3.1 Fluid inclusion analysis

Microthermometric measurements were performed with a Linkam THMS600 heating–freezing stage mounted on a BX-51 Olympus microscope at the Geosciences Environnement Toulouse (GET) laboratory and the GeoRessources laboratory (Nancy, France). Temperatures of the following phase changes were measured: eutectic melting (T_e), ice melting (T_m ice), and total homogenization (T_h). According to the calibration curves and the type of phase changes, temperatures are given with an accuracy of about ±5 ^∘C for T_e, and ±0.2 ^∘C for T_m ice and T_h. Fluid composition analyses of inclusions in quartz were carried out by LA-ICP-MS analyses at the GeoRessources laboratory (Nancy, France) following protocols published in Leisen et al. (2012). The LA-ICP-MS system is composed of a GeoLas excimer laser (ArF, 193 nm, Microlas, Göttingen, Germany), with a microscope for sample observation and laser beam focus onto the sample and an Agilent 7500c quadrupole ICP-MS. The sample is located inside a cylindrical ablation cell, attached to a motorized X–Y stage of an optical microscope (^®Olympus BX-41). The laser beam is focused onto the sample with a Schwarztschild reflective objective (magnification ×25). The ablated material is carried in helium gas, which is mixed with argon via a cyclone mixer prior to entering the ICP torch, following the procedures outlined by Roedder (1984) and Shepherd et al. (1985). Individual quartz and monazite crystals were double-polished, and ∼200 to 300 µm thick sections were prepared for fluid inclusion studies (Fig. 3a and b).

Figure 3Photographs and microphotographs (c, d, e , f) of cleft quartz (Qz) and monazite (Mnz) crystals and their fluid inclusions. Photographs (a, b) show contiguous quartz and monazite crystals used for fluid inclusion microanalyses (samples E2R4 et E2R2, respectively). Microphotographs (c, d) show fluid inclusions in monazite; note that primary fluid inclusions (MnzP; around 10 µm) are found in limpid domains whereas secondary fluid inclusions (MnzS; < 5 µm) form trails; microphotographs (e, f) show fluid inclusions in quartz: the two populations of primary inclusions (QzP1 and QzP2) have similar morphologies (euhedral with typical size of 10 µm) and are found in limpid quartz domains, while the secondary fluid inclusions are smaller and found aligned along trails. Black mineral fibres are meneghinite (Pb₁₃CuSb₇S₂₄; Moëlo et al., 2002).

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3.2 Zircon fission tracks (ZFTs)

In sample R1 (cleft contact), the smaller grain size (higher strain) resulted in a low zircon yield. Because many zircons were fractured, had strong uranium zoning and/or inclusions, only 7 zircons could be extracted from sample R1, compared to up to 12 zircons in the other samples (R2 and R3). Zircon grains were mounted in Teflon^® sheets, polished, and etched at 228 ^∘C for 16–40 h in a NaOH-KOH melt. Using the external detector method, all samples were irradiated together with IRMM541 dosimeter glasses and Fish Canyon Tuff age standards at the FRM II reactor in Garching, Germany. External detectors were etched for 18 min in 48 % HF at 20 ^∘C. Fission tracks were counted at the ISTerre Thermochronology laboratory with an Olympus BX2 microscope at 1250× magnification using the FTstage 4.04 system.

4.1 Fluid inclusion results

In cleft monazite, primary inclusions occur isolated or form trails of 2–3 inclusions and secondary inclusions are located in healed fracture planes (Fig. 3c and d). The size of fluid inclusions ranges from 10 µm×10 µm to less than 4 µm×4 µm for secondary inclusions. In quartz crystals, primary inclusions are also isolated or aligned along trails of 4–5 inclusions parallel to quartz growth faces, whereas secondary inclusions are located on former fissures that cross-cut the crystals from rim to core (Fig. 3e and f). Quartz populations show euhedral shapes with no evidence of deformation microstructures (Diamond and Tarentola, 2015). The size of primary fluid inclusions in quartz is mostly comparable to that of monazite ($\sim \mathrm{10}\mathrm{\mu }\mathrm{m}\times \mathrm{10}$ µm) but can reach $\sim \mathrm{20}\mathrm{\mu }\mathrm{m}\times \mathrm{30}$ µm. Secondary fluid inclusions are smaller than 5 µm×5 µm. All the fluid inclusions investigated are bi-phased, liquid and vapour (L-V type), and homogenize to liquid by heating. No solid phase was observed in the studied fluid inclusions.

Table 1Microthermometric analyses of fluid inclusions in monazite (Mnz) and quartz (Qz).

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Figure 4The T_h vs. T_m ice diagram of the fluid inclusions data. QzP1: population 1 of primary fluid inclusions in quartz; QzP2: population 2 of primary fluid inclusions in quartz; QzS: secondary fluid inclusions in quartz; MnzP: primary fluid inclusions in monazite; MnzS: secondary fluid inclusions in monazite.

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Figure 5Na∕K versus Na∕Li ratio measured in quartz fluid inclusions (QzP2). The dashed line corresponds to geothermometry calibrations from Verma and Santoyo (1997). The dataset indicates a temperature growth at 300–320 ^∘C.

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Based on the fluid inclusions petrography and microthermometric results, five different sets of fluid inclusion populations were distinguished in monazite and quartz (Fig. 4; Table 1). In cleft monazite (three crystals), primary fluid inclusions (MnzP) have T_h ranging from 283.2 to 345.3 ^∘C (N=10). Three secondary inclusions in monazite crystals (MnzS) were large enough for microthermometric measurements, which give T_h between 203.8 to 241.6 ^∘C (N=3). In cleft quartz (five crystals), two populations of primary inclusions with different T_h were found: the first (QzP1) corresponds to few inclusions with higher T_h (278.3 to 314.9 ^∘C; N=4) compared to the main second group (QzP2; 178.3 to 223.3 ^∘C, with a mode around 210 ^∘C; N=46). These two populations are morphologically similar. Secondary inclusions located in quartz samples (QzS) give lower T_h (120.6 to 147.7 ^∘C; N=8). Eutectic temperatures were determined between −21.2 and −23 ^∘C, which is in good agreement with T_e for H₂O-NaCl and H₂O-NaCl-KCl systems, respectively at −21.2 and −22.9 ^∘C (Bodnar, 2003; Hall et al., 1988). For the entire dataset, T_m ice obtained in quartz and monazite crystals overlap within uncertainty: they range from −6.0 to −9.6 ^∘C (Fig. 4), which converts to salinities between 9.2 and 13.5 wt % NaCl equivalent. There is no significant difference in salinities between the five fluid inclusion populations, although fluid inclusions trapped at lower temperature generally have lower salinities. Based on similar T_m ice and T_h values (Fig. 4), the first population of quartz inclusions (QzP1) is considered equivalent to primary inclusions in monazite (MnzP), whereas the secondary fluid inclusion population in monazite (MnzS) overlap with the main group of primary inclusions in quartz (QzP2). Isochores were calculated using equations of Zhang and Frantz (1987) assuming negligible CO₂ (Poty et al., 1974). MnzP and QzP1 fluid inclusion populations give P–T relationships at 11.0 and 11.8 bar ^∘C⁻¹, MnzS and QzP2 populations give 16.3 and 17.0 bar ^∘C⁻¹ and QzS gives 21.4 bar ^∘C⁻¹ (Fig. 6).

Table 2Compositional data of fluid inclusions (QzP2) obtained from LA-ICP-MS measurements with Na fixed at 43 300 ppm for all inclusions. Temperatures (T) were estimated using the Na∕K geothermometers of Can (2002).

DL: detection limit

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Figure 6Geothermobarometric results obtained for fluid inclusions of the Alpine cleft. Mean isochoric P–T relationships calculated from microthermometric measurements of fluid inclusions in monazite and quartz showing three main stages of growth. Fluid inclusion populations and number of analyses are indicated. P–T estimation for the main episode of quartz growth (QzP2) is obtained by the intersection of the T interval determined by the Na∕K geothermometer (300–320 ^∘C) with the QzP2 isochore. It gives a pressure estimates of 1.5–2.2 kbar for the main phase of quartz growth, that was used to determine the maximal T range (410–520 ^∘C) for monazite (MnzP) growth.

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In addition to microthermometric data, fluid composition was measured in primary quartz inclusion (Table 2) to evaluate the trace element concentrations and use the Na∕K and Na∕Li geothermometers (Can, 2002). Amongst the 47 fluid inclusions analysed in quartz, compositions could be retrieved for only 14 of them, due to the low-intensity signal and the presence of mineral inclusions. Fluid inclusion compositions were successfully obtained only in the fragments of centimetre-sized quartz (E2R3a, E2R31a, and E2R31b). In these quartz fragments, only fluid inclusions with T_h of the QzP2 group were detected and thus fluid inclusion compositions are attributed to this population (Table 1). In the fluid inclusions, only Na, K, Li, and Sr concentrations are systematically above the detection limit of the LA-ICP-MS measurements. The rare earth elements (Y, La, Ce, Pr, Nd, Sm), Th, U, As, and Ca are generally below the detection limits. The average T_m ice obtained for the QzP2 fluid inclusions (−7.3 ^∘C) was used to calculate the Na concentration of 43 300 ppm. This value of 43 300 ppm Na was used as an internal standard for the calculation of Li, K, and Sr concentrations (Table 2). Although fluid inclusions show different cationic contents, the Na∕K ratio in the fluid is equivalent for all the studied crystals (3.7 ± 0.3), confirming that equilibrium between feldspar and fluid is achieved. This Na∕K ratio corresponds to T of 280–330 ^∘C with most values comprised between 300 and 320 ^∘C (Table 2). A similar temperature interval can be obtained based on the Na∕K and Na∕Li ratios (Fig. 5) using the geothermometer of Verma and Santoyo (1997). Intersection of this T interval with the QzP2 isochore yields a trapping pressure at 1.5–2.2 kbar (Fig. 6).

4.2 Zircon fission track (ZFT) results

The R1 sample gave a ZFT central age of 10.3 ± 1 Ma (Fig. 7a), with an age range between 7.2 and 16.7 Ma. The R2 sample gave a ZFT central age of 16.3 ± 1.9 Ma (Fig. 7b), for an age range between 10.2 and 50.4 Ma. The R3 sample gave a ZFT central age of 14.3 ± 1.6 Ma (Fig. 7c), with individual grain ages between 5.2 and 20.1 Ma. Normally, the objective is to date at least 20 grains per sample, which was not possible because of poor sample quality with few zircons available, and many grains with strong U zoning and inclusions. Therefore only a rather limited number of grains could be analysed per sample, resulting in a relatively high dispersion in the grain age distributions and low χ² values. Unfortunately, no significant number of horizontal track lengths could be measured to determine the degree of partial annealing in all three samples. Nonetheless, the fission tracks in sample R1 appeared shorter than in the other two samples. Because of all these limitations the ZFT ages provided here should be viewed with caution; particularly the two samples collected further away from the cleft may have been affected by partial annealing only. Nonetheless, the observed single grain ages and central ages indicate that these zircons experienced cooling since the mid-Miocene.

Figure 7Radial plots of zircon ages in the granite host rock. (a) R1 sample (cleft hanging wall); (b) R2 sample (∼30 m from the cleft); (c) R3 sample (∼100 m from the cleft).

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5.1 Hydrothermal activity in the Lauzière Alpine cleft

Alpine clefts correspond to fissures that opened up under deformation related to exhumation of the ECM (Mullis et al., 1994; Poty et al., 1974). In Alpine-type clefts, hydrothermal minerals generally precipitate under retrograde metamorphic conditions at temperatures and pressures in the range of 150–450 ^∘C and 1.4–3.0 kbar, respectively (Fabre et al., 2002; Mullis et al., 1994; Poty et al., 1974). Cleft mineralogy and geochemistry show the equilibrium with the neighbouring host rocks (e.g. Sharp et al., 2005). Furthermore, fluid inclusions (e.g. Mullis et al., 1994) and mineral dating (e.g. Janots et al., 2012) indicate in general episodic crystallization pulses rather than continuous precipitation due to progressive cooling. In this study, homogenization temperatures and petrographic observations (primary or secondary) of the fluid inclusions indicate three main stages of fluid inclusion entrapment (Fig. 4):

1. MnzP + QzP1 with highest homogenization temperatures (278–345 ^∘C),

2. QzP2 + MnzS with intermediate homogenization temperatures (178–242 ^∘C),

3. QzS with the lowest homogenization temperatures (121–148 ^∘C).

All fluid inclusions have equivalent T_m ice (equivalent salinities) suggesting equilibrium between the fluid and the host rock. Equilibrium is also supported by the consistency of the Na∕K ratio measured in the quartz fluid inclusions (QzP2).

Opposite to paragenetic observations that generally suggest a late monazite crystallization (e.g. Gnos et al., 2015), the petrological observation of quartz lying on monazite and primary fluid inclusions in monazite (MnzP, N=10; Fig. 4) indicate that monazite precipitates in an early hydrothermal growth stage in the Alpine cleft studied here. This first precipitation stage is independently confirmed by few primary fluid inclusions in quartz (QzP1, N=4).

Figure 8Pressure–temperature (P–T) path for the Lauzière granite host rock and in the hanging wall of the Alpine-type cleft. Metamorphic peak conditions are not well constrained in the Lauzière massif, and are based on a compilation of metamorphic and geochronological data in the external massifs from the Western Alps (Rolland et al., 2003; Rossi et al., 2005; Simon-Labric et al., 2009; Bellanger et al., 2015). Geochronological constrains provided in this study are represented by stars. The zircon fission track (ZFT) thermochronometer fixes the cooling of the granite (R2-R3) and the cleft (R1) below 240–280 ^∘C. Microthermometric and geochronological results on monazite (Mnz) are assumed to represent the onset of hydrothermal activity (red box), while the main hydrothermal growth phase is based on a microthermometric study of the fluid inclusions in quartz (QzP2).

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The main episode of quartz growth (QzP2, N=46) is recorded by secondary fluid inclusions in monazite (MnzS, N=3; Fig. 4), indicating that monazite growth had ceased by then (Fig. 8). This second episode of hydrothermal growth may be caused by oversaturation reached due to cooling, pressure changes, fluid mixing, or deformation (e.g. discussion and references in Bergemann et al., 2017). Geothermometry based on the Na∕K ratio (and Na∕Li) in quartz fluid inclusions (QzP2) indicate that main quartz growth occurred at around 300–320 ^∘C (Fig. 5) at P= 1.5–2.2 kbar (Fig. 6). The pressure interval of 1.5–2.2 kbar determined from this group is coherent with previous micrometric studies on fluid inclusions in the ECM (Fabre et al., 2002; Mullis et al., 1994; Poty et al., 1974), and the crustal depth estimated from thermochronological data and assumed geothermal gradients for the Belledonne and surrounding massifs (Fügenschuh and Schmid, 2003; Glotzbach et al., 2011; Seward and Mancktelow, 1994).

Alpine-cleft monazite precipitation temperature can be evaluated from the pressure determined for the main phase of quartz growth (QzP2) assuming no major exhumation occurred in the relatively short duration episode (< 3 Myr, see hereafter) between the main phases of monazite (MnzP) and quartz growth (QzP2). Using the pressure interval of 1.5–2.2 kbar determined with QzP2, intersection with the MnzP isochores indicates a maximal T interval of 410–520 ^∘C for monazite growth (Fig. 6). This is one of the rare cases where fluid inclusions document a significantly higher fluid temperature compared to the metamorphic host-rock temperatures (Mullis et al., 1994; Boutoux et al., 2014). This hot fluid circulation would have been overlooked based on quartz fluid inclusions, the number of QzP1 being unrepresentative compared to the QzP2 trapped during the main phase of quartz growth (Fig. 4). To our knowledge, this is one of the first times that fluid inclusions are measured in monazite and it opens a possible avenue to determine early (possibly hot) stages of hydrothermal precipitation in Alpine clefts. Early precipitation of monazite compared to quartz, may be attributed to its low solubility, but this has to be confirmed in other clefts or for other low-solubility accessory minerals.

5.2 Impact of advective heating on ZFT age resetting

Novel ZFT ages obtained in the Lauzière massif (samples R2 and R3) confirm that metamorphic temperatures were high enough to anneal the pre-Alpine ZFT age signature (Yamada et al., 1995). The ZFT dataset indicates that the Lauzière granite cooled below 240–280 ^∘C at around 14–16 Ma (Figs. 7 and 8), in good agreement with previous ZFT data in the Belledonne massif (8–15 Ma; Fügenschuh and Schmid, 2003; Seward and Mancktelow, 1994; Fig. 1b). In the cleft hanging wall, the rejuvenation of the ZFT age at 10.3 ± 1.0 Ma indicates a resetting of the ZFT geochronological system by advective heating due to the hot hydrothermal fluid penetrating the cleft. This result is similar to previous conclusions reached in the Nojima fault, where modification of the zircon fission track lengths are interpreted as consequences of ancient thermal overprints by heat transfer or dispersion via fluids in the fault zone (Tagami and Murakami, 2007). As in our study, the effect appears extremely local since it is not seen in samples taken in the vicinity of the fault, especially in the footwall (< 0.1 m).

It is difficult to quantify the spatial impact of advective heating linked to fluid circulation at the outcrop scale. In the present study, the two samples taken at quite some distance from the cleft (30 and 100 m) have ZFT ages older than monazite growth in the cleft, indicating no significant resetting by related cleft fluid circulation. However, along the eastern margin of the Lauzière granite, fluid circulation is not restricted to clefts but is evidenced in outcrops by retrogression and growth of mica-like phyllosilicates, as shown in the Mont Blanc massif by Rolland and Rossi (2016), and the formation of centimetre-scale veins. It is thus not possible to rule out that the ZFT dataset of the deformed Lauzière granite was not affected by fluid circulation during the deformation phase responsible of the cleft formation or even during an earlier ductile deformation stage.

5.3 Geodynamic evolution and fluid circulations of the ECM

The ECMs consist of relatively anhydrous and low-permeability rocks underthrusted at mid-crustal levels during the Oligo-Miocene and then exhumed to form some of the highest Alpine relief today (> 4000 m high peaks). In the Lauzière granite, the metamorphic peak, which is not well constrained but estimated at temperatures of < 400 ^∘C, is assumed to have taken place at around 24–26 Ma (Nziengui, 1993) and more generally at around 24–30 Ma in the ECM. Exhumation in the ECM is normally assumed to start around 18 Ma (Boutoux et al., 2016). Indeed, the new ZFT ages may indicate that the Lauzière granite had cooled below the ZFTclosure temperature of about 240–280 ^∘C during the mid-Miocene (Fig. 8).

The closure temperature is considered as the temperature at which the fission-track system closes to the loss of fission tracks by annealing and is applicable in the case of monotonic cooling (Dodson, 1973). The idea is that no fission tracks are preserved in the zircon crystals at elevated ambient temperatures, but start to be retained as soon as the crystal cools below the effective closure temperature. The actual value of the closure temperature for the zircon fission-track systems depends on the rate of cooling and the amount of accumulated radiation damage (Bernet, 2009; Brandon et al., 1998; Rahn et al., 2004; Reiners and Brandon, 2006; Tagami et al., 1998). For natural, radiation-damaged zircons and typical Alpine cooling rates of 10–20 ^∘C Myr⁻¹ the closure temperature is about 240 ± 5 ^∘C, whereas for zircons with no or very low amounts of radiation damage the closure temperature is about 340 ± 10 ^∘C for the same cooling rates (Reiners and Brandon, 2006). The closure temperature should not be confused with the partial annealing zone, which is the temperature range over which fission tracks are partially but not fully annealed, either during reheating or during very slow cooling through this temperature range (e.g. Reiners and Brandon, 2006). If heating during the hydrothermal event was sufficiently high to anneal fission tracks completely in the zircons analysed in this study, then the ZFT cooling ages will reflect post-hydrothermal event cooling mainly related to exhumation, given that the hydrothermal heating event is relatively short-lived and with limited thermal impact of the surrounding country rock on a regional scale. Assuming a general regional geothermal gradient of ∼25 ^∘C km⁻¹ and a surface temperature of ∼10 ^∘C the rocks of the Lauzière granite may have been exhumed from crustal depths of < 10 km since 14–16 Ma, (Fig. 8).

In the Alpine cleft, the well resolved Th-Pb monazite age of 12.4 ± 0.1 Ma (N=86) indicates a short-duration growth of the millimetric monazite crystals (Grand'Homme et al., 2016). Combined with the new microthermometric data, the monazite age is interpreted to date the first stage of hydrothermal growth following the fracturing and infiltration of a hot fluid (410–520 ^∘C) into the host rock that had already cooled below 240–280 ^∘C (Fig. 8). In other words, there is a 150–250 ^∘C temperature difference between the fluid from which monazite precipitates and the fracturing host rock, in which fluid infiltrates. Assuming an undisturbed crustal geothermal gradient, it implies that the source of the fluid was at least 6–10 km deeper than the host rock. Based on the structural data of the sub-vertical cleft (Fig. 2), this age records the circulation of hot fluids through fractures created during a regional phase of dextral transpression in the ECM (Bergemann et al., 2017; Gasquet et al., 2010). Within the Alpine cleft, quartz growth occurred during a second hydrothermal growth stage at T= 300–320 ^∘C, comprised between the monazite growth at 12.4 ± 0.1 Ma and the cooling of the cleft hanging wall (sample R3) below 240–280 ^∘C at 10.3 ± 1 Ma (Fig. 8). The difference between these two ages constrains the time range between the infiltration of the hot fluid and cooling down of the cleft wall to temperatures similar to the host rock, i.e. it limits the duration of advective heating to around 1–3 Myr (Fig. 8), but given the uncertainty in the ZFT age, the heating interval may have been even shorter.

Although the difference between fluid and metamorphic host-rock temperatures (150–250 ^∘C) appears unusually high compared to thermal fluid regimes previously documented by fluid inclusions in Alpine-type clefts (Mullis et al., 1994; Poty et al., 1974), deep fluid circulation has been already proposed for some other Alpine-type clefts or veins using isotopic and trace-element data (Rossi and Rolland, 2014). This thermal difference attests to the fluid channelization along pathways of high permeability for upward flow of deep mid-crustal fluids towards the surface. The required fluid channelization is expected along steeply oriented deformation structures like shear zones, faults, and fractures (e.g. Boutoux et al., 2014; Goncalves et al., 2012; Marquer and Burkhard, 1992; Oliot et al., 2014). Considering the position of the Lauzière granite, possible fluid pathways through deeper crustal levels could localize westward along the major Ornon-Roselend fault, or southward along the Fond-de-France fault (Fig. 1b). Based on the mid-crustal depth required for the fluid circulations, it is unclear whether the fluids could be originated from topography-driven circulations of meteoritic water (Diamond et al., 2018; Hofmann et al., 2004; Raimondo et al., 2013) or rather liberated by underthrust rock dehydration due to metamorphic reactions. Dehydration reactions, with a likely origin in the underthrust metasediments, could be a good candidate to account for the episodic short-duration monazite precipitation observed in Alpine clefts in the ECMs (Grand'Homme et al., 2016). Further relationships between seismicity, fluid circulation, and metamorphism could also be considered here (Putnis et al., 2017).