The North Pyrenean Metamorphic Zone (NPMZ) is a narrow zone located immediately north of the North Pyrenean Fault (NPF; Fig. 1a). This fault limits the axial zone from the NPMZ. The exact nature of this structure remains unclear in the context of the extreme thinning of the Pyrenean paleo-margin during the eastward movement of Iberia relative to Europe during Albian times (Choukroune, 1992; Choukroune et al., 1973; Olivet, 1996). Close to the NPF, on its northern side over a length of about 150 km, forty lherzolitic bodies of variable size are reported (Fabriès et al., 1998; Monchoux, 1970). A mid-Cretaceous crustal emplacement age of these bodies has been proposed (105–110 Ma; Henry et al., 1998). Serpentinization of peridotites and development of ophicalcites are abundant (Clerc et al., 2014). Locally, discrete eruptive and plutonic bodies of alkaline affinity were emplaced in the area, especially in the central and western Pyrenees (Montigny et al., 1986). During the Albian period again, intense hydrothermal activity is evidenced throughout the area by 1) numerous talc/chlorite ore deposits (Boulvais et al., 2006; Moine et al., 1989) with a mineralization age of 112–97 Ma (Schärer et al., 1999) and 2) albitization and sodic-calcic metasomatism of rocks of variable nature with ages ranging from 117 to 92 Ma (Boulvais et al., 2007; Fallourd et al., 2014; Poujol et al., 2010).

Particularly well exposed in the NPMZ are quartz – calcareous black shales of Albian age, deposited in basins north of the NPF including the Boucheville Basin (Fig. 1b). The initial geometry of the basins in the Albian is described in detail by Clerc and Lagabrielle (2014) and Chelalou et al. (this issue). Inversion of the basins caused by northward motion of Iberia began during the Upper Cretaceous (Choukroune, 1976). The Albian black shales have been variably metamorphosed up to HT-LP metamorphic conditions (3 kbar–4 kbar; 500 °C–600 °C; Golberg and Leyreloup, 1990). Ages ranging from 98 Ma to 87 Ma (Cenomanian – Turonian) have been obtained by Ar–Ar dating on minerals separates from Mesozoic metasediments (Albarède and Michard-Vitrac, 1978; Golberg and Maluski, 1988). The recent synthesis of Clerc et al. (2015) highlights the east-west thermal zonation within the NPMZ, the maximum thermal conditions being encountered in the east part. In the Boucheville Basin, Chelalou et al. (this issue) report temperatures between 530 °C and 580 °C, with a homogeneous distribution throughout the basin. Unmetamorphosed equivalents of the Albian metamorphic rocks from the Boucheville Basin occur a few kilometres further north, in the Saint-Paul-de-Fenouillet and Bas-Agly Basins.

Samples from the Boucheville Basin have been collected in transects along road outcrops named CARY, DSX, SOU, VIR and BOUZ (Fig. 1b). For comparison, samples of both Albian metamorphic rocks (TAU) and unmetamorphosed Albian shales (AXAT and CDF) have been collected to the west and to the north of the Boucheville Basin (Fig. 1), respectively. Numerous calcite veins with large euhedral crystals formed in association with Late Cretaceous–Eocene reverse faulting are not considered in this study.

The unmetamorphosed Albian sediments are quartz (silty) – calcareous black shales. They are comprised, in variable amounts, of calcite, quartz, clays (chlorite and sericite have been identified by X-Ray diffraction; Ravier, 1959) and organic matter. In Fig. 2, their heterogeneity is highlighted by the large range in CaO and CO₂ contents, which can be interpreted by mixing between calcite and a silicate detrital phase. The mineralogical and geochemical heterogeneity of the Albian sediments is also observed in their metamorphic counterparts (Fig. 2); the metamorphic rocks have lower CO₂ contents for a given CaO content. This is likely related to the release of CO₂ through decarbonation reactions during metamorphism. The Albian metamorphic rocks comprise variable amounts of Cal, Qtz (from 10% to 50%), An-rich plagioclase (Pl) (from 0% to 45%), biotite (Bt) (from 0% to 15%), graphite (Gr), minor amounts of tourmaline, sulphides and titanite. Scapolite is present in calcite-rich marble layers.

The veins are composed of calcite and quartz, in variable amounts (Table 1). The length of veins ranges from 1 m to 10 m and their width from 0.5 cm to 10 cm. They are generally deformed, folded (Fig. 3a) and/or transposed parallel to the foliation; none cross-cuts foliation at high angle. Foliation is sometimes visible in veins (Fig. 3b). Taken together, all this information shows that veins formed early during the deformation history of the basin, this deformation being associated with lithospheric stretching during mid-Cretaceous times (Clerc et al., 2015). The calcite content of veins seems related to the calcite content of the local host rocks: for example, the veins with the lowest calcite content (samples SOU 01-160a and SOU 01-163a) are hosted in rocks with a very low content of Cal, lower than 20 wt% (Table 1). In host rocks, a selvage is commonly observed at the contact with veins (Fig. 3c), which corresponds to a decrease in the calcite content. Biotite and sulphides sometimes develop in veins (Fig. 3c), in which case these minerals are also present in significant amounts in the host rock. From their structure and mineralogy, we can infer that the veins developed early in the regional deformation event and at elevated temperatures, sufficient to crystallize biotite, in relation to the NPM. This conclusion was also reached by Ravier (1959) and Choukroune (1976).

The O and C isotope compositions were measured using a VG SIRA 10 triple collector mass spectrometer at the University of Rennes-1, on the CO₂ released during the reaction of calcite with anhydrous H₃PO₄ at 25 °C (McCrea, 1950). The calcite content was calculated by measuring with an Hg manometer the amount of CO₂ released. The absolute precision of calcite content estimate is ± 5%. When sulphides are present in samples, the H₂S liberated during reaction was removed by reaction with Ag₃PO₄ at around 50 °C for 5 min by forming silver sulphate. For the oxygen isotope analysis of the silicate fraction of veins and black shales, the carbonate fraction was removed through leaching with 0.15 N HCl for 3 h. Oxygen was then extracted from the residual silicate fraction following the method of Clayton and Mayeda (1963). BrF₅ was used as an oxidising agent. NBS 19 (limestone), NBS 28 (Qtz) and internal-lab standard references materials were continuously measured during the course of this work. NBS 19 measured values (n = 12) were δ¹⁸O = 28.70 ± 0.11‰ (vs SMOW) and δ¹³C = 1.91 ± 0.06‰ (vs PDB). No correction was added to carbonate results. NBS 28 value was δ¹⁸O = 9.21 ± 0.09‰ (n = 4). A correction was therefore made accordingly on the oxygen composition of silicates. Replicate analyses of materials gave a reproducibility better than 0.1‰ for both oxygen and carbon isotope composition of carbonates and of about 0.2‰ for oxygen of silicates. Strontium analyses were performed on the soluble fraction of rocks, which is presumed to represent carbonate, in 0.15 N HCl for 1 h. Sr isotope ratios were measured on a Finnigan MAT 262 mass spectrometer at the University of Rennes-1. ⁸⁷Sr/⁸⁶Sr ratios were normalised to ⁸⁶Sr/⁸⁸Sr = 0.1194. NBS 987 is used as a routine standard and give a long-term value of ⁸⁷Sr/⁸⁶Sr = 0.710250. The Sr blank was 30 pg and was considered negligible.

The O, C and Sr isotope compositions of Albian black shales and veins are reported in Table 1. The Albian metamorphic rocks of the Boucheville Basin show a limited range of δ¹⁸O_Cal values from 25 to 21‰ and a more wide range of δ¹³C from +3‰ to –6‰ (Fig. 4a), whereas unmetamorphosed Albian rocks have δ¹⁸O_Cal values between 23.6‰ and 25.9‰ and δ¹³C values between 0.6‰ and 1.5‰, values equal to or higher than their metamorphic equivalents (Fig. 4a). Samples TAU are undistinguishable from the samples from the Boucheville Basin. Both δ¹⁸O_Cal and δ¹³C of metamorphic rocks show a good correlation with the weight amount of calcite (Fig. 4b). In the Albian metamorphic rocks, the calcite fraction has a higher δ¹⁸O value than the silicate fraction by about 1‰ to 1.5‰. Using the descriptions of Ravier (1959) we have estimated a mean modal composition for this silicate fraction in the Albian metamorphic rocks: 50% Qtz, 10% Bt and 40% Pl (bytownite). The theoretical equilibrium curves between calcite and such a silicate component are given in Fig. 5, using the fractionation factors of Zheng (1993a,b). We see that the calcite and the silicate phase of the Albian metamorphic rocks are in isotopic equilibrium at temperatures about 200 °C–300 °C. The Albian unmetamorphosed black shales consistently plot well below the curves.

The veins display a covariation of δ¹⁸O and δ¹³C that mimics the Albian metamorphic host rocks (Fig. 4a), veins having lower δ¹⁸O_Cal (Δ¹⁸O_{vein–HR(mean)} = –0.3‰; Fig. 6a) and higher δ¹³C (Δ¹³C_{vein–HR(mean)} = +1.5‰; Fig. 6b) than their host rock. In veins, calcite has lower δ¹⁸O values than quartz by about 1.5‰. This fractionation is consistent with isotopic equilibrium between calcite and quartz at temperatures of about 200 °C–300 °C (Fig. 5).

At a small-scale contact where the calcite content decreases toward the vein, δ¹⁸O_Cal and δ¹³C values vary significantly towards the vein (Fig. 7b,c). Interestingly, δ¹⁸O_Cal values increase in that direction, whereas the vein has a lower value than the host rock and δ¹³C values decrease, whereas the vein has a higher value than the host rock. None of these variations can result therefore from a simple diffusional exchange between the vein and the host rock.

The Sr isotope compositions of calcite from four veins-host rock pairs are reported in Table 1 and in Fig. 8 together with the weight amount of calcite. Two of the host rocks are biotite-rich whereas the two are biotite-poor. The Albian metamorphic rocks have (⁸⁷Sr/⁸⁶Sr)_Cal ratios between 0.7074 in biotite-poor and calcite-rich samples and 0.7094 in biotite-rich and calcite-poor ones. The lowest value is identical to that of mid-Cretaceous seawater (⁸⁷Sr/⁸⁶Sr = 0,7074; Veizer, 1989). As a rule, veins have lower (⁸⁷Sr/⁸⁶Sr)_Cal ratios than their host rock, the difference being more important in biotite-rich and calcite-poor host rocks. Along the SOU 01-160 profile (Fig. 7d), the (⁸⁷Sr/⁸⁶Sr)_Cal ratio increases slightly in the host rock towards the vein, from 0.70944 to 0.70985.

6.1 Open vs closed-system

6.2 Mode of vein formation

The Sr isotope compositions of the Albian rocks and veins also argue for a derivation of the vein material from the local host rock. For two of the four vein–host rock pairs analysed (samples BOUZ 01-141 and TAU 01-81), the ⁸⁷Sr/⁸⁶Sr ratio is nearly identical (⁸⁷Sr/⁸⁶Sr = 0.70737 ± 0.0004; Table 1, Fig. 8). No radiogenic Sr has been incorporated in the calcite phase since crystallization and no input of radiogenic Sr accompanied calcite precipitation. For the two other pairs (VIR 01-137, SOU 01-160), the Sr isotope ratio of the vein is lower than the one of the host rock (Table 1, Fig. 8). In Fig. 7d, the ⁸⁷Sr/⁸⁶Sr ratio of samples SOU 01-160b increases towards the vein. This is incompatible with an isotopic exchange between Sr of the Albian rock and Sr from the vein material as the latter has lower ⁸⁷Sr/⁸⁶Sr. We argue that the difference between veins and hosts are related to mode effects. The two samples SOU 01-160b and VIR 01-137b are biotite-rich (Table 1). This mineral formed by recrystallization of clays including sericite during metamorphism – see the equation of Goujou et al. (1988) recalled above. K-rich sericite has a high Rb/Sr ratio and consequently is characterized by an increase of its ⁸⁷Sr/⁸⁶Sr with time. During metamorphic recrystallization, isotopic equilibrium between unradiogenic Sr from calcite and radiogenic Sr from sericite and biotite is facilitated by ⁸⁷Sr diffusion through the fluid phase and recrystallization. At the time of vein formation, Cal veins freeze their ⁸⁷Sr/⁸⁶Sr ratio whereas calcite in the host rock maintains its isotopic equilibrium with the whole rock through ⁸⁷Sr exchange with Bt. The rate of ⁸⁷Sr/⁸⁶Sr increase in Cal host rock is even faster after vein formation because the amount of Cal is decreased in the host. This model of closed-system Sr isotope evolution, consistent with the information obtained from stable isotopes, is summarized in Fig. 9.

6.3 Consequences for heat transfer in the Boucheville Basin

The isotopic analysis of the quartz-calcareous Albian black shales, metamorphosed during the North Pyrenean Metamorphism, in the French Pyrenees, and the associated syn-tectonic veins network suggest the following conclusions.

1/Veins composed of calcite and quartz were formed during the NPM. Host rocks exert a control on the mineralogy of the veins. A significant portion of the elements contributing to calcite growth in veins came from the local host rocks.

2/Veins do not record any infiltration of externally derived fluids as their oxygen and carbon isotope compositions are controlled by those of the local host rocks, in a rock-dominated fluid-rock interaction system.

3/Veins represent the escape pathway of fluids generated during metamorphism through dehydration–decarbonation reaction of the Albian rocks. Metamorphism is a cause, not a consequence, of fluid circulation.

4/Fluids generated within the Boucheville Basin may have acted to homogenize the temperature field in the basin.