A Paleozoic history of Armorica recounted through anorogenic to vaugneritic magmatism

Martial Caroff, Bernard Le Gall, Nicolas Estèves, Jean-Alix Barrat, Jessica

Langlade, Céline Liorzou, Philippe Nonnotte, Marie-Laure Rouget

► To cite this version:

Martial Caroff, Bernard Le Gall, Nicolas Estèves, Jean-Alix Barrat, Jessica Langlade, et al. A Paleozoic history of Armorica recounted through anorogenic to vaugneritic magmatism. Gondwana Research, 2025, 142, pp.130-151. 10.1016/j.gr.2025.02.024. hal-05046940

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# A Paleozoic history of Armorica recounted through anorogenic to vaugneritic magmatism

Martial Caroff<sup>a, *,</sup>, Bernard Le Gall<sup>a</sup>, Nicolas Estèves<sup>b</sup>, Jean-Alix Barrat<sup>c</sup>, Jessica Langlade<sup>d</sup>, Céline Liorzou<sup>a</sup>, Philippe Nonnotte<sup>a</sup>, Marie-Laure Rouget<sup>d</sup>

## ARTICLE INFO

Handling Editor: Andrea Festa

Keywords:

Vaugnerite

Anorogenic magmatism

Granite

European Variscan belt

Armorica

## ABSTRACT

The Plouaret-Commana-Huelgoat (PCH) vaugnerite-granite complex, emplaced at around 315 Ma on both sides of the North Armorican Shear Zone (NASZ) in the Armorican Massif (France), can be regarded as the “axial pivot” around which a large part of the history of the Armorican magmatism was revolving. Indeed, four main magmatic cycles can be distinguished in the Ordovician/Early Permian time-span over much of the considered areas, i.e. North and Central Armorican Domains (N/CAD): (1) several anorogenic mafic complexes (plutons, dykes and/or lavas) were successively emplaced from the Ordovician to Early Carboniferous; (2) near the Bashkirian/Moscovian time-boundary, when the N/CAD started to record the effects of the Variscan collision, magmatism became predominantly lamprophyric/vaugneritic (K-rich West-Armican kersantites and PCH vaugnerites), prior to (3) the generalized granitic peak during the Moscovian-Kasimovian ages, in turn followed by (4) a K-rich intermediate magmatic activity, including again vaugnerites and the last granitic intrusions, during the late-/post-collision stages in the upper Pennsylvanian-Early Permian time-period. The sharp geochemical break evidenced in the present work between an early anorogenic magmatic group and a syn- to post-collisional system in the N/CAD emphasizes the original magmatic history recorded by part of Armorica, including probably the Central Iberian Zone (CIZ), during the nearly entire Paleozoic era compared to those commonly applied to the rest of the European Variscan orogen. This evolution is assumed to result from: (1) large-scale asthenospheric upheavals due to the downward straightening and retreat of variously plunging slabs during the Paleozoic times, (2) partial melting of metasomatized lithospheric/asthenospheric mantle due to fluid release from fragments of buried continental crust, and (3) crustal anatexis.

## 1. Introduction

Numerous consolidated studies about the European Variscan belt have been published over the past decades (Matte, 2001; Winchester et al., 2002; Maierová et al., 2016; Franke et al., 2017; Torsvik and Cocks, 2017), as well as geodynamical investigations on more regional case studies (e.g., Schaltegger et al., 1996; Sandeman et al., 2000; Ballèvre et al., 2009; Faure et al., 2009; Guillot et al., 2020; Vanderhaeghe et al., 2020; Caroff, 2023).

According to Maierová et al. (2016), the Variscan events occurred over a c. 90 Ma-lasted time-period from continental subduction (c.

400–360 Ma) to collision (c. 360–335 Ma) and continental underthrusting (c. 335–310 Ma) processes. But, given the non-cylindrical nature of the Variscan orogen, which is composed of a mosaic of microplates (blocks) squeezed between the Gondwana and Laurussia lithospheric plates, these geodynamical events took place diachronously, as exemplified about French Variscan segments. In the Massif Central, the sequence typically includes: (i) high-pressure metamorphism in the Late Silurian/Early Devonian, (ii) nappe displacement and migmatization in the Middle Devonian (385–380 Ma), (iii) shearing and intermediate P-T metamorphism in the Late Devonian/Early Carboniferous (360–350 Ma), (iv) shearing in the south and synorogenic

<footnote><sup>a</sup> UMR/CNRS n° 6538 « Laboratoire Geo-Ocean CNRS/IFREMER/UBO/UBS », Université de Brest, Institut Universitaire Européen de la Mer (IUEM), Place Nicolas Copernic, 29280 Plouzané, France</footnote>

<footnote><sup>b</sup> CRPG, Université de Lorraine/CNRS, 15, rue Notre Dame des Pauvres, 54501 Vandøuvre-lès-Nancy, France</footnote>

<footnote><sup>c</sup> UMR/CNRS 6539 « Laboratoire des Sciences de l'Environnement Marin », LIA BeBEST, Université de Brest, Institut Universitaire Européen de la Mer (IUEM), Place Nicolas Copernic, 29280 Plouzané, France</footnote>

<footnote><sup>d</sup> UAR 3113, Université de Brest, Institut Universitaire Européen de la Mer (IUEM), Place Nicolas Copernic, 29280 Plouzané, France</footnote>

<footnote>* Corresponding author.
E-mail address: caroff@univ-brest.fr (M. Caroff).</footnote>
extension in the north during the Visean and, finally, (v) late orogenic extension in the Late Carboniferous (Faure et al., 2009). Vanderhaeghe et al. (2002) suggested a southward propagation of the deformation from c. 345 to c. 310 Ma, reflecting the southward retreat of a northerly-subducting slab. In the Vosges, the transition from oceanic to continental subduction would occur at c. 345 Ma, followed by the onset of the colliding process at c. 340 Ma and then by the southward propagation of thrusting, coupled with slab retreat and delamination of the overriding mantle lithosphere at c. 335 Ma (Guillot et al., 2020). Schaltegger et al. (1996) also mentioned a Visean extension episode in the Southern Vosges between 345 and 340 Ma, “while convergent tectonics were still ongoing in the external part of the Variscan orogeny”.

The difficulties in correlating all the spatially/temporarily distinct Variscan geodynamical events are amplified by two additional potential biases concerning the corresponding magmatic systems. Firstly, most proposed geodynamical models did not benefit from recently improved age dating methods. That is illustrated in the Armorican segment where, for instance, (i) the North Armorican doleritic swarm, previously dated at 330 ± 10 Ma (K/Ar, Perroud et al., 1986), recently yielded ages of 363.4 ± 5.8 Ma (U-Pb on apatite; Pochon et al., 2016) and (ii) the age of the Huelgoat granite in the Central Armorican Domain (CAD) has been rejuvenated from 340 ± 9 Ma (Rb-Sr; Georget, 1986) to 314.4 ± 2.5 Ma Ma (U-Pb on zircon; Ballouard, 2016). Plate interpretations can be distorted by the use of such ancient datings.

A second potential source of errors concerns the geodynamical significance of specific magmatic bodies and their interpretation as pre-, syn- or post-collisional markers. Again, these difficulties are exemplified about Armorican granitic complexes, including the Plouaret-Commana-Huelgoat one (PCH) under study, which might comprise coeval intrusions showing various mineralogical/geochemical types, e.g., the Aber-Ildut/North-Ouessant complex in the Léon metamorphic dome, consisting of mica-free metaluminous granites together with muscovite-rich strongly peraluminous granites and cogenetic leucominettes (Caroff et al., 2015). Inversely, granitic complexes with a similar mineralogy are known to have been emplaced at various times in different settings, e.g., the 314-old Huelgoat cordierite-granite in the CAD, which is mineralogically and geochemically comparable to the 292 Ma-old Brignogan “S-type” cordierite-granite in the Léon (Georget, 1986), which in turn is geographically enclosed within the contemporaneous North Armorican “I-type” cordierite/muscovite-free “red” granitic belt (Georget et al., 1986). Thus, the granite typology alone, reflecting generally various local crustal source compositions (e.g., metasedimentary/metaigneous sources for muscovite-/cordierite-bearing peraluminous granites, respectively: Moyen et al., 2017) with modest mantle-derived inputs (Frost et al., 2001), appears to be quite unsuitable for proposing interpretative lithospheric transects of great magnitude, as proposed by Ballouard et al. (2017).

Furthermore, certain types of sparse magmatic rocks present in Armorica are often seen as typical markers of specific magmatic affinities and contexts. This is the case for K-rich mafic-to-intermediate intrusives, including lamprophyres and vaugnerites, which are classically considered as orogenic magmas typical of late-/post-collisional settings (e.g., Awdankiewicz, 2007; Couzinié et al., 2014; Moyen et al., 2017; Dijkstra and Hatch, 2018; Soder and Romer, 2018; Pouclet et al., 2022). However, lamprophyres and vaugnerites may also occur in syn-collisional settings (Von Raumer et al., 2014; Guillot et al., 2020; Krmíček et al., 2020), sometimes even at an early stage (Caroff et al., 2021; Le Gall and Caroff, 2023). At last, post-collisional magmatic complexes can display anorogenic affinities (Krmíček et al., 2020; Krmíček and Chalapathi Rao, 2021). Consequently, compiling routinely K-rich magmatic systems in continental-scale contexts where affinities and sometimes ages are mingled (e.g., Turpin et al. 1988; Von Raumer et al., 2014; Dijkstra and Hatch, 2018; Pouclet et al. 2022), may result in misinterpretations.

About the Armorica microplate under study, sometimes likened to the Armorican Terrane Assemblage (ATA; e.g., Shail and Leveridge,

2009; Franke et al., 2017), additional problems relate to its definition in terms of nature, age, location of its constitutive tectono-metamorphic domains and those of its boundaries (sutures). In the numerous and mutually incompatible published geodynamical models, Armorica is variously interpreted as being composed of either: (i) the Central/North Armorican Domains (CAD/NAD), the North Vosges, and the North Bohemian Massif (Guillot et al., 2020); (ii) a large part of the Iberian Peninsula, the whole French Armorican and Central Massifs, and the Teplá-Barrandian Zone in the central part of the Bohemian Massif (Franke et al., 2017); or (iii) the Central Iberian Zone and the CAD/NAD (Ballèvre et al., 2009; Caroff, 2023). Another point of controversy is the structural significance of the Léon metamorphic dome, in the north-western part of the Armorican segment. It is generally regarded as an exotic domain (Le Corre et al., 1989), further correlated laterally with either the Saxo-Thuringian Zone (Ballèvre et al., 2009; Vanderhaeghe et al., 2020) or the Mid-German Crystalline Rise (Faure et al., 2010; Franke et al., 2017; Moreira et al., 2019). According to a third hypothesis recently proposed by Le Gall and Caroff (2023), and briefly presented below, the Léon metamorphic dome and the CAD are parts of a single Variscan tectono-magmatic domain in the Armorica microplate.

The aim of this work is to decipher the geodynamic evolution of the north/central part of the Armorican Variscan segment from the Ordovician times up to Early Permian by: (i) focusing on the Plouaret-Commana-Huelgoat (PCH) vaugnerite-bearing granitic complex, located on both sides of the North Armorican Shear Zone (NASZ) and regarded as the “axial pivot” around which a part of the Armorican magmatic history revolved, since it illustrates the passage from a first mantle-derived vaugneritic episode to crustal granites, (ii) carrying out a geochemical review of selected Paleozoic Armorican (including the Central Iberian Zone) anorogenic mafic intrusives/lavas and lamprophyric/K-rich intermediate magmatic complexes which benefited from reliable datings and thus can be used to constrain the spatio-temporal pattern of magmatism in the Armorican belt from basinal to orogenic stages, and (iii) elaborating a lithosphere/asthenosphere-scale model addressing the magma source issue in a revised geodynamical framework of the Armorica microplate. It will be argued that the Paleozoic history of Armorica does not readily fit within the general framework traditionally proposed for the European Variscan belt.

## 2. Geological setting and age framework

### 2.1. Regional structural arrangement

The area under study gathers the Léon metamorphic dome (LMD), the Central Armorican (CAD) and North Armorican (NAD) domains at the western extremity of the Armorican massif (Western France), which represents the onshore part of the Armorica microplate (Fig. 1). North of the NASZ, the NAD preserves the best records of Cadomian orogenic events (610–540 Ma) in NW Europe (D’lemos et al., 1990; Brun et al., 2001; Le Gall et al., 2021). South of the NASZ, the ca. 60 km-wide CAD is composed of a > 5 km-thick pile of low-grade metasediments, ranging in age from the Lower Ordovician to Lower Carboniferous, and moderately strained during the Variscan orogeny (Caroff et al., 2020; Le Gall and Caroff, 2023; see also section 6.1). The CAD is tectonically juxtaposed to the south, along the Nort-sur-Erdre suture zone, to Gondwana palaeomargin terranes in the South Armorican Domain (SAD) (Bitri et al., 2003; Faure et al., 2008; Ballèvre et al., 2009), in the western prolongation of the north-eastern Massif Central (Guillot et al., 2020; Vanderhaeghe et al., 2020; Fig. 1a).

Special attention should be paid to the structural relationships between the CAD and LMD terrains, since they deal with the problem of the northern boundary of Armorica. In most paleogeographic reconstructions of the European Variscan belt (e.g., Alvaro et al., 2020; Schulmann et al., 2022), the LMD is viewed as an exotic Variscan domain, either (i) translated over a distance of c. 400 km from the SAD
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Geological setting. (a) Structural map and main tectonostratigraphic units of the European Variscan belt. Adapted from Ballèvre et al. (2009), Franke et al. (2017), Soder and Romer (2018), Authemayou et al. (2019), Moreira et al. (2019), Guillot et al. (2020), and Caroff et al. (2021). (b) Sketch geological map of the Armorican Massif, showing the studied magmatic units (from Barrière, 1977; Cabanis et al., 1982; Cabanis, 1985; Galerne et al., 2006; Guillet et al., 1985; Georget, 1986; Georget et al., 1986; Le Corre et al., 1989; Lahaye et al., 1995; Le Gall, 1999; Caroff, 2023; Caroff et al., 1996, 2009, 2011, 2016, 2020, 2021; Chauris et al., 1998; Coint et al., 2008; Barboni et al., 2013; Guennoc et al., 2015; Pochon et al., 2016, 2023; Pochon, 2017). See text for explanations. NAD: North Armorican Domain; CAD: Central Armorican Domain; L-CAD: Léon and Central Armorican domains gathered; SAD: South Armorican Domain; PGSZ: Porspoyer-Guissény Shear Zone (the only significant sinistral shear zone); ESZ: Elorn Shear Zone; MASZ: Monts d'Arrée Shear Zone; MNSZ: Montagne Noire Shear Zone; NASZ: North Armorican Shear Zone; SASZ: South Armorican Shear Zone. (c) Main granitic complexes in the L-CAD and NAD. Colors express their intrusion timing (from Carric et al., 1979; Peucat et al., 1984; Georget et al., 1986; Chauris et al., 1998; Marcoux et al., 2009; Capdevila, 2010; Égal et al., 2011; Tartèse et al., 2011; Le Gall et al., 2014; Caroff et al., 2015; Guennoc et al., 2015; Ballouard, 2016; Ballouard et al., 2017). Bg, PL, Pm, PCH: see Fig. 1b; AI: Aber-Ildut/North-Ouessant complex; SR: Saint Renan leucogranite; SC: Sainte Catherine leucogranite; Q: Quintin complex; R: Rostrenen complex; P: Pontivy complex; L: Lizio granite.</figcaption></figure>

along a major N50°E dextral strike slip fault (Balé and Brun, 1986; Le Corre et al., 1989), or (ii) thrusted over the W-CAD Paleozoic series along a crustal-scale discontinuity, i.e. the Elorn/Le Conquet suture zone (Faure et al., 2005). According to the last model, the LMD is part of an intervening crustal block, referred to as the Normannian Terrane (Nutman et al., 2023) or the Armorican Terrane Assemblage (Shail and Leveridge, 2009), between Armorica and Laurussia, and further correlated with the Mid German Crystalline Rise or the Saxo-Thuringian Zone in Germany/Bohemia (Ballèvre et al., 2009; Faure et al., 2010; Vanderhaeghe et al., 2020) and with the Finisterra Terrane in Iberia (Moreira et al., 2019) (Fig. 1a). However, these assumptions have been challenged from the line of evidence briefly listed below. The thorough structural analysis of the W-CAD/LMD transition zone shows that both domains share common deformation patterns (a single major ductile

strain episode), but with marked differences attributed to the late domal structure of the LMD and to regional strain partitioning (Le Gall et al., 2014). On one other hand, the metamorphic conditions and P/T paths recorded by the dominantly micaschist/gneiss pile on the southern limb of the LMD have been evaluated by Jones (1994) and Schulz (2013). Their geothermobarometric results demonstrated that the metamorphic series exposed at the W-CAD/LMD transition zone experienced P/T conditions increasing regularly northwards, i.e. towards the structurally lower part of the succession, without any gap or reverse position of metamorphic gradients. These results, combined with our structural data, contradict previous “suture-type” models as showing that the metamorphic pile exposed at the W-CAD/LMD transition zone represents a quasi-continuous and coherent crustal section composed of an infra- (LMD) and a supra- (W-CAD) crustal portions, both being parts of
Armorica. That agrees with the identification of a Visean volcanic basin with a clear CAD affinity in the LMD-attached Ouessant island (Caroff et al., 2020; Caroff, 2024) (Fig. 1). Consequently, the Léon dome and the CAD will be hereinafter gathered under the acronym “L-CAD” (Fig. 1a).

The major strain event recorded by this continuous lithostratigraphic pile in the L-CAD is thought to coincide with the main metamorphism peak at c. 315 Ma (Schulz, 2013; Authemayou et al., 2019; Caroff et al., 2021; see also Supplementary Data, Fig. S1). Le Gall and Caroff (2023) therefore suggest that the main convergent tectonic events (folding and shearing) in the L-CAD coincides approximately with the Carboniferous Bashkirian/Moscovian ages. It has been shown by Caroff (2023) that the L-CAD/NAD and the Central Iberian Zone (CIZ) were strictly attached in Pre-Variscan times and as there is no equivalent demonstration for the North Vosges and the Teplá-Barrandian Zone, we prefer not to use here the “Armorican Terrane Assemblage” label of Shail and Leveridge (2009) and instead consider these domains as parts of Armorica (Fig. 1a).

## 2.2. Pre-Serpukhovian mafic intrusives and lavas

The main mafic complexes pre-dating the main Variscan orogenic events in the L-CAD/NAD range in age from the Ordovician to Visean (Fig. 1b). They include: (i) the Saint-Jean-du-Doigt gabbroic body (SJ), Tournaision in age (Coint et al., 2008; Caroff et al., 2011; Barboni et al., 2013; Pochon et al., 2023), (ii) various basalt/andesite lava fields comprising sometimes subordinate dolerites, such as the Katian complex in the Crozon peninsula (Cz: Cabanis, 1985; Caroff et al., 2009; Caroff, 2023), the Devonian basalts and dolerites from Bolazec (Bo: Cabanis et al., 1982; Cabanis, 1985), the Tournaision/Visean basalts and andesites from the South Ouessant, Morlaix, Châteaulin and Laval basins (TVB: Caroff et al., 1996, 2016, 2020), and (iii) numerous and widely distributed doleritic swarms ranging in age from the Floian in the Bréhec area (Br: Suire et al., 1991; Galerne et al., 2006), the Devonian in the Brest Bay (BB: Caroff et al., 2021), up to the Famennian in the North/ Central Armorican domains (NC: Lahaye et al., 1995; Pochon et al., 2016).

All these mafic bodies are evenly distributed throughout the areas concerned (Fig. 1b). The chronological framework mainly relies on paleomagnetic and/or stratigraphic constraints (references in Fig. 1b and Supplementary Data, Table S1 and S2). The more recent absolute datings are: 363.4 ± 5.8 Ma for the NC dyke swarm (LA-ICP-MS U-Pb on apatite: Pochon et al., 2016); c. 347 Ma for the SJ gabbro (ID-TIMS U-Pb on zircon: Barboni et al., 2013); 336.3 ± 2.3 Ma for the South-Ouessant TVB lavas (LA-ICP-MS U-Pb on zircon: Caroff et al., 2016).

## 2.3. A multistage Variscan granitic system

The emplacement of granites in the L-CAD/NAD occurred via successive syn-, late- and post-collisional pulses, at c. 315, 303 and 294 Ma, respectively (Supplementary Data, Fig. S1). The corresponding intrusions are shown in Fig. 1c. Below are listed the L-CAD/NAD Variscan granitic complexes (or simple plutons) for which reliable absolute datings are available:

(1) the 317–310 Ma syn-collisional stage: Lizio leucogranite, 316.4 ± 5.6 Ma (U-Pb on zircon, Tartèse et al., 2011); Saint-Renan leucogranite, 316.0 ± 2.0 Ma (LA-ICP-MS U-Pb on zircon, Le Gall et al., 2014); Pontivy-Rostrenen granitic complex, from 316.7 ± 2.5 Ma to 310.3 ± 4.7 Ma (LA-ICP-MS U-Pb on zircon, Ballouard et al., 2017); Huelgoat granite (PCH), 314.4 ± 2.5 Ma (LA-ICP-MS U-Pb on zircon, Ballouard, 2016; see section 2.4);

(2) the 305–302 Ma late-collisional stage: Pontivy-Rostrenen leucogranites, 304.7 ± 2.7 Ma (LA-ICP-MS U-Pb on zircon, Ballouard et al., 2017); Aber-Ildut/North-Ouessant granitic complex (including the M Aber-Ildut leucominettes studied here), 303.8 ± 0.89 Ma (LA-ICP-MS U-Pb on zircon, Caroff et al., 2015);

Sainte-Catherine leucogranite, 302.7 ± 0.3 (whole rock Ar-Ar, Marcoux et al., 2009);

(3) the 295–291 Ma post-collisional stage: Ploumanac’h diorite-gabbro complex (Pm), 295 ± 3 Ma (LA-ICP-MS U-Pb on zircon, Guennoc et al., 2015); Brignogan monzogranite-vaugnerite complex (Bg), 292 ± 15 Ma (whole-rock Rb-Sr, Georget et al., 1986); Quintin granitic complex, 291 ± 9 Ma (whole rock Rb-Sr, Peucat et al., 1984).

## 2.4. Syn-collisional lamprophyres and vaugnerites (and associated rocks)

During the Bashkirian/Moscovian contraction stage, the westernmost part of the CAD was intruded by two main lamprophyric/vaugneritic complexes: (i) the West-Armorican kersantite network (WAK), thoroughly studied by Caroff et al. (2021) and Le Gall and Caroff (2023), and (ii) the PCH vaugnerite-bearing granitic complex (Fig. 1b), previously studied by Guillet et al. (1985), Georget (1986), and Castaing et al. (1987), and reviewed in the present work.

The PCH complex is dextrally shifted (ca. 20 km) and deformed along the North Armorican Shear Zone (NASZ) separating the NAD and CAD (Fig. 1b). It is composed of three distinct massifs, in turn divided into eight magmatic units (Fig. 2).

(1) The Plouaret Massif, the only one to be located in the NAD (Fig. 1b and 2a), is made up of two units (Garreau, 1978; Bambier et al., 1984; Guillet et al., 1985; Chantraine et al., 1999) (Fig. 2b). Vaugnerites chiefly occur as meter-sized enclaves in the northern unit (Unit 1: Fig. 2b).

(2) The Commana-Plounéour Massif is composed of three units, all located in the CAD (Fig. 2): (i) the Lannéanou Unit along the trace of the NASZ, (ii) the Plounéour and (iii) the Commana Units, both located in the CAD between the NASZ and a satellite structure to the south, the Monts d’Arrée Shear Zone (MASZ) (Chantraine et al., 1981; Babin et al., 1982; Guillet et al., 1985; Castaing et al., 1988). Tectonic overprint of ductile wrenching in the Lannéanou Unit and the northern part of the Plounéour Unit is locally expressed by highly crushed and sheared fabrics along the NASZ (Chantraine et al., 1981; Fig. 3), as well as in the south-eastern boundary of the Commana Unit along the MASZ (Chantraine et al., 1981; Castaing et al., 1988; Fig. 2a).

(3) The Huelgoat Massif is made up of three units: (i) the La Feuillée Unit in the north, (ii) the Huelgoat s.s. (in a strict sense) Unit in the south and (iii) the Le Cloître Unit in the central part of the previous one (Castaing et al., 1984; Georget, 1986; Castaing et al., 1987, 1988; Castaing and Debéglia, 1990; Fig. 2b). The north-western edge of the La Feuillée Unit is affected by the MASZ (Castaing et al., 1987, 1988).

The syn-tectonic emplacement of the WAK intrusions in the deformed Paleozoic succession in the Brest Syncline (Fig. 1b), possibly in the time-range 320–310 Ma, has been recently documented (Le Gall and Caroff, 2023). The Plouaret Massif has been dated at 329 ± 5 Ma (whole rock Rb-Sr: Peucat et al., 1984), but this age appears questionable since the isochron has been calculated from several non-cogenetic petrological facies. Indeed, Peucat et al. (1984) analyzed the diversity of the lithologies exposed in the Plouaret Massif and obtained a straight line in a 87Sr/86Sr vs. 87Rb/86Sr diagram. Our new initial Sr-Nd isotopic compositions demonstrate that these rocks are not cogenetic (see further in the text), an essential condition for validating an isochrone. The y-intercept of this “pseudo-isochrone” is controlled by vaugnerites, while the slope is largely controlled by the most siliceous (granitic) rocks with the most radiogenic isotopic compositions. Thus, the slope of the “pseudo-isochrone” is certainly steeper than expected from strictly cogenetic samples. It follows that the true age of the Plouaret Massif is necessarily younger than that calculated by Peucat et al. (1984). The more recent absolute datings for the Huelgoat rocks are: 314.8 ± 2.0 Ma
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Geological features of the Plouaret-Commana-Huelgoat (PCH) complex. (a) Sketch geological map, modified after Guillet et al. (1985). (b) Location of the PCH samples (numbers alone refer to CP samples in Supplementary Data, Table S1).</figcaption></figure>

for the Le Cloître Unit and 314.0 ± 2.8 Ma for the La Feuillée Unit (LA-ICP-MS U-Pb on zircon: Ballouard, 2016). This narrow range of ages will be considered here as applying to the whole PCH complex.

## 2.5. Late-/post-collisional K-rich intermediate rocks

K-rich intermediate intrusives from four Armorican late-/post-collis-ional complexes are also considered in the present study (Fig. 1b): (i) the Kasimovian/Gzhelian leucominette swarm (M) belonging to the
<figure><img src="image_6.png" /><figcaption>Fig. 3. Tectonized samples from the PCH Lannéanou granitic unit: (a) crushed sample CP-2; (b, c) ductilely deformed sample CP-43.</figcaption></figure>

Aber-Ildut/North-Ouessant granitic complex (Caroff et al., 2015); (ii) the Asselian Plounévez-Lochchrist vaugnerites (PL; Chauris et al., 1998); (iii) monzodiorites/diorites of the Asselian Ploumanac'h diorite-granite complex (Pm; Guennoc et al., 2015); and (iv) vaugnerites of the Sak-marian Brignogan monzogranite-vaugnerite complex (Bg; Georget, 1986).

Among these four complexes, the M-bearing Aber-Ildut/North-Ouessant one is the only one which was ductilly trained as a late-Variscan intrusion, along the Porspoder-Guissény sinistral Shear Zone (PGSZ, Fig. 1b, c). The three other complexes are devoid of any deformation and thus typically represent post-collisional intrusions.

Together with the M complex (Caroff et al., 2015), the Morlaix Bay and Flamanville granites (Capdevila, 2010), the Ploumanac'h intrusion (Pm), specially sampled for the present study, is part of the North Armorican late-/post-collisional high-K red granitic belt (Fig. 1c), typically alkali-calcic to calc-alkaline in affinity (Caroff et al., 2015). Pm monzodioritic/dioritic outcrops are rare (mainly in Poul Palud and the Sainte-Anne Bay, Trégastel: Barrière, 1977; Supplementary Data, Table S1), but from gravimetric and aeromagnetic records, they are assumed to constitute about a quarter of the entire complex at depth (Barrière, 1977; Guennoc et al., 2015).

The PL vaugnerites yielded ages of 296 ± 3 Ma (U-Pb on zircon: Chauris et al., 1998). About the M, Pm and Bg complexes, see section 2.3.

# 3. Petrology of the PCH and Pm complexes

## 3.1. *Sampling, nomenclature and texture/structure*

Several units belonging to two distinct complexes have been sampled and analyzed for this study: (i) the eight units constitutive of the PCH (40 samples in total, distributed as follows: 23 for the Plouaret Massif; 10 for the Commana-Plounéour Massif; 7 for the Huelgoat Massif; Fig. 2b and 3), and (ii) the monzodioritic/dioritic rocks enclosed within the Pm complex (6 samples). The nomenclature of the corresponding rocks is based on the Q' vs. ANOR diagrams of Fig. 4a (new data) and Fig. 4b (published data), derived empirically by Streckeisen and Le

Maitre (1979) from the IUGS classification of Streckeisen (1976) and considered as its CIPW normative equivalent. These diagrams are based on the normative parameters 100Q / (Q + Ab + Or + An), representing silica saturation, and 100An / (An + Or) that reflects the relative abundance of calcium and potassium in the rocks.

The term “vaugnerite” (from the locality Vaugneray in the French Massif Central) was first used by Fournet (1837) and Delesse (1851) to designate biotite-rich, amphibole-bearing dark “micaceous diorites” that generally occur as enclaves within granites. Sabatier (1991) applied the term to K-rich micaceous intermediate coarse-grained rocks (“diorite” in the broad sense), mainly composed of biotite, amphibole and plagioclase, with subordinate quartz, K-feldspar and apatite. Authors such as Rock (1991) and Errandonea-Martin et al. (2018) consider the vaugnerites as plutonic equivalents to the hypabyssal lamprophyres (comprising five rock-types following the IUGS nomenclature of Le Maitre (2002) and Le Bas (2007): minettes, kersantites, spessartites, vogesites and camptonites). However, it should be noted that the vaugnerites have less mafic compositions than most lamprophyres (e.g., Scarrow et al., 2008; Krmíček et al., 2020). All these features can be used to classify the Plouaret monzonites, monzodiorites and quartz-monzodiorites of the diagram in Fig. 4 within a vaugneritic group. This Variscan syn-collisional magmatic set will be compared here to the post-collisional PL and Bg Armorican vaugnerites, rightfully designated as such by Chauris et al. (1998), together with two groups of syn-/late-collisional Armorican lamprophyres: the WAK (see section 2.4) and Aber-Ildut leucominettes (M) (see sections 2.3 and 2.5) (Fig. 4b). By contrast, the more mafic monzodiorites and diorites from the Pm complex (Fig. 4a) cannot be considered as vaugnerites because of the systematic presence of olivine, a lesser abundance in mica and less potassic compositions (Supplementary Data, Table S1).

All the studied Pm/PCH magmatic rocks have a coarse-grained texture, more or less porphyritic (see Supplementary Data, Table S1 for details). The Pm (monzo)diorites are undeformed, whereas the PCH rocks sampled near the main shear zones (NASZ and MASZ) locally display tectonic deformations. This is especially the case for samples collected in the Lannéanou Unit along the NASZ, which either are highly crushed (CP-2, Fig. 3a) or display typical dextral strike-slip-related S/C
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Q' vs. ANOR diagram for the PCH/Pm rocks (Streckeisen and Le Maitre, 1979), considered as a CIPW normative equivalent to the IUGS classification of Streckeisen (1976). The trends of Whalen and Frost (2013), corresponding to the four plutonic rock series defined by Frost et al. (2001), are also indicated: A: alkali; A-C: alkali-calcic; C-A: calc-alkalic; and C: calcic. U1: Plouaret Unit 1; U2: Plouaret Unit 2; PV-BPM: Plouaret vaugnerites and Bégard/Ploubezre monzogranites. CIPW norms have been calculated with Fe₂O₃/FeO = 0.35 for the vaugnerites (MgO > 3 wt%) and Fe₂O₃/FeO = 0.50 for the granitoids (MgO < 3 wt%) (Middlemost, 1989). (a) New PCH/Pm data. Sample labels are indicated (numbers alone refer to CP samples in Supplementary Data, Table S1). (b) Fields for the previously published PCH data (Babin et al., 1982; Bambier et al., 1984; Castaing et al., 1984; Georget, 1986; Chantraine et al., 1999), together with the Brignogan (Bg) / Plounévez-Lochrist (PL) vaugnerites (Georget, 1986; Chauris et al., 1998). Number of samples in brackets.</figcaption></figure>

tectonic fabrics (CP-43, Fig. 3b, c).

## 3.2. Mineralogy

The studied PCH magmatic rocks are moderately/weakly altered, whereas the Pm samples are fresh. Plagioclase is frequently damouritized; the cordierite megacrysts from the Commana granites are systematically replaced, partially or completely, by a muscovite-chlorite-clay association (pinite), whereas the cordierite megacrysts from the Huelgoat granites are generally fresh (except along cracks); the clinopyroxene crystals from the Plouaret vaugnerites and Bégard/Ploubezre monzogranites (referred here to as PV-BPM) are partly or

completely pseudomorphosed into amphibole; secondary quartz and calcite crystals occur occasionally in some granitic samples. The analytical methods for microprobe data are shown in Supplementary Data, Appendix A.

In addition to centimeter-sized megacrysts of plagioclase (≤ 10 vol %) in rare samples and exceptional 3–6 mm-long primo- or xenocrysts of quartz (≤ 10 vol% in CP-37), the Plouaret vaugnerites are constituted of, in order of decreasing abundance: groundmass biotite (Fig. 5c; Supplementary Data, Table S3) and plagioclase (Supplementary Data, Fig. S2a, b); glomerophyric amphibole (5–20 vol%), magnesiohornblende to actinolite in composition (Fig. 5a; Supplementary Data, Table S4), formed through pseudomorphic replacement (autometasomatism) of
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Mineral discrimination diagrams for the PCH vaugnerites and granitoids; apfu: atoms per formula unit. U1: Unit 1; U2: Unit 2; PV-BPM: Plouaret vaugnerites and Bégard/Ploubezre monzogranites. (a) Chemical variations of amphiboles in the Mg/(Mg + Fe2 + ) vs. Si diagram. All values in cations per formula unit calculated to 23O and 13 CNK (total cations except Ca, Na, K and Ba = 13). Nomenclature from Leake et al. (1997). Data in Supplementary Data, Table S4. (b) Chemical variations of clinopyroxenes in the Ca-Mg-Fe diagram. All values in cations per formula unit calculated to 6O and 4 cations. Nomenclature from Morimoto et al. (1988). Data in Supplementary Data, Table S5. A field for the W-Armorican kersantites (Caroff et al., 2021) is shown for comparison. (c) Chemical variation of biotites in the Al-Mg-Fe diagram. All values in cations per formula unit calculated to 22O. Nomenclature from Speer (1984). Data in Supplementary Data, Table S3. A field for the W-Armorican kersantites (Caroff et al., 2021) is shown for comparison. (d, e) Chemical variation of biotites in Abdel-Rahman (1994) diagrams: Al₂O₃ vs. MgO (d) and Al₂O₃ vs. FeO (e). A: biotites in anorogenic alkaline suites; Peral.: biotites in peraluminous suites; and Calc-alk.: biotites in calc-alkaline orogenic suites.</figcaption></figure>

diopside primocrysts (Fig. 5b; Supplementary Data, Table S5, Fig. S2c, d), still present in a few samples (≤ 10 vol%); Fe-Ti oxides, titanite, apatite, zircon and allanite (Fig. 6a, b and Supplementary Data, Table S1).

The Bégard/Ploubezre monzogranites (Fig. 4) present a mineralogy very close to that of the Plouaret vaugnerites (both being merged under the acronym PV-BPM), except that all of them are feldspar-phyric and contain less plagioclase and biotite, and more alkali feldspar (Supplementary Data, Table S1). Amphibole, clinopyroxene (sometimes amphibolitized: Supplementary Data, Fig. S2c, d) and biotite compositions are identical in both cases (Fig. 5).

The other granitoids from the PCH complex plot in the monzogranite, syenogranite or Afs granite fields in the diagrams of Fig. 4,

regardless of their massif/unit provenance (Supplementary Data, Table S1). They are sometimes porphyritic (especially the Huelgoat facies) and made up, in order of decreasing abundance, of alkali feldspar, plagioclase, biotite, Fe-Ti oxides, apatite and zircon. The granitoids from Plouaret and a part of those from the Commana-Plounéour Massif contain muscovite (≤ 10 vol%) (Fig. 6c), whereas all the samples from Huelgoat (except the La Feuillée Unit) and the remainder of the Commana-Plounéour granitoids enclose centimeter-sized megacrysts of cordierite (Fig. 6d). Note that the Lannéanou crushed/sheared granite has a mineralogy close to that of the Plounéour one in its northern part (feldspar, biotite and less than 5 % muscovite: e.g., CP-11 in Supplementary Data, Table S1), which would suggest that both units are in continuity.
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Photomicrographs of four PCH samples. (a) Plouaret vaugneritic monzodiorite CP-25; (b) Plouaret vaugneritic monzonite CP-31; (c) Croaz-Ilies syenogranite CP-35a; (d) Huelgoat syenogranite H-1. Ap. Apatite; All. Allanite; Bi. Biotite; Cord. Cordierite; Di. Diopside; Fe-Ti ox. Fe-Ti oxides; Mhb: Magnesiohornblende; Mu. Muscovite; Or. Orthose; Pl. Plagioclase; Qz. Quartz; Tit. Titanite; Zir. Zircon.</figcaption></figure>

The biotite composition of the PV-BPM, very restricted for both groups, is less aluminous than that of the other granites (Fig. 5c). The first set plots in the calc-alkaline field in the Abdel-Rahman (1994) diagrams of Fig. 5d, e, whereas the second one plots in the peraluminous field.

The Pm monzodiorites and diorites, labelled as norites and gabbros by Guennoc et al. (2015), have a medium/coarse-grained equigranular texture (average length of the crystals ≤ 1 mm for PLM-2 and −5; ≤ 2 mm for PLM-6; ≤ 3 mm for PLM-3 and −4) and they contain, in variable proportions, plagioclase (≥ 20 vol%), biotite (≥ 15 vol%), clinopyroxene (≥ 10 vol%), orthopyroxene (≤ 10 vol%; absent in PLM-2 and −5); olivine (3–18 vol%; absent in PLM-6), green hornblende (≤ 10 vol%; absent in PLM-2 and −3), Fe-Ti oxides and apatite (Supplementary Data, Table S1).

# 4. Geochemistry of the PCH and Pm complexes

## 4.1. Analytical methods and geochemical data

The analytical methods are shown in Supplementary Data, Appendix A. All the PCH/Pm compositions are listed in Supplementary Data Table S1 (new data), whereas the rocks belonging to the other Armorican groups studied here, formerly classified as gabbroic/dioritic rocks, lavas, dolerites and lamprophyres, appear in Supplementary Data, Table S2 (older data). There is, however, an exception: one M leuco-minette sample has been newly analyzed (Supplementary Data Table S1).

## 4.2. LOI and major elements

The loss on ignition (LOI) values are relatively low (Pm: from 0.39 to 3.96 wt%, with an average of 1.12 wt% out of 40 samples; Pm: from 0.54
to 1.11 wt%, with an average of 0.70 wt% out of 6 samples). These values reflect the moderate/weak alteration features observable under the microscope (Supplementary Data, Table S1).

The ASI (aluminum saturation index) vs. SiO₂ diagram in Fig. 7a, where the new PCH/Pm data (symbols) are shown together with previously published PCH ones (fields), can be used to discriminate peraluminous (ASI ≥ 1) from metaluminous rocks (ASI < 1 and Na + K < Al, not shown). By including P₂O₅ (Fig. 7a), the expression of the ASI takes apatite into account (Frost et al., 2001). If ASI ≥ 1.1, rocks can be

considered as strongly peraluminous (Sylvester, 1998). SiO₂ increases roughly with the ASI. Four groups can be identified in the diagram in Fig. 7a: (i) the low-ASI Pm (monzo)diorites, (ii) the metaluminous Plouaret vaugnerites, (iii) the Bégard/Ploubezre monzogranites, positioned near the ASI = 1 line, and (iv) the other PCH granitoids, most of them plotting in the strongly peraluminous field.

The K₂O vs. SiO₂ diagram in Fig. 7b shows that the PCH and Pm plutonic rocks correspond to high-K/shoshonitic magmas, this latter designation concerning chiefly the Plouaret vaugnerites, all of them

<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Discrimination diagrams based on major elements for the PCH vaugnerites/granitoids and Pm (monzo)diorites (symbols: new data; fields: previously published data; references in Fig. 4b), together with other Armorican kersantitic/vaugneritic fields. U1: Unit 1; U2: Unit 2; PV-BPM: Plouaret vaugnerites and Bégard/Ploubezre monzogranites. (a) Plot of ASI (aluminum saturation index) vs. SiO₂, showing the limits between metaluminous, moderately peraluminous and strongly peraluminous rocks (Frost et al., 2001). The W-Armorican kersantites (Caroff et al., 2021), the Aber-Ildut leucominettes (Caroff et al., 2015) and the vaugnerites from Brignogan (Georget, 1986) and Plounévez-Lochrist (Chauris et al., 1998) are also shown as fields (number of samples in brackets). (b) K₂O vs. SiO₂ diagram of Peccerillo and Taylor (1976), showing also the lamprophyric field of Rock (1987) and the other Armorican lamprophyric/vaugneritic fields (Georget, 1986; Chauris et al., 1998; Caroff et al., 2015, 2021).</figcaption></figure>
being positioned in the lamprophyre field of Rock (1987).

### 4.3. Trace elements and isotopes

The analytical methods for geochemistry are shown in Supplementary Data, Appendix A. In the Rare Earth Element (REE) chondrite-normalized diagram of Fig. 8a, the PV-BPM (Plouaret vaugnerites and Bégard/Ploubezre monzogranites) display very close parallel patterns, with a sharp negative slope. The monzogranites have slightly lower REE

<figure><img src="image_11.png" /><figcaption>**Fig. 8.** CI-chondrite normalized trace element patterns of studied rocks (normalization values: Barrat et al., 2012). Number of samples in brackets for the fields. (a) PCH Plouaret vaugnerites and Bégard/Ploubezre monzogranites (PV-BPM), together with the West-Armorican kersantites (WAK) (Caroff et al., 2021), shown as a field. (b) Other granitoids from the Plouaret massif (PCH), together with the Plouaret vaugnerites (PCH) again and Brest Bay dolerites (BB) (Caroff et al., 2021), shown as fields. (c) Huelgoat/Plounéour/Commana granitoids, together with the Plouaret vaugnerites (PCH) again and Ploumanac'h (monzo)diorites (Pm), shown as fields. The four numerated sectors concern only the sample CP-13, affected by a tetrad effect (see text).</figcaption></figure>

contents and show a stronger negative anomaly in Eu than the vaugnerites. All the other PCH granitoids, whatever their type, are relatively homogeneous, except two REE-poor samples (CP-21 and CP-13, see below). They present characteristics comparable to those of the two aforementioned groups (parallel patterns, negative slope, negative anomaly in Eu), but they plot below (Fig. 8b, c). The Pm samples have a slightly smoother slope and present no anomaly in Eu (Fig. 8c). In the spider diagrams normalized to primitive mantle (Supplementary Data, Fig. S3a), all the PCH samples are characterized by negative anomalies in Ba, Nb, Ta and Ti, and a slight positive anomaly in Pb. The Pm samples show roughly comparable features, in addition to a negative anomaly in Rb (Supplementary Data, Fig. S3b).

Two samples have anomalous REE-poor spectra (Fig. 8b, c and Supplementary Data, Fig. S3b). The first sample (CP-21) displays a concave curve with a minimum for Ho, possibly resulting from massive amphibole fractionation (e.g., Caroff et al., 1999). The second one (CP-13) has a REE pattern with four sectors, numerated from 1 to 4 in Fig. 8c, and delimited by positions at Nd, Gd and between Ho and Er. Such features are typical of a tetrad effect, as defined by Peppard et al. (1969). The tetrad effect is generally interpreted as the result of REE complexation during the opening of a magmatic system at the final evolutive stages, involving interaction of the residual melts with F-/Cl-rich hydrothermal/external fluids (Irber, 1999; Monecke et al., 2002; Caroff et al., 2015).

The Th/Nb vs. major elements diagram of Fig. 9 can be used to clearly distinguish (i) the PCH granitoids ($SiO_2 > 63$ wt% and MgO < 3 wt%) from the Plouaret vaugnerites ($SiO_2 < 63$ wt% and MgO > 3 wt%), and (ii) both the PCH granitoids and vaugnerites ($Th/Nb > 0.7$) from the Pm (monzo)diorites ($0.5 < Th/Nb \le 0.8$).

Strontium and neodymium isotopic compositions are shown in Fig. 10 for the PCH/Pm rocks. All the corresponding data plot below the line corresponding to the Chondritic Uniform reservoir (CHUR) value (initial $\varepsilon_{Nd} = 0$). The Plouaret vaugnerites have low initial $^{87}$Sr/$^{86}$Sr (<0.7056) and slightly negative initial $\varepsilon_{Nd}$ values (from -1.55 to -0.80), while the PCH granitoids have $^{87}$Sr/$^{86}$Sr$_i > 0.706$ and $\varepsilon_{Ndi} \le -2$. Note that the only Bégard/Ploubezre monzogranitic sample analyzed for the isotopes plots close to the vaugneritic group ($^{87}$Sr/$^{86}$Sr$_i = 0.705897$ and $\varepsilon_{Ndi} = -1.70$). Finally, the Pm (monzo)diorites have $\varepsilon_{Ndi}$ values comparable to those of vaugnerites and slightly higher $^{87}$Sr/$^{86}$Sr$_i (>0.7056)$.

### 5. Geochemical evolution of the L-CAD/NAD mafic to K-rich-intermediate magmas through time

In addition to the PCH/Pm complexes, many other Paleozoic mafic to K-rich-intermediate magmatic bodies are geochemically investigated in the present study. All the corresponding geochemical data, previously published for most of them, are shown in Supplementary Data, Table S2 (except the geochemical composition of one M leucomette sample, newly measured: Supplementary data, Table S1).

In the ASI vs. $SiO_2$ diagram of Fig. 7a, the WAK, M, Bg/PL lamprophyric/vaugneritic fields form a positive trend line from low-Si/metaluminous to high-Si/peraluminous compositions, respectively. Note that the metaluminous Plouaret vaugnerites are part of the trend, near the intersection of the three aforementioned fields.

In the K$_2$O vs. $SiO_2$ diagram in Fig. 7b, the WAK, the M and the least siliceous samples among the Bg/PL group plot in the Rock's (1987) lamprophyre field, together with the Plouaret vaugnerites. These fields present high-K/shoshonitic characteristics, with the M one located at a high position above the basal shoshonitic boundary line. Note that the Plouaret vaugnerites split into the WAK (low-Si) and Bg/PL (high-Si) fields.

In both the REE chondrite-normalized diagram (Fig. 8a) and spider diagram normalized to primitive mantle (Supplementary Data, Fig. S3a), the WAK fields are overlaid by the patterns of the PV-BPM. The BB REE field encloses more flat patterns, with low values in LREE
<figure><img src="image_12.png" /><figcaption>**Fig. 9.** Th/Nb vs. major elements for all the studied rocks. Number of samples in brackets for the fields. The lower grey band corresponds to pre-Serpukhovian anorogenic mafic data. The data for the CIZ (Central Iberian Zone) fields correspond to the following sites: for tholeiitic/alkaline mafics, Farminhão (480 Ma) from Cotrim et al. (2021); Tenzuela (474 Ma), Almadén (435–410 Ma) and La Codosera (375 Ma) from Villaseca et al. (2022); for vaugnerites, Bayo-Vigo (328 Ma), Tormes (316 Ma) and Sierra Bermeja (305 Ma) from Villaseca et al. (2022). References for the other previously published data in Supplementary Data, Table S2. Location of the sites in Fig. 1. (a) Th/Nb vs. SiO₂. (b) Th/Nb vs. MgO.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Fig. 10.** Initial εNd vs. initial ⁸⁷Sr/⁸⁶Sr isotope diagram showing PCH (PV-BPM: Plouaret vaugnerites and Bégard/Ploubezre monzogranites) and Pm new data, together with SJ (Caroff et al., 2011), TVB (Caroff et al., 2016), and WAK (Caroff et al., 2021) older data, the latter as a field (number of samples in brackets). The data for the CIZ fields correspond to the following sites: for mafics, Farminhão (480 Ma, tholeiitic) from Cotrim et al. (2021); Tenzuela (474 Ma, tholeiitic) and Almadén (435 Ma, alkaline) from Villaseca et al. (2022); for vaugnerites, Bayo-Vigo (328 Ma), Tormes (316 Ma) and Sierra Bermeja (305 Ma) from Villaseca et al. (2022). Acronym meaning in Fig. 9. UR (Uniform reservoir) and CHUR (Chondritic Uniform reservoir) from Dijkstra and Hatch (2018).</figcaption></figure>

(Fig. 8b). The BB field in the extended REE spider diagram of Supplementary Data, Fig. S3a is also flat and characterized by negative anomalies in Rb, K, and Pb and the absence of Nb-Ta negative anomalies.

Th/Nb ratio is an excellent tracer which enables mafic to K-rich-intermediate magmas to be drastically split as a function of their chronology. Indeed, all the mafic bodies (lavas, dykes, plutons) emplaced in pre-Serpukhovian times have a Th/Nb ratio inferior to 0.22; all the intermediate plutonic rocks intruded in post-Serpukhovian times have a Th/Nb ratio superior to 0.51 (Fig. 9). Note that the Pm (monzo)diorites belong clearly to the second group, but with a ratio close to the lower

limit (0.537 < Th/Nb < 0.805: Supplementary Data, Table S1). This repartition is almost similar for the CIZ samples: Th/Nb < 0.22 for the tholeiitic/alkaline mafic rocks with ages ranging from 480 to 375 Ma, Th/Nb > 1 for the Serpukhovian/Bashkirian vaugnerites (328–316 Ma), and a Th/Nb range for the Kasimovian CIZ vaugnerites (305 Ma) like that of the Asselian Pm monzodiorites/diorites (c. 295 Ma).

This chronological pattern is also visible in the Sr-Nd isotopic diagram of Fig. 10. The analyzed pre-Serpukhovian L-CAD/NAD/CIZ rocks have a low initial ⁸⁷Sr/⁸⁶Sr (< 0.707) and initial εNd values > 2, unlike the other studied rocks, all of them being characterized by negative εNd values (excepts the Kasimovian CIZ vaugnerite: εNd ≈ 1).

## 6. Discussion

### 6.1. Structural evolution of Central/North Armorica in Paleozoic times

The structural evolution of the L-CAD/NAD at three successive stages in Paleozoic times is shown in Fig. 11. During the Eifelian/Givetian stage, early deformation was restricted to the proto-North Léon area to the NW where granitoids were emplaced at c. 391 ± 7 Ma (evap-TIMS U-Pb on zircon, Marcoux et al., 2009; Fig. 11a). Further south, the W-CAD sedimentary basinal area, bounded by the Élorn and Montagne Noire proto-faults (which would become later the Élorn and Montagne Noire Shear Zones, ESZ and MNSZ, respectively; see Fig. 1b), was not yet affected by tectonics, but was instead the locus of basalt eruption (Bo). During the Famennian/Visean interval, the intrusion of the NNW/SSE doleritic dyke swarm (NC) in the rigid Cadomian basement is kinematically compatible with crack opening in response to a nearly horizontal extension during a regional NNW-SSE compression (Fig. 11b). During the Tournaisian/Visean, the proto-North Léon area experienced an eclogitic event (Fig. 11b). The emplacement of both the Tournaisian SJ pluton and the Visean TVB dykes/lavas was coeval with ongoing extension in the W-CAD basin. The youngest pre-convergence event in the L-CAD/NAD is the effusion of the TVB-type basaltic lavas in the South-Ouessant basin at c. 336 Ma (Caroff et al., 2016). During the Bashkirian, prior to the wide-scale emplacement of syn-collisional granite intrusions (c. 315 Ma, Fig. 1c), the South-Léon/CAD/NAD area was deformed in a transpressional context partly accommodated by localized dextral shearing along the ESZ, MASZ and MNSZ (Darboux and Le Gall, 1988; Gumiaux et al., 2004), coeval to a heterogeneous magmatism including lamprophyres/vaugnerites (WAK and PCH) (Fig. 11c).

The emplacement sequence of the eight units forming the PCH
<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /><figcaption>**Fig. 11.** Structural evolution of the L-CAD and NAD at three successive Paleozoic stages. (a) Eifelian/Givetian (390–380 Ma). (b) Famennian/Visean (365–335 Ma). (c) Bashkirian/Moscovian (320–310 Ma). Explanations in the text.</figcaption></figure>

complex can be resolute from the field relations between the intrusions. They were successively emplaced, with an overall time-related progression to the south. The northern ones (Units 1 and 2 of the Plouaret Massif) present concentric structures, with nested intrusions (Fig. 2). The oldest unit is the northernmost one. After the intrusion of the Tonquédec, then Saint-Carré granitoids into the Bégard/Ploubezré one (Fig. 2a), Unit 2 was emplaced further south (Fig. 12): the Pouthou intrusion first, then the Guerlesquin, Loguivy-Plougras and Croaz-Illiès ones (Figs. 2 and 12). Unit 2 intruded the northern one while still being in the viscous state (Guillet et al., 1985). Several deep (> 7 km) feeding zones have been documented in the southern part of the Plouaret Massif from structural data and gravimetric records. Unit 1 is made of 2–3 km-wide laccoliths, whereas Unit 2 displays cone-shaped structures (Guillet et al., 1985). During their emplacement, the southernmost PCH units experienced ductile shearing along the E-W-trending NASZ (Lannéanou Unit) and along the SW-NE-trending MASZ (between the Commanal Plounéour and Huelgoat Massifs, Fig. 12). The shearing resulted in the shift of southern PCH units to the west over a distance of c. 15 km (Fig. 12).

## 6.2. Insights into the syn-collisional Plouaret vaugnerites and West-Armoricán kersantites

The Dy/Yb vs. K/(1000Yb) diagram of Fig. 13a can be used to constrain the mineralogical composition of the melting source, and especially to discriminate between the spinel and the garnet stability fields. Melts produced in the garnet stability field generally have Dy/Yb ratios > 2.5, whereas melting in the spinel one gives lower Dy/Yb ratios < 1.5 (Duggen et al., 2005; Ma et al., 2014). The Plouaret vaugnerites have Dy/Yb ratios ranging from 2.1 and 2.4 (Supplementary Data, Table S1; Fig. 13a), within the 1.9–2.9 range defined for the WAK (Caroff et al., 2021). This implies that partial melting for both magma types took place in the spinel-garnet transition zone, assumed to be located between 75 and 85 km depth (Robinson and Wood, 1998; Ma et al., 2014), likely with phlogopite ± amphibole as accessory hydroxyl minerals (Caroff et al., 2021).

Another point of convergence between the Plouaret vaugnerites and W-Armoricán kersantites is that they likely derived from a source bearing crust/sediment material. This is especially well documented through the log(Ce/Pb) vs. log(Ce) diagram in Fig. 13b. Both rock-types form a trend (more marked for the WAK) from the OIB field toward the average continental crust and pelagic sediment fields. This evolution is comparable to that of CIZ vaugnerites (Fig. 13b) and of other European Variscan lamprophyres (Caroff et al., 2021). The fact that the Ce/Pb ratio is not correlated with any differentiation parameter, such as SiO₂ contents or mg# index, precludes any significant assimilation coupled

<figure><img src="image_17.png" /><figcaption>**Fig. 12.** Schematic view (in map) of the different emplacement stage of the PCH complex, modified after Guillet et al. (1985). Explanations in the text.</figcaption></figure>
<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>

(caption on next page)
**Fig. 13.** Trace element diagrams for the studied rocks. (a) Dy/Yb vs. K/(1000Yb) diagram for PCH vaugnerites and W-Armorican kersantites (WAK) (Caroff et al., 2021). Melting curves from Duggen et al. (2005). Non-modal, point-average fractional partial melting curves are presented for garnet lherzolite (Ga), garnet-facies phlogopite lherzolite (brown dashed lines), garnet-facies amphibole lherzolite (red dashed lines), spinel-garnet transition zone lherzolite (Ga + Sp), and spinel lherzolite (Sp). Percentages indicate degrees of partial melting. Partial melting was modelled by Duggen et al. (2005) with various K contents in the amphibole- and phlogopite-bearing lherzolites (indicated in ppm in the brown and red curves). For distribution coefficients, source composition, lherzolite modal fractions and normative weight fractions of minerals in the partial melts, see Duggen et al. (2005). (b) Ce/Pb vs. Ce plotted on a log scale for PCH and Pm (this study), Tournaisian/Visean basin basalts (TVB) (Caroff et al., 2016), Saint-Jean-du-Doigt gabbro (SJ) (Caroff et al., 2011), West-Armorican kersantites (WAK) (Caroff et al., 2021), Brest Bay dolerites (BB) (Caroff et al., 2021) and CIZ mafics and vaugnerites (grey dashed fields, n: number of samples; Cotrim et al., 2021; Villaseca et al., 2022). Mantle Ce/Pb ratio of 25 ± 5 (in gray lines), MORB and OIB data from Hofmann et al. (1986); references for continental lithospheric mantle, lower and average continental crust, and pelagic sediments: Sims and De Paolo (1997) and Owen (2008). (c) Ternary classification diagram: 2A/CNK (molar Al₂O₃/[CaO + Na₂O + K₂O] ratio); Na₂O/K₂Owt.% ratio; 2(FeOt + MgO)wt.% * (Sr + Ba)wt.% (FMSB), initially proposed for late-Archean granitoids by Laurent et al. (2014), and then used to discriminate between granitoids and vaugnerites from the French Massif Central by Laurent et al. (2017). Each pole is likely to represent an end-member source: (1) high-K crustal felsic source (2A/CNK); (2) low-K mantle mafic source (Na₂O/K₂O); and (3) mantle metasomatized with fluids rich in incompatible elements, especially LILE (FMSB). The hybrid samples plot in the convergence zone of the other groups. Full colored fields for the previously published PCH data (references in Fig. 4b); empty dashed fields for the pre-Serpukhovian mafic rocks (references in Supplementary Data, Table S2), the W-Armorican kersantites (WAK: Caroff et al., 2021) and the Aber-Ildut leucominettes (M: Caroff et al., 2015). Number of samples in brackets.

with fractional crystallization (AFC) process (De Paolo, 1981; Caroff et al., 2021). In this diagram, other samples plot below the mantle array: the pre-tectonic SJ gabbros (AFC: Caroff et al., 2011), the syn-collisional PCH granitoids (crustal melting), the late-collisional M leucominettes (hybridization between crustal-derived intermediate melts and mantle-derived lamproitic magmas: Caroff et al., 2015), and, lastly, the post-collisional Pm (monzo)diorites (see below).

The ternary diagram 2A/CNK-Na₂O/K₂O-FMSB in Fig. 13c (Laurent et al., 2014) can be used to discriminate the pre-Serpukhovian anorogenic Armorican mafic rocks (low-K, mafic source) from both the syn-collisional PCH granites (high-K felsic source) and the syn-/late-/post-collisional WAK/PCH/M/Pm high-K lamprophyric/intermediate rocks (LILE-rich mantle source). Note that the Bégard/Ploubezre monzogranites plot in the hybrid field, between the high-K and the LILE-rich ones, which likely denotes the occurrence of a hybridization process between PCH vaugneritic and granitic melts.

## 6.3. From anorogenic mafic to orogenic K-rich magmas in Armorica

One of the most notable features of the pre-Variscan (Paleozoic) magmatism in most part of Armorica is the permanence of an anorogenic mafic signal (low-slope REE spectra, Fig. 8b; Th/Nb < 0.22, Fig. 9; initial εNd > 2, Fig. 10; see also Fig. 13c) for a long time-period spanning from at least the Ordovician to the Early Carboniferous (Fig. 1b, 9). The term “anorogenic” is here based on geochemical criteria. It implies that the mantle source of the corresponding magmas was mainly asthenospheric (e.g., Hofmann et al., 1986), without prejudice to a synchronous specific tectonic context, especially since it can take millions of years for a geotectonic context to reflect in terms of geochemistry (Coulon et al., 2002). In any case, anorogenic magmas could not be derived from a metasomatized lithospheric source, nor intrude a partially molten (migmatized) warm crust without losing their specific geochemical features (e.g., Krmíček et al., 2020; Caroff et al., 2021). At a nascent stage of the Variscan orogenic events in the area under study, prior to the granitic peak, magmatism became predominantly lamprophyric/vaugneritic. Such type of K-rich magmas later reappeared during the late-/post-collisional times, in the L-CAD/NAD, such as in the CIZ (Fig. 1b, 9). There is a pronounced geochemical break between the group of pre-Serpukhovian mafic rocks and that of the Pennsylvanian/Cisuralian (syn-/late-/post-collisional) lamprophyric/K-rich intermediate products (Figs. 9, 10 and 13c). This contrast is especially apparent in the Th-Hf-Nb/2 discrimination diagram of Krmíček et al. (2011), since the corresponding L-CAD/NAD and CIZ data split up into both anorogenic and orogenic fields, respectively (Fig. 14a). A comparison with data from other sites worldwide shows that the lamprophyric/K-rich intermediate rocks magmas were preferentially emplaced in post-collisional times (e.g., Awdankiewicz, 2007; Couzinié et al., 2014; Dijkstra and Hatch, 2018; Soder and Romer, 2018; Pouclet et al., 2022; and references in Fig. 14b), with the notable exception of the Bohemian and Armorican Massifs that

enclose both syn- and post-collisional K-rich rocks (Fig. 14b). However, the post-collisional Bohemian lamprophyres/lamproites have an anorogenic signature contrary to the post-collisional Armorican/CIZ diorites/vaugnerites, (Fig. 14b and c).

Whatever the period during which they were emplaced, all the pre-Serpukhovian Armorican/CIZ mafic rocks display a similar OIB- or MORB-type geochemical signature (Figs. 9, 10 and 13b; note that the shift of the SJ out of the OIB field in Fig. 13b results from an AFC process: Caroff et al., 2011). Given both the very long duration of the phenomenon (at least 130 Ma, Fig. 14c) and its erratic spatial distribution (Fig. 1b), it would be not realistic to relate this magmatic sequence of anorogenic events to a plume-derived hot spot pattern. In any case, the classical hot spot theory, as reviewed by Frisch et al. (2022), has become increasingly contested in the last decades, especially in continental contexts. As an example, Foulger (2010) questioned the plume theory and confronted it with the “Plate hypothesis”, its exact opposite, which instead attributes melting anomalies to shallow effects directly related to plate tectonics. A very attracting model, intermediate between the “plume” and the “plate” theories was proposed by Zuo et al. (2017). In their view, slab rollbacks could trigger large-scale thermal perturbations in the asthenosphere for several tens of millions of years, resulting in mantle upwelling and extension-related magmatism away from the subduction zones. Such a model likely accounts for the long-lasting emplacement of the pre-Serpukhovian Armorican mafic rocks over a large area, as will be depicted in the following section. At the beginning of the Avalonia/Armorica collision, comparable rollback processes, eventually coupled with delamination (and then thinning) of the overriding lithosphere (Ma et al., 2014; Li et al., 2016), supplemented with (i) horizontal extension in the nearby back-arc area (Zuo et al., 2017) and (ii) lithospheric metasomatism due to fluid release from fragments of buried continental crust (Caroff et al., 2021), can lead to the production of H₂O- and LILE-rich high-K lamprophyric/vaugneritic melts at a depth not exceeding 85 km, as suggested by the geochemical indicator of spinel + garnet in the melting source (Fig. 13a; Robinson and Wood, 1998).

Our interpretation of the Armorican/Iberian post-collisional vaugnerites (PL, Bg, CIZ-305 Ma) and high-K, high-Mg intermediate rocks (Pm) is in line with the model proposed by Gómez-Frutos et al. (2023) and Gómez-Frutos and Castro (2023). These authors have shown that post-collisional silicic and intermediate magmas are not always produced by recycling old crustal igneous rocks but can derive from metasomatized mantle sources. They also suggested that the relationship between high-Mg mafic/intermediate products and their host felsic magmas could be, in some cases, primarily a differentiation process. Among the post-collisional Armorican/Iberian intermediate rocks studied here, the 295 Ma-old NAD Pm (monzo)diorites share with the 305 Ma-old CIZ vaugnerites a weak orogenic geochemical signal with respect to the other K-rich intermediate intrusives (less steep REE slopes, Fig. 8c; Th/Nb ≤ 0.8, Fig. 9; -1.13 < initial εNd < 1.23, Fig. 10; see also
<figure><img src="image_21.png" /><figcaption>**Fig. 14.** Comparison between Armorican mafic to K-rich-intermediate rocks and other magmatic sites worldwide. (a, b) Th-Hf-Nb/2 discrimination diagram (Krmíček et al., 2011) for (a) Armorican/Iberian mafic, lamprophyric and K-rich intermediate magmatic rocks (n: number of samples), compared with (b) Bohemian/ worldwide K-rich/lamprophyric magmatic rocks. The data corresponding to the Bohemian lamprophyric/lamproitic fields are from Krmíček et al. (2020); the Tertiary W-Alps lamprophyric field, from Owen (2008); the Tertiary Mediterranean lamproitic field, from Prelević et al. (2008); the Miocene/Pliocene W-Mediterranean lamproitic field, from Duggen et al. (2005); and the Miocene S-Tibet potassic volcanic rocks, from Tian et al. (2020). (c) Initial εNd vs. age (Ma) for Armorican/ Iberian/Bohemian mafic to K-rich-intermediate magmatic rocks (number of samples in brackets). U–Pb geochronology for the CIZ plutonic rocks from Bea et al. (2006), Zeck et al. (2007), Villaseca et al. (2011) and Orejana et al. (2012). See text for explanations.</figcaption></figure>
Fig. 14a). These features probably reflect a gradual return to an unme-
tasomatized mantle source after the orogenic peak. The culmination of
this process can be observed in the Bohemian Massif where the post-
collisional (younger) lamprophyres show an anorogenic signature
(Fig. 14 b, c).

6.4. Plates through time

A discussion at a wider plate scale implies reviewing the still
debatable definition of Armorica and its associated suture zones. A
consensus exists about the nature and location of the southern boundary
of Armorica, along the Nort-sur-Erdre faults (Bitri et al., 2003; Faure
et al., 2008; Ballèvre et al., 2009). By contrast, its northern limit is much

<figure><img src="image_22.png" /></figure>

Fig. 15. Cross-sectional geodynamic model of the NW European Variscan belt through time. (a) A 480 km-long crustal-scale cross-section obtained by combining the SWAT seismic dataset (BIRPS and ECORS, 1986) with onshore geology in the L-CAD to the south and in SW England to the north. Mantle-involving geodynamic reconstructions in the (b) Upper Ordovician, (c) Eifelian/Givetian, (d) Tournaisian/Visean, (e) Serpukhovian/Bashkirian, (f) Gzhelian/Asselian, based on/adapted from Matte (2001), Ballèvre et al. (2009, 2012)), Duretz et al. (2011), Guillot and Replumaz (2013), Guillot et al. (2020), Vanderhaeghe et al. (2020), and Caroff et al. (2021). The thermal profiles of the asthenosphere are based on Zuo et al. (2017). The arrows indicate the slab sinking and the extension resulting from rollback traction. MEO: Medio-European Ocean.
more controversial. According to a first type of models, this boundary
occurs onshore at the western extremity of the Armorican massif as a
major thrust/suture zone (Elorn/Le Conquet) between the W-CAD
Paleozoic series and the Léon metamorphic terrains (Fig. 1b) (Faure
et al., 2005; Ballèvre et al., 2009). These interpretations have been
contradicted above (Section 2.1), hence leading to identify the northern
boundary of Armorica in a more northern position. Alternative models
interpreted the Lizard ophiolitic complex in Cornwall as the trace of a
major suture zone between Armorica and Laurussia (BIRPS and ECORS,
1986; Le Gall, 1990; Bois et al., 1990). Other authors argued from
geochemical and metamorphic attributes that the Lizard complex was
rather a narrow and short-lived structure (Barnes and Andrews, 1984;
Cook et al., 2002), which did not correspond to a wide and long-lived
ocean (Nance and Linnemann, 2008; Kroner and Romer, 2013). Ac-
cording to this view, the Rheic suture zone might occur further south in
the buried basement of the English Channel (work in progress). This
issue being beyond the scope of the present work, it will not be devel-
oped further here.

The SWAT seismic reflection dataset recorded through the English Channel (BIRPS and ECORS, 1986) combined with onshore geology in the LMD-CAD to the south and in SW England to the north can be used to depict the crustal structure of the Variscan belt in NW Europe on the 480 km-long crustal-scale cross-section shown in Fig. 15a. This structural transect shows two significant geodynamical markers, the Lizard ophiolite and the Léon eclogitic material, both associated to northerly-directed thrust structures (Le Corre et al., 1989; Le Gall, 1990; Floyd et al., 1983). The so-called Léon thrust is considered as a minor structure, together with the associated eclogitic rocks, believed to have originated from the intrusion of mafic magmas into the crust (Vanderhaeghe et al., 2020) via a potential deep tectonic structure, as proposed by Kusbach et al. (2012) for the Bohemian Massif. If one assumes that the Lizard thrust corresponds to a suture zone, the resulting structural zoning leads us to distinguish three tectonically juxtaposed blocks which are from south to north: (i) the South Léon-CAD block in the hangingwall block of the Léon thrust, (ii) the North Léon-English Channel block forming the hangingwall of the Lizard thrust and (iii) the SW England block extending as far north as the Variscan front (Fig. 15a). The South Léon-CAD and North Léon-English Channel sectors are regarded as parts of a single crustal segment, i.e. the Armorica microplate (Fig. 15a). If the Rheic suture zone is further south, in the buried basement of the English Channel, the pattern is broadly the same except that the North Léon-English Channel block has to be slightly lessened to the north.

The spatio-temporal pattern of Variscan strain along the crustal-scale
transect in Fig. 15a can be attempted form available geochronological
dataset. The oldest tectonic events are recorded by the thrust emplace-
ment of the c. 397 Ma-old Lizard ophiolitic complex onto the southern
edge of the Laurussia/Avalonia continental margin (Sandeman et al.,
2000). At that stage, granitoids were emplaced in the proto-North Léon
area (c. 391 Ma, Marcoux et al., 2009; Fig. 11a), possibly in response to
early regional thrust-related crustal thickening.

The following tectono-metamorphic events were more extensively
distributed. To the south, deformation migrated southwards in the South
Léon-CAD sector, i.e. in a piggyback fashion, and expressed by syn-
cleavage folding, coeval to dextral ductile shearing along regional-scale
discontinuities (ESZ, MASZ, MNSZ in Fig. 1b and 11c), in a transpres-
sional context (Le Gall and Caroff, 2023). To the north, the onset of
strain propagated northwards, i.e. in a forward fashion, and resulted in a
thin-skinned fold-thrust pattern involving Devonian-Carboniferous se-
ries detached above a major decollement level as far north as the
Variscan front (Coward and Smallwood, 1984).

The plate reconstructions shown in Fig. 15 b-f illustrate the evolution
of Armorica and its margins during the Paleozoic Era, after the Cambro-
Ordovician voluminous silicic volcanic event during the lithospheric
extension stage preceding the breakup of northern margin of the
Gondwana continent (Rodríguez et al., 2022). In the Upper Ordovician,

the retreat of the southward plunging Iapetus slab (Vanderhaeghe et al., 2020) is supposed to disturb the asthenospheric isotherms (Zuo et al., 2017) (Fig. 15b). The consequential large-scale horizontal extension coupled with local mantle upwellings could be at the origin of (i) rift collapses and/or lithospheric breakages in the hot regions, and (ii) both the alkaline magmatism in the extended Gondwana margin and the Cz/ Br intrusives/volcanics in Armorica (Figs. 1, 15b). From the Devonian to the Lower Carboniferous periods, the subduction of both Medio-European and Rheic ocean slabs below the south and north margins of Armorica, respectively, is likely to have comparable consequences and to be at the origin of the Bo, SJ and TVB anorogenic magmatism (Fig. 15c, d). The colliding events in the CAD began during the Bash-kirian (Fig. 11c). In depth, the two main corresponding processes are assumed to be the rollback of the subducted northern lithospheric slab and the delamination of the overriding Armorica lithospheric mantle (Caroff et al., 2021) (Fig. 15e). Such a delamination mechanism could be the result of negative buoyancy of the heated and ductilized mantle lithosphere (Li et al., 2016). Adiabatic rise of the underlying astheno-sphere and fluid release from fragments of buried continental crust led to lithospheric metasomatism and production of lamprophyric/vaug-neritic magmas (WAK, PCH) (Fig. 15e). From the Moscovian to the Asselian, concomitantly with the main crust-derived granitic stages, the asthenospheric mantle, possibly contaminated by a myriad of stirred small continental fragments, was the likely source of late/post-collisional various K-rich intermediate products (M, Bg, PL, Pm) and some associated felsic magmas (Fig. 15f).

7. Concluding remarks

(1) One of the most notable features of the pre-/early Variscan Armorican magmatism is the permanence of an anorogenic mafic signal for a long period of time from at least the Ordovician to the Visean. Whatever the period during which they were emplaced, all these pre-Serpukhovian mafic rocks display a similar OIB-type geochemical signature. They are interpreted via a model involving large-scale thermal perturbations in the asthenosphere for several tens of millions of years, generated by slab rollbacks, inducing mantle upwelling and extension-related magmatism away from the subduction zones.

(2) At a nascent stage of the Variscan orogenic events in this area, before the granitic peak, magmatism became predominantly lamprophyric/vaugneritic. The PCH vaugnerites and the sub-contemporaneous West-Armorican kersantites (WAK) have a comparable mantle source, i.e. the base of the overriding delaminated lithosphere (spinel-garnet transition zone, c. 80 km in depth), metasomatized through LILE-rich fluid released from fragments of buried continental crust.

(3) The post-collisional stage is marked by the reappearance of vaugnerites and other K-rich intermediate products, in association with granites. Some of them display less marked orogenic geochemical features, possibly reflecting a gradual return to an unmetasomatized mantle source after the orogenic peak.

(4) Finally, all these magmatic events are replaced in a sequence of plate reconstructions, illustrating the evolution of Armorica and its margins during the Paleozoic Era from the Upper Ordovician to the Cisuralian. During that latter epoch, the asthenospheric mantle was likely contaminated by a myriad of stirred small continental fragments.

CRediT authorship contribution statement

**Martial Caroff:** Writing – review & editing, Writing – original draft, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Conceptualization. **Bernard Le Gall:** Writing – review & editing, Methodology, Investigation. **Nicolas Estèves:** Resources, Investigation. **Jean-Alix Barrat:** Resources,
Methodology, Investigation. **Jessica Langlade**: Resources. **Céline Liorzou**: Resources. **Philippe Nonnotte**: Resources. **Marie-Laure Rouget**: Resources.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This project was funded by the Armorique Regional Nature Park (PNRA) / Armorique UNESCO global Geopark, Le Faou, and by the Geo-Ocean Laboratory, IUEM, Brest (France). Field studies were conducted in 2020 and 2021. We thank A. Caro (President of the PNRA), J. Bourdoulous (Director of natural heritage, PNRA), P. Tieffenbach (thin sections), and J.-P. Oldra (sample preparation). Very detailed and constructive comments by Dr. A. Castro and two anonymous reviewers helped us a lot to improve the first version of the text. The authors thank Dr. A. Festa for his editorial assistance.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.gr.2025.02.024.

Data availability

Data will be made available on request.

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