Outline
Comptes Rendus

Petrology, Geochemistry (Geochronology)
The Montalet granite, Montagne Noire, France: An Early Permian syn-extensional pluton as evidenced by new U-Th-Pb data on zircon and monazite
Comptes Rendus. Géoscience, Volume 343 (2011) no. 7, pp. 454-461.

Abstracts

Dating the magmatism in the Montagne Noire gneiss dome in the southern French Massif Central is a key point for understanding the Late Palaeozoic evolution of this part of the Variscan belt, which is characterised by compressive tectonics during the Carboniferous and extensional tectonics during Stephanian-Permian times. The Montalet granite crops out in the north-western part of the dome and was first considered as an early syntectonic intrusion related to compressive deformation. More recently, it has been dated at 327 Ma and considered as contemporaneous with the diapiric ascent of the Montagne Noire gneiss dome before the Stephanian-Permian extension. We show that in fact, this pluton was emplaced 294 ± 1 Ma ago and is therefore contemporaneous with the Stephanian-Permian extension. This age is consistent with the interpretation of the Montagne Noire Massif as an extensional gneiss dome.

La datation du magmatisme dans le dôme gneissique de la Montagne Noire est un point-clé pour la compréhension de l’évolution de cette partie de la chaîne varisque pendant le Paléozoïque supérieur, laquelle est caractérisée par une tectonique compressive pendant le Carbonifère, puis par une tectonique extensive pendant le Stéphano-Permien. Le granite du Montalet qui affleure dans la partie nord-ouest du dôme a d’abord été considéré comme une intrusion syntectonique reliée à une tectonique compressive précoce. Il a été daté à 327 Ma et considéré comme contemporain de l’ascension diapirique du dôme, avant l’extension stéphano-permienne. Nous montrons qu’en fait ce granite se met en place à 294 Ma et est donc contemporain de l’extension stéphano-permienne. Cet âge est cohérent avec l’interprétation de la Montagne Noire comme un dôme gneissique extensif.

Metadata
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Accepted:
Published online:
DOI: 10.1016/j.crte.2011.06.002
Keywords: Montagne Noire, Extensional, Montalet granite, U-Pb geochronology, Zircon, Monazite, Variscan, France
Mot clés : Montagne Noire, Extension, Granite du Montalet, Géochronologie U-Pb, Zircon, Monazite, Varisque, France

Jean-Charles Poilvet 1; Marc Poujol 1; Pavel Pitra 1; Jean Van Den Driessche 1; Jean-Louis Paquette 2

1 Université Rennes 1, Géosciences Rennes, UMR CNRS 6118, 35042 Rennes cedex, France
2 Université Blaise-Pascal, Laboratoire Magmas et Volcans, UMR CNRS 6524, 63038 Clermont-Ferrand cedex, France
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     author = {Jean-Charles Poilvet and Marc Poujol and Pavel Pitra and Jean Van Den Driessche and Jean-Louis Paquette},
     title = {The {Montalet} granite, {Montagne} {Noire,} {France:} {An} {Early} {Permian} syn-extensional pluton as evidenced by new {U-Th-Pb} data on zircon and monazite},
     journal = {Comptes Rendus. G\'eoscience},
     pages = {454--461},
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%0 Journal Article
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%A Marc Poujol
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%A Jean Van Den Driessche
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Jean-Charles Poilvet; Marc Poujol; Pavel Pitra; Jean Van Den Driessche; Jean-Louis Paquette. The Montalet granite, Montagne Noire, France: An Early Permian syn-extensional pluton as evidenced by new U-Th-Pb data on zircon and monazite. Comptes Rendus. Géoscience, Volume 343 (2011) no. 7, pp. 454-461. doi : 10.1016/j.crte.2011.06.002. https://comptes-rendus.academie-sciences.fr/geoscience/articles/10.1016/j.crte.2011.06.002/

Version originale du texte intégral

1 Introduction

The timing and tectonic settings of the Late Paleozoic uplift of the Montagne Noire gneiss dome in the southern French Massif Central (MC) are still the subject of controversy. Three main tectonic interpretations have been proposed so far (Charles et al., 2009; Van Den Driessche and Brun, 1992 and references therein): diapiric ascent, compression resulting in a regional anticline, and extension resulting in a core complex. The two first interpretations consider that the dome rose during the Late Carboniferous compression before the regional Permian extensional tectonics. In the third interpretation, the extension started as early as the Late Carboniferous, resulting in the dome uplift, and continued during the Permian. Synmetamorphic ductile deformation that accompanied the dome uplift is attributed to diapirism or compression for the first and second interpretations, respectively, whereas it resulted from deep-seated extension that overprinted previous compression in the third interpretation. Magmatism is attributed to partial crustal melting consecutive to crustal thickening in the three models, but deformation of the plutons, when present, is interpreted in accordance with each of the mechanisms proposed for the dome uplift.

The age of the plutons is therefore a key datum that can help to discriminate between these different models. The Montalet granite is one of these plutons, with a deformation history that is still controversial (Brun and Van Den Driessche, 1996; Demange, 1996).

According to Demange et al. (1995) “the precise dating of this granite would undoubtedly solve many problems of the regional geology on the Montagne Noire”.

2 The Montalet granite

The Montalet granite crops out along the northwestern edge of the Montagne Noire gneiss dome (Fig. 1). The granite is composed of sills or laccoliths ranging in thickness from a few tens of metres to several hundreds metres (Demange et al., 1995). The deformation of the granite increases toward the major tectonic contact that limits the gneiss dome to the north and northwest. This contact is referred to as the Lacaune fault (Demange, 1996) or the Espinouse detachment (Brun and Van Den Driessche, 1994), respectively. According to Demange (1996), a magmatic foliation developed first and was then crosscut by a second foliation when approaching the Lacaune fault, as both planar structures are related to early thrusting events. For Brun and Van Den Driessche (1996), these two planar structures correspond to S-C structures (Berthé et al., 1979) (Fig. 2a) that developed during a single extensional shearing deformation along the Espinouse detachment. But in both cases, these authors consider that the Montalet granite is syn-kinematic in nature. We chose to sample this granite at the Col de Picotalen (or Piquotalen) (Fig. 2a), the location that these two conflicting interpretations are based upon, as it shows a well-developed deformation, (43° 41′ 16. 53″ N, 2° 39′ 33. 38″E).

Fig. 1

Structural map of the southern French Massif Central (MC) showing the relationships between the Montagne Noire gneiss dome, the Stephanian-Permian basins, and the Variscan thrusts and nappes. P.: Col of Picotalen (sampling site). 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 (MC) montrant les relations entre le dôme gneissique de la Montagne Noire, les bassins stéphano-permiens et les nappes et chevauchements varisques. P. : Col de Picotalen (site d’échantillonnage). 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 : Rhénohercynien. L : Lyon ; M : Montpellier ; R : Rennes.

Fig. 2

The Montalet granite. a: Outcrop of the Montalet granite at the Col of Picotalen. Orientation SW-NE.; b: Microscopic aspect in plane polarised light (thin section parallel to the λ1λ3 plane of deformation). Orientation SW-NE. Feldspar crystals, quartz ribbons and a large biotite crystal define the foliation S. Discrete NE-dipping planes marked by fine grained crystals including chlorite are the C-planes.

Le granite du Montalet. a : affleurement du granite du Montalet au col de Picotalen. Orientation SW-NE ; b : aspect en microscopie en lumière polarisée non analysée (lame mince parallèle au plan de déformation λ1λ3). Orientation SW-NE. Les cristaux de feldspath, les rubans de quartz et le grand cristal de biotite définissent la foliation S. Les plans discrets soulignés par des cristaux de petite taille, dont la chlorite, sont les plans C.

The Montalet granite is a garnet-bearing, two-mica leucogranite. The selected sample is composed of K-feldspar, plagioclase, quartz, biotite, muscovite, garnet and accessory minerals. The main foliation S is marked by the shape preferred orientation of feldspar porphyroclasts (up to 10 mm long), micas (up to 3 mm) and quartz aggregates (Fig. 2b). Subhedral K-feldspar generally lacks strong internal deformation and displays Carlsbad twin planes, generally parallel to the foliation S. The feldspars include quartz droplets and biotite and muscovite crystals inherited from the early magmatic stage. Very small (0.01–0.03 mm) grains of quartz and micas are contained within regular narrow bands, spaced by 5 to 10 mm in general, interpreted as shear bands C (Figs. 2a and b). In the vicinity of these shear bands, biotite and garnet are locally replaced by chlorite. Quartz ribbons affected by these shear bands display progressive reorientation and grain size reduction towards the shear band. The foliation strikes east-west and dips ca. 20° to the north. The stretching lineation L underlined by the long axis of quartz, micas and feldspar clasts plunges about 15° NE. The C-planes dip ca. 50° to the north, some 20 to 30° steeper than the foliation, indicating a deformation associated with a normal movement to the northeast. They bear fine striae plunging about 45° to the northeast.

The preferred orientation of the subhedral feldspar crystals may be attributed to deformation affecting a partly crystallised magma. Pervasive S-C type structures suggest intense, solid state deformation. Myrmekites that developed at the K-feldspar high-pressure sides suggest diffusion and consequently high-temperature deformation (Simpson and Wintsch, 1989). Quartz constitutes polycrystalline ribbons parallel to the foliation. Recrystallised grains are slightly elongated, forming an internal shape fabric oblique to S. Lobate boundaries suggest some recrystallisation by grain boundary migration, common at high temperatures (e.g. Gapais and Barbarin, 1986). Biotite crystals have typical fish-like shapes and tend to be replaced by chlorite, which is interpreted as a consequence of cooling. The grain size evolution from the foliation to the shear bands also suggests progressive deformation at decreasing temperature.

The S-C structures indicate a top to the northeast shear as supported by internal quartz schistosity and mica fishes. This means that the Montalet granite was deformed syntectonically during its emplacement into the footwall of a normal, ductile shear zone.

3 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 < 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 color camera available in Géosciences Rennes.

U-Th-Pb geochronology of zircon and monazite was conducted by in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) 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 91,500 zircon (Wiedenbeck et al., 1995) or the Moacir 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 Table 1 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-Th-Pb obtenues par LA-ICP-MS sur des zircons et monazites de l’échantillon ES5. Les erreurs sont données à 1 sigma.

AGES
Analysis [Pb] ppm [U] ppm [Th] ppm 207Pb/235U Err 206Pb/238U Err rho 208Pb/232Th Err 207Pb/206Pb Err 206Pb/238U Err 207Pb/235U Err 208Pb/232Th Err
ES5 Monazites
07120410a 1177 2673 82,272 0.326 0.006 0.0464 0.0006 0.67 0.0144 0.0002 238 42 292 4 286 5 289 3
08120410a 977 2580 66,302 0.388 0.007 0.0473 0.0006 0.70 0.0145 0.0002 586 38 298 4 333 5 292 3
09120410a 819 4245 50,155 0.330 0.005 0.0471 0.0006 0.76 0.0144 0.0002 233 36 297 4 289 4 289 3
10120410a 880 2289 59,156 0.323 0.006 0.0468 0.0006 0.67 0.0147 0.0002 199 43 295 4 284 5 295 3
11120410a 1081 3598 71,034 0.329 0.006 0.0467 0.0006 0.75 0.0146 0.0002 243 37 294 4 289 4 293 3
12120410a 1035 5368 61,578 0.331 0.005 0.0469 0.0006 0.79 0.0148 0.0002 249 33 296 4 291 4 298 3
13120410a 913 1500 63,069 0.332 0.007 0.0482 0.0006 0.60 0.0148 0.0002 197 49 303 4 291 6 298 3
14120410a 1019 1370 72,100 0.344 0.008 0.0483 0.0006 0.58 0.0147 0.0002 269 51 304 4 300 6 294 3
18120410a 1135 3982 74,153 0.329 0.006 0.0461 0.0006 0.75 0.0146 0.0002 280 36 290 4 289 4 292 3
19120410a 856 1566 59,623 0.340 0.008 0.0477 0.0006 0.59 0.0146 0.0002 273 50 300 4 297 6 293 3
20120410a 1011 2339 69,365 0.516 0.009 0.0485 0.0006 0.75 0.0144 0.0002 1125 32 305 4 422 6 290 3
21120410a 1093 6883 63,307 0.328 0.005 0.0464 0.0006 0.81 0.0144 0.0002 251 32 293 4 288 4 290 3
22120410a 1074 2562 72,339 0.333 0.006 0.0474 0.0006 0.68 0.0147 0.0002 239 41 298 4 292 5 295 3
23120410a 915 6262 52,282 0.322 0.005 0.0463 0.0006 0.80 0.0143 0.0002 214 33 292 4 283 4 286 3
24120410a 998 5730 59,651 0.368 0.006 0.0465 0.0006 0.81 0.0143 0.0002 508 31 293 4 318 4 287 3
28120410a 1272 2248 89,612 0.329 0.007 0.0467 0.0006 0.63 0.0144 0.0002 249 45 294 4 289 5 289 3
29120410a 849 6263 46,830 0.323 0.005 0.0462 0.0006 0.79 0.0143 0.0002 228 33 291 4 284 4 288 3
30120410a 1200 3034 80,608 0.336 0.006 0.0472 0.0006 0.70 0.0146 0.0002 270 39 297 4 294 5 293 3
31120410a 1052 3031 70,357 0.331 0.006 0.0470 0.0006 0.68 0.0145 0.0002 245 41 296 4 290 5 291 3
32120410a 1210 4462 76,896 0.326 0.005 0.0470 0.0006 0.75 0.0147 0.0002 213 37 296 4 287 4 295 3
33120410a 1199 2604 81,655 0.331 0.006 0.0466 0.0006 0.68 0.0146 0.0002 268 41 294 4 291 5 294 3
34120410a 1028 1808 72,787 0.329 0.008 0.0463 0.0006 0.54 0.0143 0.0002 261 56 292 4 288 6 287 3
35120410a 879 3079 56,592 0.322 0.006 0.0470 0.0006 0.68 0.0146 0.0002 179 41 296 4 283 5 293 3
38120410a 951 2748 63,216 0.342 0.007 0.0471 0.0006 0.67 0.0145 0.0002 314 42 297 4 299 5 291 3
39120410a 1053 3077 69,110 0.326 0.006 0.0467 0.0006 0.69 0.0147 0.0002 228 40 294 4 287 5 295 3
40120410a 1320 4158 86,214 0.331 0.006 0.0473 0.0006 0.72 0.0146 0.0002 228 38 298 4 290 4 293 3
41120410a 999 1602 69,532 0.338 0.008 0.0473 0.0006 0.59 0.0146 0.0002 277 50 298 4 296 6 293 3
43120410a 1186 2065 81,776 0.337 0.007 0.0475 0.0006 0.61 0.0146 0.0002 264 47 299 4 295 5 294 3
44120410a 1224 3685 80,665 0.327 0.006 0.0465 0.0006 0.69 0.0146 0.0002 245 40 293 4 288 5 292 3
45120410a 1212 2394 83,074 0.334 0.007 0.0471 0.0006 0.62 0.0146 0.0002 260 46 297 4 293 5 293 3
AGES
[Pb] ppm [U] ppm 207Pb/235U Err 206Pb/238U Err rho 207Pb/206Pb Err 207Pb/206Pb Err 206Pb/238U Err 207Pb/235U Err
ES5 Zircons
 03130410b 59 1525 0.337 0.004 0.0454 0.0005 0.93 0.0538 0.0006 363 26 286 3 295 3
 04130410b 42 1019 0.372 0.005 0.0514 0.0006 0.92 0.0524 0.0006 304 26 323 4 321 3
 05130410b 43 1026 0.375 0.005 0.0513 0.0006 0.92 0.0530 0.0006 330 25 323 4 324 3
 10130410b 19 507 0.352 0.005 0.0451 0.0005 0.87 0.0565 0.0007 472 27 284 3 306 3
 13130410b 35 935 0.354 0.004 0.0448 0.0005 0.92 0.0573 0.0007 503 25 282 3 308 3
 17130410b 75 2042 0.335 0.004 0.0464 0.0005 0.95 0.0524 0.0006 304 25 292 3 294 3
 18130410b 57 1438 0.336 0.004 0.0465 0.0005 0.93 0.0525 0.0006 307 26 293 3 294 3
 24130410b 44 688 0.635 0.008 0.0796 0.0009 0.90 0.0579 0.0007 525 26 494 5 499 5
 25130410b 51 812 0.650 0.008 0.0788 0.0009 0.91 0.0598 0.0007 597 25 489 5 508 5
 26130410b 43 1057 0.341 0.004 0.0472 0.0005 0.89 0.0524 0.0006 301 26 297 3 298 3
 30130410b 32 794 0.332 0.005 0.0453 0.0005 0.83 0.0531 0.0007 334 29 286 3 291 3
 34130410b 122 3233 0.334 0.004 0.0466 0.0005 0.90 0.0520 0.0006 286 27 294 3 293 3
 35130410b 17 352 0.423 0.006 0.0565 0.0007 0.79 0.0543 0.0008 383 31 354 4 358 4
 37130410b 44 1121 0.341 0.004 0.0468 0.0005 0.90 0.0529 0.0006 323 27 295 3 298 3
 38130410b 23 589 0.337 0.005 0.0458 0.0005 0.81 0.0533 0.0007 342 31 289 3 295 4
 39130410b 93 2511 0.340 0.004 0.0462 0.0005 0.88 0.0533 0.0007 342 27 291 3 297 3
 40130410b 53 1428 0.343 0.004 0.0446 0.0005 0.88 0.0557 0.0007 441 26 281 3 299 3
 44130410b 148 4044 0.337 0.005 0.0443 0.0005 0.84 0.0552 0.0007 422 28 279 3 295 3
 45130410b 50 1248 0.336 0.004 0.0467 0.0005 0.89 0.0521 0.0006 291 27 294 3 294 3
 46130410b 22 587 0.341 0.005 0.0439 0.0005 0.83 465 29 277 3 298 4
 48130410b 48 1284 0.342 0.004 0.0446 0.0005 0.88 438 27 281 3 299 3
 50130410b 11 245 0.397 0.007 0.0516 0.0006 0.70 443 36 324 4 339 5

3.2 Results

Monazite grains were generally euhedral and yellowish. Thirty grains were analysed and the data are reported in Table 1. In a 206Pb/238U versus 208Pb/232Th diagram, they all plot in a concordant to sub-concordant position (Fig. 4a) and define a Concordia age (Ludwig, 1998) of 293.66 ± 0.96 Ma (MSWD = 18), which is within the error with the mean average 208Pb/232Th date of 291.9 ± 3.3 Ma (MSWD = 0.89).

Fig. 4

206Pb/238U versus 208Pb/232Th Concordia diagram for monazites (A) and Terra Wasserburg diagram (B) for zircons from sample ES5. All ellipses are represented at 1 sigma level. All ages are quoted at 2 sigma level.

Diagramme Concordia 206Pb/238U versus 208Pb/232Th pour les monazites (A) et diagramme Terra Wasserburg pour les zircons (B) de l’échantillon ES5. Toutes les ellipses d’erreurs sont représentées à un sigma. Tous les âges sont donnés à deux sigmas.

Zircons were translucent to pinkish, euhedral and generally elongated. Cathodoluminescence imaging of the grains revealed a bright core with magmatic zoning surrounded by darker rims (Fig. 3). Twenty-two analyses were performed on twenty grains (Table 1). During the course of the analyses, several zircons showed the presence of common Pb, but no correction was applied. Plotted in a Terra Wasserburg diagram (Fig. 4b), the data do not define a simple trend with apparent dates ranging from ca. 500 Ma down to 295 Ma. A first group with two analyses (24130410b and 25130410b) defines a date around 496 ± 15 Ma. A second group with three analyses (04130410b, 05130410b and 50130410b) defines a date of 323 ± 5 Ma. The last group defined by six concordant analyses allows one to calculate a Concordia age of 294.4 ± 2.6 Ma (MSWD = 0.51; Fig. 4b, insert). The remaining points plot in a sub-concordant to discordant position. We believe that their position can be linked to the combined effect of the presence of “common” Pb incorporated in some of the grains and a slight Pb loss. The presence of initial “common” Pb in some of the grains is attested by the fact that several zircon grains present a slight (few tens of counts) positive 204Pb value. This common Pb might have been present in some small inclusions not detected during imaging or related to fractures or areas of radiation damage. In the Terra Wasserburg diagram (Fig. 4b), these data, together with the previous six concordant allow a discordia to be drawn with a lower intercept of 293.7 ± 2.3 (MSWD = 0.88). In light of the present data, which are all consistent within error regardless of whether common Pb was detected or not, we thus conclude that zircon crystallisation took place 294 ± 3 Ma ago. In this scenario, the remaining older ages must be attributed to inheritance. This age of 294 ± 3 Ma is also identical to the age of 294 ± 1 Ma found for the monazites. Consequently, we conclude that the Montalet granite was emplaced during the Early Permian, ca. 294 Ma ago.

Fig. 3

Cathodoluminescence images of some of the zircon grains dated in this study.

Photos en cathodoluminescence de certains des zircons datés lors de cette étude.

4 Discussion – conclusion

Petrological and structural data show that the Montalet granite is a syntectonic intrusion that cooled progressively during a continuous deformation from magmatic to solid states. The 294 ± 1 Ma age of the Montalet granite demonstrates that it was emplaced during the Late Paleozoic extension in the ductile lower crust, contemporaneously with the: (1) ductile shearing event related to the Espinouse detachment and; (2) sedimentation within the Stephanian-Permian Graissessac-Lodève basin.

Indeed, similar 40Ar/39Ar ages of 297 ± 3 Ma have been obtained by Maluski et al. (1991) on biotite and muscovite from sheared orthogneisses, in the footwall of the Espinouse detachment, and were also interpreted as S-C mylonites (Beaud, 1985; Burg et al., 1994; Echtler and Malavieille, 1990; Van Den Driessche and Brun, 1992). Bruguier et al. (2003) published an age of 295 ± 5 Ma for zircons extracted from a volcanic ash layer interbedded in the Late Carboniferous sedimentary fill of the Graissessac-Lodève basin, developed in the hangingwall of the Espinouse detachment (Fig. 1). Similar ages, ranging from 295 to 300 Ma have also been reported by the same authors in other Late Carboniferous extensional basins of the southern French MC, demonstrating an intense magmatic activity during this period (Bruguier et al., 2003).

On the other hand, Bé Mézème (2005) and Faure et al. (2010) have analysed monazites and zircons from the same pluton, but which was sampled in three sites located several kilometres farther to the south-west (Fig. 1).

Zircons were dated at 324 ± 3 Ma by SIMS and monazites at 327 ± 7 Ma by EPMA. At this stage, it has to be noted that four concordant to sub-concordant zircons analysed by Faure et al. (2010) plot in a “younger” position than the nine grains used to calculate the age of 324 ± 3 Ma (Figure 12C of Faure et al., 2010). A mixing discordia between common Pb and radiogenic Pb for these four “younger” points yields a date of 305 ± 29 Ma (MSWD = 0.78). The significance of these four points has not been discussed in the paper. However, it is interesting to note that this date of ca. 305 Ma is within the error of the age of 294 ± 3 Ma found for the zircons in this study. Furthermore, we also demonstrate the existence of inherited cores within our zircon population that are dated at ca. 323 Ma.

As for the monazites, Faure et al. (2010) analysed three different samples of the Montalet granite. Two of them yielded EPMA ages of 333 ± 4 Ma and 327 ± 7 Ma, respectively. For this second sample, it is interesting to note the discrepancy between the U-Pb age (289 +43/–49 Ma) and the Th-Pb age (349 +28/–25 Ma) and the fact that the theoretical isochron is barely within the error envelope. The last sample yielded an EPMA age of 499 ± 6 Ma. This indubitably shows the presence of inherited monazites within at least one of their samples (which they acknowledge on page 665). Because of the intrinsic principles of the EPMA technique, one cannot therefore exclude the presence of inherited Pb (208Pb?) within some of the monazites from the remaining two samples, which would have resulted in a meaningless older age.

Another explanation to account for the difference between the ages published by Faure et al. (2010) and the ages found in this study could be that we dated two different intrusions corresponding to two magmatic pulses that are separated in time, yet mapped as a single intrusion known as the Montalet granite.

Nevertheless, irrespective of the origin of the discrepancy between these ages, the age of 294 ± 1 Ma found for the syntectonic Montalet granite confirms the existence of a major magmatic event that accompanies crustal extension during the Late Palaeozoic in the southern French MC (Van Den Driessche and Brun, 1989).

The cause and the modes of the extension that is supposed to be responsible for the decay of the Variscan belt are still debated, but all studies agree that the period between 330 Ma and 290 Ma is a critical one: it corresponds to the vanishing of plate convergence and the onset of crustal extension (e.g. Burg et al., 1994). The present study emphasizes the absolute need for precise dating to refine this evolution and decipher the possible causal relation between these two major tectonic processes.


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