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Home All issues Volume 195 (2024) BSGF - Earth Sci. Bull., 195 (2024) 27 Full HTML
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BSGF - Earth Sci. Bull.
Volume 195, 2024
Article Number 27
Number of page(s) 16
DOI https://doi.org/10.1051/bsgf/2024023
Published online 20 December 2024

© S. Couzinié et al., Published by EDP Sciences 2024

Licence Creative CommonsThis is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1 Introduction

Within the framework of plate tectonics theory, sutures, i.e. zones along which oceanic lithospheres were subducted, are key to deciphering the evolution of orogens (Dewey, 1987). Their identification remains challenging in the case of old and deeply eroded collisional orogenic systems such as the European Variscan belt, formed through the convergence of Laurussia and Gondwana during the late Paleozoic (Edel et al., 2018; Matte, 1986; Stampfli et al., 2013). Indeed, the diagnostic suture rock associations (high-pressure metamorphic belts and ophiolite sequences) have been largely dismantled by early erosion and intensely reworked by collisional processes, leading to conflicting views on the number of suture zones and their associated oceans (see discussions in Franke et al., 2017; Kroner and Romer, 2013). Paleomagnetic data and paleobiogeography (Domeier, 2016; Paris and Robardet, 1990; Van der Voo et al., 1980) suggest that the formation of the Variscan belt followed the closure of a single >1000 km-wide Ordovician oceanic domain called the Rheic Ocean. On the other hand, petrological evidence coupled with geophysical data collectively indicate the existence of at least three sutures (Fig. 1a, Schulmann et al., 2022). The subduction of an Ordovician-early Devonian “Mid-Variscan” ocean (Galicia/Southern Brittany/Tepla ocean of Matte, 1986) would have initiated in the mid- to late-Devonian period (390–360 Ma). The narrower Saxothuringian ocean would have closed shortly after in the early Carboniferous (360–340 Ma). Lastly, subduction of the Devonian Lizard–Rhenohercynian Ocean would have occurred at 340–330 Ma (Zeh and Gerdes, 2010).

Delineating the suture zones along the belt is of marked importance to unravel the architecture of the European continental crust and the evolution of the Variscan orogeny. However, accurately correlating their locations and geometries in the eastern French Massif Central (eFMC, Fig. 1a) has proven challenging due to a lack of comprehensive seismic survey and the absence of ophiolite sequences (Schulmann et al., 2022). In the eFMC, it is commonly inferred (Lardeaux et al., 2014; Matte, 1986; Pin, 1990; Vanderhaeghe et al., 2020) that the Mid-Variscan suture is stamped by distinctive heterogeneous lithological formations referred to as “Leptynite–Amphibolite Complexes” (LACs, see review in Santallier et al., 1988). The latter encompass mafic meta-igneous rocks occurring as metric to kilometric boudins embedded in paragneisses and/or are intimately associated with metarhyolites/metagranites of crustal derivation (Briand et al., 1995, 1991; Chelle-Michou et al., 2017; Pin and Marini, 1993). The metabasites often bear relics of Devonian (385–360 Ma) high-pressure metamorphism defining a P–T gradient consistent with subduction (Lardeaux, 2023). Ultramafic bodies representing subducted mantle material are also present (Gardien et al., 1990). Early geochronological studies suggested that both mafic and felsic igneous protoliths were emplaced contemporaneously during the Cambrian–Ordovician period (Pin and Lancelot, 1982). This has led many authors to interpret the wide variety of geochemical signatures displayed by mafic rocks, ranging from MORB-type tholeiites to back-arc basalts and OIB (Briand et al., 1995, 1991; Downes et al., 1989; Giraud et al., 1984; Piboule and Briand, 1985; Pin and Marini, 1993) as reflecting an emplacement in an attenuated continental lithosphere at the incipient stages of oceanization (Lardeaux et al., 2014 and references therein). In this context, the LACs were proposed to represent fragments of the passive margin to ocean–continent transition zone of the Mid-Variscan ocean, which were subsequently buried through oceanic subduction and later exhumed as tectonic mélanges along major thrust zones in the orogenic wedge (Burg and Matte, 1978; Lardeaux et al., 2014, 2001), or possibly extruded through the upper plate (de Hoÿm de Marien et al., 2023; Keppie et al., 2010; Maierová et al., 2021).

Following the recognition of the main nappe architecture in the eFMC during the mid-1980s, the various LACs were grouped into a single litho-tectonic unit referred to as “the LAC” (singular) and were assumed to share the same geodynamic significance (Bouchez and Jover, 1986; Lardeaux et al., 2014; Matte, 1986; Vanderhaeghe et al., 2020). This broad correlation was questioned by Santallier et al. (1988), who pointed out the lack of age data on the protoliths and the uncertainty on the respective petrogenesis of individual LACs. This concern is echoed in the adjacent Maures Massif, where three LACs were initially identified, yet only two are currently regarded as delineating a suture zone (Bellot et al., 2010; Briand et al., 2002; Jouffray et al., 2023; Schneider et al., 2014). Therefore, a reexamination of individual (ultra)mafic–felsic rock associations across the eFMC appears necessary to ascertain whether each LAC record the same geodynamic events and collectively represent a single litho-tectonic unit. In this context, the increasing versatility of in situ laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon U–Pb dating offers new opportunities to unravel the protolith ages of the LAC rocks and clarify their origin (Chelle-Michou et al., 2017; de Hoÿm de Marien et al., 2023; Lotout et al., 2020; Paquette et al., 2017).

This contribution focuses on the Riverie LAC from the Monts-du-Lyonnais metamorphic complex in the eFMC. Novel petrological and geochronological data demonstrate that this mafic–felsic association consists of juvenile arc/back-arc-derived magmatic rocks of late Ediacaran age which are unrelated to the opening and closure of the Mid-Variscan ocean. These findings highlight the necessity for a reevaluation of the tectonic history of individual LACs within the eFMC.

thumbnail Fig. 1

(a) Sketch map depicting the exposed Variscan domains of western Europe and the location of the inferred suture zones, based on Schulmann et al. (2022). SXT: Saxo-Thuringian suture. (b) Geological map of the eastern French Massif Central showing the main nappe architecture. Are also depicted the locations/names of “Leptynite-Amphibolite Complex” occurrences (in black), adapted from Chantraine et al. (2003).

(a) Carte simplifiée montrant les zones où affleurent les terrains varisques et les sutures supposées. (b) Carte géologique de l’Est du Massif Central français oriental localisant les zones où affleure les "complexes leptyno-amphiboliques".

2 Geological setting

2.1 The eastern French Massif Central

The eFMC is part of the internal zone of the Variscan belt (Fig. 1), primarily composed of metamorphic nappes intruded by various Carboniferous granitoids (see review in Faure et al., 2009). In the northern part of the eFMC, metamorphic rocks are overlain by Visean sediments and volcanics indicating that the main orogenic phase occurred before 345 Ma. In contrast, compression persisted till the Namurian (corresponding to the Serpukhovian and Bashkirian pro parte, c. 325 Ma) in the foreland basin to the south. Upper Carboniferous (Stephanian, i.e. Kasimovian–Gzhelian pro parte) to Permian sediments were unconformably deposited over the metamorphic-plutonic basement in a late to post-orogenic setting. The nappe pile consists of the high-grade “Upper Gneiss Unit” overlying a crustal package of parautochtonous terrains, sometimes divided into the Lower Gneiss Unit and the (lower-grade) Parautochtonous Unit (Ledru et al., 1989). Southwards-directed thrusting of the Upper Gneiss Unit was coeval to the development of an inverted Barrovian metamorphic sequence (Burg et al., 1984) at 360–345 Ma (Chelle-Michou et al., 2017; Costa, 1989). The LACs are commonly located at the base of the Upper Gneiss Unit and mark the contact between this unit and the underlying parautochtonous terrains. Within most LACs, mafic meta-igneous rocks preserve a record of high-pressure subduction-related metamorphism with pressures exceeding 15 kbar at 385–360 Ma (de Hoÿm de Marien et al., 2023; Joanny et al., 1991; Lardeaux et al., 2001; Lotout et al., 2020, 2018). The parautochtonous terrains were locally affected by a low-pressure high-temperature Buchan-type metamorphic event coeval with crust extension and the formation of the Velay and Montagne Noire gneiss domes (Gardien et al., 2022, 1997; Ledru et al., 2001). The latter hosts mafic eclogites whose origin is disputed: they could either correspond to Upper Gneiss Unit material (and thus be subduction-related, Pitra et al., 2022) or lower crustal rocks exhumed during doming (Whitney et al., 2020). Lastly, the Brévenne unit is a bimodal volcanic sequence of late Devonian age metamorphosed under greenschist- to lower amphibolite-facies conditions and juxtaposed to the Upper Gneiss Unit during the main collisional phase (Feybesse et al., 1995).

The protoliths of the Upper Gneiss Unit and parautochtonous terrains consist of Ediacaran to Ordovician detrital (volcano-)sedimentary units, intruded by several generations of plutonic rocks (Couzinié et al., 2022, 2019). No rock older than Neoproterozoic has been described so far. Pre-Variscan magmatic rocks include: (i) latest Ediacaran (c. 545 Ma) and Cambro-Ordovician granites and rhyodacites of crustal origin, found throughout the nappe pile regardless the metamorphic grade (Chelle-Michou et al., 2017; Couzinié et al., 2022, 2017; Couzinié and Laurent, 2021); (ii) mafic (gabbroic and basaltic) rocks, mainly in the LACs of the Upper Gneiss Unit but also in the parautochtonous terrains as lava flows, pyroclastic deposits, and subvolcanic sills (Briand et al., 1995, 1992). Mafic rocks exhibit a large range of geochemical signatures ranging from tholeiitic to alkaline and calc-alkaline (Briand et al., 1992; Pouclet et al., 2017). The crust-derived magmas attest to the reworking of Ediacaran sediments (Couzinié et al., 2022, 2017), while mafic magmas would originate from the lithospheric and asthenospheric mantle, with the subordinate involvement of a garnet-bearing source (Pin and Marini, 1993; Pouclet et al., 2017). The latest Ediacaran crustal melting was possibly a distal manifestation of the Cadomian orogenic events (Couzinié et al., 2017), while Cambro-Ordovician crust-mantle magmatism should be tied to protracted lithosphere extension that prevailed at that time along the northern Gondwana margin (Chelle-Michou et al., 2017; Couzinié et al., 2022; Pin, 1990; Pin and Marini, 1993).

2.2 The Monts-du-Lyonnais metamorphic complex

The Monts-du-Lyonnais metamorphic complex (Fig. 2) belongs to the Upper Gneiss Unit. Its litho-tectonic pile consists of a set of amphibolite-(granulite-)facies metamorphic rocks including from bottom to top (Feybesse et al., 1995): (i) a lower metasedimentary unit corresponding to cordierite-bearing migmatites and garnet–sillimanite–biotite paragneisses in the northwestern and southeastern part of the complex, respectively, (ii) three (ultra)mafic–felsic rock associations cartographically corresponding to “belts” and assimilated to LACs by Chantraine et al. (2003); (iii) mildly peraluminous, locally anatectic orthogneisses; (iv) an upper metasedimentary unit, also migmatitic, with relics of high-pressure granulite-facies metamorphism. The first, southernmost (ultra)mafic–felsic belt includes 100–3000 m-long amphibolite boudins corresponding to retrogressed eclogites stretched within a paragneiss matrix and associated with garnet-bearing mantle peridotites (Blanc, 1981; Gardien et al., 1990). The second, northernmost Chaussan belt is composed of fine-grained leucogneisses (leptynites) enclosing numerous hecto- to kilometric massifs of meta-igneous mafic and felsic granulites (Dufour, 1985). Finally, the Riverie belt located in-between the previous two features an association of amphibolites and amphibole-bearing gneisses, locally leucocratic (leptynites) (Blanc, 1981).

Metasediments experienced peak metamorphic conditions of 750–800 °C at 7–9 kbar coeval to the development of the regional foliation S1-2 (Feybesse et al., 1995; Gardien et al., 1990). Anatexis initiated at that stage and persisted during exhumation down to c. 4 kbar. The age of this anatexis is poorly constrained, as the only published data consist of a whole-rock Rb–Sr isochron date of 384 ± 16 Ma, interpreted as the age of partial melting (Duthou et al., 1994). Relictual kyanite in metasediments suggests an early high-pressure evolution at P > 10 kbar (Feybesse et al., 1995). Rocks from the LACs record different metamorphic conditions. Eclogites from the southern belt experienced peak conditions of P > 15 kbar and 750–850 °C (Dufour et al., 1985; Joanny et al., 1991). One coesite grain retrieved from an eclogite sample would even suggest P > 28 kbar (Lardeaux et al., 2001). Meta-igneous granulites from the Chaussan belt record P of 8–10 kbar and T in the range 800–870 °C (Dufour, 1985). Metamorphic conditions for the Riverie belt have not been estimated but Feybesse et al. (1995) stressed that these rocks lack any evidence of a high-pressure evolution.

A late event of ductile deformation thought to have occurred at around 500 °C (Feybesse et al., 1995; Gardien et al., 1990) juxtaposed the Monts-du-Lyonnais metamorphic complex to the adjacent lower-grade Brévenne Unit (Fig. 2). The last increment of this tectonic phase was marked by the activation of NE-SW trending strike-slip shear zones and the intrusion of several synkinematic granitoids (Feybesse et al., 1995) at 345–335 Ma (Ar-Ar dating on micas and amphibole, Costa et al., 1993). Notably, the Riverie LAC is bounded by such shear zones to the other constituents of the metamorphic complex. Late brittle deformation reworked the original thrust contact between the Monts-du-Lyonnais metamorphic complex and the underlying Pilat unit (representing the parautochtonous terrains).

Little is known about the age of the Monts-du-Lyonnais metamorphic complex protoliths. There are no chronological constraints available on the depositional age of the detrital metasediments. The age of the orthogneiss protolith is estimated to be around 470 Ma (based on zircon Pb evaporation and whole-rock Rb–Sr ages, Dufour, 1982; Feybesse et al., 1995), although Duthou et al. (1984) report a significantly older whole-rock Rb–Sr isochron date of 502 ± 7 Ma. An identical date of 497 ± 8 Ma was obtained for felsic granulites and interpreted as the emplacement age of their igneous protolith (Duthou et al., 1981). Mafic meta-igneous rocks display a variety of geochemical types, ranging from N–MORB to LREE-rich tholeiites and even transitional rocks with alkaline affinities (Briand et al., 1995). They have never been dated, and a Cambro–Ordovician emplacement age has been postulated by analogy with other bimodal meta-igneous associations from the eFMC, notably in the nearby Vivarais area (Chelle-Michou et al., 2017).

In the Riverie belt, felsic rocks described as “amphibole-bearing (felsic) gneisses” and closely associated with amphibolites were first reported by Peterlongo (1958) and later studied by Blanc (1981). The relationships between amphibole-bearing felsic gneisses and amphibolites (the constituent rocks of the belt) are well-exposed in the disused “Les Roches” quarry near Riverie (Fig. 2). Amphibole-bearing felsic gneisses known in other eFMC LACs have been variously regarded as former pyroclastic/epiclastic rocks coeval to the amphibolite protoliths (Pin and Lancelot, 1982), metadiorites (Bodinier et al., 1986) or anatectic magmas formed by partial melting of the amphibolites at Variscan times (Benmammar et al., 2020).

thumbnail Fig. 2

Simplified geological map of the Monts-du-Lyonnais metamorphic complex, adapted from Feybesse et al. (1995).

Carte géologique simplifiée du complexe métamorphique des Monts-du-Lyonnais.

3 Analytical methods

Eight samples of amphibole-bearing felsic gneisses and four samples of amphibolites collected from the pit#2 of the “Les Roches” quarry (GPS coordinates: 45.5964N, 4.5879E, see Supplementary Text) were analyzed for major and trace element compositions by the ALS Global firm (details on the procedures, detection limits and accuracy/reproducibility in Supplementary text, full dataset available as Supplementary Table S1). Recalculation and plotting of the whole-rock geochemical data were performed using the GCDkit plugin for R (Janoušek et al., 2006). Mineral major element compositions were obtained using a Zeiss EVO MA15 Scanning Electron Microscope at the Central Analytical Facility of Stellenbosch University, South Africa. Mineral compositions were determined by EDX (Energy Dispersive X-ray) analysis using an Oxford Instruments® X-Max 20 mm2 detector and the Oxford INCA software. Beam conditions were 20 kV accelerating voltage, 1.5 nA probe current, a working distance of 8.5 mm and a specimen beam current of 20 nA. Analyses were quantified using natural mineral standards and representative mineral compositions are reported in the Supplementary Table S2. Biotite and amphibole structural formulas (including ferric iron contents) were calculated using the spreadsheets of Li et al., (2020a, 2020b). Two samples of amphibole-bearing felsic gneisses (RV-1 and RV-2) were selected for zircon U–Pb–Hf determinations to constrain the emplacement age and isotopic signature of their protoliths. Zircon grains were separated from the crushed rock samples at Saint-Etienne University using conventional techniques (sieving, panning, magnetic and heavy liquids separation followed by handpicking). Selected grains were subsequently cast into epoxy mounts and polished down to a subequatorial grain section. BSE and CL-imaging were performed at the Central Analytical Facility of Stellenbosch University using a Zeiss MERLIN Scanning Electron Microscope. In situ U–Pb dating and zircon Lu–Hf isotope measurements were carried out by LA–ICP–MS at the J.W. Goethe University, Frankfurt-am-Main (Germany). Plotting and calculation of Concordia dates were performed using IsoplotR (Vermeesch, 2018). Information on the analytical methods is presented in the Supplementary Text. The full datasets (standards and samples) are reported in Supplementary Tables S3 to S6.

4 Sample petrography and geochemistry

The Riverie belt occurs as a tectonic boudin bounded by dextral shear zones and syn-kinematic (mylonitic) granites (Fig. 2). In the largest pit of the disused Les Roches quarry, felsic gneisses exhibiting a vertical foliation and wrapping around 1 to 3 m-wide amphibolite bodies (Fig. 3a, b) constitute the main petrographic type. Amphibolites are particularly abundant in the SE part of the pit. Both mafic and felsic rocks are cut across by a network of decimeter-scale discordant or concordant granitic and pegmatitic veins.

The felsic gneisses are primarily composed of plagioclase, quartz, green amphibole with low amounts of brown biotite either in the matrix or rimming amphibole (Fig. 3c, d, e). The foliation is underlined by alternating layers with varying proportions of mafic minerals (Fig. 3c). The felsic rocks show a high-temperature texture with interlobate grain contacts and preservation of high-energy surfaces (Fig. 3d), with amphibole often being interstitial. Inclusions of plagioclase in amphibole and vice versa are locally observed (Fig. 3e). Amphibole is a magnesio-ferri-hornblende with mg# of 0.63 and Fe3+/Fetot ratios in the range 0.29–0.37, slightly higher than the value (0.27) estimated through wet chemistry by Blanc (1981). Biotite is also magnesian, with mg# of 0.65–0.72, and TiO2-rich (>3 wt.%). Plagioclase is chemically unzoned and has an oligoclase composition (An24-26). Accessory minerals are magnetite, apatite, zircon, and sulphides (Fig. 3c, d, e). The edenite–richterite thermometer of Holland and Blundy (1994) and the barometer of Molina et al. (2015) for paired plagioclase–amphibole compositions yielded equilibration temperatures of c. 700 °C for upper to mid-crustal pressures of 2–4 kbar.

The amphibole-bearing felsic gneisses do not deviate from the igneous trend in the FMW plot of Ohta and Arai (2007) which points to an igneous and not volcanosedimentary origin for their protoliths (Fig. 4a). They show SiO2 contents ranging from 61 to 69 wt.%, reflecting the varying mineral modes at the scale of the quarry (50 m). They are metaluminous to subaluminous with A/CNK (molar Al2O3/[CaO + Na2O + K2O]) below 1.05, and markedly poor in K2O (≤1 wt.% and mostly below 0.7 wt.%, Supplementary Table S1). Accordingly, they plot in the field of tonalites in the P–Q diagram of Debon and Le Fort (1983) (Fig. 4b). All samples show similar immobile, incompatible trace elements patterns characterized by overall low concentrations. The amphibole-bearing felsic gneisses are depleted in HREE (by a factor 0.3 to 0.6) and slightly enriched in LREE–Zr (by a factor 2 to 3) and Th (10 times) with respect to N-MORBs (Fig. 4c). Of importance are the pronounced Nb and Ti negative anomalies (with Nb/Nb*: 0.2–0.3; Ti/Ti*: 0.3–0.6), along with a weakly positive Zr anomaly (Zr/Zr*: 1.2–2.1). Th/Nb ratios are high, clustering around 0.6–0.7 (Fig. 4e). The concentrations of LILE such as Ba (100–400 ppm), Sr (200–300 ppm) and Rb (8–20 ppm) are lower than those of the Bulk Continental Crust (Rudnick and Gao, 2003). Chondrite-normalized REE patterns (Boynton, 1984) are moderately fractionated (LaN/YbN = 2.9–7.1) and show a slight depletion in mid-HREE (Dy to Tm, Fig. 4d). A weak Eu negative anomaly is observed (Eu/Eu* = 0.71–0.95). Vanadium contents are low (78–151 ppm) with V/Ti ratios close to 20 (Fig. 4f). Most trace element compositions (except Zr) show a rough decrease with the SiO2 content.

The mafic rocks have a nematoblastic texture and are dominantly composed of an equigranular assemblage of euhedral to subhedral amphibole and plagioclase (Fig. 3f) with apatite and titanite as the main accessory minerals. These amphibolites show mildly alkaline basic compositions (SiO2: 46.6–51.9 wt.%; Na2O+K2O: 4.2–5.1 wt.%, Fig. 4b). Three samples show primitive compositions (i.e. close to that of primary mantle-derived melts) as indicated by their elevated Mg# (62–63), MgO (7.5–9.0 wt.%) and Cr (270–320 ppm) contents and the lack of Eu anomaly. They are slightly enriched in Th and LREE with respect to N-MORBs, and three samples display a pronounced negative Nb anomaly (with Th/Nb>0.2, Fig. 4c, e). REE patterns are nearly flat (LaN/YbN = 1.5–2.0, Fig. 4d). Vanadium contents are moderately elevated (227–362 ppm) with V/Ti ratios between 25 and 40 (Fig. 4f).

Several observations suggest that the igneous protolith of the amphibole-bearing felsic gneisses was plutonic and coeval to or younger than that of the amphibolites: (i) the contacts between the two rock types are often intricate, with preserved evidence for intrusive relationships of the former in the latter (see white triangles, Fig. 3a); (ii) pluri-centimetric amphibole-rich clusters in the felsic rocks resemble assimilated mafic material (black triangle, Fig. 3a); (iii) the foliation of the gneisses fades in strain shadows at the edges of the mafic bodies so that the felsic rock appears isotropic (white triangle, Fig. 3b).

thumbnail Fig. 3

Field observations and petrography of the Riverie felsic gneisses and amphibolites. (a) Amphibole-rich clusters embedded within the felsic gneisses (black triangles) recalling assimilated mafic material. Veins of felsic rocks cutting across the amphibolite (white triangles). (b) Felsic gneisses with plagioclase–quartz leucocratic bands rimmed by coarse amphibole underlining the vertical foliation (black triangles) and isotropic zones between the amphibolite boudins (white triangles). (c) Polarized-light thin section scan showing the variation in amphibole mode typically observed in the felsic gneisses. (d,e) Polarized-light photomicrographs of felsic gneisses illustrating the main plagioclase–quartz–amphibole assemblage with biotite rimming amphibole. (f) Polarized-light photomicrograph of a mafic rock showing the equigranular plagioclase–amphibole assemblage.

Fig. 3. Observations de terrain et pétrographie des gneiss et amphibolites de Riverie. (a) Amas riches en amphibole dans les gneiss (triangles noirs) pouvant correspondre à du matériel mafique assimilé et veines de roches felsiques dans les amphibolites (triangles blancs). (b) Gneiss felsiques avec des bandes leucocratiques de plagioclase-quartz bordées par de l’amphibole soulignant la foliation verticale (triangles noirs) et des zones isotropes entre les boudins d’amphibolite (triangles blancs). (c) Variation de la proportion d’amphibole dans les gneiss. (d, e) Assemblage des gneiss à plagioclase-quartz-amphibole-biotite. (f) Assemblage équigranulaire plagioclase-amphibole des amphibolites.
thumbnail Fig. 4

Whole-rock geochemical data of the Riverie felsic gneisses and amphibolites. (a) FMW diagram of Ohta and Arai (2007) demonstrating the igneous origin of felsic gneisses. The M and F values characterize mafic and felsic rock sources, respectively. The W value quantifies the degree of weathering. The references of data for (presumably) meta-igneous from “Leptynite-Amphibolite Complexes” are available in the Supplementary Text. (b) P–Q classification diagram of Debon and Le Fort (1983). The dotted lines represent the contour encompassing 75% of the data (n=75) for the Velay orthogneiss formation, a coeval (meta)igneous suite from the parautochtonous terrains (Couzinié et al., 2017). (c) Incompatible, immobile element patterns of the Riverie meta-igneous rocks normalized to the composition of NMORBs (Sun and McDonough, 1989). (d) Chondrite-normalized (Boynton, 1984) Rare Earth Elements patterns. (e) Th/Yb vs. Nb/Yb diagram of Pearce (2008). Oceanic rocks plot within the MORB–OIB array. Magmas contaminated by the continental crust during ascent or which mantle source incorporated a slab-derived (SZ) component plot in the volcanic arc array. (f) V–Ti diagram of Shervais (1982). This diagram highlights the effects of the change in Vanadium redox state when the mantle is metasomatized by slab-derived fluids. Vanadium becomes more incompatible and resulting magmas exhibit high V/Ti ratios. The magnetite + amphibole fractionation trend is from Shervais (1982). Data for the Bulk Continental Crust are from Rudnick and Gao (2003).

Données géochimiques des roches totales des gneiss et amphibolites de Riverie. (a) Diagramme FMW démontrant l’origine ignée des gneiss. (b) Diagramme de classification P-Q de Debon et Le Fort (1983). (c) Profils d’éléments incompatibles normalisés à la composition des N-MORB. (d) Profils de terres rares normalisés aux chondrites. (e) Diagramme Th/Yb vs. Nb/Yb de Pearce (2008). Les roches océaniques se situent dans le domaine MORB-OIB. Les magmas contaminés par la croûte continentale lors de leur mise en place ou dont la source mantellique a incorporé un composant dérivé de la plaque subduite se situent dans le domaine des arcs volcaniques. (f) Diagramme V-Ti de Shervais (1982).

5 Zircon data

5.1 Zircon textures

Zircon grains extracted from amphibole-bearing felsic gneisses are euhedral to subhedral, ranging in length between 70 and 200 μm, and often show width-to-lengh ratios higher than 1:2 with well-developed pyramidal tips (Fig. 5a). A few grains are stubbier with lower aspect ratios down to 1:1. CL-images reveal concentric oscillatory zoning, locally associated with sector zoning (see for instance zircon 1 of sample RV-2, Fig. 5a). The zoning pattern may differ from the inner to outer part of few grains, due to varying relative growth rate of crystal faces upon crystallization (white triangles on Fig. 5a). However, there is no marked textural break such as a change in CL intensity or clear evidence for resorption. Only very thin (<10 μm-large) CL-bright overgrowths truncate the zoning pattern of some grains. Local recrystallization is also evidenced (black triangles on Fig. 5a).

thumbnail Fig. 5

Zircon textural and U–Pb–Hf data for the Riverie amphibole gneisses. (a) Representative cathodoluminescence images with the locations of laser spots (white and yellow circles for U–Pb and Lu–Hf analyses respectively) indicated along with the spot name (aXX or YYa/b). The corresponding 206Pb/238U dates are quoted with 2σ uncertainty, in Ma. All displayed analyses are concordant (except those in italic). Hf isotope data are reported using the εHf calculated at the 206Pb/238U date obtained on the same zircon domain, quoted with 2σ uncertainty. The white and black triangles highlight changes in the zoning pattern and local grain recrystallization, respectively. Laser spot sizes are 30 μm for U–Pb and 40 μm for Lu–Hf. (b) Tera-Wasserburg diagrams (238U/206Pb vs. 207Pb/206Pb). Error ellipses and ages are displayed at 95% level of uncertainty. Green ellipses are those considered for Concordia age calculations. The reported MSWDs are those of concordance plus equivalence. (c) Measured εHf(t) on magmatic zircon grains, recalculated and plotted using the 206Pb/238U date. When such value was not available, the crystallization age determined for the sample was used. The range for the Depleted Mantle reservoir is bracketed by the models of Naeraa et al., (2012) and Griffin et al., (2002). The background red shading mimics the contours of the distribution of 223 zircon analyses from crust-derived felsic (meta-)igneous rock from the French Massif Central (Chelle-Michou et al., 2017; Couzinié et al., 2017; Moyen et al., 2017). These data are taken as representative of the Hf isotopic composition of the local continental crust. Data for the Armorican Massif Cadomian juvenile arc magmatism are from Samson et al., 2003. An εHf(t)–time crust evolution array calculated with an average 176Lu/177Hf ratio of 0.015 (Griffin et al., 2002) is also depicted. (d) Available LA–ICP–MS zircon U–Pb data on meta-igneous rocks from the “Leptynite–Amphibolite Complexes” in the eastern French Massif Central (represented as Kernel Density Estimates of concordant 206Pb/238U dates (calculated with IsoplotR using an adaptative bandwith). Zircon data are from Whitney et al., (2020) for the Montagne Noire, de Hoÿm de Marien et al., (2023) for the Haut-Allier, Paquette et al., 2017 for the Lot, Lotout et al., (2020, 2018); for the Rouergue, Chelle-Michou et al., (2017) for the Vivarais, and this study for the Monts-du-Lyonnais.

Données texturales et U-Pb-Hf des zircons extraits des gneiss de Riverie. (a) Images en cathodoluminescence montrant la zonation magmatique des grains et la localisation de quelques points d’analyse. (b) Diagrammes Tera-Wasserburg. Les ellipses vertes sont celles considérées pour les calculs d’âge Concordia. (c) Valeurs de εHf des grains de zircon recalculées à l’âge de mise en place. L'arrière-plan rouge représente la composition isotopique en Hf de la croûte continentale de l’Est du Massif Central. (d) Données U-Pb sur zircons obtenues pour les roches méta-ignées des "complexes leptyno-amphiboliques" de l’Est du Massif Central français.

5.2 U–Pb results

In gneiss sample RV–1, 40 measurements were performed on 32 grains, all within zones displaying clear oscillatory zoning (Fig. 5a). Seven spots showed moderate common Pb contents (up to 3%) and were markedly discordant. These spots will not be discussed further. A total of 16 concordant spots are statistically equivalent, and a Concordia date of 545.9 ± 1.9 Ma (± 6.4 Ma after propagation of systematic uncertainties) can be calculated from them (MSWD of concordance and equivalence = 1.6). Besides, 6 analyses yielded similar 206Pb/238U dates but were discordant, presumably owing to small amounts of common Pb. Eleven analyses showed younger, slightly discordant to concordant 206Pb/238U dates ranging between 536 ± 10 and 509 ± 9 Ma.

In gneiss sample RV–2, 33 analyses were performed on 25 grains. A Concordia date of 543.9 ± 1.5 Ma (±6.3 after propagation of systematic uncertainties) can be calculated from 19 statistically equivalent concordant analyses (with a MSWD of concordance and equivalence equal to 1.4, Fig. 5b). Spot a58 yielded an identical yet discordant 206Pb/238U date owing to the presence of common Pb. Nine measurements devoid of common Pb yielded concordant to moderately discordant 206Pb/238U dates ranging from 536 ± 6 down to 520 ± 6 Ma. Two analyses (a50 and a60) characterized by minor common Pb contents (0.3–0.8%) showed discordant and younger 206Pb/238U dates of 484 ± 5 and 438 ± 7 Ma respectively. Two measurements performed on a single zircon grain yielded a concordant 206Pb/238U date of 361 ± 4 Ma and a discordant 206Pb/238U date of 354 ± 4 Ma (estimated common Pb content: 0.4%). Both also featured markedly lower Th/U ratios (0.02 and 0.04, respectively) than grains yielding 206Pb/238U dates of c. 545 Ma (always >0.15).

5.3 Lu–Hf isotopes

Twenty-eight measurements were performed on grain domains analogous to those that previously yielded concordant 206Pb/238U dates (at ca. 545 Ma) and three on undated grains texturally identical to the main zircon population. Initial Hf isotope compositions were calculated using the intrusion age determined for each sample. The 176Hf/177Hf(t) ratios of RV-1 zircons were all identical within uncertainty, ranging from 0.282709 ± 42 to 0.282752 ± 35 (at 2 s), corresponding to εHf(t) of +9.5 to +11.0 with an average value of +10.5 ± 0.9 (2 S.D. − standard deviation; n = 14; Fig. 5c). Zircons from sample RV-2 showed a similar range of 176Hf/177Hf(t) ratios, from 0.282717 ± 33 to 0.282787 ± 44 (2 S.E.) corresponding to εHf (t) of +9.7 to +12.2 and yielding an identical average εHf (t) at +10.6 ± 1.2 (2 S.D.; n = 18).

thumbnail Fig. 6

Results of mass balance calculations performed by least-square regression considering the average composition of felsic gneisses as representative of the intrusive low-K dacitic melt. Samples LY25-, RV-1 and RV-2 are modelled as residual melts, while the remaining samples are considered cumulates. Σr2 represents the sum of squared residuals.

Résultats des calculs de bilan de masse réalisés par régression des moindres carrés en considérant la composition moyenne des gneiss comme représentative du liquide dacitique initial. Les échantillons LY25-, RV-1 et RV-2 sont modélisés comme des liquides résiduels alors que les autres sont considérés comme des cumulats.

6 Discussion

6.1 Interpretation of U–Pb data

Field relationships, petrographic, and geochemical data indicate that the amphibole-bearing felsic gneisses represent former tonalites (see Sect. 4). Zircon grains show euhedral shapes and oscillatory zoning, as typically observed in zircon grown from melts (Corfu et al., 2003). Hence, the Concordia dates of 545.9 ± 1.9 (6.4) and 543.9 ± 1.5 (6.3) Ma are interpreted as late Ediacaran crystallization ages of the (meta)tonalites RV-1 and RV-2, respectively. The other, discordant, zircon U-Pb data for both samples can be explained by a combination of initial Pb incorporation and/or Pb loss from zircons that originally belonged to the dominant population of concordant, c. 545 Ma-old grains. This interpretation is consistent with the fact that all zircons show identical 176Hf/177Hf ratios within uncertainties regardless the apparent 206Pb/238U date, as disturbance of the U–Pb system does not affect Hf isotopes (Gerdes and Zeh, 2009).

Analyses a65 and a66 were both performed close to the rims of the same zircon grain and showed younger 238U/206Pb dates of c. 360 Ma, concordant for a65. CL images do not provide clear evidence for the existence of textural discontinuities associated with new zircon growth or extensive zircon recrystallization. Yet, both analyses show very low Th/U ratios, different from the main c. 545 Ma population. This suggests that upon laser ablation both analyses may have (partly or wholly) sampled narrow CL-bright overgrowths or recrystallized domains visible on a few grains (Fig. 5a). Therefore, it is unclear whether the c. 360 Ma dates correspond to the age of a Variscan metamorphic overprint, which would be in agreement with what is observed in the Upper Gneiss Unit of the eFMC (Chelle-Michou et al., 2017), or represent geologically meaningless, mixed ages.

6.2 Typology of the mafic–felsic association

At the scale of the quarry outcrop (approximately 50 m), the (meta)tonalites exhibit a range of major element compositions yet very similar mineral assemblages. Since they derive from a plutonic protolith, these characteristics may reflect different proportions between igneous minerals, resulting from variable extents of crystal accumulation versus crystal/melt separation during solidification of a mush at the emplacement site (e.g. Barnes et al., 2016; Cornet et al., 2022; Laurent et al., 2020; Lee and Morton, 2015). By considering a crystallizing low-K dacitic melt with a chemical composition similar to the average of the (meta)tonalites, the samples can be modeled as a combination of (Fig. 6): (i) residual melts resulting from the separation of 5–20 wt.% plagioclase An25 + hornblende ± magnetite from the initial melt (for samples LY25, RV-1, RV-2); (ii) cumulates, corresponding to the initial melt plus 5–25 wt.% accumulated plagioclase An25 ± hornblende ± magnetite (for samples LY23, −24, 26, −27, −28). Trace element data are consistent with this interpretation given the relatively gradual decrease in most trace elements with SiO2 contents.

In general, low-K dacitic melts are known to derive from mantle melting followed by fractionation of a primary H2O-rich basaltic melt or from dehydration melting of amphibolites (Beard, 1995; Stern et al., 1996). Zircon Hf isotope data indicate that the dacitic melt from which the Riverie felsic rocks originated had a composition overlapping with that of the Depleted Mantle (DM) reservoir at c. 545 Ma, ruling out an origin by melting of an old mafic (amphibolitic) crust (Fig. 5c). Therefore, we favor the formation of the dacitic melt through crystal fractionation from a primary mantle-derived melt, consistent with its weak depletion in mid-HREE (Dy to Tm) and the Eu negative anomaly, which are indicative of amphibole and plagioclase fractionation (Bédard, 2006; Nandedkar et al., 2016). The pronounced Nb–Ti negative anomaly (Fig. 4c) and the radiogenic Hf isotope composition (εHf(545Ma) of c. +11) of the low-K dacitic melt collectively indicate that its depleted mantle source had been metasomatized by fluids derived from an oceanic slab (Fig. 4e; Pearce and Peate, 1995). The presence of magnetite, the elevated amphibole Fe3+/Fetot as well as the low bulk-rock V/Ti ratios displayed by the (meta)tonalites are consistent with a parental melt having formed under relatively oxidizing conditions (Fig. 4f), which aligns with a metasomatized mantle source.

The country-rocks of the tonalitic intrusion corresponded to gabbros or basalts (now amphibolites). Those have near-primary mantle melt compositions which entails that their negative Nb anomalies can be regarded as pristine and not related to crustal contamination (Fig. 4e). Therefore, the igneous protolith of the amphibolites originated from a mantle having incorporated a slab-derived component, as for the parental melt of the (meta)tonalites, meaning that both mafic and felsic rocks were possibly generated from the same metasomatized mantle source.

thumbnail Fig. 7

(a) Paleogeography at 545 Ma based on the full-plate model of Merdith et al. (2021), drawn using Python 3 with opensource packages pyGMT and pyGplates. The yellow star depicts the putative location of the terrains today exposed in Upper Gneiss Unit (UGU) of the eFMC. (b) Geodynamic sketch of the northern Gondwana margin at 545 Ma highlighting the context and mechanism through which the Riverie rocks formed, inspired from Erdős et al. (2022).

(a) Paléogéographie à 545 Ma basée sur le modèle de plaque de Merdith et al. (2021). L’étoile indique l’emplacement supposé des terrains correspondant aujourd’hui à l’Unité Supérieure des Gneiss. (b) Schéma géodynamique de la marge nord du Gondwana à 545 Ma, soulignant le contexte et le mécanisme par lesquels les roches de Riverie se sont formées.

6.3 Geodynamic setting of the Riverie magmatism

As demonstrated in the previous section, the felsic rocks from the Riverie belt can be traced back to a mantle metasomatized by slab-derived fluids shortly prior to melting, namely in the Ediacaran period. At that time, the terrains currently exposed in the eFMC were situated along the northern Gondwana margin, which was significantly influenced by the Avalonian–Cadomian accretionary orogeny (Andonaegui et al., 2016; Collett et al., 2020; Garfunkel, 2015; Linnemann et al., 2014, 2008; Soejono et al., 2017). Therefore, it is likely that the Riverie magmatism tapped into a mantle imprinted by Cadomian subduction. In fact, the Riverie rocks share isotopic similarities with the late Cryogenian–early Ediacaran juvenile arc rocks of the Cadomian type-area in northern France (Samson et al., 2003; Fig. 5c).

According to full-plate tectonic models, the southwards subduction of the Iapetus/Mirovoi oceans was still ongoing at the late Ediacaran/early Cambrian transition (Fig. 7a, Merdith et al., 2021). Consistently, evidence for the existence of 0.55 Ga juvenile arcs along the northern Gondwana margin is provided by the detrital zircon record of the parautochtonous terrains in the eFMC (Couzinié et al., 2019), the Corsica basement (Avigad et al., 2018), the Upper Allochton of the Iberian Massif (Albert et al., 2015), and the Polish Góry Sowie Massif (Tabaud et al., 2021). Additionally, in the Ossa-Morena Zone of the Iberian Massif, juvenile arc-derived magmatism is represented by andesites–dacites interbedded within the late Ediacaran/early Cambrian Malcocinado/San Jerónimo formations and diorites–tonalites from the c. 555 Ma-old Mérida–Montoro igneous complex (Bandres et al., 2002; Pin et al., 2002). Both suites exhibit positive εNd(t) values along with trace element characteristics that resemble the Riverie (meta)tonalites.

In the Maures massif, the Bormes orthogneiss was recently regarded as a remnant of an Ediacaran (590–556 Ma) arc by Tabaud et al. (2023). However, the presence in their samples of 505–460 Ma-old zircon rims together with the major and trace element signature of the peraluminous high-K host rocks rather suggest that they represent Cambrian–Ordovician crust-derived granitoids related to the rifting event that led to the opening of the Variscan oceans (e.g. Couzinié et al., 2022). Arc magmatism in the Maures massif is more likely represented by the Arcs–Gassin leptynites, which have εNd(t) values ranging from +1 to +2 and yielded a protolith emplacement age of 548 +15/-7 Ma (U–Pb on zircon; Innocent et al., 2003). As this age was obtained by isotope-dilution thermal ionization mass spectrometry on multi-grain fractions, it deserves to be confirmed using in situ techniques (Paquette et al., 2017).

The Riverie rocks represent modest volumes and their V/Ti systematics suggest a distal position with respect to the Cadomian arc (Fig. 4f). Therefore, we infer that this juvenile magmatism would have occurred within the back-arc domain of the Cadomian orogen and resulted from the convective thinning of the lithospheric mantle (Fig. 7b).

6.4 Implications for the architecture of the eFMC

The Riverie mafic–felsic association represents the first unambiguous record of late Ediacaran magmatism in the Upper Gneiss Unit of the eFMC and is distinctly different from coeval magmatic rocks in the parautochtonous terrains (Fig. 4). In the latter, the c. 550–530 Ma period is marked by voluminous peraluminous felsic magmatism, which corresponds today to the Velay Orthogneiss Formation (Couzinié et al., 2017), the Montredon-Labessonié orthogneiss (Couzinié and Laurent, 2021), the Plaisance–Cammazes orthogneiss (Guérangé-Lozes et al., 2013) and the Rivernous and Sériès dacites–rhyolites in the Montagne Noire area (Lescuyer and Cocherie, 1992; Padel et al., 2017). Collectively, the geochemical (major/trace element and isotopic) signatures (Fig. 4) and zircon inheritance patterns exhibited by these rocks indicate significant crust reworking through the melting of Neoproterozoic sedimentary sequences (Chelle-Michou et al., 2017; Couzinié et al., 2017). The absence of evidence for late Ediacaran compressional deformation (Álvaro et al., 2014) suggests that this crustal melting event took place in an extensional environment, presumably in the back-arc of the Cadomian orogen (Couzinié and Laurent, 2021). The very contrasted typology of late Ediacaran magmatism between the Upper Gneiss Unit and the parautochtonous terrains indicates that they represent two distinct crust segments which had different paleogeographic positions within the late Ediacaran Cadomian back-arc.

The age of magmatism observed in the Riverie belt is unusual compared to other eFMC LACs. The compilation of LA-ICP-MS zircon U-Pb dates obtained from LACs’ meta-igneous rocks (Fig. 5d) indicates that magmatism mostly occurred between 492 and 472 Ma i.e., the Furongian–Lower Ordovician epochs (Chelle-Michou et al., 2017; de Hoÿm de Marien et al., 2023; Lotout et al., 2020, 2018; Paquette et al., 2017; Whitney et al., 2020). However, the rocks from the Riverie belt are at least 50 million years older, dating back to the late Ediacaran period. Furthermore, the meta-igneous felsic rocks of the LACs are generally potassic and display time-integrated unradiogenic Nd-Hf isotope compositions (εNd(t) < −3; εHf(t) < −1), which is interpreted as reflecting an origin through melting of the local continental crust (Chelle-Michou et al., 2017; Pin and Marini, 1993). In contrast, the felsic rocks in the Riverie belt are sodic, have radiogenic Hf isotope compositions, and ultimately originated from a juvenile source. The pressure and temperature of mineral equilibration estimated for the Riverie felsic rocks are close to the tonalite solidus (Schmidt and Thompson, 1996) at relatively low pressures (2–4 kbar), possibly reflecting the crystallization conditions of the parental low-K dacitic melt. These rocks do not record evidence for a high-pressure metamorphic evolution, and no eclogite has ever been recovered from the Riverie belt (Blanc, 1981; Feybesse et al., 1995). Collectively, petrographic, geochemical (including isotopic) and geochronological data obtained on the Riverie belt show that the igneous protoliths of this LAC did not form during the rifting of the northern Gondwana margin that resulted in the opening of the Mid-Variscan ocean and were presumably not subducted during the Devonian. These results highlight that all LACs from the Monts-du-Lyonnais, and more generally from the eFMC, are not identical in terms of age, petrogenesis and metamorphic evolution, arguing against their grouping as a single litho-tectonic unit.

7 Conclusion

The amphibole-bearing felsic gneisses from the Riverie LAC in the Monts-du-Lyonnais metamorphic complex represent a former tonalitic intrusion of late Ediacaran age (c. 545 Ma). The parental melt underwent variable amounts (5–25 wt.%) of crystal accumulation versus crystal/melt separation at the emplacement site and formed through fractionation of a mafic melt that originated from a mantle source metasomatized by slab-derived fluids. The (meta)tonalites are intimately associated with mafic (meta-)igneous rocks, which were likely derived from the same source. The Riverie mafic–felsic association is unrelated to the Furongian (late Cambrian)–Lower Ordovician rifting that resulted in the opening of the Mid-Variscan ocean and instead represents a newly identified remnant of Cadomian arc/back-arc magmatism along the northern Gondwana margin. Overall, our results challenge the assumption that all mafic–felsic associations (LACs) from the high-grade domains of the eFMC can be grouped together as a single litho-tectonic unit (“the” LAC). A detailed reexamination of undated LACs in the eFMC (e.g. the Artense, Sioule, Truyère, Marvejols massifs) using modern techniques is essential for clarifying the diversity of these associations and refining their correlations.

Acknowledgments

C. Guilbaud made the thin sections. G. Stevens and A. Gerdes granted access to the Stellenbosch CAF and Frankfurt LA–ICP–MS laboratory, respectively. A. Laurie, M. R. Franzenburg and L. Marko assisted during the SEM and LA–ICP–MS sessions. O. Vanderhaeghe provided a template for Figure 1. L.-S. Doucet drew Figure 7a. H. Bertrand introduced the quarry to SC in 2011. Thorough reviews were provided by F. Roger and an anonymous reviewer. The manuscript was handled by V. Bosse and L. Jolivet. We extend our gratitude to all of them.

This work is dedicated to the memory of J.-L. Paquette, our collaborator for a decade, for all his input on topics related to the eFMC, geochronology and granitoids.

Supplementary Material

Table S1: Whole-rock major and trace element compositions of samples from the Riverie belt Analyses performed by ALS Global

Table S2: Representative mineral analyses of the amphibole-bearing felsic gneisses RV-1 & RV-2

Table S3: Results of LA-ICP-MS U–Pb analyses of zircon standards performed during the sessions at GUF

Table S4: Results of LA-ICP-MS zircon U–Pb analyses performed during the sessions at GUF

Table S5: Results of LA-MC-ICP-MS Lu–Hf analyses of zircon standards performed during the session at GUF

Table S6: Results of LA-MC-ICP-MS Lu–Hf analyses of zircon performed during the session at GUF

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References

  • Albert R, Arenas R, Gerdes A, Sánchez Martínez S, Fernández-Suárez J, Fuenlabrada JM. 2015. Provenance of the Variscan Upper Allochthon (Cabo Ortegal Complex, NW Iberian Massif). Gondwana Res 28: 1434–1448. [CrossRef] [Google Scholar]
  • Álvaro JJ, Bauluz B, Clausen S, Devaere L, Gil Imaz A, Monceret É, Vizcaïno D. 2014. Stratigraphic review of the Cambrian-Lower Ordovician volcanosedimentary complexes from the northern Montagne Noire, France. Stratigraphy 11: 83–96. [Google Scholar]
  • Andonaegui P, Arenas R, Albert R, Sánchez Martínez S, Díez Fernández R, Gerdes A. 2016. The last stages of the Avalonian-Cadomian arc in NW Iberian Massif: isotopic and igneous record for a long-lived peri-Gondwanan magmatic arc. Tectonophysics 681: 6–14. [CrossRef] [Google Scholar]
  • Avigad D, Rossi Ph, Gerdes A, Abbo A. 2018. Cadomian metasediments and Ordovician sandstone from Corsica: detrital zircon U-Pb-Hf constrains on their provenance and paleogeography. Int J Earth Sci (Geol Rundsch) 107: 2803–2818. [CrossRef] [Google Scholar]
  • Bandres A, Eguı́luz L, Gil Ibarguchi JI, Palacios T. 2002. Geodynamic evolution of a Cadomian arc region: the northern Ossa-Morena zone, Iberian massif. Tectonophysics 352: 105–120. [CrossRef] [Google Scholar]
  • Barnes CG, Coint N, Yoshinobu A. 2016. Crystal accumulation in a tilted arc batholith. Am Mineralog 101: 1719–1734. [CrossRef] [Google Scholar]
  • Beard JS. 1995. Experimental, geological, and geochemical constraints on the origins of low-K silicic magmas in oceanic arcs. J Geophys Res: Solid Earth 100: 15593–15600. [CrossRef] [Google Scholar]
  • Bédard JH. 2006. Trace element partitioning in plagioclase feldspar. Geochim Cosmochim Acta 70: 3717–3742. [CrossRef] [Google Scholar]
  • Bellot J-P., Laverne C, Bronner G. 2010. An early Palaeozoic supra-subduction lithosphere in the Variscides: new evidence from the Maures massif. Int J Earth Sci (Geol Rundsch) 99: 473–504. [CrossRef] [Google Scholar]
  • Benmammar A, Berger J, Triantafyllou A, Duchene S, Bendaoud A, Baele J-M., Bruguier O, Diot H. 2020. Pressure-temperature conditions and significance of Upper Devonian eclogite and amphibolite facies metamorphisms in southern French Massif central. Bull Soc Géolog France 191: 28. [CrossRef] [EDP Sciences] [Google Scholar]
  • Blanc D. 1981. Les roches basiques et ultrabasiques des Monts du Lyonnais. Etude pétrographique, minéralogique et géochimique (PhD thesis). [Google Scholar]
  • Bodinier JL, Giraud A, Dupuy C, Leyreloup A, Dostal J. 1986. Caractérisation géochimique des métabasites associées à la suture méridionale hercynienne: Massif Centra français et Chamrousse (Alpes). Bull Soc Géolog France 8: 115–123. [CrossRef] [Google Scholar]
  • Bouchez JL, Jover O. 1986. Le Massif Central: un chevauchement de type himalayen vers l’Ouest-Nord-Ouest. Compt Rendus Acad Sci 302: 675–680. [Google Scholar]
  • Boynton WV, 1984. Cosmochemistry of the rare earth elements: meteorite studies, in: Henderson, P. (Ed.), Rare Earth Element Geochemistry. Amsterdam: Elsevier, pp. 63-114. [Google Scholar]
  • Briand B, Bouchardon, J.-L., Capiez P, Piboule M, 2002. Felsic (A-type)-basic (plume-induced) Early Palaeozoic bimodal magmatism in the Maures Massif (southeastern France). Geolog Mag 139. https://doi.org/10.1017/s0016756802006477 [Google Scholar]
  • Briand B, Bouchardon J-L., Ouali H, Piboule M, Capiez P. 1995. Geochemistry of bimodal amphibolite-felsic gneiss complexes from eastern Massif Central, France. Geolog Mag 132: 321–337. [CrossRef] [Google Scholar]
  • Briand B, Bouchardon J-L., Santallier D, Piboule M, Ouali H, Capiez P. 1992. Alkaline affinity of the metabasites in the gneissic series surrounding the Velay migmatitic domain. Géol France 2: 9–15. [Google Scholar]
  • Briand B, Piboule M, Santallier D, Bouchardon J-L. 1991. Geochemistry and tectonic implications of two Ordovician bimodal igneous complexes, southern French Massif Central. J Geolog Soc London 148: 959–971. [CrossRef] [Google Scholar]
  • Burg J-P., Leyreloup AF, Marchand J, Matte P. 1984. Inverted metamorphic zonation and large scale thrusting in the Variscan belt: an example in the French Massif central. J Geolog Soc London 14: 47–61. [CrossRef] [Google Scholar]
  • Burg J-P., Matte P. 1978. A cross section through the French Massif Central and the scope of its Variscan geodynamic evolution. Z. dt. geol. Ges. 129: 429–460. [Google Scholar]
  • Chantraine J, Autran A, Cavelier C, Bureau de recherches g??ologiques et mini??res (France). 2003. Carte géologique de la France à l’échelle du millionième, 6ème édition révisée, BRGM. ed. Orléans. [Google Scholar]
  • Chelle-Michou C, Laurent O, Moyen J-F., Block S, Paquette J-L., Couzinié S, Gardien V, Vanderhaeghe O, Villaros A, Zeh A. 2017. Pre-Cadomian to late-Variscan odyssey of the eastern Massif Central, France: Formation of the West European crust in a nutshell. Gondwana Res 46: 170–190. [CrossRef] [Google Scholar]
  • Collett S, Schulmann K, Štípská P, Míková J. 2020. Chronological and geochemical constraints on the pre-variscan tectonic history of the Erzgebirge, Saxothuringian Zone. Gondwana Res 79: 27–48. [CrossRef] [Google Scholar]
  • Corfu F, Hanchar JM, Hoskin PWO, Kinny PD. 2003. Atlas of zircon textures. Rev Mineral Geochem 53: 469–500. [CrossRef] [Google Scholar]
  • Cornet J, Bachmann O, Ganne J, Fiedrich A, Huber C, Deering CD, Feng X. 2022. Assessing the effect of melt extraction from mushy reservoirs on compositions of granitoids: from a global database to a single batholith. Geosphere 18: 985–999. [CrossRef] [Google Scholar]
  • Costa S. 1989. Age radiométrique 39Ar-40Ar dating of the Barrovian metamorphism of the Lot Valley series associated with the Marvejols nappe emplacement. Compt Rendus Acad Sci 309: 561–567. [Google Scholar]
  • Costa S, Maluski H, Lardeaux JM. 1993. 40Ar-39Ar chronology of Variscan tectono-metamorphic events in an exhumed crustal nappe: the Monts du Lyonnais complex (Massif Central, France). Chem Geol 105: 339–359. [CrossRef] [Google Scholar]
  • Couzinié S, Bouilhol P, Laurent O, Grocolas T, Montel J-M. 2022. Cambro-Ordovician ferrosilicic magmatism along the northern Gondwana margin: constraints from the Cézarenque-Joyeuse gneiss complex (French Massif Central). BSGF − Earth Sciences Bulletin 193. [Google Scholar]
  • Couzinié S, Laurent O. 2021. Zircon U-Pb dating of the Montredon-Labessonnié orthogneiss by LA-ICP-MS: new evidence for late Ediacaran crustal melting in the French Massif Central. Géol France 1: 24–31. [Google Scholar]
  • Couzinié, S., Laurent O, Chelle-Michou C, Bouilhol P, Paquette, J.-L., Gannoun, A.-M., Moyen, J.-F. 2019. Detrital zircon U-Pb-Hf systematics of Ediacaran metasediments from the French Massif Central: consequences for the crustal evolution of the north Gondwana margin. Precambrian Res 324: 269–284. [CrossRef] [Google Scholar]
  • Couzinié S, Laurent O, Poujol M, Mintrone M, Chelle-Michou C, Moyen J-F., Bouilhol P, Vezinet A, Marko L. 2017. Cadomian S-type granites as basement rocks of the Variscan belt (Massif Central, France): implications for the crustal evolution of the north Gondwana margin. Lithos 286-287: 16–34. [CrossRef] [Google Scholar]
  • de Hoÿm de Marien L, Pitra P, Poujol M, Cogné N, Cagnard F Le Bayon B. 2023. Complex geochronological record of an emblematic Variscan eclogite (Haut-Allier, French Massif Central). J Metamorph Geol 41: 967–995. [CrossRef] [Google Scholar]
  • Debon F, Le Fort P. 1983. A chemical-mineralogical classification of common plutonic rocks and associations. Trans Royal Soc Edinburgh 73: 135–149. [CrossRef] [Google Scholar]
  • Dewey JF. 1987. Suture, in: Structural Geology and Tectonics, Encyclopedia of Earth Science. Berlin, Heidelberg: Springer, pp. 775-784. [Google Scholar]
  • Domeier M. 2016. A plate tectonic scenario for the Iapetus and Rheic oceans. Gondwana Res 36: 275–295. [CrossRef] [Google Scholar]
  • Downes H, Bodinier JL, Dupuy C, Leyreloup AF, Dostal J. 1989. Isotope and trace-element heterogeneities in high-grade basic metamorphic rocks of Marvejols: tectonic implications for the Hercynian suture zone of the French Massif Central. Lithos 24: 37–54. [CrossRef] [Google Scholar]
  • Dufour E. 1985. Granulite facies metamorphism and retrogressive evolution of the Monts du Lyonnais metabasites (Massif Central, France). Lithos 18: 97–113. [CrossRef] [Google Scholar]
  • Dufour E. 1982. Pétrologie et géochimie des formations orthométamorphiques acides des Monts du Lyonnais (Massif central, France) (PhD thesis). [Google Scholar]
  • Dufour E, Lardeaux JM, Coffrant D. 1985. Eclogites et granulites dans les Monts du Lyonnais: une évolution métamorphique plurifaciale éohercynienne. Compt Rendus Acad Sci 300: 141–144. [Google Scholar]
  • Duthou J-L., Cantagrel J-M., Didier J, Vialette Y. 1984. Palaeozoic granitoids from te French Massif Central: age and origin studied by 87$Rb-87$Sr system. Phys Earth Planet Interiors 35: 131–144. [CrossRef] [Google Scholar]
  • Duthou J-L., Chenevoy M, Gay M. 1994. Rb-Sr middle Devonian age of cordierite bearing migmatites from Lyonnais area (French Massif Central). Compt Rendus Acad Sci 319: 791–796. [Google Scholar]
  • Duthou J-L., Piboule M, Gay M, Dufour E. 1981. Datations radiométriques Rb-Sr sur les orthogranulites des Monts du Lyonnais (Massif Central français). Compt Rendus Acad Sci 292: 749–752. [Google Scholar]
  • Edel JB, Schulmann K, Lexa O, Lardeaux JM. 2018. Late Palaeozoic palaeomagnetic and tectonic constraints for amalgamation of Pangea supercontinent in the European Variscan belt. Earth-Sci Rev 177: 589–612. [CrossRef] [Google Scholar]
  • Erdős Z, Huismans RS, Faccenna C. 2022. Wide versus narrow back-arc rifting: control of subduction velocity and convective back-arc thinning. Tectonics 41: e2021TC007086. [CrossRef] [Google Scholar]
  • Faure M, Lardeaux J-M., Ledru P. 2009. A review of the pre-Permian geology of the Variscan French Massif Central. Comptes Rendus Geoscience 341: 202–213. [CrossRef] [Google Scholar]
  • Feybesse J-L., Lardeaux JM, Tegyey M, Peterlongo J-M., Kerrien Y, Lemière B, Maurin G, Mercier F, Thiéblemont D. 1995. Notice explicative, Carte géol. France (1/50 000), feuille Saint-Symphorien-sur-Coise (721). BRGM, Orléans. [Google Scholar]
  • Franke W, Cocks LRM, Torsvik TH. 2017. The Palaeozoic Variscan oceans revisited. Gondwana Res 48: 257–284. [CrossRef] [Google Scholar]
  • Gardien V, Lardeaux JM, Ledru P, Allemand P, Guillot S. 1997. Metamorphism during late orogenic extension: insights from the French Variscan belt. Bull Soc Géolog France 168: 271–286. [Google Scholar]
  • Gardien V, Martelat J-E., Lelou P-H Mahéo G, Bevillard B, Allemand P, Monié P, Paquette J-L., Grosjean A-S., Faure M, Chelle-Michou C, Fellah C. 2022. Fast exhumation rate during late orogenic extension: The new timing of the Pilat detachment fault (French Massif Central, Variscan belt). Gondwana Res 103: 260–275. [CrossRef] [Google Scholar]
  • Gardien V, Teygey M, Lardeaux JM, Misseri M, Dufour E. 1990. Crust-mantle relationships in the French Variscan chain: the example of the Southern Monts du Lyonnais unit (eastern French Massif Central). J Metamorph Geol 8: 477–492. [CrossRef] [Google Scholar]
  • Garfunkel Z. 2015. The relations between Gondwana and the adjacent peripheral Cadomian domain—constrains on the origin, history, and paleogeography of the peripheral domain. Gondwana Res 28: 1257–1281. [CrossRef] [Google Scholar]
  • Gerdes A, Zeh A. 2009. Zircon formation versus zircon alteration — new insights from combined U-Pb and Lu-Hf in-situ LA-ICP-MS analyses, and consequences for the interpretation of Archean zircon from the Central Zone of the Limpopo Belt. Chem Geol 261: 230–243. [CrossRef] [Google Scholar]
  • Giraud A, Marchand J, Dupuy C, Dostal J. 1984. Geochemistry of leptyno-amphibolite complex from Haut Allier (French Massif Central). Lithos 17: 203–214. [CrossRef] [Google Scholar]
  • Griffin WL, Wang X, Jackson SE, Pearson NJ, O’Reilly SY, Xu X-S., Zhou X. 2002. Zircon chemistry and magma mixing, SE China: In-situ analysis of Hf isotopes, Tonglu and Pingtan igneous complexes. Lithos 61: 237–269. [CrossRef] [Google Scholar]
  • Guérangé-Lozes J, Demange M, Mouline M. 2013. Notice explicative, Carte géol. France (1/50 000), feuille Castres (986). BRGM, Orléans. [Google Scholar]
  • Holland T, Blundy J. 1994. Non-ideal interactions in calcic amphiboles and their bearing on amphibole-plagioclase thermometry. Contributions to Mineralogy and Petrology. [Google Scholar]
  • Innocent C, Michard A, Guerrot C, Hamelin B. 2003. Datation U-Pb sur zircons à 548 Ma de leptynites des Maures centrales. Signification géodynamique des complexes leptyno-amphibolitiques de l’Europe varisque. Bull Soc Géolog France 174: 585–594. [Google Scholar]
  • Janoušek V, Farrow CM, Erban V. 2006. Interpretation of whole-rock geochemical data in igneous geochemistry: introducing geochemical data toolkit (GCDkit). J Petrol 47: 1255–1259. [CrossRef] [Google Scholar]
  • Joanny V, van Roermund H, Lardeaux JM. 1991. The clinopyroxene/plagioclase symplectite in retrograde eclogites: a potential geothermobarometer. Geol Rundsch 80: 303–320. [CrossRef] [Google Scholar]
  • Jouffray F, Lardeaux J-M., Tabaud A-S., Corsini M, Schneider J. 2023. Deciphering the nature and age of the protoliths and peak P−T conditions in retrogressed mafic eclogites from the Maures-Tannneron Massif (SE France) and implications for the southern European Variscides. BSGF − Earth Sci Bull 194: 10. [CrossRef] [EDP Sciences] [Google Scholar]
  • Keppie JD, Nance RD, Murphy JB, Dostal J, Braid JA. 2010. The high-pressure Iberian-Czech belt in the Variscan orogen: extrusion into the upper (Gondwanan) plate? Gondwana Res 17: 306–316. [CrossRef] [Google Scholar]
  • Kroner U, Romer RL. 2013. Two plates — many subduction zones: the Variscan orogeny reconsidered. Gondwana Res 24: 298–329. [CrossRef] [Google Scholar]
  • Lardeaux J-M. 2023. Metamorphism and linked deformation in understanding tectonic processes at varied scales. Comptes Rendus Géosci 356: 1–25. [Google Scholar]
  • Lardeaux JM, Ledru P, Daniel I, Duchene S. 2001. The Variscan French Massif Central − a new addition ot the ultra-high pressure metamorphic “club”: exhumation porcesses and geodynamic consequences. Tectonophysics 332: 143–167. [CrossRef] [Google Scholar]
  • Lardeaux JM, Schulmann K, Faure M, Janousek V, Lexa O, Skrzypek E, Edel JB, Stipska P. 2014. The Moldanubian Zone in the French Massif Central, Vosges/Schwarzwald and Bohemian Massif revisited: differences and similarities. Geolog Soc London Spec Publicat 405: 7–44. [CrossRef] [Google Scholar]
  • Laurent O, Björnsen J, Wotzlaw J-F., Bretscher S, Pimenta Silva M, Moyen J-F., Ulmer P, Bachmann O. 2020. Earth’s earliest granitoids are crystal-rich magma reservoirs tapped by silicic eruptions. Nat Geosci 13: 163–169. [CrossRef] [Google Scholar]
  • Ledru P, Courrioux G, Dallain C, Lardeaux JM, Montel JM, Vanderhaeghe O, Vitel G. 2001. The Velay dome (French Massif Central): melt generation and granite emplacement during orogenic evolution. Tectonophysics 342: 207–237. [CrossRef] [Google Scholar]
  • Ledru P, Lardeaux JM, Santallier D, Autran A, Quenardel J-M., Floc’h J-P., Lerouge G, Maillet N, Marchand J, Ploquin A. 1989. Where are the nappes in the French Massif central? Bull Soc Géolog France 8: 605–618. [CrossRef] [Google Scholar]
  • Lee C-TA, Morton DM. 2015. High silica granites: terminal porosity and crystal settling in shallow magma chambers. Earth Planet Sci Lett 409: 23–31. [CrossRef] [Google Scholar]
  • Lescuyer J-L., Cocherie A. 1992. Single-zircon dating of the Sériès meta-dacites: evidence for a Late Proterozoic age of the “X Schists” from Montagne Noire (Southern French Massif Central). Compt Rendus Acad Sci 314: 1071–1077. [Google Scholar]
  • Li X, Zhang C, Behrens H, Holtz F. 2020a. Calculating amphibole formula from electron microprobe analysis data using a machine learning method based on principal components regression. Lithos 362-363: 105469. [CrossRef] [Google Scholar]
  • Li X, Zhang C, Behrens H, Holtz F. 2020b. Calculating biotite formula from electron microprobe analysis data using a machine learning method based on principal components regression. Lithos 356-357: 105371. [CrossRef] [Google Scholar]
  • Linnemann U, Gerdes A, Hofmann M, Marko L. 2014. The Cadomian Orogen: Neoproterozoic to Early Cambrian crustal growth and orogenic zoning along the periphery of the West African Craton—Constraints from U-Pb zircon ages and Hf isotopes (Schwarzburg Antiform, Germany). Precambrian Res 244: 236–278. [CrossRef] [Google Scholar]
  • Linnemann U, Pereira F, Jeffries TE, Drost K, Gerdes A. 2008. The Cadomian Orogeny and the opening of the Rheic Ocean: the diacrony of geotectonic processes constrained by LA-ICP-MS U-Pb zircon dating (Ossa-Morena and Saxo-Thuringian Zones, Iberian and Bohemian Massifs). Tectonophysics 461: 21–43. [CrossRef] [Google Scholar]
  • Lotout C, Pitra P, Poujol M, Anczkiewicz R, Van Den Driessche J. 2018. Timing and duration of Variscan high-pressure metamorphism in the French Massif Central: a multimethod geochronological study from the Najac Massif. Lithos 308-309: 381–394. [CrossRef] [Google Scholar]
  • Lotout C, Poujol M, Pitra P, Anczkiewicz R, Van Den Driessche J. 2020. From Burial to Exhumation: emplacement and metamorphism of Mafic Eclogitic Terranes constrained through multimethod petrochronology, case study from the Lévézou Massif (French Massif Central, Variscan Belt). J Petrol 61. https://doi.org/10.1093/petrology/egaa046 [CrossRef] [Google Scholar]
  • Maierová P, Štípská P, Gerya T, Lexa O. 2021. Trans-lithospheric diapirism explains the presence of ultra-high pressure rocks in the European Variscides. Commun Earth Environ 2. https://doi.org/10.1038/s43247-021-00122-w [CrossRef] [Google Scholar]
  • Matte P. 1986. Tectonics and plate tectonics model for the Variscan belt of Europe. Tectonophysics 126: 329–374. [CrossRef] [Google Scholar]
  • Merdith AS, Williams SE, Collins AS, Tetley MG, Mulder JA, Blades ML, Young A, Armistead SE, Cannon J, Zahirovic S, Müller RD. 2021. Extending full-plate tectonic models into deep time: linking the Neoproterozoic and the Phanerozoic. Earth-Sci Rev 214: 103477. [CrossRef] [Google Scholar]
  • Molina JF, Moreno JA, Castro A, Rodríguez C, Fershtater GB. 2015. Calcic amphibole thermobarometry in metamorphic and igneous rocks: New calibrations based on plagioclase/amphibole Al-Si partitioning and amphibole/liquid Mg partitioning. Lithos 232: 286–305. [CrossRef] [Google Scholar]
  • Moyen JF, Laurent O, Chelle-Michou C, Couzinié, S., Vanderhaeghe O, Zeh A, Villaros A, Gardien V. 2017. Collision vs. subduction-related magmatism: Two contrasting ways of granite formation and implications for crustal growth. Lithos 277: 154–177. [CrossRef] [Google Scholar]
  • Naeraa T, Schersten A, Rosing MT, Kemp AI, Hoffmann JE, Kokfelt TF, Whitehouse MJ. 2012. Hafnium isotope evidence for a transition in the dynamics of continental growth 3.2 Gyr ago. Nature 485: 627–630. [CrossRef] [Google Scholar]
  • Nandedkar RH, Hürlimann N, Ulmer P, Müntener O. 2016. Amphibole-melt trace element partitioning of fractionating calc-alkaline magmas in the lower crust: an experimental study. Contrib Mineral Petrol 171: 71. [CrossRef] [Google Scholar]
  • Ohta T, Arai H. 2007. Statistical empirical index of chemical weathering in igneous rocks: a new tool for evaluating the degree of weathering. Chem Geol 240: 280–297. [CrossRef] [Google Scholar]
  • Padel M, Álvaro JJ, Clausen S, Guillot F, Poujol M, Chichorro M, Monceret É, Pereira MF, Vizcaïno D. 2017. U-Pb laser ablation ICP-MS zircon dating across the Ediacaran-Cambrian transition of the Montagne Noire, southern France. Comptes Rendus Geosci https://doi.org/10.1016/j.crte.2016.11.002 [Google Scholar]
  • Paquette J-L., Ballèvre M, Peucat J-J., Cornen G. 2017. From opening to subduction of an oceanic domain constrained by LA-ICP-MS U-Pb zircon dating (Variscan belt, Southern Armorican Massif, France). Lithos 294-295: 418–437. [CrossRef] [Google Scholar]
  • Paris F, Robardet M. 1990. Early Palaeozoic palaeobiogeography of the Variscan regions. Tectonophysics 177: 193–213. [CrossRef] [Google Scholar]
  • Pearce JA. 2008. Geochemical fingerprinting of oceanic basalts with applications to ophiolite classification and the search for Archean oceanic crust. Lithos 100: 14–48. [CrossRef] [Google Scholar]
  • Pearce JA, Peate DW. 1995. Tectonic implications of the composition of volcanic arc magmas. Annu Rev Earth Planetary Sci 23: 251–285. [CrossRef] [Google Scholar]
  • Peterlongo J-M. 1958. Les terrains cristallins des Monts du Lyonnais (Massif central français), Annales de la Faculté des Sciences de Clermont. [Google Scholar]
  • Piboule M, Briand B. 1985. Geochemistry of eclogites and associated rocks of the southeastern area of the French Massif Central: origin of the protoliths. Chem Geol 50: 189–199. [CrossRef] [Google Scholar]
  • Pin C. 1990. Variscan oceans: ages, origins and geodynamic implications inferred from geochemical and radiometric data. Tectonophysics 177: 215–227. [CrossRef] [Google Scholar]
  • Pin C, Lancelot J. 1982. U-Pb dating of an early paleozoic bimodal magmatism in the French Massif Central and of its further metamorphic evolution. Contrib Mineral Petrol 79: 1–12. [CrossRef] [Google Scholar]
  • Pin C, Liñán E, Pascual E, Donaire T, Valenzuela A. 2002. Late Neoproterozoic crustal growth in the European Variscides: Nd isotope and geochemical evidence from the Sierra de Córdoba Andesites (Ossa-Morena Zone, Southern Spain). Tectonophysics 352: 133–151. [CrossRef] [Google Scholar]
  • Pin C, Marini F. 1993. Early Ordovician continental break-up in Variscan Europe: Nd-Sr isotope and trace element evidence from bimodal igneous associations of the Southern Massif Central, France. Lithos 29: 177–196. [CrossRef] [Google Scholar]
  • Pitra P, Poujol M, Van Den Driessche J, Bretagne E, Lotout C, Cogné N. 2022. Late Variscan (315 Ma) subduction or deceptive zircon REE patterns and U-Pb dates from migmatite-hosted eclogites? (Montagne Noire, France). J Metamorph Geol 40: 39–65. [CrossRef] [Google Scholar]
  • Pouclet A, Álvaro JJ, Bardintzeff J-M., Imaz AG, Monceret E, Vizcaïno D. 2017. Cambrian-early Ordovician volcanism across the South Armorican and Occitan domains of the Variscan Belt in France: continental break-up and rifting of the northern Gondwana margin. Geosci Front 8: 25–64. [CrossRef] [Google Scholar]
  • Rudnick RL, Gao S. 2003. Composition of the continental crust, in: Rudnick, R.L. (Ed.), The Crust, Treatise on Geochemistry. Oxford: Elsevier-Pergamon, pp. 1-64. [Google Scholar]
  • Samson SD, D’Lemos RS, Blichert-Toft J, Vervoort J. 2003. U-Pb geochronology and Hf-Nd isotope compositions of the oldest Neoproterozoic crust within the Cadomian orogen: new evidence for a unique juvenile terrane. Earth Planet Sci Lett 208: 165–180. [CrossRef] [Google Scholar]
  • Santallier D, Briand B, Ménot RP, Piboule M. 1988. Les complexes leptyno-amphioliques (C.L.A.): revue critique et suggestions pour un meilleur emploi de ce terme. Bull Soc Géolog France 8: 3–12. [CrossRef] [Google Scholar]
  • Schmidt MW, Thompson AB. 1996. Epidote in calcalkaline magmas; an experimental study of stability, phase relationships, and the role of epidote in magmatic evolution. Am Mineralog 81: 462–474. [CrossRef] [Google Scholar]
  • Schneider J, Corsini M, Reverso-Peila A, Lardeaux JM. 2014. Thermal and mechanical evolution of an orogenic wedge during Variscan collision: an example in the Maures-Tanneron Massif (SE France) Geological Society, London, Special Publications 405: 313–331. [Google Scholar]
  • Schulmann K, Edel J-B., Martínez Catalán JR, Mazur S, Guy A, Lardeaux J-M., Ayarza P, Palomeras I 2022. Tectonic evolution and global crustal architecture of the European Variscan belt constrained by geophysical data. Earth-Sci Rev 234: 104195. [CrossRef] [Google Scholar]
  • Shervais JW. 1982. Ti-V plots and the petrogenesis of modern and ophiolitic lavas. Earth Planet Sci Lett 59: 101–118. [CrossRef] [Google Scholar]
  • Soejono I, Janoušek V, Žáčková, E., Sláma J, Konopásek J, Machek M, Hanžl P. 2017. Long-lasting Cadomian magmatic activity along an active northern Gondwana margin: U-Pb zircon and Sr-Nd isotopic evidence from the Brunovistulian Domain, eastern Bohemian Massif. Int J Earth Sci (Geol Rundsch) 106: 2109–2129. [CrossRef] [Google Scholar]
  • Stampfli GM, Hochard C, Vérard C, Wilhem C, vonRaumer J. 2013. The formation of Pangea. Tectonophysics 593: 1–19. [CrossRef] [Google Scholar]
  • Stern RJ, Bloomer SH, Martinez F, Yamazaki T, Harrison TM. 1996. The composition of back-arc basin lower crust and upper mantle in the Mariana Trough: a first report. Island Arc 5: 354–372. [CrossRef] [Google Scholar]
  • Sun SS, McDonough WF. 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. Geolog Soc Lond 42: 313–345. [CrossRef] [Google Scholar]
  • Tabaud AS, Lardeaux JM, Corsini M. 2023. A vestige of an Ediacaran magmatic arc in southeast France and its significance for the northern Gondwana margin. Int J Earth Sci (Geol Rundsch) 112: 925–950. [CrossRef] [Google Scholar]
  • Tabaud AS, Štípská P, Mazur S, Schulmann K, Míková J, Wong J, Sun M. 2021. Evolution of a Cambro-Ordovician active margin in northern Gondwana: geochemical and zircon geochronological evidence from the Góry Sowie metasedimentary rocks, Poland. Gondwana Res 90: 1–26. [CrossRef] [Google Scholar]
  • Van der Voo R, Briden JC, Duff A. 1980. Late Precambrian and Palaeozoic palaeomagnetism of the Atlantic-bordering continents, in: Géologie de l’Europe, Mémoires Du BRGM. pp. 203–212. [Google Scholar]
  • Vanderhaeghe O, Laurent O, Gardien V, Moyen J-F., Gébelin A, Chelle-Michou C, Couzinié S, Villaros A, Bellanger M. 2020. Flow of partially molten crust controlling construction, growth and collapse of the Variscan orogenic belt: the geologic record of the French Massif Central. BSGF https://doi.org/10.1051/bsgf/2020013 [Google Scholar]
  • Vermeesch P. 2018. IsoplotR: A free and open tool box for geochronology. Geosci Front 9: 1479–1493. [CrossRef] [Google Scholar]
  • Whitney DL, Hamelin C, Teyssier C, Raia NH, Korchinski MS, Seaton NCA, Bagley BC, von der Handt A, Roger F, Rey PF. 2020. Deep crustal source of gneiss dome revealed by eclogite in migmatite (Montagne Noire, French Massif Central). J Metamorph Geol 38: 297–327. [CrossRef] [Google Scholar]
  • Zeh A, Gerdes A. 2010. Baltica- and Gondwana-derived sediments in the Mid-German Crystalline Rise (Central Europe): implications for the closure of the Rheic ocean. Gondwana Res 17: 254–263. [CrossRef] [Google Scholar]

Cite this article as: Couzinié S, Laurent O, Bouilhol P, Chelle-Michou C, Ganzhorn A-C, Gardien V, Moyen J-F. 2024. Late Ediacaran juvenile magmatism in the Variscan Monts-du-Lyonnais metamorphic complex (Massif Central, France), BSGF - Earth Sciences Bulletin 195: 27. https://doi.org/10.1051/bsgf/2024023

All Figures

thumbnail Fig. 1

(a) Sketch map depicting the exposed Variscan domains of western Europe and the location of the inferred suture zones, based on Schulmann et al. (2022). SXT: Saxo-Thuringian suture. (b) Geological map of the eastern French Massif Central showing the main nappe architecture. Are also depicted the locations/names of “Leptynite-Amphibolite Complex” occurrences (in black), adapted from Chantraine et al. (2003).

(a) Carte simplifiée montrant les zones où affleurent les terrains varisques et les sutures supposées. (b) Carte géologique de l’Est du Massif Central français oriental localisant les zones où affleure les "complexes leptyno-amphiboliques".
In the text
thumbnail Fig. 2

Simplified geological map of the Monts-du-Lyonnais metamorphic complex, adapted from Feybesse et al. (1995).

Carte géologique simplifiée du complexe métamorphique des Monts-du-Lyonnais.
In the text
thumbnail Fig. 3

Field observations and petrography of the Riverie felsic gneisses and amphibolites. (a) Amphibole-rich clusters embedded within the felsic gneisses (black triangles) recalling assimilated mafic material. Veins of felsic rocks cutting across the amphibolite (white triangles). (b) Felsic gneisses with plagioclase–quartz leucocratic bands rimmed by coarse amphibole underlining the vertical foliation (black triangles) and isotropic zones between the amphibolite boudins (white triangles). (c) Polarized-light thin section scan showing the variation in amphibole mode typically observed in the felsic gneisses. (d,e) Polarized-light photomicrographs of felsic gneisses illustrating the main plagioclase–quartz–amphibole assemblage with biotite rimming amphibole. (f) Polarized-light photomicrograph of a mafic rock showing the equigranular plagioclase–amphibole assemblage.

Fig. 3. Observations de terrain et pétrographie des gneiss et amphibolites de Riverie. (a) Amas riches en amphibole dans les gneiss (triangles noirs) pouvant correspondre à du matériel mafique assimilé et veines de roches felsiques dans les amphibolites (triangles blancs). (b) Gneiss felsiques avec des bandes leucocratiques de plagioclase-quartz bordées par de l’amphibole soulignant la foliation verticale (triangles noirs) et des zones isotropes entre les boudins d’amphibolite (triangles blancs). (c) Variation de la proportion d’amphibole dans les gneiss. (d, e) Assemblage des gneiss à plagioclase-quartz-amphibole-biotite. (f) Assemblage équigranulaire plagioclase-amphibole des amphibolites.
In the text
thumbnail Fig. 4

Whole-rock geochemical data of the Riverie felsic gneisses and amphibolites. (a) FMW diagram of Ohta and Arai (2007) demonstrating the igneous origin of felsic gneisses. The M and F values characterize mafic and felsic rock sources, respectively. The W value quantifies the degree of weathering. The references of data for (presumably) meta-igneous from “Leptynite-Amphibolite Complexes” are available in the Supplementary Text. (b) P–Q classification diagram of Debon and Le Fort (1983). The dotted lines represent the contour encompassing 75% of the data (n=75) for the Velay orthogneiss formation, a coeval (meta)igneous suite from the parautochtonous terrains (Couzinié et al., 2017). (c) Incompatible, immobile element patterns of the Riverie meta-igneous rocks normalized to the composition of NMORBs (Sun and McDonough, 1989). (d) Chondrite-normalized (Boynton, 1984) Rare Earth Elements patterns. (e) Th/Yb vs. Nb/Yb diagram of Pearce (2008). Oceanic rocks plot within the MORB–OIB array. Magmas contaminated by the continental crust during ascent or which mantle source incorporated a slab-derived (SZ) component plot in the volcanic arc array. (f) V–Ti diagram of Shervais (1982). This diagram highlights the effects of the change in Vanadium redox state when the mantle is metasomatized by slab-derived fluids. Vanadium becomes more incompatible and resulting magmas exhibit high V/Ti ratios. The magnetite + amphibole fractionation trend is from Shervais (1982). Data for the Bulk Continental Crust are from Rudnick and Gao (2003).

Données géochimiques des roches totales des gneiss et amphibolites de Riverie. (a) Diagramme FMW démontrant l’origine ignée des gneiss. (b) Diagramme de classification P-Q de Debon et Le Fort (1983). (c) Profils d’éléments incompatibles normalisés à la composition des N-MORB. (d) Profils de terres rares normalisés aux chondrites. (e) Diagramme Th/Yb vs. Nb/Yb de Pearce (2008). Les roches océaniques se situent dans le domaine MORB-OIB. Les magmas contaminés par la croûte continentale lors de leur mise en place ou dont la source mantellique a incorporé un composant dérivé de la plaque subduite se situent dans le domaine des arcs volcaniques. (f) Diagramme V-Ti de Shervais (1982).
In the text
thumbnail Fig. 5

Zircon textural and U–Pb–Hf data for the Riverie amphibole gneisses. (a) Representative cathodoluminescence images with the locations of laser spots (white and yellow circles for U–Pb and Lu–Hf analyses respectively) indicated along with the spot name (aXX or YYa/b). The corresponding 206Pb/238U dates are quoted with 2σ uncertainty, in Ma. All displayed analyses are concordant (except those in italic). Hf isotope data are reported using the εHf calculated at the 206Pb/238U date obtained on the same zircon domain, quoted with 2σ uncertainty. The white and black triangles highlight changes in the zoning pattern and local grain recrystallization, respectively. Laser spot sizes are 30 μm for U–Pb and 40 μm for Lu–Hf. (b) Tera-Wasserburg diagrams (238U/206Pb vs. 207Pb/206Pb). Error ellipses and ages are displayed at 95% level of uncertainty. Green ellipses are those considered for Concordia age calculations. The reported MSWDs are those of concordance plus equivalence. (c) Measured εHf(t) on magmatic zircon grains, recalculated and plotted using the 206Pb/238U date. When such value was not available, the crystallization age determined for the sample was used. The range for the Depleted Mantle reservoir is bracketed by the models of Naeraa et al., (2012) and Griffin et al., (2002). The background red shading mimics the contours of the distribution of 223 zircon analyses from crust-derived felsic (meta-)igneous rock from the French Massif Central (Chelle-Michou et al., 2017; Couzinié et al., 2017; Moyen et al., 2017). These data are taken as representative of the Hf isotopic composition of the local continental crust. Data for the Armorican Massif Cadomian juvenile arc magmatism are from Samson et al., 2003. An εHf(t)–time crust evolution array calculated with an average 176Lu/177Hf ratio of 0.015 (Griffin et al., 2002) is also depicted. (d) Available LA–ICP–MS zircon U–Pb data on meta-igneous rocks from the “Leptynite–Amphibolite Complexes” in the eastern French Massif Central (represented as Kernel Density Estimates of concordant 206Pb/238U dates (calculated with IsoplotR using an adaptative bandwith). Zircon data are from Whitney et al., (2020) for the Montagne Noire, de Hoÿm de Marien et al., (2023) for the Haut-Allier, Paquette et al., 2017 for the Lot, Lotout et al., (2020, 2018); for the Rouergue, Chelle-Michou et al., (2017) for the Vivarais, and this study for the Monts-du-Lyonnais.

Données texturales et U-Pb-Hf des zircons extraits des gneiss de Riverie. (a) Images en cathodoluminescence montrant la zonation magmatique des grains et la localisation de quelques points d’analyse. (b) Diagrammes Tera-Wasserburg. Les ellipses vertes sont celles considérées pour les calculs d’âge Concordia. (c) Valeurs de εHf des grains de zircon recalculées à l’âge de mise en place. L'arrière-plan rouge représente la composition isotopique en Hf de la croûte continentale de l’Est du Massif Central. (d) Données U-Pb sur zircons obtenues pour les roches méta-ignées des "complexes leptyno-amphiboliques" de l’Est du Massif Central français.
In the text
thumbnail Fig. 6

Results of mass balance calculations performed by least-square regression considering the average composition of felsic gneisses as representative of the intrusive low-K dacitic melt. Samples LY25-, RV-1 and RV-2 are modelled as residual melts, while the remaining samples are considered cumulates. Σr2 represents the sum of squared residuals.

Résultats des calculs de bilan de masse réalisés par régression des moindres carrés en considérant la composition moyenne des gneiss comme représentative du liquide dacitique initial. Les échantillons LY25-, RV-1 et RV-2 sont modélisés comme des liquides résiduels alors que les autres sont considérés comme des cumulats.
In the text
thumbnail Fig. 7

(a) Paleogeography at 545 Ma based on the full-plate model of Merdith et al. (2021), drawn using Python 3 with opensource packages pyGMT and pyGplates. The yellow star depicts the putative location of the terrains today exposed in Upper Gneiss Unit (UGU) of the eFMC. (b) Geodynamic sketch of the northern Gondwana margin at 545 Ma highlighting the context and mechanism through which the Riverie rocks formed, inspired from Erdős et al. (2022).

(a) Paléogéographie à 545 Ma basée sur le modèle de plaque de Merdith et al. (2021). L’étoile indique l’emplacement supposé des terrains correspondant aujourd’hui à l’Unité Supérieure des Gneiss. (b) Schéma géodynamique de la marge nord du Gondwana à 545 Ma, soulignant le contexte et le mécanisme par lesquels les roches de Riverie se sont formées.
In the text

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