Ion Microprobe Dating of Fissure Monazite in the Western Alps: Insights from the Argentera Massif and the Piemontais

Ricchi et al. Swiss J Geosci (2020) 113:15 https://doi.org/10.1186/s00015-020-00365-3

Swiss Journal of Geosciences

ORIGINAL PAPER Open Access Ion microprobe dating of fssure monazite in the Western Alps: insights from the Argentera Massif and the Piemontais and Briançonnais Zones Emmanuelle Ricchi1* , Edwin Gnos2, Daniela Rubatto3,4, Martin John Whitehouse5 and Thomas Pettke3

Abstract Ion probe 208Pb/232Th fssure monazite ages from the Argentera External Massif and from the high-pressure units of the Western Alps provide new insights on its Cenozoic tectonic evolution. Hydrothermal monazite crystallizes during cooling/exhumation in Alpine fssures, an environment where monazite is highly susceptible to fuid-mediated dissolution-(re)crystallization. Monazite growth domains visualized by BSE imaging all show a negative Eu anomaly, positive correlation of Sr and Ca and increasing cheralite component (Ca Th replacing 2REE) with decreasing xenotime (Y) component. The huttonite component (Th Si replacing REE+ and P) is very low. Growth domains record crystallization following chemical disequilibrium in a fssure+ environment, and growing evidence indicates that they register tectonic activity. Fissure monazite ages obtained in this study corroborate previous ages, recording crystallization at ~ 36 Ma, ~ 32–30 Ma, and ~ 25–23 Ma in the high-pressure regions of the Western Alps, interpreted to be respectively related to top-NNW, top-WNW and top-SW thrusting in association with strike-slip faulting. During this latter transpressive phase, younger fssure monazite crystallization is recorded between ~ 20.6 and 14 Ma in the Argentera Massif, interpreted to have occurred in association with dextral strike-slip faulting related to anticlockwise rotation of the Corsica-Sardinia Block. This strike-slip activity is predating orogen-parallel dextral strike-slip movements along and through the internal part of all other External Crystalline Massifs (ECM), starting only at ~ 12 Ma. Our combined compositional and age data for hydrothermal monazite track crystallization related to tectonic activity during unroofng of the Western Alps for over more than 20 million years, ofering chronologic insights into how diferent tectonic blocks were exhumed. The data show that fssures in the high-pressure units formed during greenschist to amphibolite facies retrograde deformation, and later in association with strike-slip faulting. Keywords: 208pb, 232th fssure monazite age, Western Alps, High pressure, Argentera Massif, Tectonic activity, Hydrothermal monazite chemistry

1 Introduction Torium-Pb dating of hydrothermal monazite-(Ce), hereafter called monazite, ofers the possibility to constrain deformation along the retrograde path of a cooling orogen. Recent studies conducted in several parts of the Editorial handling: Daniel Marty. Alpine chain and in the Mexican orogen have acquired *Correspondence: [email protected] numerous T-Pb monazite crystallization ages that have 1 Department of Earth Sciences, University of Geneva, Rue des Maraîchers proven complementary to published chronological and 13, 1205 Geneva, Switzerland thermochronological data (Bergemann et al. 2017, 2018, Full list of author information is available at the end of the article

© The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativeco source: https://doi.org/10.7892/boris.147117 | downloaded: 27.12.2020 mmons.org/licen ses/by/4.0/ . 15 Page 2 of 27 E. Ricchi et al.

2019, 2020; Berger et al. 2013; Fitz-Diaz et al. 2019; Gas- results in patchy monazite replacing prior monazite (e.g. quet et al. 2010; Gnos et al. 2015; Grand’Homme et al. Grand’Homme et al. 2016b). Each dissolution, replace- 2016a; Janots et al. 2012; Ricchi et al. 2019, 2020). Hydro- ment or growth of a monazite domain corresponds to thermal (or fssure) monazite, a Light Rare Earth Element a reaction induced by chemical disequilibrium, which 3+ 4+ 4+ 5+ 2− (LREE) phosphate ((LREE ,T ,U )P O 4) that is might be related to deformation events (see above), and commonly mm-sized, crystallizes in fuid-flled Alpine- can usually be identifed texturally (e.g. patchy texture, type fssures in a temperature range of ~ 400–200 °C growth zoning, cross cutting relationships) and chemi- (Gnos et al. 2015; Janots et al. 2019). Alpine fssures form cally (distinct T/U ratio; e.g. Grand’Homme et al. 2016a; at P–T conditions at or below 0.3–0.4 GPa lithostatic Grand’Homme et al. 2016b; Seydoux-Guillaume et al. pressure and 450–500 °C (e.g. Mullis 1996; Sharp et al. 2012). Tus, dating of hydrothermal monazite growth 2005; Stalder et al. 1998). Te fssures are usually ori- domains within a single sample has the potential to iden- ented perpendicular to the main foliation and lineation tify a sequence of tectonic events. of the host-rock (e.g. Gnos et al. 2015). When a fssure Previous studies on fssure monazite grains from opens and becomes flled by aqueous fuid, chemical dis- the French parts of the Western Alps have provided equilibrium between the fuid and host-rock wall leads weighted mean ages between 11.6 ± 0.3 and 6.7 ± 0.2 Ma to dissolution of host-rock wall minerals, resulting in a in the Mont-Blanc Massif (Bergemann et al. 2019; porous and commonly bleached alteration envelope (e.g. Grand’Homme et al. 2016a), between 9.2 ± 0.3 and Gnos et al. 2015). Within the fssure, this results in the 7.9 ± 0.3 Ma in the Aiguilles Rouges Massif (Bergemann crystallization of strongly zoned hydrothermal minerals. et al. 2019), between 12.4 ± 0.1 and 5.4 ± 0.5 Ma in the Te dissolution and reprecipitation of fssure minerals is Belledonne Massif (Gasquet et al. 2010; Grand’Homme generally a cyclic process triggered when chemical dis- et al. 2016a), 17.6 ± 0.3 Ma in the Pelvoux Massif (Gas- equilibrium occurs, commonly related to tectonic activ- quet et al. 2010), 20.6 ± 0.3 Ma in the Argentera Mas- ity (e.g. fssure propagation, exposure or delamination of sif (Grand’Homme et al. 2016a) and 23.0 ± 0.3 and of fssure wall). 32.3 ± 0.3 Ma in the Briançonnais Zone (Grand’Homme In monazite-bearing fssures, quartz and adularia typi- et al. 2016a; Fig. 1). cally form in an early stage of fssure formation (< 500– Tis study was undertaken in order to confrm with 450 °C), whereas monazite crystallizes at later stages supplementary data that fssure monazites in the Argen- (between 400–200 °C) (e.g. Gnos et al. 2015). Tus, tera Massif and the Piemontais and Briançonnais zones monazite dating does not yield the formation of the fs- are older than monazites from other parts of the Western sure but only subsequent stages of hydrothermal activity, Alps. Fissures are relatively rare in these parts of the Alps. usually caused by deformation under greenschist to sub- For this reason, only fve fssure monazites were available greenschist facies metamorphic conditions. for this study (Table 1). In addition to backscatter elec- Fissure monazite is reliably dated by the 232T-208Pb tron images, we also tried to use trace and rare earth ele- system, as it typically incorporates high T contents (e.g. ments to distinguish growth domains in the monazites. Janots et al. 2012) and no Pb (e.g. Gardés et al. 2007) during its crystallization. Moreover, T and Pb difusion are 2 Geological setting negligible under hydrothermal monazite crystallization Te Western Alps are the result of the closure of the Lig- conditions (< 400 °C) due to the high closure temperature urian part of the Tethys and the collision of the European of monazite (> 800 °C; e.g. Cherniak et al. 2004; Cher- with the Apulian plate (e.g. Lemoine et al. 1986; Schmid niak and Pyle 2008; Gardés et al. 2006, 2007). Te 232T- et al. 2004). Tis process lasted from the Cretaceous to 208Pb isotopic system can thus only be disturbed or reset the Eocene (e.g. Rubatto et al. 1999; Rubatto and Her- by chemical disequilibrium induced by the presence of mann 2001; Table 2). hydrous fuids resulting in the dissolution and/or re-pre- Tis study focuses on the westernmost part of the Alps, cipitation of monazite (e.g. Cherniak et al. 2004; Cherniak an arcuate and roughly N-S oriented segment of the and Pyle 2008; Grand’Homme et al. 2016b; Seydoux- Alpine orogen characterized by the exposure of European Guillaume et al. 2002, 2012), a process that commonly (or external) units in the west, separated tectonically

(See fgure on next page.) Fig. 1 Tectonic map of the study area modifed after Schmid et al. (2004), Sanchez et al. (2011a), Steck et al. (2013) and Bergemann et al. (2019). Stars correspond to monazite samples from this study (yellow) and from Gasquet et al. (2010), Grand’Homme et al. (2016a) and Bergemann et al. (2019) (green stars). Weighted mean age or age range of growth domains in monazite are indicated. The internal domains are labelled as follow: MR: Monte Rosa, DB: Dent Blanche, S: Sesia, GP: Grand Paradiso, Am: Ambin, DM: Dora Maira Ion microprobe dating of fssure monazite in the Western Alps Page 3 of 27 15 15 Page 4 of 27 E. Ricchi et al.

Table 1 Summary of monazite-(Ce) samples considered in this study with information on sample locality, host-rock lithology and metamorphic degree, fssure mineral association and orientation Locality Sample Domain Latitude (°N) Longitude (°E) Host-rock Alpine Association Average metamorphism fssure orientation

Margone, Valle di VIU1 Piémontais Zone 45°13.064’ 007°11.268’ gneiss Eclogite/green- Qz, Ab, Rt 181/77 Viù, Piedmont, schist facies Italy Costa Balzi Rossi, BALZI2 Briançonnais 44°11.300’ 008°14.100’ metarhyolites Blueschist/green- Qz, Ab, Ad, Ant, Block in scree Magliolo, Ligu- Zone schist facies Brk, Hem, Xnt ria, Italy Morglione, Valle MORI1 Argentera Massif 44°20.470’ 007°04.767’ gneiss Greenschist Qz, Ad, Ant 186/86 Stura, Argen- facies tera, Piedmont, Italy Vinadio, Valle VINA1 Argentera Massif 44°18.376’ 007°09.606’ gneiss Greenschist Qz, Ad, Ant 186/86 Stura, Argen- facies tera, Piedmont, Italy Monte Ray, GESS1 Argentera Massif 44°13.660’ 007°21.640’ gneiss Greenschist Cc, Ant, Syn, Xnt, Vertical Valle Gesso, facies Crt Entraque, Pied- mont, Italy

Ab albite, Ad adularia, Ant anatase, Brk brookite, Cc calcite, Crt cerussite, Hem hematite, Qz quartz, Rt rutile, Syn synchisite-(Ce), Xnt xenotime-(Y) from Penninic (or internal) units by the Penninic Front larger scale, this indicates an oblique (sinistral) collision (PF) (Fig. 1). Te external crystalline massifs (ECM) during the Eocene, followed by westward indentation derive from the European margin and consist of the Aar, of the Ivrea mantle wedge (portion of the Adria man- Gotthard, Mont Blanc, Aiguilles Rouges, Belledonne, tle lithosphere), associated with backfolding and back- Pelvoux and Argentera massifs and their correspond- thrusting (e.g. Ceriani et al. 2001; Collombet et al. 2002; ing sedimentary cover (Helvetic and Dauphiné units), Schmid et al. 2017; 2nd episode/stage on Table 2). Tis whereas the internal units formed during subduction- indentation overprinted the Eocene regime of sinistral collision of the Tethyan Ocean and the European margin transpression in the area of the Western Alps (Laubscher and consists of lower and middle Penninic units (Val- 1991) and was followed by a Miocene shortening phase ais and Briançonnais Zones) and upper Penninic units related to the exhumation of the ECM (Table 2). During (Piemontais, Schistes Lustrés and Austro-Alpine Units; the Neogene, orogen-parallel deformation developed Fig. 1; e.g. Ceriani et al. 2001; Schmid et al. 2004). Te at the expense of NW-directed thrusting (e.g. Hubbard diferent units of the Piemontais and Briançonnais Zones and Mancktelow 1992). Tis NE-trending deformation show Alpine eclogite/greenschist and blueschist/green- was associated with extensional movements along the schist metamorphism, whereas the Argentera and other Simplon normal fault due to exhumation of the Lepon- external Massifs show greenschist facies metamorphism tine structural and metamorphic dome (e.g. Bergemann (Bousquet et al. 2012). et al. 2020), accommodated by dextral strike-slip faults, In the external Alps, shortening started during the bordering and cutting the ECM, and by SW-directed main collisional phase of the orogen, at the transition thrusting along the Digne Trust (Table 2). Te top-SW between the Late Eocene and the Early Oligocene (Ceri- thrusting afecting the southern part of the Western ani et al. 2001). According to that study, the boundary Alps is related to the formation of the Apennines lead- between the external and internal units (the PF) acted ing to oroclinal bending. Simultaneously, anticlockwise frst as a transpressive suture zone that became sealed by rotation of the Corsica-Sardinia block between 20.5 and Priabonian fysch. Tis frst activation of the PF, related to 16 Ma accommodates the opening of the Liguro-Proven- the underthrusting of the ECM below the internal units, çal Basin (Gattacceca et al. 2007; Mafone et al. 2008; occurred in the Eocene and ended at ~ 35 Ma (e.g. Ceri- Schmid et al. 2017; stage 3 on Table 2). From the Pliocene ani et al. 2001; Grand’Homme et al. 2016a; 1st episode/ onwards, brittle normal faulting developed following the stage on Table 2). It was then reactivated between 35 and reactivation of the northern and southern Houiller and 25 Ma according to Schmid et al. (2017), where it acted Penninic Front (Fügenschuh et al. 1999; 3rd episode of as a WNW-directed thrust: the Roselend Trust. At Ceriani et al. 2001, Table 2). Seismological data attest that Ion microprobe dating of fssure monazite in the Western Alps Page 5 of 27 15 C, K B, B, G, W J, N, S J, F, L D, T B, C, R, S B, B, T B, G, K,B, S B, U E, K, Q K, P, V K, P, Ref - - exhumation along the PF Houiller Front in the N and Houiller Front in the S Front the Penninic Apennines and opening of Basin the Liguro-Provençal (Simplon Fault), NE-striking (Simplon Fault), dextral strike-slip (around the ECM), SW- and through trusting (Digne) directed migration of the deformation of the deformation migration front indentation of the Ivrea Overprintmantle wedge. of sinistral transpression Eocene directed head-ondirected collision in Alps the Central gence and frontal collision in gence and frontal Alps; ECM under the Central internal units below thusted the internal units and the onset of the external Alps shortening formation, composed of formation, units internal Penninic Mnz age coincides with the Related to reactivation of the to Related Related to orogeny in the orogeny to Related SW-directed normal faulting Contemporaneous westwards westwards Contemporaneous Related Related WNW-directed to Related to the end of N-S- to Related Related to N-S-directed to Related conver Related to the exhumation of to Related Related to subduction to Related prism Remarks ~ 32 Monazite age [Ma]Monazite 1st episode of B D1 to D2 phase of C D1 to 3rd episode of B 3rd Stage 3 of R 2nd episode of B / Stage 2 R End of 1st episode of B / Stage 1 R Phase - - Briançonnais thrusting and of propagation westward deformation orogen-perpendicular extenorogen-perpendicular - sion bending and anticlockwise of the Corsica-Sar rotation dinia block (20.5–15 Ma) tion of the ECM along the PF, associated to to associated along the PF, and backthrust backfolding ing top-NNW thrusting in sinistral PF activation transpression; collision continental margin Ocean Alpine collision. PF activation, Late brittle normal faulting, brittleLate normal faulting, Top-SW thrusting, oroclinal oroclinal thrusting, Top-SW Orogen-parallel deformation Orogen-parallel Shortening phase and exhuma - Top-WNW-directed thrusting Top-WNW-directed End of oblique collision Oblique collision associated to to Oblique collision associated European margin—Apulia margin—Apulia European Subduction of the European Subduction of the European Subduction of the Thetys Subduction of the Characteristics Alps Briançonnais Zone Western Alps Western Southern part of the Western SouthernWestern part of the Western Alps Western Western Alps Western Western Alps Western Western Alps Western Western Alps Western Western Alps Western Western Alps Western Western Alps Western Domain - Summary of deformation phases in the Western Alps phases in the Western Summary of deformation cene ~ 35 Late Eocene to Oligocene to Eocene Late After 5 After 25–0 Miocene—Pliocene Miocene 35–25

Eocene to earliest to Oligocene Eocene Late Eocene to Early to Oligo Eocene Late Middle Eocene Late to Cretaceous to Eocene to Cretaceous Deformation in the high-pressure units (internal domains): in the high-pressure Deformation 2 Table Large-scale history: deformation Age [Ma] Age 15 Page 6 of 27 E. Ricchi et al. A, K, M, O I, K D, H, I D, D, K C, K Ref dextral strike slip (vertical veins) dextral strike slip (vertical vein) movements along the movements Ornon-Roselend fault and the Rhone-Simplon fault (vertical veins) records the major uplift of records Massif (horizontal the Pelvoux vein) striking dextral strike slip to a given deformation deformation a given to episode Mnz ages related to NE striking to Mnz ages related Mnz ages related to NE striking to Mnz ages related Mnz ages are related to dextral to Mnz related ages are Mnz from a horizontal vein Mnz vein a horizontal from Mnz NW- to age likely related Mnz attribute age is difcult to Remarks ~ 9—8 ~ 11.5—6.5 ~ 12—5.5 ~ 17.6 ~ 20.6 ~ 23 Monazite age [Ma]Monazite . Mnz: monazite; ECM: External Cristalline Massifs; PF: Penninic Front 1999 . Mnz: Penninic ECM: External monazite; PF: Cristalline Massifs; D4 phase of C D3 phase of C Phase - forma shear zone to related veins) tion (horizontal regime and "En echelon" regime arrangement of vertical veins - thrusting (horizon to related tal veins) Alpine tilting) - forma shear zone to related veins) tion (horizontal phase) First and second shortening First Polyphased transpressive transpressive Polyphased Exhumation and cooling Normal/strike-slip faults (late First major shortening phase First Back-thrusting (main folding (main folding Back-thrusting Characteristics Mont Blanc Massif Aiguilles Rouges Massif Belledonne Massif Pelvoux Massif Pelvoux Briançonnais Zone Argentera Massif Argentera Briançonnais Zone Domain (continued) 11–10 and 7–5 ~ 23–22 and ~ 16 ~ 20–15 ~ 22.5

Miocene—Actual

Late Oligocene Late Deformation in the ECM (externalDeformation domains): 2 Table [Ma] Age and Schmid 2003 ; I: Gasquet et al. 1999 ; H: Fügenschuh et al. 2008; G: Fügenschuh et al. Dumont 1997; F: 2004 ; E: Duchêne et al. et al. 2002 ; D: Corsini et al. 2001 ; C: Collombet et al. 2019 ; B: Ceriani et al. A: Bergemann and Hermann 1999 ; Q: Rubatto et al. Rubatto 2008 ; O: Rossi 2005 ; N: Mafoneand Rolland 2014 ; P: et al. et al. 1992 ; M: Leloup 2007 ; K: and Mancktelow, et al. 2016a; L: Hubbard 2010 ; J: Gattacceca Grand’Homme et al. 2013 ; W: et al. Steck Sue V: Schmid and Kissling 2009 ; 2000; U: Simon-Labric et al. T: 2017 ; 2004 ; S: Schmid2001 ; R: Schmid et al. et al. Ion microprobe dating of fssure monazite in the Western Alps Page 7 of 27 15

the Penninic core of the Western Alps is still afected by Spatially close BSE regions displaying a comparable grey this orogen-perpendicular extension (Sue et al. 1999). shade, showing chemical grouping and providing compa- Whereas the timing of fssure formation in the external rable age spots are treated as one growth domain. Some massifs (except Argentera) is well constrained by mona- domains were too small to provide robust average ages, zite data (Gasquet et al. 2010; Grand’Homme et al. 2016a, and single spot ages may still provide hints about the crys- b; Bergemann et al. 2019; Ricchi et al. 2019), such data tallization history. Weighted mean 208Pb/232T ages were are sparse for the Argentera Massif and the high-pressure calculated for each growth domain following the approach overprinted units. of Bergemann et al. (2017, 2018, 2019, 2020) and Ricchi et al. (2019, 2020). Since fssure monazite is dissolved and 3 Methods re-precipitated under changing chemical conditions (e.g. In situ T-Pb dating of fve monazite grains was carried Grand’Homme et al. 2018), spot analyses afected by Pb c out at the SwissSIMS Ion microprobe facility, equipped (as indicated by older dates related to higher Pbc, i.e. posi- with a Cameca IMS 1280 h instrument, at University tive age-f208 correlation), or those with high uncertainty of Lausanne, Switzerland (Table 3). Preliminary sample (1σ·absolute·> 1) were removed from the dataset. However, preparation and backscattered electron (BSE) imaging analyses located on dissolution trails, generally providing was conducted for identifying distinct chemical and tex- younger dates, were considered in the age ranges because tural domains, following the procedure of Bergemann they likely record a later phase of monazite alteration. et al. (2017). Te SIMS spot measurements were distrib- Laser-Ablation Inductively-Coupled-Plasma Mass- uted on the basis of domains visible to capture the crystal- Spectrometry (LA-ICP-MS) analyses were carried out to lization duration. In order to obtain more robust growth obtain the concentration of major and trace elements in domain ages, the selected domains were large enough to the investigated fssure monazites (Table 5). Te measure- place a minimum of three measurement spots. Te instru- ments were done using a GeoLas-Pro 2006 193 nm ArF ment was run using the same procedure as Janots et al. Excimer laser system (Lambda Physik / Coherent) inter- (2012) and the same parameters as Ricchi et al. (2019, faced to a Perkin Elmer Elan DRC-e quadrupole mass 2020). Te SIMS measurements were performed focus- spectrometer at the University of Bern. Te ablation was – ing a -13 kV O 2 primary beam on the samples with an performed following procedures as documented in Berge- intensity of ca. 3nA creating a spot with a 15 μm diameter. mann et al. (2017) and Gnos et al. (2015), using beam sizes Applying a mass resolution between 4300–5000 depend- of 24 to 60 μm to measure as close as possible to the ion ing on analytical session (M/ΔM, at 10% peak height) and probe spots in the same growth domain, with an energy an energy window at 40 eV, with data collected in peak density on the sample of 4–5 J/cm2 and a laser repeti- hopping mode using an ion-counting electron multiplier. tion rate of 10 Hz. Te ICP-MS settings were optimized USGS-44069 monazite (425 Ma; Aleinikof et al. 2006) was to maximum signal to background intensity ratios with selected to standardise the data; the uncertainty on the (232T16O)+ production rates tuned to below 0.2% and standard 208Pb/232T–TO/T calibration was between robust plasma conditions as monitored by equal sensitivi- 1.01 to 2.56%. Data reduction and standardization was ties of U and T. Two reference materials were used, GSD- also carried out using the same parameters as Ricchi et al. 1G for external calibration and the synthetic SRM 610 (2020); namely, common lead Pb c correction was calcu- glass from the National Institute of Standards and Tech- lated at time zero applying the terrestrial Pb evolution nology (NIST) for quality control, employing preferred model of Stacey and Kramers (1975) using the Cameca element concentration data from GeoRem (https://geore Customisable Ion Probe Software (CIPS) and Isoplot 4.15 m.mpch-mainz.gwdg.de/). Data quantifcation used total (Ludwig 2003) software. Isoplot 4.15 was also employed for oxides for internal standardisation (i.e., summation of the weighted average age calculations and plotting. Uncertain- measured major element oxides (P2O5, CaO, Y2O3, La2O3, ties on single ages are quoted at 1 sigma level and weighted Ce2O3, Pr2O3, Nd2O3, Sm2O3, Eu 2O3, Gd2O3, Tb2O3, mean ages, hereafter called average ages, are quoted at Dy2O3, TO2) to 100 wt%; e.g. Gray 1985). Data reduction 95% confdence level throughout the text (Table 4). was performed using the SILLS software (Guillong et al. Previous studies have shown that U-T contents seem 2008) with improved calculation of the limit of detection to be the easiest way to diferentiate domains and to iden- (Pettke et al. 2012). tify altered zones (e.g., Gnos et al. 2015; Bergemann et al. 2017). Since new crystallization or alteration is associated 4 Results with a change in chemical composition and likely hap- 4.1 Alpine fssures in the Western Alps pened in equilibrium with the surrounding fuid, any age Two fssure monazites analysed in this study were col- cluster within a chemical group must be due to the simul- lected in the high-pressure regions of the Western Alps, taneous formation or alteration of those monazite parts. sample VIU1 is from the Piemontais Zone (eclogite/ 15 Page 8 of 27 E. Ricchi et al. 0.37 0.76 0.49 0.92 0.91 0.77 0.95 0.53 0.58 0.81 1σ (abs.) 0.57 0.45 0.49 0.47 0.47 0.50 0.51 0.50 0.45 0.45 0.46 0.47 0.53 0.48 0.46 0.48 0.46 0.48 0.47 0.47 0.45 Th 232 Pb/ 207—corr spot ages 208 33.55 34.96 36.36 35.90 39.88 35.48 44.92 36.28 50.12 43.10 Age (Ma) Age 40.14 28.74 31.73 30.58 30.92 32.42 33.21 32.46 29.64 29.16 29.72 30.21 31.18 30.76 29.83 30.85 28.52 31.47 30.08 30.61 29.14 1σ (%) 1.1 2.2 1.3 2.6 2.3 2.2 2.1 1.5 1.2 1.9 1.4 1.6 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.7 1.6 1.5 1.5 1.6 1.5 1.6 1.5 1.5 Th 232 Pb/ 207—corr 208 0.001661 0.001731 0.001800 0.001778 0.001975 0.001757 0.002225 0.001796 0.002483 0.002135 0.001988 0.001423 0.001571 0.001514 0.001531 0.001605 0.001645 0.001607 0.001468 0.001444 0.001471 0.001496 0.001544 0.001523 0.001477 0.001527 0.001412 0.001558 0.001489 0.001516 0.001443 1σ (abs.) 1.05 0.86 1.58 1.33 1.22 / 1.51 1.21 / 2.79 1.25 0.44 0.47 0.46 0.47 0.49 0.50 0.49 0.44 0.44 0.45 0.46 0.52 0.47 0.45 0.47 0.45 0.47 0.46 0.46 0.44 Th 232 Pb/ 33.24 34.65 35.62 35.23 39.69 35.81 44.56 36.29 50.50 48.38 40.74 28.52 31.38 30.31 30.69 32.09 32.91 32.20 29.27 28.91 29.50 30.04 30.88 30.55 29.71 30.53 28.24 31.22 29.79 30.29 28.96 204—corr spot ages 208 Age (Ma) Age 1σ (%) 3.2 2.5 4.4 3.8 3.1 / 3.4 3.3 / 5.8 3.1 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.7 1.5 1.5 1.5 1.6 1.5 1.5 1.5 1.5 Th 232 Pb/ 208 204—corr 0.001646 0.001716 0.001764 0.001745 0.001966 0.001774 0.002207 0.001797 0.002502 0.002396 0.002018 0.001412 0.001554 0.001501 0.001519 0.001589 0.001630 0.001594 0.001449 0.001431 0.001460 0.001488 0.001529 0.001513 0.001471 0.001511 0.001398 0.001546 0.001475 0.001500 0.001434 4.6 1.0 4.7 0.9 2.1 6.5 0.8 7.4 4.4 0.7 0.3 0.3 0.4 0.4 0.6 0.9 1.2 0.9 0.8 0.8 1.2 1.6 1.1 1.4 1.5 0.8 0.7 0.5 0.6 16.0 11.0 f208 from 207(%) 1.0 2.2 1.1 2.5 2.3 2.2 2.1 1.3 1.1 1.6 1.2 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.7 1.6 1.5 1.5 1.6 1.5 1.6 1.5 1.5 1.6 1σ (%) Th 232 Pb/ 0.001741 0.001749 0.002144 0.001997 0.002073 0.001774 0.002273 0.001922 0.002502 0.002306 0.002080 0.001576 0.001518 0.001537 0.001611 0.001655 0.001622 0.001485 0.001457 0.001483 0.001508 0.001563 0.001548 0.001493 0.001550 0.001434 0.001571 0.001501 0.001524 0.001452 0.001432 208 5.1 4.6 7.0 8.7 2.0 2.3 2.5 2.1 2.4 2.1 2.1 2.5 2.5 2.6 2.4 1.8 2.4 2.4 2.4 2.2 2.3 2.5 2.6 2.6 11.0 11.6 14.2 16.1 24.4 13.2 11.7 1σ (%) Pb 206 Pb/ 0.336 0.089 0.479 0.263 0.174 0.074 0.119 0.340 0.068 0.374 0.141 0.078 0.071 0.087 0.082 0.100 0.120 0.128 0.093 0.094 0.085 0.093 0.184 0.110 0.121 0.133 0.104 0.103 0.088 0.091 0.086 207 1σ (%) 55 67 25 25 42 0 95 47 0 95 95 8 10 11 9 11 11 10 14 14 16 14 9 14 13 13 11 11 11 14 13 Pb 204 Pb/ 208 704 2020 218 306 744 / 1341 594 / 938 1299 2739 3335 3274 2810 2499 2261 1588 2116 2545 2893 1791 1686 2582 1564 1547 2316 2263 2431 3043 2522 7 6 7 48 18 20 12 11 16 26 18 94 92 68 55 66 55 46 91 52 48 48 80 78 81 62 47 108 113 109 112 Th/U 4100 5800 4900 3800 5500 3900 7000 3900 4600 3800 2700 42,000 31,000 26,000 36,000 28,000 22,000 19,000 17,000 21,000 17,000 13,000 22,000 14,000 12,000 14,000 20,000 21,000 21,000 18,000 17,000 Th (μg/g) 86 320 240 320 510 250 950 150 760 210 410 390 330 230 330 250 240 280 310 320 310 280 240 270 250 290 250 270 260 290 360 U (μg/g) VIU1@01 VIU1@02 VIU1@03 VIU1@04 VIU1@06 VIU1@11 VIU1@05 VIU1@07 VIU1@08 VIU1@09 VIU1@10 VIU1@12 VIU1@13 VIU1@14 VIU1@15 VIU1@16 VIU1@17 VIU1@18 VIU1@19 VIU1@20 Ion microprobe Th-U–Pb analyses of fssure monazite of fssure analyses Th-U–Pb microprobe Ion BALZI2@31 BALZI2@44 BALZI2@43 BALZI2@42 BALZI2@40 BALZI2@33 BALZI2@39 BALZI2@32 BALZI2@37 BALZI2@36 BALZI2@34 Analysis ID Analysis 3 Table Groups A High-pressure regions High-pressure A B Ion microprobe dating of fssure monazite in the Western Alps Page 9 of 27 15 0.63 0.90 0.63 0.31 0.74 0.70 0.83 0.68 0.68 0.68 0.31 0.82 0.44 0.77 0.79 0.79 0.46 0.47 0.34 0.37 0.44 0.35 0.43 0.41 1σ (abs.) 0.54 0.48 Th 232 Pb/ 11.18 29.39 21.61 11.08 26.34 25.62 25.07 207—corr spot ages 208 25.43 24.73 24.81 27.13 26.02 28.20 26.47 24.29 24.99 31.09 29.85 31.20 29.71 29.63 31.15 29.91 30.13 Age (Ma) Age 29.90 29.65 5.6 3.1 2.9 2.8 2.8 2.7 3.3 1σ (%) 2.7 2.8 2.8 1.1 3.1 1.5 2.9 3.3 3.2 1.5 1.6 1.1 1.2 1.5 1.1 1.4 1.4 1.8 1.6 Th 232 Pb/ 0.000553 0.001455 0.001070 0.000548 0.001304 0.001268 0.001241 207—corr 208 0.001259 0.001224 0.001228 0.001343 0.001288 0.001396 0.001310 0.001203 0.001237 0.001539 0.001478 0.001545 0.001471 0.001467 0.001542 0.001481 0.001492 0.001480 0.001468 1σ (abs.) 1.29 1.54 1.07 0.61 1.12 1.01 0.88 0.93 1.06 1.13 / 0.89 1.48 0.80 0.78 0.74 / / 0.70 0.82 0.50 / 0.72 1.11 / 0.52 Th 232 Pb/ 7.43 10.96 30.37 19.07 25.92 25.38 23.26 26.11 23.70 25.75 27.24 24.16 26.05 26.52 21.73 24.43 31.37 30.20 32.74 28.21 29.34 31.47 30.00 27.22 30.29 29.51 204—corr spot ages 208 Age (Ma) Age 11.8 5.1 5.6 8.2 4.3 4.0 3.8 1σ (%) 3.6 4.5 4.4 / 3.7 5.7 3.0 3.6 3.0 / / 2.2 2.9 1.7 / 2.4 4.1 / 1.8 Th 232 Pb/ 204—corr 0.000542 0.001504 0.000944 0.000368 0.001283 0.001257 208 0.001152 0.001293 0.001173 0.001275 0.001349 0.001196 0.001290 0.001313 0.001076 0.001209 0.001553 0.001495 0.001621 0.001396 0.001453 0.001558 0.001485 0.001347 0.001500 0.001461 0.4 9.0 0.9 1.2 1.1 0.6 0.3 1.0 2.4 1.3 0.2 57.2 45.8 31.2 20.4 16.8 15.5 15.2 16.4 22.4 28.2 32.7 21.9 16.7 20.3 28.7 f208 from 207(%) 3.2 2.6 2.7 2.6 2.7 2.7 3.3 2.6 2.6 2.6 1.1 3.1 1.1 2.9 3.2 3.1 1.5 1.6 1.0 1.2 1.5 1.1 1.4 1.1 1.8 1.6 1σ (%) Th 232 Pb/ 0.001293 0.002683 0.001555 0.000688 0.001567 0.001501 0.001464 0.001506 0.001576 0.001710 0.001349 0.001915 0.001788 0.001440 0.001445 0.001551 0.001553 0.001495 0.001561 0.001480 0.001471 0.001558 0.001517 0.002091 0.001500 0.001472 208 4.1 2.2 3.2 4.8 3.8 3.9 2.8 3.5 3.3 3.0 1.9 4.8 3.2 2.6 1.7 2.7 16.8 17.3 16.5 19.1 20.0 14.8 27.7 15.0 10.7 16.1 1σ (%) Pb 206 Pb/ 0.678 0.628 0.573 0.279 0.391 0.361 0.380 0.421 0.459 0.578 0.077 0.651 0.678 0.459 0.519 0.529 0.106 0.117 0.163 0.121 0.069 0.150 0.235 0.715 0.165 0.061 207 1σ (%) 18 11 14 17 19 20 12 20 16 16 0 8 20 16 10 8 0 0 0 47 67 0 95 11 0 95 Pb 204 Pb/ 208 62 86 98 83 197 237 175 273 151 152 / 103 139 437 151 175 / / / 685 3122 / 1859 109 / 5432 67 15 68 21 17 18 36 27 28 43 65 61 66 67 74 56 54 54 90 92 74 62 100 142 120 104 Th/U 2800 3200 3100 3500 4200 3500 3100 3800 3600 3500 5600 4100 2500 8600 5500 7100 5500 3800 3600 3200 5900 1700 2100 5800 6500 6800 Th (μg/g) 42 45 85 82 86 67 25 82 96 98 71 67 35 41 14 20 63 88 210 170 250 200 140 130 130 110 U (μg/g) (continued) BALZI2@23 BALZI2@51 BALZI2@20 BALZI2@14 BALZI2@22 BALZI2@21 BALZI2@19 BALZI2@17 BALZI2@16 BALZI2@15 BALZI2@25 BALZI2@13 BALZI2@24 BALZI2@12 BALZI2@02 BALZI2@01 BALZI2@50 BALZI2@49 BALZI2@48 BALZI2@47 BALZI2@46 BALZI2@30 BALZI2@29 BALZI2@28 BALZI2@27 BALZI2@26 Analysis ID Analysis 3 Table Groups D C B 15 Page 10 of 27 E. Ricchi et al. 1σ (abs.) 0.33 0.31 0.30 0.37 0.29 0.33 0.25 0.41 0.25 0.32 0.26 0.33 0.27 0.34 0.27 0.31 0.26 0.32 0.26 0.33 0.23 0.33 0.34 0.28 0.33 0.30 0.33 0.32 0.30 0.33 Th 232 Pb/ 207—corr spot ages 208 Age (Ma) Age 15.59 15.47 15.15 15.73 14.22 15.55 16.53 15.86 16.06 15.28 16.70 15.59 16.02 15.40 16.52 15.41 16.46 14.43 16.30 15.27 14.93 14.99 15.00 14.23 15.39 15.03 14.42 15.21 15.14 15.74 1σ (%) 2.1 2.0 2.0 2.3 2.0 2.1 1.5 2.6 1.5 2.1 1.5 2.1 1.7 2.2 1.6 2.0 1.6 2.2 1.6 2.1 1.5 2.2 2.3 1.9 2.2 2.0 2.3 2.1 2.0 2.1 Th 232 Pb/ 207—corr 208 0.000772 0.000766 0.000750 0.000779 0.000704 0.000770 0.000818 0.000785 0.000795 0.000756 0.000826 0.000772 0.000793 0.000762 0.000818 0.000763 0.000815 0.000714 0.000807 0.000756 0.000739 0.000742 0.000742 0.000704 0.000762 0.000744 0.000714 0.000753 0.000749 0.000779 1σ (abs.) 0.34 0.32 0.31 0.39 0.32 0.40 0.25 0.44 0.25 0.36 0.25 0.37 0.27 0.36 0.26 0.33 0.25 0.33 0.24 0.38 0.22 0.39 0.40 0.28 0.40 0.33 0.34 0.33 0.32 0.33 Th 232 Pb/ 15.10 15.02 14.98 15.13 13.59 14.90 16.25 15.88 15.76 15.04 16.39 14.82 15.58 14.74 16.16 15.23 16.28 14.10 15.38 14.83 14.62 14.40 14.83 13.58 14.91 14.66 13.62 14.61 14.85 15.07 204—corr spot ages 208 Age (Ma) Age 1σ (%) 2.3 2.2 2.1 2.6 2.4 2.7 1.5 2.8 1.6 2.4 1.5 2.5 1.7 2.4 1.6 2.2 1.6 2.3 1.6 2.6 1.5 2.7 2.7 2.1 2.7 2.3 2.5 2.3 2.1 2.2 Th 232 Pb/ 208 204—corr 0.000747 0.000743 0.000742 0.000749 0.000672 0.000737 0.000804 0.000786 0.000780 0.000744 0.000811 0.000734 0.000771 0.000730 0.000800 0.000754 0.000806 0.000698 0.000761 0.000734 0.000724 0.000712 0.000734 0.000672 0.000738 0.000726 0.000674 0.000723 0.000735 0.000746 4.0 2.6 3.5 3.3 4.0 4.2 0.4 4.3 5.1 4.8 4.0 3.9 2.9 3.0 0.5 2.4 0.5 2.3 7.4 6.1 0.5 4.9 5.4 1.5 5.9 3.7 3.1 3.1 2.3 2.5 f208 from 207(%) 2.0 2.0 2.3 2.0 2.1 1.5 2.6 1.5 2.1 1.5 2.1 1.7 2.2 1.6 2.0 1.6 2.2 1.6 2.1 1.5 2.2 2.2 1.9 2.1 2.0 2.3 2.1 2.0 2.1 2.1 1σ (%) Th 232 Pb/ 0.000786 0.000777 0.000805 0.000734 0.000803 0.000821 0.000820 0.000837 0.000795 0.000861 0.000803 0.000817 0.000786 0.000822 0.000782 0.000819 0.000731 0.000871 0.000805 0.000743 0.000780 0.000785 0.000715 0.000810 0.000772 0.000737 0.000777 0.000767 0.000799 0.000804 208 5.3 3.8 5.5 4.0 5.8 4.4 5.5 2.5 4.9 2.0 5.4 3.8 5.3 5.2 6.2 5.1 5.7 1.5 4.7 3.6 5.4 5.4 5.5 4.2 4.4 4.5 4.2 4.6 3.7 3.4 1σ (%) Pb 206 Pb/ 0.473 0.164 0.476 0.160 0.391 0.240 0.446 0.685 0.503 0.615 0.449 0.556 0.446 0.251 0.366 0.240 0.364 0.676 0.532 0.359 0.425 0.451 0.270 0.211 0.205 0.202 0.222 0.188 0.134 0.186 207 1σ (%) 19 21 19 18 21 11 26 10 22 8 19 14 18 13 27 17 22 5 19 9 21 25 15 21 21 17 17 20 15 16 Pb 204 Pb/ 208 712 840 555 463 426 1855 792 564 609 667 450 685 541 1423 1097 2366 866 307 440 1499 443 591 609 434 637 453 555 843 586 548 24 22 47 65 97 86 90 35 29 551 369 238 904 742 777 598 699 345 528 500 706 484 595 422 665 252 212 195 167 1185 Th/U 4800 1700 3500 1200 2400 3000 3700 3500 5000 4600 3400 3400 2800 2700 2400 2500 3300 3700 4400 4400 1900 1600 29,000 12,000 20,000 11,000 20,000 20,000 23,000 37,000 Th (μg/g) 9 9 4 6 8 71 54 10 32 15 29 10 21 10 28 10 34 35 14 31 13 14 14 54 51 38 51 49 55 55 U (μg/g) VINA1@17 VINA1@20 VINA1@22 VINA1@23 VINA1@24 VINA1@25 VINA1@26 VINA1@18 (continued) MORI1@06 MORI1@22 MORI1@07 MORI1@23 MORI1@08 MORI1@09 MORI1@10 MORI1@11 MORI1@12 MORI1@13 MORI1@14 MORI1@15 MORI1@16 MORI1@17 MORI1@18 MORI1@01 MORI1@02 MORI1@03 MORI1@04 MORI1@05 MORI1@20 MORI1@21 Analysis ID Analysis 3 Table Groups Argentera Massif Argentera A A B Ion microprobe dating of fssure monazite in the Western Alps Page 11 of 27 15 0.41 0.33 0.30 1σ (abs.) 0.21 0.23 0.26 0.25 0.24 0.24 0.24 0.29 0.27 0.33 0.31 0.30 0.28 0.25 0.24 0.25 0.27 0.25 0.32 Th 232 Pb/ 16.23 15.39 12.19 207—corr spot ages 208 Age (Ma) Age 13.04 14.58 15.81 15.55 14.68 15.12 15.01 16.01 14.83 14.71 15.60 15.65 15.65 15.09 14.23 14.36 14.30 14.14 15.57 2.6 2.1 2.5 1σ (%) 1.6 1.5 1.6 1.6 1.6 1.6 1.6 1.8 1.8 2.3 2.0 1.9 1.8 1.6 1.7 1.8 1.9 1.8 2.0 Th 232 Pb/ 207—corr 208 0.000803 0.000762 0.000603 0.000646 0.000722 0.000782 0.000769 0.000727 0.000748 0.000743 0.000792 0.000734 0.000728 0.000772 0.000775 0.000775 0.000747 0.000704 0.000711 0.000708 0.000700 0.000771 1σ (abs.) 0.66 0.34 0.50 0.20 0.22 0.26 0.25 0.24 0.24 0.24 0.37 0.35 0.37 0.33 0.36 0.30 0.26 0.25 0.27 0.28 0.27 0.31 Th 232 Pb/ 16.36 14.81 11.06 12.74 14.26 15.51 15.31 14.32 14.72 14.63 15.78 14.64 14.30 15.12 15.20 15.40 14.69 14.09 14.06 14.07 13.79 14.97 204—corr spot ages 208 Age (Ma) Age 1σ (%) 4.1 2.3 4.6 1.6 1.6 1.7 1.6 1.7 1.6 1.6 2.3 2.4 2.6 2.2 2.4 2.0 1.8 1.8 1.9 2.0 1.9 2.1 Th 232 Pb/ 204—corr 208 0.000810 0.000733 0.000547 0.000631 0.000706 0.000768 0.000758 0.000709 0.000728 0.000724 0.000781 0.000725 0.000708 0.000749 0.000752 0.000762 0.000727 0.000697 0.000696 0.000697 0.000682 0.000741 4.4 1.5 3.0 0.7 0.7 0.8 0.8 0.7 9.9 1.6 1.6 0.8 3.8 1.2 0.6 0.6 0.8 0.9 0.8 3.2 30.3 19.1 f208 from 207(%) 2.3 2.1 2.3 1.6 1.5 1.6 1.6 1.6 1.6 1.6 1.8 1.8 2.3 2.0 1.9 1.8 1.6 1.7 1.7 1.9 1.8 2.0 1σ (%) Th 232 Pb/ 0.000797 0.000746 0.000655 0.000744 0.000788 0.000775 0.000732 0.000754 0.000748 0.000879 0.000746 0.000740 0.000778 0.000805 0.000784 0.000751 0.000708 0.000716 0.000714 0.000706 0.000796 0.001153 208 4.0 3.6 2.5 2.9 6.0 5.9 6.4 6.4 6.7 3.3 7.5 8.2 9.1 5.3 7.9 8.0 8.4 8.3 7.9 8.4 3.3 2.5 1σ (%) Pb 206 Pb/ 0.398 0.439 0.383 0.594 0.190 0.209 0.203 0.209 0.194 0.508 0.130 0.132 0.122 0.274 0.164 0.113 0.116 0.140 0.160 0.136 0.305 0.591 207 1σ (%) 16 16 8 11 19 20 19 19 19 15 30 33 28 23 33 24 31 28 27 27 12 13 Pb 204 Pb/ 208 481 145 1019 760 1538 1741 1211 1132 1192 346 816 903 1008 590 1390 1189 1905 1344 1327 1168 557 130 51 65 79 88 256 320 786 290 295 326 235 252 121 119 169 144 150 124 139 258 119 152 Th/U 4900 1200 8200 8300 7100 6800 7300 2800 1900 1800 2500 2400 2700 4200 4100 3700 3900 3600 7200 1500 32,000 19,000 Th (μg/g) 19 23 24 28 28 22 29 29 23 29 23 21 14 19 28 33 27 15 30 47 17 100 U (μg/g) VINA1@19 VINA1@21 VINA1@06 VINA1@07 VINA1@08 VINA1@09 VINA1@10 VINA1@11 VINA1@12 VINA1@13 VINA1@14 VINA1@15 VINA1@16 VINA1@01 VINA1@02 VINA1@03 VINA1@04 VINA1@05 GESS1@27 GESS1@24 GESS1@25 GESS1@26 (continued) Analysis ID Analysis 3 Table Groups B C1 C2 A 15 Page 12 of 27 E. Ricchi et al. 0.27 0.29 0.25 0.27 1σ (abs.) 0.33 0.39 0.29 0.40 0.31 0.28 0.28 0.33 0.33 0.31 0.33 0.30 0.32 0.41 0.33 0.28 0.33 0.28 0.32 Th 232 Pb/ 12.23 12.05 12.84 12.77 207—corr spot ages 208 Age (Ma) Age 15.05 15.53 14.32 14.63 15.72 14.19 14.47 15.17 15.24 15.31 14.78 15.20 14.94 14.80 13.56 13.26 13.84 13.66 13.78 1σ (%) 2.2 2.4 1.9 2.1 2.2 2.5 2.1 2.7 1.9 1.9 1.9 2.2 2.2 2.0 2.2 2.0 2.2 2.8 2.4 2.1 2.4 2.0 2.3 Th 232 Pb/ 207—corr 208 0.000605 0.000596 0.000636 0.000632 0.000745 0.000769 0.000709 0.000724 0.000778 0.000702 0.000716 0.000751 0.000754 0.000758 0.000732 0.000752 0.000739 0.000732 0.000671 0.000656 0.000685 0.000676 0.000682 1σ (abs.) 0.31 0.36 0.25 0.28 0.43 0.52 0.34 0.44 0.31 0.27 0.28 0.36 0.35 0.34 0.36 0.33 0.35 0.42 0.41 0.31 0.35 0.28 0.40 Th 232 Pb/ 11.94 11.80 12.60 12.68 14.54 15.33 13.86 13.80 15.38 13.71 14.29 14.42 14.98 14.85 14.56 14.79 14.52 13.60 12.63 13.02 13.51 13.30 12.94 204—corr spot ages 208 Age (Ma) Age 1σ (%) 2.6 3.1 2.0 2.2 3.0 3.4 2.4 3.2 2.0 2.0 2.0 2.5 2.3 2.3 2.5 2.2 2.4 3.1 3.2 2.4 2.6 2.1 3.1 Th 232 Pb/ 208 204—corr 0.000584 0.000624 0.000628 0.000720 0.000759 0.000686 0.000683 0.000761 0.000679 0.000707 0.000714 0.000741 0.000735 0.000721 0.000732 0.000718 0.000673 0.000625 0.000644 0.000669 0.000658 0.000640 0.000591 2.9 0.9 0.8 3.2 4.4 1.7 3.5 4.0 0.9 1.3 2.4 1.8 2.1 2.2 8.3 2.5 2.1 1.7 1.1 6.3 2.1 14.9 17.2 f208 from 207(%) 2.4 1.9 2.1 2.2 2.5 2.0 2.7 1.9 1.9 1.9 2.2 2.2 2.0 2.2 2.0 2.1 2.8 2.4 2.1 2.4 2.0 2.3 2.2 1σ (%) Th 232 Pb/ 0.000614 0.000641 0.000637 0.000769 0.000805 0.000721 0.000751 0.000811 0.000708 0.000726 0.000769 0.000768 0.000774 0.000748 0.000884 0.000806 0.000885 0.000689 0.000671 0.000697 0.000684 0.000729 0.000618 208 7.4 6.5 7.5 6.7 7.5 5.9 3.1 6.0 5.3 4.7 5.1 4.9 6.3 1.9 2.7 2.2 7.4 6.9 6.7 5.7 4.5 7.8 10.0 1σ (%) Pb 206 Pb/ 0.160 0.190 0.144 0.162 0.209 0.126 0.224 0.437 0.162 0.235 0.112 0.093 0.104 0.104 0.531 0.316 0.418 0.137 0.178 0.166 0.099 0.215 0.116 207 1σ (%) 32 22 47 33 32 30 22 13 17 22 20 28 24 36 9 13 10 25 31 27 23 19 36 Pb 204 Pb/ 208 609 1425 2697 599 531 798 428 633 922 1520 535 1089 755 1062 225 355 161 415 982 954 1030 319 884 30 83 42 49 69 28 23 27 26 75 39 36 47 45 59 31 21 43 127 105 185 141 142 Th/U 700 890 3900 2700 2300 3600 7800 1200 3800 3300 3300 1400 1200 1500 1100 3200 2400 2100 1200 1500 2300 1200 7700 Th (μg/g) 23 31 26 28 86 17 21 23 23 50 52 55 42 43 61 59 19 26 25 74 59 160 180 U (μg/g) GESS1@07 GESS1@13 GESS1@14 GESS1@01 GESS1@02 GESS1@03 GESS1@06 GESS1@10 GESS1@11 GESS1@12 GESS1@15 GESS1@16 GESS1@17 GESS1@18 GESS1@19 GESS1@20 GESS1@23 GESS1@05 GESS1@08 GESS1@09 GESS1@21 GESS1@22 GESS1@04 (continued) Analysis ID Analysis 3 Table Groups B Ion microprobe dating of fssure monazite in the Western Alps Page 13 of 27 15

Table 4 Summary of fssure monazite growth domains weighted mean ages and spot age ranges Sample domain Figure Zoning of the grains Weighted mean domain MSWD Number Spot 208Pb/232Th age 208Pb/232Th ages (Ma, 1σ) of analyses range of entire grain ± (Ma, 1σ) ± VIU1—B 3a Regular 29.9 0.5 3.7 14 33.2 0.5—28.5 0.5 ± ± ± BALZI2—A 3b Patchy border 36.0 0.6 0.79 5 50.1 0.6—11.2 0.6 ± ± ± BALZI2—B 30.3 0.5 2.80 10 ± BALZI2—C 25.4 0.5 0.69 7 ± MORI1—A 3c Regular 15.2 0.3 2.3 13 15.9 0.4—14.2 0.3 ± ± ± MORI1—B 15.1 0.4 2.6 9 ± VINA1—A 3d Regular 16.4 0.2 0.97 7 16.7 0.3—13.0 0.2 ± ± ± VINA1—B 15.2 0.6 3.3 5 ± VINA1—C1 15.4 0.5 3.0 6 ± VINA1—C2 14.4 0.5 2.4 5 ± GESS1—B 3e Patchy border 14.9 0.3 2.2 14 16.2 0.4—12.1 0.3 ± ± ± greenschist facies) and sample BALZI2 from the Brian- mean 208Pb-232T ages (right) for growth domains. Ion çonnais Zone (blueschist/greenschist facies). Samples probe and LA-ICP-MS spot locations are indicated on MORI1, VINA1 and GESS1 were collected in the Argen- BSE images as solid and dashed circles, respectively. tera Massif (greenschist facies) (Table 1; Fig. 1). All the dated grains come from vertical fssures hosted by gneiss, 4.2.1 Grains from high‑pressure regions except BALZI2 sample for which information on fs- VIU1 is a ~ 700 μm-long grain (Fig. 3a) composed of sure orientation is missing (metarhyolite block in scree; two major chemical domains. Domain A comprises the Table 1). bright bottom part in BSE image with T/U ratio close to Field images showing fssures in high-pressure over- 100. Domain B, comprising the major part of the grain, printed regions are displayed in Fig. 2b–f, between the is characterised by a darker colour in BSE image, related Saas Fee locality in the north and the Costa Balzi Rossi to lower T and U contents of ~ 12,000–22,200 and 240– locality in the south (Fig. 1). Alpine fssures from these 260 µg/g respectively (Table 3). A weighted mean age regions are all oriented subvertically with a strike rang- of 29.9 ± 0.5 Ma (MSWD = 3.7, n = 14) is calculated for ing between N145-180 (Fig. 2a), as seen in Fig. 2b–e. domain B (Table 4). Te Costa Balzi Rossi area (Fig. 2f) is a famous locality BALZI2 (Fig. 3b) is a complex grain that underwent for REE minerals (Bracco et al. 2012). As the analysed several alteration episodes as indicated by the presence monazite was found in a block in scree (R. Bracco, pers. of dissolution trails (red-dashed lines on Fig. 3b) and comm.), the original orientation of the fssure is not pores (indicated by red arrows on Fig. 3b) visible on the known. Samples VIU1 and BALZI2 were collected in the BSE image. Te oldest part of the mineral is mainly pre- Valle di Viù (Fig. 2d) and Costa Balzi Rossi (Fig. 2f) local- served in the lower-right part of the grain (BALZI2-A ities, respectively. domain): it yields a distinct T/U ratio of between 6 and In the Argentera Massif, Alpine fssures are also ori- 50 and providing a weighted mean age of 36.0 ± 0.6 Ma ented vertically (Fig. 2 g–h) with a strike in the range (MSWD = 0.79, n = 5; Table 4). Spots 34 to 40 of BALZI2- between N005-N020 (Fig. 2a). Samples MORI1 and A domain display older and scattered dates, which may VINA1 were collected in the Stura Valley near the Sam- indicate a complex early history of the grain. Most analy- buco locality and GESS1 grain further south, in the Gesso ses located in the rim and in the upper part of the grain, Valley (Fig. 1; Table 1). BALZI2-B, C and D domains, display T/U ratios ranging between 20 and 140 overlapping with domain A chem- 4.2 Fissure monazite dating istry (Table 3). BALZI2-B and C domains are chemically In this section, chemical, textural and chronological comparable, but provide distinct weighted mean ages of data are presented in detail for each grain. Figure 3 gives 30.3 ± 0.5 Ma (MSWD = 2.80, n = 10) and 25.4 ± 0.5 Ma information on T-U content obtained by ion probe (left) (MSWD = 0.69, n = 10; Table 4). A few analyses (spot and on chemical domains distinguishable on BSE images 1, 2, 12, 14, 23, 24, 25 and 31, open symbols) sit on or (centre). Tese combined data were used to defne min- close to dissolution trails and give younger ages (Table 3). eral growth domains, allowing calculation of weighted Tese analyses are excluded from weighted mean age 15 Page 14 of 27 E. Ricchi et al. ± 700 ± 300 ± 700 ± 860 ± 250 ± 170 ± 50 ± 200 ± 130 ± 8 ± 9 ± 60 ± 240 ± 0.03 ± 1 ± 13 ± 7 ± 16 ± 0.1 ± 0.2 ± 0.2 ± 1 ± 4 ± 1 33,010 292,700 137,200 313 0.6 503 108,400 6600 0.9 2920 < 0.64 122,530 21,770 1544 14,020 1190 3550 301 346 19 4430 49 0.15 2.7 < 350 < 0.18 < 0.36 47,48,49 MORI1—A 29 ± 3800 ± 3800 ± 2200 ± 2100 ± 130 ± 830 ± 710 ± 350 ± 160 ± 50 ± 80 ± 220 ± 790 ± 0.05 ± 7 ± 55 ± 18 ± 17 ± 0.1 ± 0.2 ± 0.3 ± 1 ± 3 ± 1 ± 0.2 33,290 287,500 125,200 333 0.4 730 111,200 9040 0.9 4050 < 0.40 126,600 23,190 1720 14,420 1310 4430 394 484 28 4830 74 0.16 4 < 214 0.7 < 0.23 43,44,45,46 MORI1–A 24 ± 2400 ± 1100 ± 1200 ± 270 ± 180 ± 490 ± 570 ± 430 ± 100 ± 60 ± 50 ± 170 ± 390 ± 0.03 ± 6 ± 22 ± 22 ± 25 ± 5 ± 0.1 ± 0.2 ± 1 ± 3 ± 0.2 ± 3 ± 0.1 33,000 275,100 120,700 347 1 949 111,900 9630 0.9 4230 < 0.31 132,740 27,460 2090 18,830 1650 5280 445 475 23 3570 53 0.18 2.5 < 207 5 1 40,41,42 MORI1—B 116 ± 1700 ± 640 ± 1800 ± 1200 ± 220 ± 170 ± 170 ± 250 ± 170 ± 20 ± 30 ± 60 ± 1070 ± 0.04 ± 3 ± 32 ± 12 ± 11 ± 5 ± 0.1 ± 0.1 ± 0.9 ± 0.1 ± 1 ± 2 ± 0.2 32,910 272,400 119,750 349 0.5 871 111,600 1 11,020 3780 < 0.41 133,000 27,920 2230 19,430 1720 5450 494 559 28 5420 66 0.17 3.1 < 233 0.8 0.4 36,37,38,39 MORI1—B 121 ± 1400 ± 2600 ± 800 ± 1800 ± 190 ± 340 ± 260 ± 120 ± 590 ± 210 ± 1040 ± 0.02 ± 27 ± 206 ± 460 ± 55 ± 21 ± 26 ± 0.1 ± 0.3 ± 0.2 ± 1 ± 4 ± 18 33,110 307,100 144,800 376 0.3 382 106,000 0.9 3500 890 < 1 121,900 17,140 1630 9720 783 2350 194 219 11 3670 29 0.16 1.5 < 390 < 0.25 < 0.52 74,75,76,77 BALZI2—B 48 ± 1600 ± 2000 ± 200 ± 1200 ± 80 ± 657 ± 400 ± 400 ± 110 ± 60 ± 50 ± 180 ± 490 ± 2 ± 12 ± 19 ± 26 ± 0.5 ± 0.1 ± 0.1 ± 0.2 ± 2 ± 5 ± 9 ± 4 32,500 294,800 141,500 367 0.7 473 0.7 7700 < 0.91 1360 19,470 2500 11,980 1120 4020 395 531 31 3210 90 0.2 5.2 107,500 < 380 6 < 0.4 71,72,73 BALZI2—C 93 123,300 ± 7400 ± 18,400 ± 1800 ± 10,800 ± 1200 ± 3600 ± 2270 ± 170 ± 2100 ± 550 ± 470 ± 240 ± 990 ± 1350 ± 29 ± 111 ± 159 ± 26 ± 0.1 ± 0.1 ± 0.1 ± 9 ± 55 ± 1 115,300 31,000 289,900 154,300 363 0.4 1540 0.7 7800 < 0.37 4020 18,900 10,910 2270 1040 3890 394 567 37 2980 119 0.2 7 107,700 < 580 < 0.22 < 0.57 68,69,70 BALZI2—C 85 ± 5500 ± 4000 ± 7800 ± 200 ± 490 ± 2000 ± 1650 ± 2200 ± 500 ± 610 ± 410 ± 190 ± 510 ± 0.03 ± 13 ± 136 ± 32 ± 23 ± 12 ± 0.1 ± 0.2 ± 0.1 ± 0.1 ± 1 ± 2 119,900 29,310 301,200 121,200 296 0.4 485 0.8 5000 0.7 3090 23,600 17,130 4780 1510 4120 282 245 10 234 15,000 22 0.14 1.1 106,500 < 341 < 0.19 < 0.34 13,14,15,16 VIU1—B ± 640 ± 540 ± 900 ± 800 ± 110 ± 160 ± 30 ± 1500 ± 70 ± 230 ± 12 ± 30 ± 0.1 ± 0.04 ± 0.05 ± 4 ± 39 ± 8 ± 3 ± 6 ± 3 ± 0.2 105,320 28,170 319,260 139,600 252 < 1 677 3090 < 0.42 3460 < 3 18,300 3430 11,310 887 2300 154 134 5 259 15,400 10.6 0.19 0.54 103,900 < 1290 < 0.7 < 1 10,11,12 VIU1—B ± 4100 ± 2200 ± 5700 ± 1700 ± 610 ± 810 ± 310 ± 2700 ± 430 ± 390 ± 100 ± 10 ± 240 ± 0.01 ± 11 ± 58 ± 22 ± 22 ± 97 ± 0.1 ± 0.03 ± 0.1 ± 2 ± 1 119,300 29,150 298,400 114,500 296 0.3 630 4150 0.8 4060 < 0.47 22,600 4400 15,780 1300 3560 234 189 7 349 23,700 15 0.14 0.7 109,100 < 149 < 0.16 < 0.24 7,8,9 VIU1—A LA-ICP-MS analyses of analyses LA-ICP-MS in concentrations element that domains (μg/g). Note monazite and Yb to correspond Lu maximum concentrations 5 Nd146 Pr141 Ce140 La139 As75 Ba137 Sr88 Y89 Mo95 Ca43 Ti49 Sm147 Eu151 Gd157 Tb159 Dy163 Ho165 Er167 Tm169 U238 Th232 Yb173 W182 Lu175 P31 Si29 Na23 B11 spot # Table during measurement interferences GdObecause of potential and TbO Sample domain / Element (isotope) Ion microprobe dating of fssure monazite in the Western Alps Page 15 of 27 15 ± 3050 ± 500 ± 2600 ± 700 ± 530 ± 240 ± 400 ± 110 ± 10 ± 80 ± 320 ± 170 ± 500 ± 0.06 ± 9 ± 12 ± 5 ± 133 ± 0.2 ± 0.02 ± 4 ± 1 ± 2 ± 0.1 130,190 GESS1—B 34 27,650 63,64,65,66 1980 < 0.67 16,370 < 0.38 1300 < 760 3620 107,300 286 1380 310 < 1 15 332 36 332 1.8 6700 0.2 1 1370 0.44 132,300 287,500 33,580 ± 1600 ± 500 ± 2100 ± 200 ± 660 ± 540 ± 160 ± 80 ± 50 ± 120 ± 320 ± 140 ± 530 ± 0.03 ± 10 ± 9 ± 5 ± 43 ± 0.2 ± 0.2 ± 0.1 ± 2 ± 1 ± 2 127,300 GESS1 – B 36 23,900 60,61,62 1760 < 0.43 15,520 < 0.19 1270 < 380 3640 107,900 297 1720 326 < 0.55 17 328 40 339 2.1 6350 0.15 0.9 2350 0.4 135,500 290,200 33,300 value" represent measurements below LOD below measurements represent value" < ± 430 ± 80 ± 800 ± 1000 ± 330 ± 380 ± 34 ± 50 ± 50 ± 170 ± 60 ± 440 ± 320 ± 0.02 ± 17 ± 18 ± 5 ± 13 ± 0.1 ± 0.3 ± 3 ± 1 ± 2 ± 2 129,550 GESS1—B 40 27,130 57,58,59 1680 < 0.53 16,200 4 1280 < 780 3440 106,000 253 2500 244 < 1 11 329 25 559 1.3 5870 0.13 1.1 8070 < 0.36 130,700 286,500 33,480 ± 1200 ± 400 ± 1900 ± 2000 ± 1100 ± 1030 ± 140 ± 100 ± 100 ± 370 ± 710 ± 590 ± 770 ± 4 ± 31 ± 32 ± 8 ± 31 ± 0.2 ± 0.1 ± 0.3 ± 0.3 ± 2 ± 5 ± 7 138,400 GESS1—B 118 32,100 54,55,56 2230 < 0.75 22,170 6 1840 < 840 5230 107,000 398 2880 390 < 2 17 357 36 655 1.7 8600 0.2 0.9 5030 0.8 118,700 273,300 33,930 ± 2100 ± 1100 ± 3100 ± 4600 ± 1230 ± 1340 ± 160 ± 220 ± 150 ± 470 ± 290 ± 790 ± 250 ± 36 ± 43 ± 5 ± 143 ± 0.2 ± 7 ± 2 ± 6 ± 0.2 141,500 GESS1—B 93 32,470 50,51,52,53 2430 < 0.68 22,430 < 0.51 1850 < 810 5250 105,900 407 1860 410 < 0.65 18 363 38 460 1.7 8490 < 0.14 1 2440 < 0.57 118,500 275,200 34,450 ± 6800 ± 1800 ± 7100 ± 2970 ± 2070 ± 640 ± 3200 ± 660 ± 20 ± 250 ± 340 ± 1100 ± 220 ± 620 ± 0.02 ± 86 ± 28 ± 29 ± 15 ± 0.2 ± 0.2 ± 0.6 ± 37 ± 3 ± 2 129,300 VINA1—A 57 26,130 33,34,35 1360 < 0.51 14,170 < 0.48 898 1500 1940 101,900 122 4700 104 < 1 4 325 10 1140 0.5 2610 0.13 1.1 37,300 1.4 129,400 274,500 32,750 ± 800 ± 12,000 ± 3500 ± 6800 ± 1010 ± 1270 ± 2100 ± 1110 ± 130 ± 130 ± 470 ± 1010 ± 370 ± 950 4 ± 0.03 ± 47 ± 63 ± 17 ± 0.2 ± 4 ± 12 ± ± 1 ± 1 VINA1 – A 21,610 29,30,31,32 1430 < 0.54 12,850 < 0.34 1050 < 670 3010 106,300 241 4840 273 < 1 15 285 39 1040 2 5440 0.18 0.8 11,000 1 24 149,100 287,400 31,640 116,200 ± 1000 ± 960 ± 630 ± 1200 ± 330 ± 330 ± 2100 ± 180 ± 20 ± 20 ± 70 ± 120 ± 80 ± 0.02 ± 4 ± 10 ± 4 ± 56 ± 0.1 ± 0.2 ± 0.2 ± 1 ± 2 ± 2 ± 0.1 VINA1—A 22,190 25,26,27,28 1570 < 0.36 14,250 < 0.19 1150 < 390 3240 106,000 255 3850 276 0.5 14 303 36 861 1.9 5180 0.14 0.8 18,200 1 39 138,780 282,760 32,240 123,000 ± 900 ± 800 ± 400 ± 600 ± 230 ± 220 ± 80 ± 20 ± 20 ± 80 ± 160 ± 170 ± 690 ± 0.03 ± 0.04 ± 7 ± 10 ± 2 ± 31 ± 0.1 ± 0.1 ± 0.1 ± 1 ± 2 ± 1 VINA1—B 22,580 21,22,23,24 1740 < 0.16 13,460 < 0.12 1160 < 203 3300 109,000 266 2420 310 0.4 17 314 45 543 2.3 5870 0.15 0.8 8040 0.48 33 139,800 285,300 32,550 124,400 ± 1000 ± 2900 ± 2000 ± 1800 ± 610 ± 580 ± 160 ± 70 ± 70 ± 200 ± 120 ± 400 ± 210 ± 0.03 ± 0.04 ± 18 ± 22 ± 8 ± 18 ± 0.1 ± 0.1 ± 0.1 ± 1 ± 3 ± 4 VINA1—C2 24,640 17,18,19,20 2070 < 0.34 16,080 < 0.23 1260 < 380 3410 106,600 255 1140 274 0.5 14 321 33 284 1.6 5400 0.13 0.8 3830 0.32 32 140,900 284,000 33,170 129,500 (continued) 5 Table Sample domain / Element (isotope) detection. entries " below Data but are analysed In115 and Bi209 were Mn55, Li7, Al27, each element. Fe57, the number following to correspond isotopes analysed The Sm147 spot # Eu151 B11 Gd157 Na23 Tb159 Si29 Dy163 P31 Ho165 Ca43 Er167 Ti49 Tm169 As75 Yb173 Sr88 Lu175 Y89 W182 Mo95 Th232 Ba137 U238 La139 Ce140 Pr141 Nd146 15 Page 16 of 27 E. Ricchi et al.

calculations. Two additional spot analyses (20 and 51, to ~ 16 Ma. One analysis, spot 24 (open red circle), sits on open blue symbols), located in the upper part of the a dissolution trail and give a younger date (Table 3). Te grain, are also sitting on dissolution trails and provide the second domain (GESS1-B) constitutes the major part of youngest dates for this grain at around ~ 11 Ma. Tese the crystal and is chemically and isotopically heteroge- youngest dates, labeled BALZI2-D, most likely record the neous with a T/U ratio ranging between 20 and 180. A latest phase of monazite alteration. weighted mean age of 14.9 ± 0.3 (MSWD = 2.2, n = 14) is calculated for this second domain (Table 4), but scattered 4.2.2 Monazites from the Argentera Massif spot ages as young as ~ 12 Ma are also observed (blue Sample MORI1 (Fig. 3c) displays two distinct T and U open circles in Fig. 3e; Table 4), likely recording a later clusters ranging between 1200 and 5000 and 4–70 µg/g, phase of monazite alteration that has barely afected the respectively labelled as MORI1-A and B domains chemistry. (Table 3). No clear textural evidence allows distinguish- ing the diferent domains without chemical information: 4.3 Fissure monazite chemistry but the low and high U contents can be used to distin- Trace element LA-ICP-MS analyses of dated fssure guish two groups (A = 5–20 µg/g and B = 40–70 µg/g). monazite domains are listed in Table 5. However, due to Identical weighted mean ages of 15.2 ± 0.3 (MSWD = 2.3, the larger spot size (24–60 µm) of the LA-ICP-MS, some n = 13) and 15.1 ± 0.4 Ma (MSWD = 2.6, n = 9) were cal- of the dated domains were too small to be analyzed. To- culated for domains A and B, respectively (Table 4). rium and U contents measured by ion probe and LA- Te crystal VINA1 (Fig. 3d) is nearly homogeneous in ICP-MS are comparable, which confrms the chemical U content (29 µg/g on average), but displays decreasing groups described in Fig. 3. T contents from core to rim (Table 3), i.e. from domain CI-chondrite-normalised (McDonough and Sun 1995) A (~ 11,200–37,000 µg/g, red symbols), to domain fssure monazite REE patterns are displayed in Fig. 4. Te B (~ 1800–8400 µg/g, orange symbols), to domain C patterns are generally comparable with some variation in (~ 3600–4200 µg/g, green and blue symbols), also visible the slope of the HREE: sample VIU1 and GESS1 having on the BSE image as decreasing brightness. Weighted a lower content in HREE than BALZI2, MORI1. Tere mean ages of 16.4 0.2 (MSWD 0.97, n 7), 15.2 0.6 are also minor diferences in the negative Eu anomaly ± = = ± 0.5 (MSWD = 3.3, n = 5), 15.4 ± 0.5 Ma (MSWD = 3.0, n = 6) (Eu/Eu* = EuN / (SmN x GdN) McLennan 1989): aver- and 14.4 ± 0.5 Ma (MSWD = 2.4, n = 5) were calculated age values of 0.72, 0.46, 0.28, 0.27 and 0.27 for grains for domains A, B, C1 and C2 (Table 4). Spot analyses VIU1, BALZI2, MORI1, VINA1 and GESS1. Again, sam- 18, 19 and 21 located in domain A (red open symbols), ple VIU1 is distinct with respect to the other grains by record a younger age that can be explained by the pres- a less pronounced Eu anomaly (Fig. 4). Only the MORI1 ence of a dissolution trail/pores, probably corresponding grain clearly shows diferent REE patterns in its two age to a younger alteration episode. domains (see Additional fle 1: Figure S1; Table 5). Te grain GESS1 (Fig. 3e) is composed of a patchy and Distinct chemical trends of fssure monazites from this inclusion-rich core (GESS1-A domain), characterized study are presented in bivariate plots in Fig. 5a–c. T and by T/U ratios ranging between 260 and 90 (Table 3), U contents of BALZI2, MORI1 and GESS1 grains clus- and providing the oldest spot dates of the grain of up ter below 10,000 µg/g T and below 130 µg/g U, whereas

(See fgure on next page.) Fig. 2 a Stereographic projection of planes and their poles of subvertical Alpine fssure in the Western Alps. Localities are indicated in Fig. 1. Fissures in high pressure regions and in the Argentera Massif are presented in b-f and g-h, respectively. b NNE (N020) striking vertical fssure in amphibolite facies metabasalt of the Zermatt-Saas high-pressure zone. At the locality of Mittaghorn, Saas Fee, Switzerland, the vertical fssure shown contains crystals of albite (pericline), quartz and chlorite (the wall of the fssure is indicated by the blue arrow). c Subvertical, SSE (N145) striking fssure in Permo-Carboniferous metaconglomerates at Montvalezan, Savoie, France (yellow-dashed lines), associated with strike-slip faulting (note horizontal lineation). The fssure is located in greenschist-facies overprinted blueschist facies rocks. d N (N000) striking, subvertical fssures (green-dashed lines) in greenschist facies overprinted eclogite facies metasedimentary rocks at Margone, Val di Viù, Piedmont, Italy. Sample VIU1 is from this locality. e Meta-granitoid rocks at Montoso, Piedmont, exploited as "Pietra di Luserna". The steeply oriented, NE (N050) striking fssures (indicated by red-dashed lines) cutting the horizontal foliation caused bleaching (dissolution of biotite) in the adjacent host rock. Fissure monazite was reported from these quarries (Finello et al. 2007). f Permian metarhyolites and metasedimentary rocks forming the mountains of Costa Balzi Rossi, Magliolo, Liguria, Italy. The dated monazite sample BALZI1 derives from fssures located in the wall-forming metarhyolites (Bracco et al. 2012), but was collected in the scree. g Vertical N-S (N005) striking fssures (indicated by orange-dashed lines) at the locality Sambuco, Valle Stura, Argentera Massif. Monazite is reported from fssures of this area. Monazites from Vinadio (VINA1) and Moriglione (MORI1) in Valle Stura are from comparable fssures. h NNE (N020) striking fssure in metamorphic Permian siltstones of the Argentera Massif, France, ca. 1.5 km north of Saint-Dalmas de Tende, showing milky quartz crystals on fssure wall (violet arrow). Ion microprobe dating of fssure monazite in the Western Alps Page 17 of 27 15

a b c

High pressure regions: Argentera Massif: Saas Fee Sambuco locality Montvalezan North of Saint-Dalmas Valle di Viù de Tende Luserna

d e

f g h 15 Page 18 of 27 E. Ricchi et al.

Fig. 3 Chemical, textural, and geochronological information for the fve fssure monazite grains analysed in this study. Colour-flled and open-dashed circles on BSE images, respectively, correspond to ion probe and LA-ICP-MS spot locations. The defned growth domains (A, B, C…) are indicated on BSE images with a distinct colour code (red, orange, blue …) and referred to as groups in Table 3. Spot ages considered in the weighted mean age calculations are indicated by colour-flled bars whereas spot ages only considered in the age range are indicated by open bars (bar length representing the spot age plus its 2 σ uncertainty). Red arrows point to pores indicating dissolution-reprecipitation reactions Ion microprobe dating of fssure monazite in the Western Alps Page 19 of 27 15

Argentera High pressure Massif: regions: MORI1

REE CI Chondrite Norm. (McDonough and Sun, 1995) VINA1 VIU1 GESS1 BALZI2 Gnos et al. 2015

La Ce Pr Nd Sm Eu Gd Tb Dy YHoErTmYbLu Fig. 4 Chondrite-normalised REE element patterns of studied fssure monazites

VIU1 and VINA1 grains show higher values and distinct growth domains. Moreover, our data do not show sys- average T/U ratios close to 65 and 500 respectively tematic compositional changes with age (Fig. 5 and Addi- (Fig. 5a). A positive correlation of Sr and Ca is observed tional fle S1). Tus, as described above, growth domains for all the grains (Fig. 5b) whereas Y is correlated nega- discrimination was essentially based on the combination tively with Ce (Fig. 5c). of textural observations and U-T contents variation. Te main chemical variations present in the cationic site of the studied fssure monazites are displayed in a 5 Discussion Ca-Y-T triangular plot (Fig. 5d). T and Ca increase is 5.1 Fissure monazite crystallization ages related to Y decrease, this corresponds to an increase In the Briançonnais Zone, fssure monazite ages record 2+ 4+ 5+ 2− of the cheralite component, (Ca ,T(U) )(P O 4)2, crystallization at ~ 32 and ~ 23 Ma (Grand’Homme et al. and to a decrease of the xenotime-(Y) component 2016a) and at ~ 36, ~ 30 and 25 Ma in the BALZI2 grain 3+ 5+ 2− (Y P O 4). Te lack of Si in the studied monazites (Figs. 1, 3 and 6). Te distinct age domains recorded in (below detection limit of 500 µg/g on average) indicates BALZI2 capture a crystallization duration of ~ 11 Ma, that the T pole of the Ca-Y-T triangular plot corre- possibly up to ~ 40 Ma if the isolated dates in the core 4+ 4+ 2− sponding to the huttonite (T(U) Si O 4) component of the grain are considered (Fig. 3; Table 4). Older spot is very low (Table 5). Te Y pole represents the xeno- ages around ~ 45 Ma may indicate that fssure formation 3+ 5+ 2− time-(Y) component. Te chernovite-(Y), Y As O 4 occurred already during deformation at high-pressure component is negligible due to low As content (aver- greenschist facies conditions in this area (Bousquet et al. age As content: 330 µg/g; Table 5). In summary, VINA1 2012). Te VIU1 monazite grain from the Piemontais and VIU1 grains display a higher cheralite component Zone records monazite growth mostly at ~ 30 Ma, possi- (2 REE3+ substituted by T 4+ and Ca 2+) whereas the bly with an earlier episode already at ~ 32 Ma (Figs. 1, 3 other grains (BALZI2, MORI1 and GESS1) tend to a and 6; Table 4). Our data thus confrm that fssure forma- higher xenotime component (REE3+ substituted by Y3+) tion occurred in the Briançonnais and Piemontais zones (Fig. 5d). In some cases, REE and trace elements show mainly in association with the retrograde metamorphic clear diferences between diferent growth domains, but overprinting of the high-pressure rocks during its exhu- in many cases diferences are too small to distinguish mation, and later in association with strike-slip faulting. 15 Page 20 of 27 E. Ricchi et al.

40000 1600 A Th/U = 500 C

/U = 65 30000 Th 1200 A

B A (µg/g) 20000 A 800 A Sr (µg/g) Th B A B B B A A B C A 10000 B B 400 B A B B C2 C2 C 0 0 0100 200300 400500 01000200030004000500060007000 U (µg/g) Ca (µg/g)

Ca (apfu) cd12000

B 10000 A

B C 8000 ralite che C B A 0.5 A 0.5 6000 C2 B A B B Y (µg/g) A B A A A B B A 4000 A B B C2 B B C 2000 A (Y)

xenotime- 0 huttonite 260000 280000300000320000 Y (apfu) 0.5 Th (apfu) Ce (µg/g)

Samples: VIU1 BALZI2 MORI1 VINA1 GESS1 Fig. 5 a–c Compositional bivariate plots of studied fssure monazites. Each point represents the average of a growth domain. d Ca-Y-Th triangular diagram (atomic proportions) displaying main cationic variation of monazite from this study. Note that A, B, etc. labels correspond to sample domains reported in Fig. 3 and Table 5.

In the ECM of the Western Alps, fssure monazite yield all other ECM. Our monazite data indicate that strike- a wide range of ages: (i) ~ 17.6 Ma for the Pelvoux Massif slip faulting ceased in the Argentera Massif at ~ 14 Ma, (Gasquet et al. 2010), (ii) ~ 20.6 and ~ 16–14 Ma for the whereas dextral strike-slip movements along the west- south-western (Grand’Homme et al. 2016a) and northern ECM started at ~ 11–12 Ma (e.g., Steck and Hunziker eastern (MORI1, VINA1 and GESS1 grains) border of 1994; Bergemann et al. 2017). the Argentera Massif, (iii) ~ 12–11 and ~ 8–5 Ma for the Belledonne Massif (Gasquet et al. 2010; Grand’Homme et al. 2016a) and (iv) ~ 12–7 Ma for the Aiguilles Rouges 5.2 Comparison with deformation, cooling ages and Mont Blanc Massifs (Bergemann et al. 2019; and tectonic evolution of the Western Alps Grand’Homme et al. 2016a; Figs. 1 and 6). However, In order to interpret the newly obtained 208Pb/232T the ~ 17.6 Ma fssure monazite from the Pelvoux Mas- monazite crystallization ages (Fig. 6) in a tectonic con- sif (Gasquet et al. 2010) is related to the formation of text, they are compared to available crystallization/ horizontal fssures that formed in association with the deformation and cooling/exhumation ages for the areas development of a steeply oriented foliation, while in of interest, i.e. Argentera Massif, Briançonnais Zone and the Argentera Massif only vertical fssures developed in Piemontais Zone. association with strike-slip faulting. Tis shows that the Alpine tectonic evolution of Argentera is diferent to Ion microprobe dating of fssure monazite in the Western Alps Page 21 of 27 15

Western ECM High pressure regions

AIGUILLES ROUGES - OISANS - BELLEDONNE ARGENTERA BRIANÇONNAIS PIEMONTAIS MONT-BLANC PELVOUX SW NE

MORI1 BALZI2 Ma VINA1 MTV ISO GESS1 MTC VIU1 0 S PLIOCENE Mess. 10 Tort. (3) Serr. onset of ssure formation in Lang. association with

MIOCENE late dextral strike-slip Burd. faulting 20 Aquit.

onset of ssure Chatt. formation in association with dextral strike-slip faulting (2) 30 OLIGOCEN E Rup. N

onset of early Priab. dextral strike-slip faulting minimum onset of ssure formation Bart. during 40 exhumation (1) Lut. EOCENE

50 Ypr.

Alpine ssure ages : Crystallization ages: Cooling / exhumation ages:

Th-Pb monazite from this study Mean Th-Pb xenotime [2] K-Ar white mica [4] AFT [8] Th-Pb monazite spot date range from this study Ar-Ar adularia [3] Ar-Ar white mica [5] ZFT [9] Th-Pb monazite [1] Rb-Sr white mica [6] Apatite (U-Th)/He [10] (1) Top-NNW thrusting; (2) Top-WNW thrusting; (3) Strike-slip and top-SW thrusting Mean Th-Pb allanite [7] Modi ed after Grand’Homme et al. (2016a) Fig. 6 Summary of fssure monazite ages from this study compared to published crystallization and cooling ages.[1] Bergemann et al. (2019); Gasquet et al. (2010); Grand’Homme et al. (2016a); [2] Grand’Homme et al. (2016a); [3] Gasquet et al. (2010); Leutwein et al. (1970); Marshall et al. (1998); Rossi and Rolland (2014); [4] Gasquet (1979); [5] Agard et al. (2002); Bellanger et al. (2015); Corsini et al. (2004); Crespo-Blanc et al. (1995); Kirschner et al. (2003); Lanari et al. (2014); Rolland et al. (2008); Sanchez et al. (2011a); Simon-Labric et al. (2009); Strzerzynski et al. (2012); Villa et al. (2014); [6] Freeman et al. (1997, 1998); [7] Cenki-Tok et al. (2014); [8] Beucher (2009); Beucher et al. (2012); Bigot-Cormier et al. (2006); Bogdanof et al. (2000); Fügenschuh et al. (1999); Fügenschuh and Schmid (2003); Lelarge (1993); Leloup et al. (2005); Malusà et al. (2005); Sabil (1995); Sanchez et al. (2011b); S. Schwartz et al. (2007); Seward and Mancktelow (1994); Seward et al. (1999); Soom 1990; Tricart et al. (2007); van der Beek et al. (2010); [9] Beucher (2009); Bigot-Cormier et al. (2006); Carpéna (1992); Crouzet et al. (2001); Fügenschuh and Schmid (2003); Glotzbach et al. (2011); Malusà and Vezzoli (2006); Schwartz et al. (2007); Schwartz (2000); Seward and Mancktelow (1994); Soom (1990); van der Beek et al. (2010); Vance (1999); [10] Sanchez et al. (2011b). AFT: apatite fssion track, ZFT: zircon fssion track, N: north, S: south. Square brackets to the left delimit the main periods of monazite growth discussed in the text: (1) Top-NNW thrusting, (2) Top-WNW thrusting, (3) Top-SW thrusting and orogen parallel deformation

5.2.1 Top‑NNW thrusting (> 35 Ma) the development of the main foliation during emplace- Te oldest fssure monazite age of the Western Alps is ment of the Siviez-Mischabel Nappe dated at 41–36 Ma recorded at ~ 36 Ma by BALZI2 grain from the Brian- (Markley et al. 1998). Ar–Ar dating of syn-kinematic çonnais Zone (Figs. 1 and 6) is interpreted to be related phengite from the Modane-Aussois area, west of Ambin to Late Eocene—earliest Oligocene top-NNW thrusting (Am on Fig. 1; Strzerzynski et al. 2012), suggest that (episode (1); Table 2) which started at or before 38 Ma top-NNW thrusting ended at around ∼37 Ma during (Cardello et al. 2019). Tis deformation was coeval with decompression, giving way to WNW-directed thrusting 15 Page 22 of 27 E. Ricchi et al.

with the development of SW-directed normal fault- (episode (2) on Fig. 6; Table 2) from ∼35 Ma down to at ing along the Simplon Fault and SW-directed thrusting least ∼32 Ma. A xenotime grain from a fssure located further north in the same high-pressure terrane records along the Digne Trust (Hubbard and Mancktelow 1992; an average age of ~ 35 Ma (Grand’Homme et al. 2016a; Grasemann and Mancktelow 1993). Fig. 6). According to Freeman et al. (1997), the onset Fissure monazite crystallization recorded at ~ 23 Ma of backthrusting in the Briançonnais is constrained by Grand’Homme et al. (2016a) related to shear zones at ~ 34 Ma by Rb–Sr dating of syn-kinematic white mica activity the Briançonnais Zone Houillère (MTV in Figs. 1 formed under mid-greenschist facies conditions near the and 2c) occurred during the westward indentation of the western and northern border of the Grand Paradiso (GP) Adriatic plate constrained to between 26.8 ± 0.7 Ma and internal massif (Fig. 6). Te fssure monazite domain 20.5 ± 0.3 Ma by Ar–Ar dating of syn-kinematic phen- age constrains the minimal onset of exhumation in the gite from the Argentera Massif (Sanchez et al. 2011a; Briançonnais Zone at ~ 36 Ma, but isolated spot ages in Fig. 6) and the onset of the Liguro-Provençal Basin ocean BALZI2 point at an even earlier start at ~ 45 Ma. In the spreading starting during the Aquitanian (e.g. Gattacceca Piemontais Zone, greenschist conditions are constrained et al. 2007). between ~ 40 and 35 Ma by Ar–Ar dating of phengite Te grain BALZI2 records a domain age at ~ 25 Ma (Agard et al. 2002; Fig. 6). Note that fssure monazite (Figs. 1 and 6). We suggest that this monazite domain and xenotime ages from high-pressure terranes are older crystallized at ~ 25 Ma also in association with shear zone than the zircon fssion track (ZFT) record, suggesting activity related to the earliest stages of ocean spreading that these fssure minerals crystallized above a maximum in the Liguro-Provençal Basin (e.g. Gattacceca et al. 2007; temperature of ~ 380–190 °C (e.g. Glotzbach et al. 2010; Sanchez et al. 2011a; Schmid et al. 2017). Ricchi et al. 2019). At the South-Western border of the Argentera Massif, younger monazite crystallization is dated at ~ 20.6 Ma 5.2.2 Top‑WNW thrusting (35–25 Ma) (ISO grain from Grand’Homme et al. 2016a), inter- Slightly younger fssure monazite crystallization ages preted to be related to NW-striking dextral shear zones are recorded in the Briançonnais Zone between ~ 32 and (Grand’Homme et al. 2016a; Fig. 1). Tis age is in the 30 Ma (BALZI2 grain from this study and MTC grain range of synkinematic phengite of 26.8 ± 0.7 Ma and from Grand’Homme et al. 2016a) and in the Piemontais 20.5 ± 0.3 Ma dated by Sanchez et al. (2011a). Tis fs- Zone at ~ 30 Ma (VIU1 grain; Figs. 1, 3a, b and 6). Tese sure monazite age is contemporaneous with the onset ages correspond to a rapid uplift episode, i.e. exhumation of the anticlockwise rotation of the Corsica-Sardinia of high-pressure units, that afected the Dora Maira inter- block constrained at 20.5 Ma (e.g. Gattacceca et al. 2007; nal massif between ~ 32 and 30 Ma (e.g. Dumont et al. Schmid et al. 2017; stage 3 in Table 2). Most of the Cor- 2012; Rubatto and Hermann, 2003; Schmid et al. 2017). sica-Sardinia anticlockwise rotation (30°), accommodat- Tis uplift episode occurred in response to the WNW- ing the opening of the Liguro-Provençal Basin, occurred directed indentation of the Ivrea mantle wedge (Table 2). between 20.5 and 18 Ma and was active until 16–15 Ma, Rb–Sr dating of syn-kinematic phengites from Free- resulting in a total rotation of 45° (Gattacceca et al. 2007). man et al. (1997, 1998) show that this second episode of Tis age range corresponds almost exactly with the WNW-directed thrusting started at ~ 34 Ma in the east- activity of the orogen-parallel stretching related to the ern part of the Briançonnais Zone (along the NW border Simplon Fault (Grasemann and Mancktelow 1993; Cam- of the GP Massif), migrated westward, reaching the cur- pani et al. 2010). Contemporaneous with this rotational rent western border of the Briançonnais Zone at ~ 31 Ma phase, deformation during uplift of the Pelvoux Massif is (see also Egli et al. 2016), and ended between ~ 27 and recorded at ~ 17.6 Ma by a fssure monazite from a hori- 23 Ma (Fig. 6). An 40Ar/39Ar synkinematic muscovite zontal vein (Gasquet et al. 2010). In the north-eastern from the Argentera Massif indicates that dextral strike- part of the Argentera Massif, younger fssure monazite slip faulting started at 33.6 0.6 Ma during this period. ages of 16–14 Ma are interpreted to constrain dextral ± fault activity related to the end of the Corsica-Sardinia 5.2.3 Strike slip and top‑SW thrusting (25–0 Ma) anticlockwise rotation as described by Gattacceca et al. Te top-WNW-directed thrusting along the PF led to (2007), and mark the onset of (re)-activation of dextral backfolding, backthrusting, and later to the exhumation strike-slip faulting along and through the internal border of the ECM together with westward migration of the of the western ECM (Figs. 1 and 6). deformation front (Table 2). NE-striking dextral strike- Te middle Miocene dextral strike-slip record move- slip movements started to develop around and through ment afecting the western ECM, occurred earlier in the the internal border of the western ECM. Tis orogen- Argentera Massif and shifted then to the other ECMs. parallel deformation episode ((3) on Fig. 6) also coincides Indeed, the fssure monazite ages recording fault activity Ion microprobe dating of fssure monazite in the Western Alps Page 23 of 27 15

along the BSZ or VSZ dextral shear zones in the Argen- or carbonate as a source (Gnos et al. 2015; Fig. 5b). Sam- tera Massif (Fig. 1) are between ~ 16 and 14 Ma (MORI1, ples from the Argentera Massif (MORI1, VINA1 and VINA1 and GESS1 grains), followed by dextral faulting GESS1) have nearly identical Sr/Ca ratios (average Sr/ between ~ 12 and 11 Ma and between ~ 8 and 5 Ma in the Ca = 0.22), suggesting a similar source for these elements. Belledonne Massif (Gasquet et al. 2010; Grand’Homme, In contrast, samples from the high-pressure regions et al. 2016a), and between ~ 12 and 7 Ma in the Aiguilles have average Sr/Ca = 0.39 (BALZI2) and 0.17 (VIU1). Rouges and Mont Blanc massifs (Bergemann et al. 2019; An increase in Ce correlated with a decrease in Y sug- Grand’Homme, et al. 2016a; Figs. 1 and 6). Note that gests that more monazite or allanite, (Ce3+,La3+ ,Nd3 + 2+ 3+ 3+ 2+ 3+ fssures-bearing monazite related to dextral strike-slip ,Ca ,Y )2(Al ,Fe ,Fe )3(SiO4)3(OH), was dissolved movements from the Aiguilles Rouges, Mont-Blanc and with respect to xenotime (Fig. 5c). Finally, an increase in Belledonne massifs are sub-vertical and E-W striking cheralite component (Ca + T replacing 2REE) related to (Bergemann et al. 2019; Grand’Homme et al. 2016a). a decrease of xenotime component (Y) is observed for However, NNE-striking vertical fssures are observed grains VINA1 and VIU1 (Fig. 5). Only in few cases REE between the southern termination of the Argentera Mas- and trace elements show sufcient variation helping to sif and Saint-Dalmas de Tende locality (Fig. 2 h), suggest- separate growth domains in the hydrothermal monazites ing that these fssures may have had a similar orientation, studied here. Tere is also no systematic compositional but underwent 45°- anticlockwise rotation caused by the change over time. opening of the Liguro-Provençal Basin. Finally, an overall decreasing trend of cooling ages is observed from the internal high-pressure regions toward 6 Conclusions the western ECM (Fig. 6) implying that exhumation frst Te oldest ages recorded by fssure monazite found in afected the internal units before progressively propa- the Briançonnais Zone are related to exhumation along gating toward the external units, (e.g. Fügenschuh and the PF during top-NNW thrusting in sinistral transpres- Schmid 2003). In the high-pressure regions, the minimal sion. Tis frst exhumation episode was overprinted by onset of exhumation (i.e. the maximal age of the start of subsequent top-WNW thrusting recorded between ~ 32 exhumation) is constrained at ~ 36 Ma by fssure mona- and 30 Ma in fssure monazites from the Briançonnais zite (this study) and phengite crystallization (Agard et al. and Piemontais zones. During top-SW thrusting, fssure 2002) under greenschist facies conditions. Tese ages monazite crystallization is recorded in association with are older than ZFT cooling ages (Fig. 6), whereas in the strike-slip faulting in the Briançonnais Zone Houillère western ECM the minimal onset of exhumation follows at ~ 23 Ma, likely related to the progressive opening of ZFT ages, and fssure monazite crystallization is system- the Liguro-Provençal Basin. Later, at ~ 20.6 Ma, fssure atically observed between ZFT and AFT cooling ages. monazite from a fault zone located in the south-western Tese observations suggest slower cooling rates in the border of the Argentera Massif is attributed to the onset internal units with respect to the external ones. of the anticlockwise rotation of the Corsica-Sardinia Block. By contrast, monazite crystallization recorded 5.3 Fissure monazite composition at ~ 16–14 Ma in a fault zone of the north-eastern Argen- Te composition of fssure monazites provides hints on tera Massif likely constrains the end of the Corsica- dissolved host-rock minerals and oxidation condition, in Sardinia block rotation. Contemporaneous with this the fssure. Trace element analyses of fssure monazite rotation, deformation during the exhumation of the Pel- show a negative Eu anomaly, most likely resulting from voux Massif is recorded at ~ 17.6 Ma. Finally, successively co-crystallization of albite. Te diference in negative Eu/ younger episodes of dextral strike-slip related to increas- Eu* is interpreted to be inherited from the host rock and ing concentration of deformation along the Rhone-Sim- is distinct in VIU1 (Piemontais Zone) compared to the plon/Penninic Front fault system are recorded at ~ 12–11 other grains (Fig. 4). Generally, the REE content of fssure and ~ 8–5 Ma in the Belledonne Massif and at ~ 12–7 Ma monazite does not change systematically over time, but in the Mont-Blanc and Aiguilles Rouges massifs, mark- seems mainly controlled by dissolution of available REE- ing the jump from thrusting to dextral strike-slip faulting bearing accessory minerals. toward the northern ECM of the Western Alps. Te high T/U content of the VINA1 grain indicates a Chemical observations of the investigated fssure mon- strongly oxidizing environment of crystallization in asso- azites suggest a similar source of Sr and Ca for the grains ciation with hematite, as described by Gnos et al. (2015) from the Argentera Massif, generally higher amounts (Fig. 5a). It has indeed been mentioned that hematite is of monazite or allanite dissolution from the host-rock not rare in fssures in this area (Piccoli 2002). Sr and Ca with respect to xenotime and corroborate previous correlation indicates dissolution of host-rock plagioclase 15 Page 24 of 27 E. Ricchi et al.

observations of extremely high T/U content formed Bergemann, C. A., Gnos, E., & Whitehouse, M. J. (2019). Insights into the tectonic history of the Western Alps through dating of fssure monazite in under oxidizing conditions. the Mont Blanc and Aiguilles Rouges Massifs. Tectonophysics, 750(2018), 203–212. https://doi.org/10.1016/j.tecto.2018.11.013. Acknowledgements Berger, A., Gnos, E., Janots, E., Whitehouse, M., Soom, M., Frei, R., et al. (2013). We thank Gaspare Maletto, Roberto Bracco and Francis Guichon for providing Dating brittle tectonic movements with cleft monazite: Fluid-rock the samples and valuable information on the sample localities. This study was interaction and formation of REE minerals. Tectonics, 32(5), 1176–1189. funded by Swiss National Foundation Grant 200020-165513. https://doi.org/10.1002/tect.20071. Beucher, R. (2009). Evolution Néogène de l’arc alpin sud-occidental: Approches Authors’ contributions sismotectonique et thermochronologique. Fissure monazite samples were organized by EG and selected for ion probe Beucher, R., Van Der Beek, P., Braun, J., & Batt, G. E. (2012). Exhumation and relief dating by EG and ER according to tectonic settings and activity of the study development in the Pelvoux and Dora-Maira massifs (western Alps) area. Sample preparation and BSE imaging was carried out by ER. Ion probe assessed by spectral analysis and inversion of thermochronological age and LA-ICP-MS data acquisition and reduction was performed by ER under transects. Journal of Geophysical Research: Earth Surface, 117(3), 1–22. the supervision of DR, MJW and TP respectively. The manuscript was prepared https://doi.org/10.1029/2011JF002240. by ER during her PhD project under the supervision of EG with contributions Bigot-Cormier, F., Sosson, M., Poupeau, G., Stéphan, J. F., & Labrin, E. (2006). The from all co-authors. All authors read and approved the fnal manuscript. denudation history of the Argentera Alpine external crystalline massif (Western Alps, France-Italy): An overview from the analysis of fssion Funding tracks in apatites and zircons. Geodinamica Acta, 19(6), 455–473. https:// This study was fnanced by the Swiss National Foundation Grant doi.org/10.3166/ga.19.455-473. 200020–165513. Bogdanof, S., Michard, A., Mansour, M., & Poupeau, G. (2000). Apatite fssion track analysis in the Argentera massif: Evidence of contrasting denuda- Availability of data and materials. tion rates in the external crystalline massifs of the Western Alps. Terra All data generated or analysed during this study are included in this published Nova, 12(2), 117–125. https://doi.org/10.1111/j.1365-3121.2000.00281.x. article and its additional fles. Bousquet, R., Oberhänsli, R., Schmid, S. ., Berger, A., Wiederkehr, M., Robert, C., et al. (2012). Metamorphic framework of the Alps: Map 1: 1 000 000. 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