Structural and microstructural analyses are carried out in two sites of the Argentera-Mercantour Massif, Valscura and Val du Haut Boréon, where swarms of lamprophyres intruded into Variscan migmatites and early Permian granitoids. Efforts aim at defining the structural relationships between lamprophyres and country rocks, and at constraining the structural and metamorphic evolution the dykes record. Mesoscale structural data are synthesised in geologic maps originally surveyed at 1/10 000 scale, supported by form-surface maps at 1/100 scale. The lamprophyres are magnesian, calc-alkalic to alkali-calcic, and metaluminous; they emplaced at very shallow crustal levels intersecting three generations of ductile structures in the host migmatites (D1, D2, D3). Epidote- and actinolite-bearing mineral assemblages result from late-intrusive hydrothermal circulation that has not affected the host rocks. Mylonitic shear zones of Alpine age (D4) are continuous through migmatites, granites, and lamprophyres: in these latter, they are supported by albite, actinolite, biotite, chlorite, epidote, phengite, and titanite. This detailed multi-scale structural analysis, coupled with major and trace elements geochemistry, highlights two main results: i) the lamprophyres, which post-date both the late- to post-collisional “high-Mg” and the “low-Mg” granitoids, reflect the last magmatic event in the Argentera-Mercantour Massif related to the Permian-Triassic lithospheric thinning; ii) the metamorphic assemblages that support the Alpine shear zones in the lamprophyres are consistent with the transition between the greenschist and amphibolite facies conditions.

Des analyse structurales et microstrucrurales ont menées dans deux secteurs clés du massif de l’Argentera-Mercantour, la zone de Valscura et la zone du Val du Haut Boréon, où des réseaux de filons de lamprophyres intrudent des migmatites Varisques et des granites d’âge Permien inférieur. Nous présentons et discutons les résultats d’une analyse détaillée, d’une part, des relations entre filons de lamprophyres et roches encaissantes et, d’autre part, des évolutions structurales et métamorphiques de ces filons. Les données structurales observables à la mésoéchelle sont synthétisées dans des cartes géologiques obtenues à l’échelle du 1/10 000, appuyées par des analyses géométriques détaillées à l’échelle du 1/100. Les lamprophyres sont des roches magnésiennes, calco-alcalines à alcalino-calciques et moyennement alumineuses, qui sont affectées par un évolution hydrothermale tardi-magmatique marquée par le remplacement statique des phases magmatiques par des associations à épidote et actinote. Dans les deux zones considérées, les lamprophyres se sont mis en place à un niveau structural très superficiel et recoupent trois générations de structures superposées identifiées dans le complexe migmatitique (D1, D2, D3). Les zones mylonitiques Alpines (D4) qui sont localisées recoupent et décalent les filons de lamprophyres. Ces structures sont soulignées par une association minéralogique à albite, amphibole, biotite, chlorite, epidote, phengite et sphène. Cette analyse structurale détaillée et multi-échelles, associée à l’étude de la géochimie des éléments majeurs et des éléments en traces, met en évidence deux résultats principaux : i) les lamprophyres représentent le dernier événement magmatique enregistré dans le Massif de l’Argentera-Mercantour. Il est postérieur aux granitoïdes tardi a post-collision « à forte » comme « à faible » teneur en Mg, et est lié à l’amincissement lithosphérique d’âge Permo-Triasique ; ii) les assemblages métamorphiques qui soulignent les zones de cisaillement Alpines dans les lamprophyres sont compatibles avec la transition entre les faciès des Schistes Verts et des Amphibolites.

Introduction

Mafic dykes are structural and metamorphic markers of the first order for distinguishing between superposed tectonic and/or metamorphic cycles in orogenic systems, as highlighted in pioneering to recent contributions (e.g.Myers, 1978; Mørk, 1985; Williams, 1985; Spalla et al., 2000; Baletti et al., 2012; Gosso et al., 2015; Zanoni et al., 2016). In the Argentera-Mercantour Massif, a portion of deep Variscan crust reactivated during the Alpine collision, swarms of pre-Alpine lamprophyres are known for over sixty years (Faure-Muret, 1955; Bortolami and Sacchi, 1968; Malaroda et al., 1970; Compagnoni et al., 2010). Since these lamprophyres intruded into Variscan migmatites and early Permian granitoids and were subsequently deformed and metamorphosed, they are appropriate markers to distinguish between Variscan and Alpine syn-metamorphic structures in the basement rocks of the Argentera-Mercantour Massif.

The purpose of this paper is to present and discuss the multi-scale structural analysis we have carried out in two localities, Valscura and Val du Haut Boréon, where the lamprophyres are widespread. Major and trace element compositions are investigated to frame the Argentera-Mercantour lamprophyres in the record of the Permian and Triassic magmatism of the Southern Variscan belt.

Geological outline

The late- to post-collisional magmatism in the Southern Variscan belt

Two main magmatic cycles of late Carboniferous to Triassic ages are described in the Southern Variscan belt. The first cycle comprises calc-alkaline tonalites, granodiorites, and monzogranites, as well as metaluminous to peraluminous rhyolites and dacites (Cortesogno et al., 1998; Atzori et al., 2000; Paquette et al., 2003; Traversa et al., 2003; Gaggero et al., 2007; Romano et al., 2011; Rossi et al., 2015). The second cycle is of alkaline affinity and includes peralkaline to slightly peraluminous A-type granites and rhyolites, as well as alkaline diorites, andesites, basalts, and minor lamprophyres (Cortesogno et al., 1998; Traversa et al., 2003; Cocherie et al., 2005; Gaggero et al., 2007). Mafic and ultramafic layered complexes and transitional to tholeiitic basalts and andesites are generally ascribed to the alkaline cycle, although their ages are partially in overlapping with those of the calc-alkaline suites (Paquette et al., 2003; Cocherie et al., 2005).

Field relationships highlight that, in most of the cases, alkaline magmas intruded into the calc-alkaline magmatic suites and bimodal alkaline lavas are the youngest in the volcano-sedimentary sequences (Leroy and Cabanis, 1993; Cortesogno et al., 1998; Traversa et al., 2003; Dallagiovanna et al., 2009; Rossi et al., 2015). Moreover, the alkaline magmatism locally lasted up to Triassic times (Vatin-Pérignon et al., 1972; Vatin-Pérignon et al., 1974; Buffet and Aumaitre, 1979; Gaggero et al., 2007; Barca et al., 2010; Cirrincione et al., 2014; Cirrincione et al., 2016). A mantle contribution in the genesis of these magmatic rocks is undisputed (e.g.Rottura et al., 1998; Traversa et al., 2003; Gaggero et al., 2007; Renna et al., 2007; Duchesne et al., 2013; Cirrincione et al., 2016).

Composite calc-alkaline intrusives emplaced in Corsica at 308–304 Ma (U-Pb zircon dating, Paquette et al., 2003; Rossi et al., 2015) and were followed by calc-alkaline monzogranites, calc-alkaline volcano-sedimentary sequences, and transitional to tholeiitic mafic intrusives, dated between 293 and 279 Ma (U-Pb zircon dating, Rossi et al., 2002; Paquette et al., 2003: Cocherie et al., 2005; Renna et al., 2007; Rossi et al., 2015). Similarly, the A-type granites of Corsica are dated between 290 and 278 Ma (U-Pb zircon dating, Cocherie et al., 2005; Rossi et al., 2015).

In Sardinia, the calc-alkaline cycle started between 321 and 313 Ma, with apex at 311–307 Ma (U-Pb zircon dating; Casini et al., 2012). Here, calc-alkaline granodiorites and diorites emplaced between 295 to 286 Ma, whereas transitional to alkaline basalts came after at 254 to 248 Ma (hornblende 40Ar/39Ar dating, Gaggero et al., 2007).

Alkali-calcic and calc-alkalic granites emplaced in the Maures-Tanneron Massif at around 300 Ma and were intruded by slightly younger calc-alkaline dolerite dykes (biotite and muscovite 40Ar/39Ar dating, Morillon et al., 2000; U-Pb zircon dating, Duchesne et al., 2013). A-type rhyolites and younger transitional tholeiitic lavas and dykes in the volcano-sedimentary sequence of the adjacent Estrél basin are dated between 278 and 264 Ma (40Ar/39Ar feldspar dating, Zheng et al., 1992; Poitrasson and Pin, 1998; Lapierre et al., 1999); whereas, in the Toulon basin, Middle Permian transitional tholeiitic to alkaline basalts post-date Lower to Middle Permian calc-alkaline rhyolites (Leroy and Cabanis, 1993). Middle to Upper Triassic alkaline to transitional basalts and late Triassic to early Jurassic tholeiites are also reported in Calabria and Sicily (Barca et al., 2010; Cirrincione et al., 2014; Cirrincione et al., 2016).

In the Alpine area, very few lamprophyres intruding ferriferous alkali-calcic granites of late Carboniferous ages are described in the Mont-Blanc and Aiguilles Rouges Massifs (von Raumer, 1987; Bussy et al., 2000). In the Pelvoux Massif, minor swarms of alkaline mafic dykes intersect Carboniferous granites and calc-alkaline diorite dykes, whereas alkaline volcaniclastics are interlayered in the Triassic sedimentary sequences (Vatin-Pérignon et al., 1972; Vatin-Pérignon et al., 1974; Buffet and Aumaitre, 1979). In the Ligurian basement, calc-alkaline rhyolites, andesites, and rhyodacites of 286 to 272 Ma are post-dated by alkaline rhyolites dated at 258.5 ± 2.8 Ma (Buzzi and Gaggero, 2008; U-Pb zircon dating, Dallagiovanna et al., 2009). Permian and Triassic calc-alkaline to alkaline intrusives and volcanics are also abundant in the Southern Alps (Giobbi Origoni et al., 1988; Sloman, 1989; Rottura et al., 1998; Garuti et al., 2001; Cassinis et al., 2007; Schaltegger and Brack, 2007; Locmelis et al., 2016; Casetta et al., 2018; Zanoni and Spalla, 2018; Storck et al., 2019).

The Argentera-Mercantour Massif

The crystalline basement of the Argentera-Mercantour Massif (Fig. 1) consists of migmatitic orthogneisses, paragneisses, and amphibolites that include pre-anatectic relics of eclogites, high-pressure mafic granulites, ultramafites and calc-silicates (Faure-Muret, 1955; Malaroda et al., 1970; Blasi, 1971; Bortolami et al., 1974; Bogdanoff and Ploquin, 1980; Bogdanoff, 1986; Latouche and Bogdanoff, 1987; Paquette et al., 1989; Rubatto et al., 2001; Ferrando et al., 2008; Compagnoni et al., 2010; Rubatto et al., 2010). The widespread migmatisation, dated at 323 ± 12 Ma, came after the emplacement of monzonite dykes at 332 ± 3 Ma (U-Pb zircon dating, Rubatto et al., 2001). These monzonites are part of “high-Mg number” suites of the External Crystalline Massifs (Debon and Lemmet, 1999), which are interpreted as products of a late-collisional event that involved melting of an enriched mantle source triggered by slab window or slab break-off (Banzet, 1987; von Raumer et al., 2014).

The late- to post-Variscan exhumation of the migmatitic complex is testified by the structural and metamorphic evolution of the Valletta Shear Zone (VSZ, Fig. 1), a mayor dextral transpressive mylonite that has been active between 330 and 314 Ma during retrograde metamorphism (Faure-Muret, 1955; Musumeci and Colombo, 2002; Corsini et al., 2004; Carosi et al., 2016; Simonetti et al., 2018). The surface exposition of the Argentera-Mercantour Massif is constrained by late Carboniferous to Permian siliciclastic sequences that unconformably rest on the migmatites (Faure-Muret, 1955; Malaroda et al., 1970; Bortolami et al., 1974). At the same time, sets of brittle structures accommodated the emplacement of the Central Granite (Boucarut, 1967; Compagnoni et al., 2010), whose age (292 ± 10 Ma, Rb-Sr muscovite dating, Ferrara and Malaroda, 1969; 299–296 Ma, 40Ar/39Ar muscovite dating, Corsini et al., 2004) and composition match those of late- to post-Variscan “low-Mg number suites” of the External Massifs (Debon and Lemmet, 1999). However, rhyolites, dacites, and basalts in the sedimentary covers testify that magmatic activity in the Argentera-Mercantour Massif lasted, at least, through the Permian (Faure-Muret, 1955; Malaroda et al., 1970; Romain and Vernet, 1978). In the stratigraphic record, syn-extensional siliciclastic sequences of Permian age (Aicard et al., 1968; Delteil et al., 2003) were firstly followed by Middle to Upper Triassic evaporites and shallow-water limestones, and lately by the Lower Jurassic deepening successions of the European passive margin (Faure-Muret, 1955; Malaroda et al., 1970; Lemoine et al., 1986; Dardeau, 1988).

In the Alpine framework, the Argentera-Mercantour Massif is a segment of thinned European crust shortened during the continental collision (Bogdanoff et al., 2000; Bigot-Cormier et al., 2006; Schwartz et al., 2007; Schreiber et al., 2010; Lardeaux, 2014). The massif attained its maximum burial in the early Oligocene, by overthrusting of internally derived nappes (Kerckhove, 1969; Merle and Brun, 1984; Evans and Elliot, 1999; Ford et al., 1999; Simon‐Labric et al., 2009; Sanchez et al., 2010). The VSZ was severely re-activated as a dextral strike-slip mylonite and several inverse shear zones, mainly E-W striking, developed in response to Alpine transpression (Corsini et al., 2004; Sanchez et al., 2011a; Simonetti et al., 2018).

An Alpine metamorphic event affecting Carboniferous covers and early Permian granitoids is described since Boucarut (1967). However, in the lack of structural and/or chronological markers, the Alpine metamorphic overprint in migmatites is hardly distinguishable from that coming along with the late-Variscan exhumation, as Bortolami et al. (1974) point out. 40Ar/39Ar ages of phengite crystals supporting greenschist facies mylonites constrain the Alpine metamorphism between 34 and 20 Ma (Corsini et al., 2004; Sanchez et al., 2011a). In particular, the oldest Alpine shear zones (34 Ma) developed at T = 375 ± 25 °C and P = 0.7–0.9 GPa, whereas the youngest (22–20 Ma) at 375 ± 25 °C and 0.4–0.6 GPa (Sanchez et al., 2011a). Thermobarometric estimates on Permian-Triassic meta-sedimentary rocks at the southern edge of the Argentera-Mercantour Massif confirm T > 300 °C and P > 0.3 GPa for the Alpine metamorphic event (Attal, 2000). The same shear zones, still keeping the same kinematics under brittle-ductile conditions, are accountable for the final exhumation of the basement rocks: exhumation below 150 °C occurred at 8 Ma (Apatite FT dating) and this stage was followed by slower exhumation rates driven by the transcurrent tectonic context (Tricart et al., 2004; Bigot-Cormier et al., 2006; Baietto et al., 2009; Sanchez et al., 2010; Sanchez et al., 2011b; Leclère et al., 2014; Bauve et al., 2014).

Principal rocks associations: high-resolution mapping

Valscura site

The basement rocks of Valscura comprise migmatitic paragneisses, meta-granitoids and cordierite-bearing anatexites, with minor migmatitic amphibolites (Fig. 2). Some outcrops, in which lamprophyres are widely surfacing, are mapped in detail for highlighting structural and petrogenetic relationships with the country rocks (Fig. 3).

Millimetre-thick films of biotite, fine-grained trails of garnet and sillimanite, and quartz-feldspar leucosomes in centimetre-thick bands outline the migmatitic foliation in paragneisses (Fig. 4a). The migmatitic foliation in meta-granitoids is, instead, supported by anastomosing films of biotite, and quartz-feldspar leucosomes occupy microlithons and strain shadow domains around porphyroclasts of igneous K-feldspar. Veins of cordierite-bearing leucosome crosscut the migmatitic foliation in both paragneisses and meta-granitoids. The anatexites are leucocratic rocks mainly composed of quartz + feldspar ± cordierite leucosomes, which result from high degrees of partial melting. Disarticulated and convolute films of biotite support the migmatitic foliation in the anatexites. Chlorite, titanite, white-mica, and rare epidote replace migmatitic mineral assemblages in all the rock types.

Leucosomes and granitoids of different textures and compositions are hosted in the migmatites: fine-grained biotite-bearing granites outcrop between M. Malinvern and Laghi Superiori di Valscura, whereas coarse-grained two-mica-bearing granites (i.e. the Central Granite) surface at C.ma del Tavels (Fig. 2). Afterwards, migmatites and granites were intruded by aplitic to pegmatitic dykes, consisting of plagioclase, quartz, K-feldspar, white mica with rare biotite and/or garnet, and, lastly, by swarms of lamprophyres.

The intrusive contacts between lamprophyres and country rocks are sharp and outlined by chilled margins (Fig. 4b). The lamprophyres that are less than a metre-thick are spessartites, with euhedral phenocrysts of amphiboles into a greenish aphanitic groundmass (Fig. 4c). Amphibole phenocrysts in spessartites may show shape-preferred orientation (SPO) tracking a magmatic foliation; in other cases, they are randomly oriented, often in glomeroporphyric aggregates. Millimetre-sized vesicles are common in the external parts of the porphyritic dykes, and they can be either empty or filled by aggregates of plagioclase and minor amphibole, or by calcite. The thickest lamprophyres, classified as appinites according to IUGS (Le Maitre et al., 2002), are phaneritic, with randomly oriented crystals of amphibole, plagioclase, minor K-feldspar, and rare quartz. Sharp to lobate fragments of appinite may be enclosed into leuco-appinites, which consist of plagioclase, K-feldpsar, quartz, and minor amphibole: such internal structures suggest that mingling and magmatic brecciation occurred during their emplacement. Aggregates of chlorite and epidote widely replace igneous minerals in the lamprophyres (Fig. 4d), as well as early formed mineral assemblages in the country rocks close to the dyke walls. Very localised mylonitic shear zones crosscut migmatites, granitoids and lamprophyres. In the migmatites, these structures are supported by chlorite, phengite, minor biotite, and very rare actinolite (Figs. 2, 3 and 4e).

Val du Haut Boréon site

The migmatitic paragneisses of Val du Haut Boréon are locally interlayered with metre-thick layers of amphibolites and grade into diatexites and anatexites towards the eastern edge of the mapped area (Fig. 5); structures and supporting mineral assemblages are similar to those described in Valscura. Heterogeneously deformed migmatitic orthogneisses with granulite relics, already described as meta-volcanics by Colombo et al. (1993) and Rubatto et al. (2001), outcrop around C.ma Ghilié (Fig. 5). Cordierite-bearing pegmatites, white mica-bearing granite pockets, and aplite dykes are widespread in the migmatitic complex: all these intrusives are crosscut by the lamprophyres. Structural study of outcrops, where the lamprophyres display clear structural relationships with the country rocks, requests mapping at 1/100 scale (Fig. 6).

In Val du Haut Boréon, the lamprophyres show higher textural and compositional heterogeneities in comparison with those from Valscura, although they are characterised by chilled margins as well (Fig. 7a). Very rare grains of clinopyroxene, locally in glomeroporphyric aggregates, are found together with phenocrysts of amphibole in up to metre-thick and NW-SE striking spessartites. Vesicles, mainly filled by feldspar, igneous amphibole, or by calcite, are arranged in layers parallel to the dyke walls. A single E-W-striking dyke of 10 metres of thickness run through all the Val du Haut Boréon from its northwestern edge to Cime de l’Agnel (Fig. 5): at Lacs Bessons (Figs. 5 and 6) it displays a symmetrical structure with two metre-thick bands of melanocratic spessartites along its margins (Figs. 7a and 7b) and appinites and leuco-appinites at core (Fig. 7c). Melanocratic spessartites are formed by rounded to angular amphibole-rich domains of millimetre to centimetre size enclosed into a mesocratic matrix, and include several layers of vesicles. In the central part, lobate to angular enclaves of appinites are mingled together with leuco-appinites, as a consequence of different, but concomitant, magmatic pulses (Fig. 7c). Comb layers, outlined by phenocrysts of amphibole of centimetre size interlayred with fine-grained domains, divide the appinites from the melanocratic spessartites and define two metre-thick bands (Figs. 6 and 7d). Amphibole phenocrysts in comb layers are arranged in fan-shaped aggregates, which are almost orthogonal with respect to the dyke walls. Locally, melanocratic spessartites are directly in contact with the appinites: here, clasts of comb layered appinites and melanocratic spessartites, which locally preserve chilled margins, are included in the central part of the dyke (Fig. 7e). NE-SW striking spessartite dykes intersect the external parts of the main dyke at the Lacs Bessons and intrusive contacts are lobate (Fig. 6); conversely, they seem to be coalescent with appinites and leuco-appinites at its core. Centimetre-thick leucocratic veins follow the margins of the main dyke and crosscut the melanocratic spessartites and, in few cases, the leuco-appinites (Figs. 7e and 7f). As well as in Valscura, igneous minerals are widely replaced by fine-grained aggregates of epidote and chlorite, which grew in spotted domains, mainly localised in coarse-grained appinite or in and around the leucocratic veins (Figs. 7c and 7f): such pervasive pseudomorphosis is not observed in the migmatites away from the dyke walls. Migmatites, granitoids, and lamprophyres are intersected by ductile to cataclastic shear zones (Figs. 7g and 7h): the mineral assemblages that support mylonitic and stylolitic foliations in lamprophyres will be the topic of the next paragraphs.

Finite strain pattern

Valscura site

The pervasive structure developed in the migmatitic complex of Valscura is the composite fabric S1+2 that defines the boundaries between migmatitic paragneisses and migmatitic meta-granitoids. Relics of S1 and earlier formed veins of leucosome are preserved as hinges of tight to isoclinal D2 folds characterised by sub-horizontal and N- to NW-trending axes (Fig. 8). S1 and S2 axial plane foliation mainly dip to SW or to NE. S2 is locally affected by grain size reduction, as it occurs between Laghi Inferiori and Laghi Superiori di Valscura (Figs. 2 and 4b). Since newly formed cordierite-bearing leucosome veins crosscut D2 fold limbs and leucosomes fill saddle-reefs in D2 fold hinges, D2 developed under migmatitic conditions.

S2 was refolded during D3, which resulted in open to tight folds of metric wavelength (Fig. 4a), characterised by sub-vertical axes and by steep axial planes dipping NW or N (Fig. 8). A chlorite and phengite-bearing axial plane foliation (S3), dipping NW or SE (Fig. 8), is locally coupled with D3 folds. D3 folds are crosscut by granitoids and vertical to sub-vertical, NE-SW striking, lamprophyres (Figs. 4c and 8). Local deviation from the average striking likely reflects primary irregularities of the lamprophyre dyke walls (cf.Fig. 3).

Afterwards, sub-vertical NW-SE and minor E-W striking D4 mylonitic shear zones developed in the basement rocks (Fig. 8). NW-SE striking D4 mylonites are characterised by strike-slip dextral shearing, as suggested by S-C geometries, by sub-horizontal mineral lineation on shear planes, and by the offset of the lamprophyre dyke walls. The chilled margins were re-activated as shear surfaces during D4 (Fig. 4f) and stylolitic films, coupled with mineralised tension veins, developed. Field relationships, shear sense indicators, and supporting mineral assemblages allow correlating the D4 shear zones to the regional scale structures of Alpine age (Corsini et al., 2004; Baietto et al., 2009; Sanchez et al., 2011a; Leclère et al., 2014).

Val du Haut Boréon site

The dominant structure in the basement rocks of Val du Haut Boréon is the composite fabric S1+2, which results from the superposition of D2 tight to isoclinal folds on S1. Both D1 and D2 structures developed under migmatitic conditions, as suggested by leucosome in microlithons, and by leucosome veins crosscutting D2 hinges and limbs. D2 folds are generally isoclinal and fold limbs are parallel to SW-dipping D2 axial planes (Fig. 8). D2 stretching lineation in L to S-L syn-migmatitic mylonites in migmatitic orthogneisses between Lac Guillié and Téte du Lacs Bessons, dips 40–70° to SW. S2 is bent by D3 open folds of metric wavelength (Fig. 6), characterised by axes plunging to SW and axial surfaces dipping either to NW or to SE. D3 folds are intersected by white mica-bearing granite and aplite dykes.

All these rocks and structures are crosscut by sub-vertical NE-SW striking lamprophyres and by the E-W trending dyke of Lacs Bessons. The intrusive contacts between lamprophyres and country rocks are sharp and made angulated by syn-intrusive fracturing. Dykes are locally arranged in en-echelon systems, indicating emplacement contemporaneous to sinistral shearing (Fig. 6). Migmatites, granites and lamprophyres are, in turn, displaced by the NW-SE striking ductile to cataclastic D4 shear zones, characterised by S-C surfaces and sub-horizontal stretching lineations consistent with dextral sense of shear (Figs. 5 and 8). During D4, the intrusive contacts of lamprophyres were locally reactivated as sinistral shear planes, as testified by S-C foliations along the chilled margins (Fig. 7h).

Whole rock geochemistry

Lamprophyres are analysed for major and trace elements at SARM, “Centre de Recherche Petrographiques et Geochimiques” (CNRS, Vandoeuvre-les-Nancy, France) by ICP-OES and ICP-MS, respectively. Information on the procedure, precision and accuracy is specified by Carignan et al. (2001). Analyses are carried out on 11 unfoliated samples from both the studied sites that do not appear significantly metamorphosed at meso- and micro-scale. Major and trace element compositions are reported in Table 1.

All the analysed samples share a calc-alkaline high-K character. Silica content of lamprophyres varies from 51 to 60 %wt and the transition to the more silicic magmas is associated with decreasing of MgO, Fe2O3 and CaO, as well as increasing of Al2O3 and K2O amount. The MALI indexes plot either in the calc-alkalic or in the alkali-calcic fields (Fig. 9a); A/CNK ratios from 0.66 to 1.15 correspond to metaluminous to slightly peraluminous melts (Fig. 9b). Lamprophyres are classified as magnesian in the Fe-number diagram of Frost et al. (2001) (Fig. 9c), whereas in the Mg-number-B classification scheme of Debon and Le Fort (1988) they plot in close proximity to the “critical line” that divides the “magnesian” from the “ferriferous” field.

The MORB-normalised spider diagram highlights humped patterns: the lamprophyres are markedly enriched in Sr, K, Rb, Ba, and Th, whereas the Ti and Y contents plot close to MORB values (Fig. 9d). The concentrations of Ni and Cr, which likely testify the mantle influence on the genesis of the melts, widely range from 83 to 6 ppm and from 290 to 15 ppm, respectively, the content of V decreases from 332 to 68 ppm with the increase of the silica content from 50.96 to 56.17 %wt. The geochemical signatures of the Argentera-Mercantour lamprophyres are thus consistent with crustal contamination of mantle-derived magmas.

Lamprophyres microstructures

Igneous microstructures

Lamprophyres are mainly constituted by igneous amphibole and plagioclase. AmpI1 is brown, euhedral to subhedral, and commonly twinned (mineral abbreviations are from Whitney and Evans, 2010). It displays oscillatory compositional zoning (Fig. 10a) and, only in few cases, preserves dark brown cores with corroded boundaries. PlI1 is in subhedral crystals or small laths, usually forming simple or polysynthetic twins. Glomeroporphyric aggregates of clinopyroxene occur in the spessartites from Val du Haut Boréon (Fig. 10b), whereas aggregates of chlorite, titanite, and biotite entirely replaced euhedral crystals likely of igneous biotite or clinopyroxene in both aphanitic and phaneritic rocks. Accessories apatite, magnetite, and ilmenite are included within AmpI1 and PlI1 in the phaneritic rock types (Fig. 10a), or lie within the groundmass in the spessartites.

In appinites and leuco-appinites, up to centimetre-sized crystals of AmpI1 are randomly oriented and associated with smaller subhedral to interstitial grains of PlI1, KfsI1, and rare QzI1. Mingling structures between appinites and leuco-appinites are highlighted by modal variations of AmpI1 and PlI1.

Spessartites are highly heterogeneous in term of textures: sub-millimetric phenocrysts of AmpI1 typically occur within a microcrystalline groundmass, or together with twinned laths of PlI1. In few cases, phenocrysts of AmpI1 are partly to entirely enclosed in up to centimetre-sized crystals of PlI1, conveying a sub-ophitic to ophitic textures. Strain-free phenocrysts of AmpI1 and laths of PlI1 with SPO support the magmatic foliation in spessartites. Approaching the chilled margins, the SPO of crystals supporting the magmatic foliation is progressively more pervasive and homogeneous.

In melanocratic spessartites, irregular-shaped amphibole-rich domains of centimetre size are enclosed in a more leucocratic groundmass. Two types of amphibole-rich domains are recognised, both with anhedral to subhedral interstitial PlI1 crystals: in the former, crystals of AmpI1 are dark-brown and highly prolate, whereas, in the latter, AmpI1 occurs in brown stocky grains. AmpI1 crystals in comb layered appinites display a seriate crystal size distribution: centimetre-sized phenocrysts of AmpI1 with interstitial PlI1 are iso-oriented and, locally, in fan-shaped aggregates, whereas millimetre- and sub-millimetre-sized AmpI1 crystals are randomly oriented and completely to partially included in PlI1.

Vesicles, which commonly occur in spessartites and melanocratic spessartites, are tangentially bounded by undeformed crystals of AmpI1 (Fig. 10c) and are locally filled either by PlI1 and AmpI1 radially oriented, by hydrothermal mineral assemblages, or by twinned crystals of calcite. Along the chilled margins, small phenocrysts of AmpI1 are recognisable within the cryptocrystalline groundmass (Fig. 10d). The igneous mineral assemblages filling leucocratic veins comprises AmpI1, PlI1, KfsI1, and minor QzI1.

Dark-green AmpI2 rims AmpI1 in Valscura lamprophyres (Fig. 10e) and partially replaces AmpI1 in and around the leucocratic veins in Val du Huat Boréon (Fig. 10f). AmpI2 is interpreted as late-magmatic phase since it is replaced by hydrothermal mineral assemblages (M1, described below).

Subsolidus microstructures

Microstructures developed under subsolidus conditions and supporting mineral assemblages are similar in both the investigated areas. However, in Val du Boréon, syn-D4 mineral assemblages are significantly developed only in intermediate to high strain domains.

M1 is the earliest post-magmatic mineral assemblage retained by the lamprophyres, which comprises AbM1 + ActM1 + ChlM1 + EpM1 + KfsM1 + TtnM1 ± BtM1 ± CalM1 ± QzM1 in polygonal aggregates with rational grain boundaries and without SPO (Fig. 11a). Up to millimeter-sized randomly-oriented epidote grains are diagnostic for M1. Rare aggregates of ZoM1 may occupy the same microstructural domains of EpM1, as observed in few samples from Val du Haut Boréon. OpqM1, hematite and pyrite, locally occur. AmpI1 and AmpI2 are partially to totally replaced by aggregates of anhedral and un-twinned AbM1, colourless ActM1, ChlM1, EpM1, TtnM1, and rare QzM1 (Figs. 10e and 11b11d); a similar mineral assemblage, but without ActM1, replaced PlI1. BtM1 rims KfsI1.

M2 mineral assemblages support stylolitic and mylonitic D4 foliations in lamprophyres (Figs. 11c11f). Igneous phenocrysts and aggregates of EpM1 are wrapped by D4 mylonitic foliation marked by SPO of AbM2 + BtM2 + ChlM2 + TtnM2 + PhM2 ± ActM2 ± EpM2 ± OpqM2 ± QzM2 (Figs. 11d and 11e), and are partially dissolved at strain caps or along stylolitic films mainly supported by microgranular aggregates of TtnM2 (Figs. 11c, 11e and 11f). Pale-green ActM2 fills pressure shadows and boudin necks of AmpI1 porphyroclasts (Figs. 11d and 11f). Newly formed grains of ActM2 rarely mark the D4 mylonitic foliation. KfsM2 exclusively fills tensional veins at dyke margins, where is associated with AbM2 + ActM2 + BtM2 + ChlM2 (Fig. 11b). In low-strain domains, AmpI1 phenocrysts, partially to totally overgrown by M1 assemblages, are rimmed by discontinuous coronae of ActM2. M2 mineral assemblages, which support D4 structures in lamprophyres, are consistent with the transition between greenschist and amphibolite facies conditions (Maruyama et al., 1982; Maruyama et al., 1983; Spear, 1993).

Discussion

Structural and metamorphic relationships between lamprophyres, granitoids, and migmatites allow an accurate discrimination between superposed Variscan and Alpine tectono-metamorphic stages in the crystalline basement of the Argentera-Mercantour Massif. During D2, which was coeval with the Variscan migmatisation, deformation was highly heterogeneous, as indicated by D2 mylonites through domains where the dominant fabric is the folded S1 foliation. Late-Variscan D3 event is responsible for folds, subsequently crosscut by acidic dykes and pockets, and by younger NE-SW striking lamprophyres.

The Argentera-Mercantour lamprophyres are spessartites and appinites, formed by AmpI1, PlI1, and minor KfsI1. Igneous clinopyroxene is observed in few spessartite dykes, whereas QzI1 is rare in phaneritic rock types. Rims of late-magmatic AmpI2 may occur, in particular at or near sets of late leucocratic veins. Mingling and magmatic breccia textures are common in spessartites, appinites, and leuco-appinites, which are locally divided by comb layers. These structures point out that the dykes have collected compositionally heterogeneous magmas during their ascent.

Chilled margins and vesicles, the latter interpreted as resulting from degassing (Vernon, 2018), suggest that the Argentera-Mercantour Massif had been already exhumed at shallow depths before the emplacement of the lamprophyres. Furthermore, brittle structures accomodated their emplacement. M1 assemblages (AbM1 + ActM1 + ChlM1 + EpM1 + KfsM1 + TtnM1 ± BtM1 ± CalM1 ± QzM1 ± OpqM1 ± QzM1), which exclusively developed in the lamprophyres and along their margins, reflect localised hydrothermal circulation that took place during the latest emplacement stages, unrelated with the Alpine tectono-metamorphic evolution.

Lamprophyres are crosscut by dextral shear zones of Alpine ages (D4), striking from E-W to NW-SE, the latter of which characterised by sub-horizontal mineral lineations. M2 assemblages (AbM2 + BtM2 + ChlM2 + TtnM2 + PhM2 ± ActM2 ± EpM2 ± KfsM2 ± OpqM2 ± QzM2) support D4 foliation in lamprophyres, fill D4 tensional veins at dyke walls, and rim both igneous and M1 assemblages in low strain domains. Therefore, the lamprophyres of the Argentera-Mercantour Massif retain Alpine metamorphic conditions compatible with the transition between the greenschist and amphibolite facies. Since M2 mineral assemblages are similar in Valscura and Val du Haut Boréon, similar Alpine metamorphic conditions are assumed for the two tectonic slices divided by the Fremamorta Shear Zone (FMZ in Fig. 1). Furthermore, no changes in dyke attitudes between the two areas imply that only minor relative rotation has affected these two slices since the lamprophyre emplacement up to the present day, despite of the interposition of the FSZ.

In the Argentera-Mercantour Massif, rapid exhumation of the thickened Variscan crust in late-collisional setting triggered decompression melting at around 323 ± 12 Ma (Rubatto et al., 2001; Ferrando et al., 2008), which culminated with the surface exposure of the lower crust from late Carboniferous times (Faure-Muret 1955; Malaroda et al., 1970; Bortolami et al., 1974). It occurred after the slab break-off (Rubatto et al., 2001; von Raumer et al., 2014) and concurrently with the emplacement of the Central Granite (Boucarut, 1967; Ferrara and Malaroda 1969; Corsini et al., 2004). After tens of million years of migmatisation and successive emplacement of early Permian peraluminous granitoids in the retrogressed and exhumed migmatites, texturally heterogeneous lamprophyres, resulting from crustal contamination of mantle-derived magmas, emplaced at shallow crustal levels. A Permian-Triassic age for the Argentera-Mercantour lamprophyres is suitable for the following reasons: i) the lamprophyres only intruded into Variscan migmatites and early Permian granitoids, but not into the Mesozoic to Tertiary sedimentary sequences (Faure-Muret 1955; Malaroda et al., 1970); ii) the lamprophyres record a prograde metamorphism, reaching metamorphic conditions at the transition between the greenschist and amphibolite facies during the Alpine deformation. Furthermore, dacites and basalts are reported in the Permian siliciclastic sequences (Faure-Muret, 1955; Malaroda et al., 1970; Romain and Vernet, 1978).

The tectonic setting accountable for the Permian-Triassic magmatism in the southern Variscan belt area is controversial. Both the calc-alkaline and the subsequent alkaline cycles, comprising transitional to tholeiitic mafic rocks, have been intepreted either as a result of Variscan collapse, or crustal delamination, or post-orogenic extension (e.g.Lardeaux and Spalla, 1991; Bussy et al., 2000; Cannic et al., 2002; Cocherie et al., 2005; Bussien et al., 2008; Buzzi and Gaggero, 2008; Dallagiovanna et al., 2009; Spalla et al., 2014; Rossi et al., 2015). Numerical models performed to explore the transition from Variscan convergence to Permian-Triassic extension indicate that forced extension (at least 2 cm/yr) is necessary to produce successive pulses of mantle-derived magma in Permian-Triassic times: such a magmatic activity could not result from the gravitational evolution characterising the end of the Variscan subduction-collision (e.g.Marotta et al., 2009; Marotta et al., 2018; Roda et al., 2018). Taking into accounts these model predictions, the Argentera-Mercantour lamprophyres are more easily interpreted as magmatic records of the Permian-Triassic lithospheric thinning, announcing the Jurassic rifting on the European margin, instead of the igneous witness of the late-Variscan collapse.

Conclusion

The multiscale structural analysis carried out in the southeastern portion of the Argentera-Mercantour Massif highlights the following significant steps of its geodynamic evolution, from Variscan to Alpine times:

  • texturally heterogeneous lamprophyres, magnesian, calc-alkalic to alkali-calcic and metaluminous in composition, emplaced at shallow crustal levels crosscutting three generations of superposed Variscan structures in the migmatitic complex and early Permian granitoids;

  • lamprophyres were places of hydrothermal circulation immediately after their intrusion;

  • mineral assemblages supporting the strike-slip mylonitic shear zones of Alpine ages are consistent with the transition between the greenschist and amphibolite facies.

Syn-intrusive structures and metamorphic evolution are thus reasonably consistent with the lamprophyres of the Argentera-Mercantour Massif interpreted as evidence of Permian-Triassic lithospheric thinning in the External Crystalline Massifs of Western Alps.

Acknowledgement

The authors acknowledge funding by University of Milano: Linea 2, Azione A – fondi giovani ricercatori “Analisi strutturale delle catene collisionali” (PSR2018_DZANONI). Achille Blasi has driven our attention to the dykes of val du Haut Boréon, making available to G.G. a wealth of his samples and unpublished 1/10 000 maps from the sixties; inclusion of data from master theses by Luigi Cinquegrani and Valentina Montani is gratefully acknowledged. Curzio Malinverno prepared quality thin sections. Part of this work supported compilation of the candidacy Dossier on the area “Alpes de la Méditerranée” as UNESCO world heritage site. Facilities for fieldwork logistics were provided by “GECT Alpi Marittime – Mercantour”. For these reasons, the paper is framed in the MIUR project “Dipartimenti di Eccellenza 2017 – Le geoscienze per la società: risorse e loro evoluzione” (work-package 4, task 3). Two anonymous reviewer are thanked for constructive comments.

Cite this article as: Filippi M, Zanoni D, Gosso G, Lardeaux J-M, Verati C, Spalla MI. 2019. Structure of lamprophyres: a discriminant marker for Variscan and Alpine tectonics in the Argentera-Mercantour Massif, Maritime Alps, BSGF - Earth Sciences Bulletin 190: 12.

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