Outline
Comptes Rendus

Petrology, geochemistry
Early Permian extensional shearing of an Ordovician granite: The Saint-Eutrope “C/S-like” orthogneiss (Montagne Noire, French Massif Central)
Comptes Rendus. Géoscience, Volume 344 (2012) no. 8, pp. 377-384.

Abstracts

Dating the magmatic events in the Montagne Noire gneiss dome is a key point to arbitrate between the different interpretations of the Late Carboniferous–Early Permian tectonics in this southern part of the Variscan belt. The Saint-Eutrope orthogneiss crops out along the northern flank of the dome. We show that the protolith of this orthogneiss is an Ordovician granite dated at 455 ± 2 Ma (LA-ICP-MS U-Pb dating on zircon). This age is identical to that previously obtained on the augen orthogneiss of the southern flank, strongly suggesting that both orthogneiss occurrences have the same Ordovician protolith. The Saint-Eutrope orthogneiss experienced intense shearing along the Espinouse extensional detachment at ca. 295 Ma (LA-ICP-MS U-Pb-Th on monazite), an age close to that determined previously on mica by the 39Ar-40Ar method and contemporaneous with the emplacement age of the syntectonic Montalet granite farther to the west. This normal sense shearing reworked previous fabrics related to Variscan thrusting that can be still observed in the augen orthogneiss of the southern flank, and is responsible for the spectacular “C/S-like” pattern of the Saint-Eutrope orthogneiss. This work also shows that care is needed when dealing with C/S-type structures, since they can develop not only in syntectonic intrusions, but also in orthogneisses affected by an intense secondary deformation, at decreasing temperature.

Dater les événements magmatiques dans le dôme gneissique de la Montagne Noire est un point-clé pour départager les différentes interprétations de la tectonique de la transition Carbonifère–Permien dans la partie méridionale de la chaîne varisque. L’orthogneiss de Saint-Eutrope affleure le long du flanc nord du dôme. Nous montrons que le protolithe de cet orthogneiss est un granite ordovicien daté à 455 ± 2 Ma (datation U-Pb par LA-ICP-MS sur zircon). Cet âge est identique à celui obtenu auparavant sur les orthogneiss œillés du flanc sud. Cela suggère fortement que les deux orthogneiss avaient un protolithe ordovicien commun. L’orthogneiss de Saint-Eutrope a été affecté par un intense cisaillement le long du détachement extensif d’Espinouse à ca. 295 Ma (datation U-Pb-Th par LA-ICP-MS sur monazite). Cet âge est proche de celui déterminé auparavant sur micas par la méthode 39Ar-40Ar et contemporain avec la mise en place du granite syntectonique du Montalet, situé plus à l’ouest. Ce cisaillement en faille normale a repris la fabrique antérieure, associée au chevauchement varisque et qui peut encore être observée dans les orthogneiss œillés du flanc sud. Il est responsable de l’apparence de l’orthogneiss de Saint-Eutrope, spectaculairement similaire aux granites C/S. Ce travail montre qu’il est important de traiter avec précaution des structures de type C/S, car elles peuvent se développer non seulement dans des intrusions syntectoniques, mais aussi dans des orthogneiss ayant subi une intense déformation secondaire, à température décroissante.

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DOI: 10.1016/j.crte.2012.06.002
Keywords: Montagne Noire, Extensional tectonics, S/C mylonite, Orthogneiss, U-Pb geochronology, Variscan orogen, France
Mot clés : Montagne Noire, Tectonique extensive, Mylonite C/S, Orthogneiss, Géochronologie U-Pb, Orogène varisque, France
Pavel Pitra 1; Marc Poujol 1; Jean Van Den Driessche 1; Jean-Charles Poilvet 1, 2; Jean-Louis Paquette 3

1 Géosciences Rennes, UMR 6118, Université Rennes 1 and CNRS, 35042 Rennes cedex, France
2 Laboratoire Chrono-environnement, UMR 6249, Université de Franche-Comté, 25030 Besançon, France
3 Laboratoire Magmas et Volcans, Université Blaise-Pascal, UMR CNRS 6524, 63038 Clermont-Ferrand cedex, France
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     author = {Pavel Pitra and Marc Poujol and Jean Van Den Driessche and Jean-Charles Poilvet and Jean-Louis Paquette},
     title = {Early {Permian} extensional shearing of an {Ordovician} granite: {The} {Saint-Eutrope} {{\textquotedblleft}C/S-like{\textquotedblright}} orthogneiss {(Montagne} {Noire,} {French} {Massif} {Central)}},
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Pavel Pitra; Marc Poujol; Jean Van Den Driessche; Jean-Charles Poilvet; Jean-Louis Paquette. Early Permian extensional shearing of an Ordovician granite: The Saint-Eutrope “C/S-like” orthogneiss (Montagne Noire, French Massif Central). Comptes Rendus. Géoscience, Volume 344 (2012) no. 8, pp. 377-384. doi : 10.1016/j.crte.2012.06.002. https://comptes-rendus.academie-sciences.fr/geoscience/articles/10.1016/j.crte.2012.06.002/

Version originale du texte intégral

1 Introduction

The tectonic interpretation of the Montagne Noire gneiss dome remains disputed. Numerous divergent models have been proposed ranging from compressive anticline, through diapiric uplift, to extensional gneiss dome (Van Den Driessche and Brun, 1992). Although the debate remains active, it is behind the scope of the present paper to discuss in detail these structural models. Intense shearing deformation has been described along the northern flank of the dome for a long time, especially in the orthogneiss that crops out in this area (Latouche, 1968). We call this specific orthogneiss the “Saint-Eutrope orthogneiss”, named after a 14th century chapel built on these rocks (Fig. 1). The texture of this orthogneiss led Latouche (1968) and Bogdanoff et al. (1984) to compare it to the “gneiss minuti” of the Gran Paradiso massif in the Alps. For Demange (1975), the protolith of this orthogneiss is comagmatic with that of the augen orthogneisses found on the southern flank of the Montagne Noire. Latouche (1968) and Demange (1975) related the deformation of the Saint-Eutrope orthogneiss to the compressive Variscan tectonics. Beaud (1985), Van Den Driessche and Brun (1992) and Brun and Van Den Driessche (1994) interpreted this orthogneiss as a C/S mylonitic leucogranite emplaced within the detachment shear zone that would be responsible for the dome uplift during extensional tectonics. According to Gapais (1989), C/S structures (Berthé et al., 1979b) typically develop in syn-kinematic intrusions. During extension, the emplacement of granitic bodies within an extensional detachment zone is supposed to be fast enough (Davy et al., 1989) so that dating such bodies provides a close estimate for the age of the deformation. For this reason we decided to date the emplacement age of the Saint-Eutrope orthogneiss using the U-Th-Pb LA-ICP-MS method on both zircon and monazite.

Fig. 1

Structural map of southern French Massif Central showing the relationships between the Montagne Noire gneiss dome, Late Carboniferous-Permian basins, and Variscan thrusts and nappes (modified after Brun and Van Den Driessche, 1994). Ed: Espinouse extensional detachment; G: Graissessac basin; HG: Héric gorges; Lo: Lodève basin; P: Col de Picotalen (location of the sample of the Montalet syntectonic leucogranite, Poilvet et al., 2011); SE: Saint-Europe chapel. Inset shows the location of the study area within the European Variscan belt (modified from Pitra et al., 2010). A: Alps; AM: Armorican Massif; BM: Bohemian Massif; MC: Massif Central; B: Teplá-Barrandian; Mo: Moldanubian; ST: Saxothuringian; RH: Rhenohercynian. L: Lyon, M: Montpellier, R: Rennes.

Schéma structural du Sud du Massif central montrant les relations entre le dôme gneissique de la Montagne Noire, les bassins stéphano-permiens et les nappes et chevauchements varisques (modifié d’après Brun et Van Den Driessche, 1994). Ed : détachement d’Espinouse ; G : bassin de Graissessac ; HG : gorges d’Héric ; Lo : bassin de Lodève ; P : col de Picotalen (localisation de l’échantillon du leucogranite syntectonique de Montalet, Poilvet et al., 2011) ; SE : chapelle de Saint-Europe. L’encart montre la position de la zone d’étude dans la chaîne varisque européenne (modifié d’après Pitra et al., 2010). A : Alpes ; AM : Massif armoricain ; BM : Massif de Bohême ; MC : Massif central ; B : Teplá-Barrandien ; Mo : Moldanubien ; ST : Saxothuringien ; RH : Rhenohercynien. L : Lyon ; M : Montpellier ; R : Rennes.

2 Petrography and texture of the Saint-Eutrope orthogneiss

The Saint-Eutrope orthogneiss is a peraluminous, leucocratic, fine-grained (0.1–0.5 mm), locally porphyric, muscovite ± biotite-bearing granitic rock. It commonly contains large (up to 5 cm) porphyroclasts of K-feldspar (Fig. 2a). In some samples, tourmaline and garnet are present. The orthogneiss contains veins of tourmaline + muscovite-bearing pegmatites that have a foliation parallel to the main foliation of the orthogneiss. The main foliation is defined by flattening of feldspar, quartz-feldspar aggregates and shape preferred orientation of micas. K-feldspar porphyroclasts have Carlsbad twins and flame perthites. The porphyroclasts are in general also parallel to the rock foliation and asymmetric recrystallization tails developed on their short sides. Smaller equant grains formed by dynamic recrystallization at the rim of K-feldspar porphyroclasts. Quartz is mixed with other minerals and ribbons, if present, are short, without an internal grain shape orientation. Quartz grains are commonly equant or slightly elongated parallel to the foliation and their boundaries suggest bulging recrystallization (Fig. 2d). The foliation strikes east-west and dips about 20–30° N. The dominant structures are relatively pervasive discrete regular shear planes striking east-west and dipping ca. 40–50° N, about 20° steeper than the foliation (Fig. 2a, c, d). They bear fine striae plunging about 40° to the north-east (Figs. 4 and 5 in Van Den Driessche and Brun, 1992 for detailed structural maps). This indicates shear deformation associated with a normal movement to the north-east. Locally, the foliation is nearly parallel to the shear planes and the development of steep shear bands can then be observed, consistent with the normal sense of shear. The analysed sample was taken near the Saint-Eutrope chapel (43°39′41.48′′N, 2°59′′20.27″E).

Fig. 2

a: outcrop photograph of the Saint-Eutrope orthogneiss displaying large porphyroclasts of K-feldspar, elongated parallel to the foliation, and conspicuous shear bands, indicating a normal movement to the north-east; b: outcrop photograph of the Héric gorges augen orthogneiss. The foliation is subvertical; c, d: photomicrographs of the Saint-Eutrope orthogneiss in PPL (c) and XPL (d). Muscovite (mu) is oriented parallel to S and locally affected by north-east dipping shear bands. Note the lack of quartz ribbons and grain-boundary migration recrystallization. kfs: K-feldspar; pl: plagioclase; q: quartz.

a : affleurement de l’orthogneiss de Saint-Eutrope avec de grands porphyroclastes de feldspath K, allongés parallèlement à la foliation, et des bandes de cisaillement indiquant un mouvement normal vers le nord-est ; b : affleurement de l’orthogneiss œillé des gorges d’Héric. La foliation est subverticale ; c, d : photographies au microscope polarisant de l’orthogneiss de Saint-Eutrope en LPNA (c) et LPA (d). Les cristaux de muscovite (mu) sont parallèles à la foliation S et localement affectés par des bandes de cisaillement à pendage nord-est. Notez l’absence de rubans de quartz et de recristallisation par migration des joints de grains. kfs : feldspath K ; pl : plagioclase ; q : quartz.

The rock bears textural similarities with a syntectonic C/S granite, but the presence of K-feldspar augen, uncommon in such context, suggests that it may rather be a porphyric orthogneiss re-deformed in a ductile normal shear zone. The lack of quartz ribbons displaying an internal shape fabric and grain-boundary migration recrystallization, typical of C/S granitoids (Berthé et al., 1979a; Gapais and Barbarin, 1986; Pitra et al., 1994), also points towards this interpretation.

3 U-Th-Pb LA-ICP-MS dating

3.1 Analytical techniques

A classic mineral separation procedure has been applied to concentrate minerals suitable for U-Th-Pb dating using the facilities available at Géosciences Rennes. Rocks were crushed and only the powder fraction with a diameter less than 250 μm has been kept. Heavy minerals were successively concentrated by Wilfley table and heavy liquids. Magnetic minerals were then removed with an isodynamic Frantz separator. Zircon and monazite grains were carefully handpicked under a binocular microscope and embedded in epoxy mounts. The grains were then hand-grounded and polished on a lap wheel with a 6 μm and 1 μm diamond suspension successively. Zircons were imaged by cathodoluminescence (CL) using a Reliotron CL system equipped with a digital colour camera available at Géosciences Rennes.

U-Th-Pb geochronology of zircon and monazite was conducted by in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at the Laboratoire Magmas et Volcans in Clermont-Ferrand, France. Ablation spot diameters of 26 μm and 7 μm with repetition rates of 3 Hz and 1 Hz were used for zircon and monazite, respectively. Data were corrected for U-Pb and Th-Pb fractionation and for the mass bias by standard bracketing with repeated measurements of the 91500 zircon (Wiedenbeck et al., 1995) or the Moacyr monazite standards (Gasquet et al., 2010). Repeated analyses of GJ-1 zircon (Jackson et al., 2004) or Manangoutry monazite (Paquette and Tiepolo, 2007) standards treated as unknowns were used to control the reproducibility and accuracy of the corrections. Data reduction was carried out with the GLITTER® software package developed by the Macquarie Research Ltd. (Jackson et al., 2004). Concordia ages and diagrams were generated using Isoplot/Ex (Ludwig, 2001). All errors given in Tables 1 and 2 are listed at one sigma, but where data are combined for regression analysis or to calculate weighted means, the final results are provided with 95% confidence limits. Further information on the instrumentation and the analytical technique is detailed in Hurai et al. (2010).

Table 1

Données U-Pb obtenues par LA-ICP-MS sur des zircons de l’échantillon ES2.

Grain (zircon) [Pb] (ppm) [U] (ppm) Th/U 207Pb/235U ± 1 σ 206Pb/238U ± 1 σ 207Pb/206Pb ± 1 σ Rho Ages Conc. (%)
207Pb/235U 206Pb/238U 207Pb/206Pb ± 1 σ
1.1a 56 767 0.09 0.872 0.011 0.0899 0.0011 0.0703 0.0008 0.91 637 555 938 24 59
1.2a (*) 39 682 0.06 0.572 0.007 0.0732 0.0009 0.0567 0.0007 0.91 459 455 478 26 95
2a (*) 19 323 0.13 0.577 0.009 0.0736 0.0009 0.0569 0.0008 0.77 463 458 486 32 94
4a (*) 22 380 0.14 0.580 0.009 0.0735 0.0009 0.0572 0.0008 0.78 464 457 498 31 92
6a (*) 25 458 0.06 0.554 0.008 0.0710 0.0008 0.0566 0.0008 0.83 448 442 476 29 93
7.1a 36 400 0.18 0.992 0.013 0.1091 0.0013 0.0659 0.0008 0.90 700 668 805 24 83
8.1a 16 179 0.64 0.853 0.012 0.0979 0.0012 0.0632 0.0009 0.81 627 602 716 29 84
8.2a 19 367 0.13 0.528 0.008 0.0665 0.0008 0.0576 0.0009 0.74 430 415 515 33 81
10a (*) 33 588 0.06 0.559 0.007 0.0726 0.0009 0.0559 0.0007 0.88 451 452 447 26 101
12.1a 16 283 0.07 0.579 0.008 0.0711 0.0008 0.0591 0.0008 0.81 464 443 572 29 77
12.2a 25 323 0.51 0.888 0.012 0.0817 0.0010 0.0788 0.0010 0.84 645 506 1167 26 43
13a (*) 45 695 0.54 0.561 0.008 0.0730 0.0009 0.0558 0.0007 0.86 452 454 442 27 103
14.1a 34 657 0.07 0.527 0.007 0.0670 0.0008 0.0570 0.0007 0.84 430 418 491 29 85
14.2a (*) 14 245 0.11 0.569 0.009 0.0733 0.0009 0.0563 0.0008 0.78 458 456 465 32 98
15.2a 16 322 0.09 0.485 0.009 0.0629 0.0008 0.0559 0.0010 0.65 401 393 449 39 88
18.1a (*) 19 329 0.09 0.578 0.008 0.0740 0.0009 0.0566 0.0008 0.82 463 460 476 29 97
18.2a 17 212 0.15 0.835 0.012 0.1002 0.0012 0.0604 0.0008 0.82 616 615 619 29 99
19a 21 351 0.17 0.618 0.011 0.0752 0.0009 0.0596 0.0010 0.68 489 467 591 36 79
20.1a 34 301 0.27 1.223 0.016 0.1344 0.0016 0.0660 0.0008 0.86 811 813 806 26 101
21.1a 32 254 0.53 1.289 0.018 0.1385 0.0016 0.0675 0.0009 0.82 841 836 853 27 98
24a (*) 15 255 0.07 0.563 0.009 0.0727 0.0009 0.0562 0.0008 0.74 454 452 460 33 98
27a (*) 32 520 0.28 0.565 0.008 0.0732 0.0009 0.0560 0.0007 0.83 455 455 451 29 101
29a (*) 8 140 0.23 0.567 0.010 0.0726 0.0009 0.0566 0.0010 0.67 456 452 476 38 95
31a (*) 28 472 0.24 0.555 0.008 0.0708 0.0008 0.0568 0.0008 0.81 448 441 485 30 91
34a (*) 12 207 0.06 0.582 0.010 0.0753 0.0009 0.0561 0.0009 0.68 466 468 457 37 102
Table 2

Données U-Th-Pb obtenues par LA-ICP-MS sur des monazites de l’échantillon ES2.

Grain [Pb] (ppm) [U] (ppm) [Th] (ppm) Th/U 206Pb/238U ± (1σ) 207Pb/235U ± (1σ) 208Pb/232Th ± (1σ) Ages
206Pb/238U 207Pb/235U 208Pb/232Th ± (1σ)
06120410c 1171 2464 79,494 32 0.04862 0.00058 0.36096 0.00686 0.0146 0.0001 306 313 293 3
07120410c 1067 3143 69,358 22 0.04849 0.00057 0.34415 0.00625 0.0147 0.0001 305 300 296 3

3.2 Results

Numerous zircon and only two monazite grains were recovered from this sample. Zircons were usually lightly pink, euhedral and elongated with nicely defined magmatic zoning (Fig. 3a). Some, however, were euhedral squat grains with inherited dark cores surrounded by brighter rims (Fig. 3b). Monazite grains were subhedral and slightly yellow in colour.

Fig. 3

Cathodoluminescence images of some of the zircon grains dated in this study: a: euhedral and elongated crystals with nicely defined magmatic zoning; b: euhedral squat crystals with evidence of inherited dark cores surrounded by brighter rims. The white circles represent the spot analyses and the numbers correspond to the 207Pb/206Pb ages found. Zr number corresponds to the grain number in Table 1.

Photographies en cathodoluminescence de certains des zircons datés lors de cette étude : a : cristaux allongés et automorphes avec une zonation magmatique bien développée ; b : cristaux trapus automorphes avec des évidences de cœurs sombres hérités, entourés par des bordures plus claires. Les cercles blancs représentent la position des analyses ponctuelles et les chiffres correspondent aux âges 207Pb/206Pb trouvés. Les numéros Zr correspondent aux numéros des grains dans le Tableau 1.

Twenty-five analyses were performed on 20 zircon grains (Table 1). Plotted in a Tera-Wasserburg concordia diagram (Fig. 4a), zircon grains plot on a concordant to discordant position with apparent dates ranging from ca. 810 Ma down to 400 Ma. Two groups of ellipses (shaded) define two dates, ca. 810 Ma and 620 Ma, respectively. A group of thirteen concordant points (unfilled ellipses) allows one to calculate a concordia date (Ludwig, 1998) of 455.2 ± 2.2 Ma (Fig. 4a insert, grey-filled ellipse). The position of the dash-lined ellipses (Fig. 4a) can be attributed to variable degrees of Pb loss.

Fig. 4

A. Tera-Wasserburg 207Pb/206Pb versus 238U/206Pb concordia diagram for zircons analyzed in this study. Inset: close-up for the thirteen concordant points. B. 206Pb/238U versus 208Pb/232Th diagram for the monazite analyzed in this study. N refers to the number of analyses. All ellipses are represented at one sigma level. All ages are quoted at two sigma level.

A. Diagramme concordia de Tera-Wasserburg 207Pb/206Pb vs 238U/206Pb pour les zircons analysés dans cette étude. L’encart montre un agrandissement pour les treize points concordants. B. Diagramme 206Pb/238U vs. 208Pb/232Th pour les monazites analysées dans cette étude. N correspond au nombre d’analyses. Toutes les ellipses d’erreur sont représentées à un sigma. Tous les âges sont donnés à deux sigmas.

The two monazite grains have been analysed (Table 2). In a 206Pb/238U versus 208Pb/232Th concordia diagram (Fig. 4b), they plot in a slightly reverse discordant position, which could be attributed to excess 206Pb due to excess 230Th as described by Schärer (1982). To avoid calculating an artificially older age if 206Pb excess is confirmed, we favour dealing with the 208Pb/232Th ratios. They allow to calculate a mean 208Pb/232Th date of 294.4 ± 4 Ma.

4 Interpretation and geological significance of the geochronological data

Two interpretations can be drawn from the geochronological data. Either all zircon grains are inherited and monazite dates the emplacement and the contemporaneous deformation of the rock, or the date of ca. 455 Ma yields the age of emplacement of the protolith and monazite dates a subsequent deformation event at ca. 294 Ma. Two arguments are in favour of the second hypothesis. First, in the syntectonic Montalet leucogranite (Poilvet et al., 2011), emplaced along the Espinouse detachment farther west, not only monazite but also zircon crystallized from the magma during the emplacement, and the same behaviour would be expected if the Saint-Eutrope orthogneiss was a syntectonic intrusion. Second, as argued before, the porphyric character of the Saint-Eutrope orthogneiss is not expected in syntectonic leucogranites. Therefore, we interpret the concordant date of 455.2 ± 2.2 Ma, yielded by a large proportion of the analysed zircon grains, as the age of the emplacement of the orthogneiss protolith. This age is identical to the mean average 207Pb/206Pb age of 455 ± 2 Ma found for the protolith of the augen orthogneisses from the southern flank of the Montagne Noire dome (Roger et al., 2004). This agrees with the comagmatic origin of these orthogneisses, as previously suggested by Demange (1975). It also implies that the rocks from the Saint-Eutrope locality, despite the textural resemblance, are not a syntectonic leucogranite as proposed by Beaud (1985), Van Den Driessche and Brun (1992) and Brun and Van Den Driessche (1994), but instead an orthogneiss reactivated in a ductile normal shear zone. The ductile extensional shearing was sufficiently intense to strongly rework the previous fabrics that developed during the Variscan compressive tectonics and that can be still observed in the orthogneisses of the southern flank, unaffected by the extensional shearing (Héric gorges, Figs. 1, 2b). The same orthogneiss displays therefore a strikingly contrasting deformation imprint on the northern and southern flank of the Montagne Noire dome, respectively. This asymmetric character of the deformation at the scale of the dome is characteristic for detachment-controlled gneiss domes developed in extensional settings (Brun and Van Den Driessche, 1994; Brun et al., 1994; Buck, 1988; Davis and Coney, 1979; Wernicke and Axen, 1988) and supports this interpretation for the origin of the Montagne Noire gneiss dome (Van Den Driessche and Brun, 1992). This interpretation is also consistent with the Late Carboniferous post-thickening LP-HT metamorphism, which caused partial melting in the core of the Montagne Noire gneiss dome and with the subsequent exhumation of the dome below the Espinouse detachment (Brun and Van Den Driessche, 1994).

In order to understand the C/S character of the Saint-Eutrope orthogneiss, one may remember that true pervasive C/S structures only develop during retrograde deformation histories (Gapais, 1989). This is typically the case for syntectonic intrusions, but should also apply to hot orthogneisses deformed subsequently at decreasing temperature. This would explain the “C/S-like” mesoscopic deformation patterns observed in the Saint-Eutrope orthogneiss (Fig. 2). Moreover the presence of K-feldspar augen in a granitoid is symptomatic of a two-stage crystallisation process that is rather difficult to conciliate with the rapid emplacement and cooling of a syntectonic leucogranite during an extensional tectonic event. This confirms that C/S structures are not exclusively indicative of syntectonic intrusions.

Since the date of 455 Ma is considered as the age of the Saint-Eutrope granitic protolith, the dates of ca. 810 Ma and 620 Ma are interpreted as the ages of the source material inherited in the original granite. This inheritance can be attributed either to a Proterozoic basement or to sediments, which recycled such a basement. As the Saint-Eutrope orthogneiss is muscovite-bearing, the protolith was probably a S-type granite. Therefore we favour a sedimentary inheritance, although the presence of a Proterozoic basement cannot be completely ruled out.

Although the zircons did not record any younger event (as in Roger et al., 2004), two monazite grains allowed to calculate a mean 208Th/232Pb date of 294.4 ± 4 Ma. This date is interpreted as the age of the deformation (and probable associated fluid circulation) of the protolith of the Saint-Eutrope orthogneiss along the Espinouse detachment. This age agrees within the uncertainty with the 297 ± 2.8 Ma obtained on muscovite extracted from the same orthogneiss (Maluski et al., 1991) and interpreted as marking the end of the ductile normal movement. This age is also identical to the 294.4 ± 2.6 Ma dating the emplacement of the C/S syntectonic Montalet leucogranite (Brun and Van Den Driessche, 1994; Poilvet et al., 2011), emplaced along the Espinouse detachment farther to the west (Fig. 1). Finally, Bruguier et al. (2003) published an age of 295 ± 5 Ma for zircons extracted from a volcanic ash layer interbedded in the Late Carboniferous–Early Permian sedimentary fill of the Graissessac-Lodève basin. This hemi-graben is situated immediately to the north-east of the Saint-Eutrope orthogneiss, in the hanging wall of the Espinouse detachment (Fig. 1). The contemporaneity of ductile deformation in the basement and deposition of the ash layer confirms that the Espinouse detachment controlled the development of the basin (Van Den Driessche and Brun, 1989; Becq-Giraudon and Van Den Driessche, 1993), which is best explained by the interpretation of the Montagne Noire in the framework of large scale crustal extension.

5 Conclusion

The protolith of the Saint-Eutrope orthogneiss from the northern flank of the Montagne Noire is an Ordovician granitic intrusion emplaced 455 ± 2 Ma ago. This protolith age is identical to that of the augen orthogneisses from the southern flank, suggesting that they are comagmatic. However, deformation patterns differ between the two occurrences; the C/S patterns are only found within the Saint-Eutrope orthogneiss. They are related to the intense normal shearing that took place ca. 295 Ma ago along the Espinouse detachment and that did not affect the southern part of the Montagne Noire dome. This asymmetric character of the deformation at the scale of the dome is characteristic for detachment-controlled gneiss domes developed in extensional settings, supporting this interpretation for the Montagne Noire. Finally, this work also shows that care is needed when dealing with C/S-type structures, since they can develop not only in syntectonic intrusions, but also in orthogneisses affected by an intense later deformation at decreasing temperature.

Acknowledgments

We are indebted to Jean-Pierre Burg and an anonymous reviewer for swift and constructive reviews. Marc Chaussidon is thanked for his efficient editorial handling of the manuscript.


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