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).

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.

3.1 Analytical techniques

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.

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.

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.

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.