Denis Thiéblemont
email: d.thieblemont@brgm.fr

Jean-Baptiste Jacob

Philippe Lach

Catherine Guerrot

Mathieu Leguérinel

l'unité cristalline de Serre Chevalier (domaine briançonnais, France)

Keywords: Western Alps, Briançonnais, Precambrian, Ediacarian, Alpin orogen, Cadomian Orogen Alpes occidentales, Briançonnais, Précambrien, Edicarien, Orogène Alpin, Orogène Cadomien

teaching and research institutions in France or abroad, or from public or private research centers.

First report of an Ediacarian basement in the Western Alps: the Serre Chevalier crystalline unit (Briançonnais domain, France) Découverte d'un socle édiacarien dans les Alpes occidentales :

First report of an Ediacarian basement in the Western

Alps: the Serre Chevalier crystalline unit (Briançonnais domain, France)

Denis Thiéblemont, Jean-Baptiste Jacob, Philippe Lach, Catherine Guerrot,

Mathieu Leguérinel

To cite this version:

Denis Thiéblemont, Jean-Baptiste Jacob, Philippe Lach, Catherine Guerrot, Mathieu Leguérinel. First report of an Ediacarian basement in the Western Alps: the Serre Chevalier crystalline unit (Briançonnais domain, France). Bulletin de la Société Géologique de France, In press, 10.1051/bsgf/2023011. hal-04332029

Introduction

The mainly Variscan age of the Alpine basement is widely documented and diverse models have been proposed trying to insert this prealpine basement in a plate tectonic model (see for example Guillot et al., 2009;Guillot and Ménot, 2009;[START_REF] Faure | Reconstructing the Variscan Terranes in the Alpine Basement: Facts and Arguments for an Alpidic Orocline[END_REF][START_REF] Von Raumer | Ediacaran to Cambrian oceanic rocks of the Gondwana margin and their tectonic interpretation[END_REF][START_REF] Ballèvre | Pre-Alpine (Variscan) inheritance: A key for the location of the future Valaisan Basin (Western Alps)[END_REF][START_REF] Ballèvre | A step towards unraveling the paleogeographic attribution of pre-Mesozoic basement complexes in the Western Alps based on U-Pb geochronology of Permian magmatism[END_REF][START_REF] Siegesmund | Cadomian to Cenerian accretionary orogenic processes in the Alpine basement: the detrital zircon archive[END_REF][START_REF] Jacob | HT overprint of HP granulites in the Oisans-Pelvoux massif: Implications for the dynamics of the Variscan collision in the external western Alps[END_REF]Vanardois et al., 2022). This topic has led to the acquisition of a large amount of geochronological data by more and more precise techniques providing a large panel of ages indicating emplacement in the case of magmatic protoliths and source inheritance in the case of (meta)sedimentary rocks. To date, occurrences of Precambrian rocks have been recovered only in some places of the Central and Eastern Alps (see review in Von [START_REF] Von Raumer | Ediacaran to Cambrian oceanic rocks of the Gondwana margin and their tectonic interpretation[END_REF][START_REF] Siegesmund | Cadomian to Cenerian accretionary orogenic processes in the Alpine basement: the detrital zircon archive[END_REF]. In the Western Alps, the possible occurrence of a Precambrian basement has been postulated in the 1970's based on field and petrographical considerations [START_REF] Pêcher | Présence de gneiss du faciès granulite dans le noyau précambiren du massif des Ecrins-Pelvaoux[END_REF] apparently supported by « total-Pb » zircon ages ranging from 430 to 800 Ma [START_REF] Biju-Duval | Etude pétrologique des terrains cristallins de la région du Sirac (sud du massif des Ecrins-Pelvoux-Haut dauphiné) -Alpes-France[END_REF]; Barféty and Pêcher, 1984). In particular, the occurrence of high-grade metamorphic rocks (granulite facies) in this basement was considered as a determining argument in favour of a pre-Hercynian age [START_REF] Pêcher | Présence de gneiss du faciès granulite dans le noyau précambiren du massif des Ecrins-Pelvaoux[END_REF]. Recent work indicating an Ordovician age (c. 480 Ma) for granulite protolith and Carboniferous age for HT metamorphism [START_REF] Jacob | HT overprint of HP granulites in the Oisans-Pelvoux massif: Implications for the dynamics of the Variscan collision in the external western Alps[END_REF] clearly discards the previous interpretation. In fact, the best argument for the possible occurrence of Precambrian rocks in the Western Alps is provided by the presence of Neoproterozoic zircons in metasedimentary formations thought to derive from the erosion of the pre-Alpine basement [START_REF] Chu | Detrital zircon U-Pb ages and Hf isotopic constraints on the terrigeneous sediments of the western Alpes and their paleogeographic implications[END_REF][START_REF] Fréville | Structural, metamorphic and geochronological insights on the Variscan evolution of the Alpine basement in the Belledonne Massif (France)[END_REF]. It has been shown that the pre-Alpine basement of the Central and Eastern Alps is partly made of an Ediacarian arc system [START_REF] Von Raumer | Ediacaran to Cambrian oceanic rocks of the Gondwana margin and their tectonic interpretation[END_REF], but no such basement has been evidenced

to date in the Western Alps.

In this paper, we present new geochronological data obtained in a restricted crystalline terrain located in the Serre Chevalier area, about 5 km to the west of Briançon city. These data provide the first evidence of the existence of a Precambrian (Cadomian ?) event in the pre-Alpine basement of the Western Alps and its local reworking in a metasedimentary sequence of Brioverian (i.e. Upper Ediacarian to lower Cambrian) age.

Geological outline

The studied area is located in the Briançonnais domain (simply named « Briançonnais » in the following text), one of the main tectonic domains of the internal Western Alps (fig. 1).

At the latitude of Briançon (fig. 1 intensively deformed and metamorphosed under high pressure -low temperature metamorphic conditions in the blueschist/eclogite facies (e.g. [START_REF] Bousquet | Metamorphic framework of the Alps[END_REF].

At the transition between the Briançonnais and Liguro-Piemontese domains a specific Piemontese domain is classically defined (Barféty et al., 1995) which is considered as the continental margin making the transition between continent and ocean. Alternatively, this domain may be considered as part of the Briançonnais [START_REF] Ballèvre | A step towards unraveling the paleogeographic attribution of pre-Mesozoic basement complexes in the Western Alps based on U-Pb geochronology of Permian magmatism[END_REF] with no need to define an additional paleogeographic domain. Following this point of view, we distinguish an external Briançonnais to the west and an internal Briançonnais to the east, the former being further divided into a western and an eastern subdomain (Barféty et al., 1995;1996).

The external Briançonnais, well exposed in the eponym Briançon region (Barféty et al., 1995), is mainly characterized by well-developed Carboniferous to Permian sedimentary and volcanosedimentary basins overlained by a thick Triassic sedimentary sequence, a restricted Jurassic to lower Cretaceous interval and an upper Cretaceous to Eocene sequence with dominant flysch deposits. The lowest term of the supracrustal succession is the Cristol detrital formation (conglomerates, sandstones) of supposed Namurian age (Barféty et al., 1996). The basement below this formation is unknown. The whole domain is affected by west-verging thrusts and was metamorphosed under upper-greenschist facies conditions during the Eocene-Oligocene Alpine collision.

The internal (eastern) part of the Briançonnais domain, thrusted over the external one (fig. 2 and 3), is dominated by east verging late stage backfolds and backthrusts that often invert the original nappe stack, bringing Briançonnais-derived thrust sheets over Liguro-Piemontese ophiolites and associated sediments (« schistes lustrés ») [START_REF] Ballèvre | A step towards unraveling the paleogeographic attribution of pre-Mesozoic basement complexes in the Western Alps based on U-Pb geochronology of Permian magmatism[END_REF]. The metamorphism is of blueschist facies with calculated temperature and pressure of 300°C and 6-10 kb respectively (Barféty et al., 1995). The basement of the internal Briançonnais (i.e.

« Piemontese domain » according to Barféty et al., 1995) is not visible in the Briançon region but may be observed to the north in Vanoise, being part of the group of the Internal Crystalline Massifs which represents the easternmost edge of the European basement (fig. 1).

The Serre Chevalier crystalline terrain

For convenience the studied rocks are designated as the Serre Chevalier terrain but correspond to a group of three closely spaced outcrops of kilometric size (fig. 2) designated by three different names (Eychauda, Crête du Mal Parti, Serre Chevalier) (Barféty et al., 1996).

These crystalline rocks are mapped as a unique formation on the geological map (symbol ξ) (Barféty et al., 1996) but exhibit a large range of facies (see below) which could not be distinguished at the 1/50,000 scale (Barféty et al., 1996). These outcrops are the largest though not the only occurrences of crystalline rocks in the external Briançonnais. Another one, located at the summit part of the Chaberton peak, is represented on fig. 2 and3. Other occurrences are of too limited extension to be reported on these figures. Collectively, they have been included in a specific lithotectonic unit considered as the uppermost part of the external Briançonnais nappes stack (Termier, 1903) now designated as the intermediate slices (Barféty et al., 1995 and refs. therein). This situation is well exhibited in the Serre Chevalier area where the crystalline terrains lie at the top of the mountain range, a position placing them at the upper part of the nappes stack (fig. 3).

To the east, the intermediate slices unit continues into the Prorel outcrop (fig. 2) which is mainly composed of a coarse-grained and polygenic sedimentary breccia including crystalline fragments at the top (Barféty et al., 1995). More to the east (fig. 2 and3), the intermediate slices unit marks the transition between the external and internal Briançonnais, being mapped as a west-verging tectonic slice backthrusted over the Triassic of the Chaberton peak on its eastern side (fig. 3). The isolated outcrop of crystalline terrains mapped at the summit of that peak corresponds to the easternmost extension of the intermediate slices unit.

In the Serre Chevalier area (fig. 4), the crystalline rocks reach an apparent maximal thickness of about 300 m and overlies different sedimentary units (fig. 3) (Barféty et al., 1996): a dark flysch (« flysch noir ») at the base, grading upward into an olistostrome locally overlain by a polygenic breccia with abundant fragments derived from the overlying crystalline unit (Eychauda breccia, fig. 4).

All those units have been proven to be in sedimentary contact (Barféty et al., 1992;1995).

Microfossils at the base of the succession (dark flysch) have yielded a lower Bartonian age suggesting an upper Eocene age for the whole sequence (Barféty et al., 1992). This led Barféty et al. (1992) to reinterpret the upper part of the Serre Chevalier succession as an olistostrome precursor to the emplacement of the intermediate slices and other Briançonnais nappes in a context of continental coliision.

A more straightforward interpretation is to consider the intermediate slices unit as a unique olistostrome including a large range of olistoliths of either sedimentary or crystalline nature emplaced in the course of the dark flysch deposition. A Briançonnais origin would appear as the most probable for those olistoliths.

The Briançon geological map (1/50,000 scale) (Barféty et al., 1995(Barféty et al., , 1996) ) is the main document giving some information on the Serre Chevalier crystalline terrain and no additional work seems to have been published since that time. The unit is mapped as a composite formation associating dominant micaschist and other crystalline rocks with no atttempt to make cartographical distinctions between the different lithologies (Barféty et al., 1996). It is described as a succession of micaschist and amphibolite beds with sparse lenses of orthogneiss, scarce pegmatites and also chlorite schists and dark schists. Massive amphibolites showing hornblende with brown core and barroisite rim are also described. Brown hornblende belongs to an early stage metamorphic assemblage also including garnet, brown and white mica, and rutile. Late stage minerals include green amphibole (barroisite), chlorite and white mica.

Chlorite is frequently observed as pseudomorphs after biotite or garnet.

Toward the base of the unit, a progressive mylonisation and retrogression affect the rocks with apparition of chlorite schists (Barféty et al., 1995). Nevertheless the contact with the basal breccia is described as stratigraphic. For instance, the retrogressed facies observed at the bottom of the basement unit also occur as blocks within the undelying breccia. This is one of the arguments which led Barféty et al. (1995) to conclude to an emplacement of the basement slices as olistoliths. After exhumation at surface, the basement has been fragmented and incorporated as blocks and panels of plurikilometric size in the Eocene sedimentary succession. For the authors, the metamorphism affecting the basement is clearly of ante-Permian age, but the retrogression could be related to exhumation and only barroisite may be considered as an alpine metamorphic mineral.

Our observations, including the petrographical study of representative facies (Supplementary materials, S1), are consistent with that of Barféty et al. (1996) with some significant additional observations. We do not attempt to draw a precise map as it probably requires to map at the 1/10,000 scale due to the short-scale transitions between lithologies.

The dominant lithologies are metasedimentary with a predominance of micaschist alternating with fine-grained gneiss richer in plagioclase. Transition may be gradual or sharp. In this latter case, gneiss occurs as distinct bands parallel to the main foliation. Quartzite may be locally abundant as submetric bands alternating with micaschist. Amphibolite and fine-grained felsic gneiss (leptynites) appear as decimetric to plurimetric bands (fig. 5) intercalated within metasediments. Amphibolite and leptynite may alternate as very regular beds with sharp contacts. Amphibolite is generally coarse-grained with well visible amphibole and garnet. The textural association of these large secondary minerals is suggestive of a former subvolcanic (i.e. doleritic) texture. A conglomeratic layer intercalated within the micaschists and paragneisses has also been observed (fig. 6) with pebbles and boulders of granite some of them reaching an almost metric size.

This new investigation of the Serre Chevalier basement let us to select four rocks for a geochronological study: (i) a micaceous quartzite (sample BRI0321); (ii) a leptynite (finegrained leucocratic gneiss) (sample BRI0058); (iii) a boulder of granite (sample BRI0334); (iv) a massive garnet-amphibolite (sample BRI0057A). Their petrographical descriptions and geographical coordinates are provided as Supplementary materials (Table S1).

Analytical techniques

The sample preparation, including crushing, zircon extraction and thin section, has been operated in the BRGM laboratories (Orléans, France). In situ U-Pb analyses were obtained on an Agilent 8900 ICP-MS for BRI0321 and BRI0057 and on ThermoScientific ICP-MS X séries II for BRI0058 and BRI0334, both coupled to a Cetac Excite 193nm laser ablation system. The sample is located in a dual-volume laser ablation cell with Helium gas flow. The detailed analytical parameters for the ICP-MS and the laser ablation system are listed in Table S2-1 (Supplementary materials).

The occurrence of common Pb in the sample can be monitored by the evolution of the with repeated measurements of the 91500 zircon standard [START_REF] Wiedenbeck | Three natural zircon standards for U-Th-Pb, Lu-Hf, trace element and REE analysis[END_REF].

Reproducibility and accuracy of the corrections are controlled through repeated measurements of the Plešovice zircon standard (338 ± 1 Ma, Slama et al., 2008) (Supplementary materials, Table S2-2 and fig. S2). Data reduction was carried out with Gliter® (van Achterbergh et al., 2001). Concordia ages and diagrams were generated using Isoplot/Ex version 4.15 [START_REF] Ludwig | ISOPLOT A geochronological toolkit for Microsoft Excel[END_REF]. Uncertainties given in the Tables for individual analyses (ratios and ages) are at 2σ level. Results for age calculation and uncertainties are shown in the Figures at 2σ. The error correlation coefficient (ρ) is calculated according to [START_REF] Schmitz | Derivation of isotope ratios, errors, and errors correlations for U-Pb geochronology using 205 Pb-235 U-( 233 U)-spiked isotope dilution thermal ionization mass spectrometric data[END_REF]:

ρR75-R68 =((σ75/R75) 2 +(σ68/R68) 2 -(σ76/R76) 2 )/ (2*(σ75/R75)*(σ68/R68))
where R75 = 207 Pb/ 235 U; R68 = 206 Pb/ 238 U; R76= 207 Pb/ 206 Pb , σ = uncertainty of the isotopic ratio.

Results

Zircon cathodoluminescence images of all the dated zircons are provided as Supplementary materials (fig. S3-1 to S3-4). U/Pb analyses of the four studied samples are reported in Table S4-1 to Table S4-4).

Micaceous quartzite -Sample BRI0321

This sample shows a rather heterogeneous zircon population. Crystals are very small (< 100 µm), often broken (fig. 7). They are colorless to pinky beige, translucent to opaque, subeuhedral to rounded. This wide diversity is consistent with the sedimentary nature of the sample. SEM image also reveals a highly heterogeneous zircon population in terms of zoning patterns, core-rim relationships, luminescence etc (supplementary materials, fig. S3-1).

105 analyses have been performed on 105 zircons of varied shape and colour. These analyses show a large spread of ages between 500 and 3300 Ma (Fig. 8).

4 analyses are rejected because of a 206 Pb/ 204 Pb ratio lower than 1000 indicative of substantial common lead. 42 analyses are rejected for the statistics because they show a degree of discordance higher than 4%. A unique analyse attests for a very old age (> 3300 Ma). Age components deconvolution is done for the 58 remaining analyses using the Sambridge-Compston algorithm [START_REF] Ludwig | ISOPLOT A geochronological toolkit for Microsoft Excel[END_REF] 11b), the 22 analyses plot along the concordia and amongst then 18 define a well-constrained age of 597 ± 4 Ma (fig. 11).

Granite boulder -Sample BRI0334

This sample shows rare zircons of variable size (~100-500 µm) (fig. 12). Crystals are generally stubby, light beige, translucent and euhedral to subhedral with frequent bubble inclusions and some of them (Supplementary materials, fig. S3-3) exhibit a distinct core with an irregular shape and bright CL luminescence surrounded by a well-crystallized and oscillatory zoned rim with darker CL luminescence (fig. 13).

45 analyses have been performed on 40 zircons. 4 analyses have been discarded because of a 206 Pb/ 204 Pb ratio lower than 1000 indicative of substantial common lead. The remaining 41 analyses are very scattered in the Concordia diagram (fig. 14a). 34 plot along the concordia between 750 and 400 Ma with two significant events at 580-600 Ma (13 analyses) and 490-500 Ma (18 analyses) (fig. 14b). An age of 582 ± 5 Ma may be calculated from 5 concordant analyses of the first group (fig. 15b) which corresponds mostly to crystal cores (spots 2, 7, 13, 25, 37) and display low U/Th ratios (< 5). An age of 492 ± 4 Ma may be calculated from 9

analyses of the second group (fig. 15d) which correspond to crystal rims (spots 1, 4, 11, 29, 30, 34, 35, 38, 40) and display high U/Th ratios (> 5). Despite the mild accuracy of the probabilty associated with this age (MSWD = 5.4), the grouping of points confers it a reasonnable level of confidence. An example is displayed on fig. 13, with a single zircon

showing a core at 584 ± 8 Ma ( 206 Pb/ 238 U age) and U/Th ratio of 1 and a rim at 471 ± 5 Ma and U/Th ratio of 12.

Plot of 206 Pb/ 238 U age against Th/U (fig. 16) shows a clear discrimination between two groups :

(i) a young group corresponding mostly to analyses performed at zircon rims. Except for 3

analyses, the zircons of this group show Th/U ratios < 0.15 and 11 out of them show Th/U < 0.1 ; (ii) an old group corresponding only to analyses performed in zircon cores. The Th/U ratio in this group ranges from 0.2 to 1 being generally higher than that of the other group. Note that all analyses with a Th/U ratio < 0.15 and age < 500 Ma are from the rims, nevertheless some zircons display rims with Th/U > 0.15 and age > 500 Ma. This indicates that young rims are not present around all zircons.

Differences of Th/U ratios between magmatic and metamorphic zircons have been frequently

observed with the conclusion that magmatic zircon generally display high Th/U relative to metamorphic ones (Pystina and Pystin, 2019 and refs. therein). Based on a regional compilation of U-Th data from igneous and metamorphic zircons from Western Australia, [START_REF] Yakymchuk | Th/U ratios in metamorphic zircon[END_REF] showed that less than 1% of the igneous zircons from their data base display Th/U ratios < 0.1, whereas such low values are commonly observed in the metamorphic zircons. Nevertheless, the rule is not absolute; an unambiguous example of igneous zircons with low Th/U ratios has been reported by [START_REF] Lopez-Sanchez | An example of low-Th/U zircon overgrowths of magmatic origin in a late orogenic Variscan intrusion: the San Ciprián massif (NW Spain)[END_REF]. Those zircons of Permian occur as rims surrounding zircon cores of Ediacarian age.

According to [START_REF] Hoskin | The composition of zircon and igneous and metamorphic petrogenesis[END_REF] The geochemical analyse of BRI0334 (Supplemtary materials, Table S5) indicates a moderate SiO2 content (68.1%) and high TiO2 content (= 0.74%) suggestive of relatively high temperature (e.g. [START_REF] Hayden | Rutile saturation in hydrous melts and its bearing on Tithermometry of quartz and zircon[END_REF]. U and Th contents are respectively of 2.13 ppm and 16.3 ppm. U(zircon/whole rock) and Th(zircon/whole rock) fractionation factors have been calculated for the two age-populations of zircons (492 Ma and 582 Ma) from the mean U and Th contents measured at each spot. U(zircon/whole rock) and Th(zircon/whole rock) for the young rims are of 317 and 3 respectively, against 259 and 15 for the c. 582 Ma-old zircons. Values for the rims would suggest a temperature of c. 800°C if calculated after U and more than 1200°C if estimated after Th. This is clearly inconsistent, indicating that zircons with the U and Th contents of zircon rims of BRI0334 cannot crystallize from a magma having the composition of BRI0334.

The temperature obtained from the mean U and Th contents of the c. 582 Ma-old zircons are of c. 850°C when calculated from U and c. 1000°C when calculated from Th. This is more consistent and shows that zircon with the U and Th contents of the c. 582 Ma-old zircons may actually crystallize from a magma having the composition of BRI0334. Note that the temperatures estimated for BRI0334 fom three different methods (zircon solubility, Watson and Harisson, 1983; rutile solubility, [START_REF] Hayden | Rutile saturation in hydrous melts and its bearing on Tithermometry of quartz and zircon[END_REF]monazite solubility, Montel, 1993) are similar and equal to 900°C.

Coarse-grained amphibolite -BRI0057A

The zircon extracted from this amphibolite are very small, brittle, translucent, of pale pink color and euhedral to anhedral (fig. 17). Under CL images (Supplementary materials, fig. S3-4) the zoning pattern of the grains is often "striped", diffuse or patchy, rather than clearly oscillatory.

Such features are quite common in zircons from mafic rocks [START_REF] Laurent | Protracted, coeval crust and mantle melting during Variscan late-orogenic evolution: U-Pb dating in the eastern French Massif Central[END_REF]

Discussion

Chronology of the Serre Chevalier crystalline terrain

The results obtained on the micaceous quartzite BRI0321, with a well-defined age of 610 ± 3

Ma corresponding to more than half of the zircon population and a subordinate age of 939 ± 8

Ma, clearly point to the dominant contribution of Neoproterozoic material in the original sediment. Minimal ages of 580 Ma obtained from the zircons of this rock fixes a maximal age for the deposition of the original sediment.

The quartzite alternates with the more common micaschist lithology which has not been analyzed but constitutes the matrix of the conglomerate with granitic pebbles, one of them (sample BRI0334) giving two main zircon ages populations: 492 ± 4 Ma and 582 ± 5 Ma.

As pointed earlier, the younger age is obtained from zircon rims which are clearly distinct from the core and present lower Th/U ratios (< 0.15). From this low Th/U ratio and considerations about the fractionation factors of U and Th relative to the host rock (granitic boulder BRI0334)

we came to the conclusion that these rims cannot be of magmatic origin.We suggest that the zircon rims grown during a metamorphic event at 492 Ma, while zircon cores had crystallized at 582 Ma from the granitic magma.

Taken together, the maximal sedimentation age of 580 Ma obtained from the micaceous quartzite and the age of 582 Ma age obtained for the granitic boulder imply the deposition of the sedimentary to have occurred after ~ 580 Ma.

The wide spectrum of ages obtained for the micaceous quartzite attests for multiple sources.

A dominant Ediacarian age is indicated by the well-defined peak at 610 ± 3 Ma. An other peak at 939 ± 8 Ma witnesses another Neoproterozoic contribution whereas the remaining zircon populations attest for Paleoproterozoic supplies.

The well defined age of 597 ± 4 Ma obtained for the leptynite alternating with paragneiss, with only one analysis giving an older date of c. 2.1 Ga is again compatible with a Neoproterozoic age of the Serre Chevalier crystalline terrain. Two hypotheses may be envisaged: (i) the leptynite had a magmatic origin and emplaced as lava interbedded in the sedimentary sequence or as dyke cutting accross it; (ii) the leptynite derived from a sedimentary protolith and could correspond to an arkosic bed with a composition very close to that of its magmatic precursor. This latter hypothesis is the most consistent. Knowing that the maximal age of deposition of the sedimentary sequence is 580 Ma presence of a 597 Ma-old lava or intrusion in this sequence is actually inconsistent.

Another data to be discussed is the age of 517 ± 3 Ma obtained on the garnet-amphibolite BRI0057A. This rock is clearly of magmatic origin and intercalated within metasediments. Initial emplacement may have been extrusive (lava) or intrusive (lode). A volcanic origin would implie the deposition of the sedimentary series to have taken place at 517 Ma and an intrusive origin that it took place after 582 Ma and before 517 Ma. An intrusive origin is more compatible with the coarse texture of the amphibolite but this fact is not really conclusive. In the volcanic hypothesis, the lack of any zircon with age close to 517 Ma in the metasediments would implie the volcanic material to have never been reworked in the sediments. This is again more compatible with an intrusive origin but not totally conclusive. In any case the available constraints make sure that the emplacement of the Serre Chevalier unit occurred in the interval 582-517 Ma.

Finally, as discussed earlier, we interpret the 492 Ma age obtained on the zircon rims of the granite pebble BRI0334 as reflecting a later metamorphic event, which presumably affected the whole Serre Chevalier crystalline basement. Evidence of amphibolite-facies metamorphism is indeed found in several lithologies of the Serre-Chevalier unit. In particular, the mafic amphibolite (sample BRI0057A) displays a well-developped amphibolite-facies assemblage with coexistence of green hornblende, garnet and rutile. Considering the coeval stability of rutile and garnet, this metamorphism presumably occurred at relatively high pressure >10 kbar [START_REF] Green | Activitycomposition relations for the calculation of partial melting equilibira in metabasic rocks[END_REF]. This metamorphism is clearly pre-Alpine (i.e. pre-Permian) (Barféty et al., 1985) but remains to be precisely dated.

Regional perspective

Precambrian to lower Cambrian rocks are rather scarce in the Alpine basement. Occurrences of rocks older than 500 Ma reported in Table 1 show that they occur mainly in the Eastern and Central Alps. To our knowledge, the rocks of Serre Chevalier are the first Ediacarian formations dated so far in the Western Alps.

In the external crystalline Massifs (ECM), the oldest known formation is the ophiolite complex of Chamrousse which has been dated around 500 Ma [START_REF] Ménot | 496 My age of plagiogranites in the Chamrousse ophiolite complex (external crystalline massifs in the French Alps): evidence of a Lower Paleozoic oceanization[END_REF][START_REF] Ch | A Sm-Nd isotopic study of 500 Ma old oceanic crust in the Variscan belt of Western Europe: the Chamrousse ophiolite complex, Western Alps (France)[END_REF] and is therefore significantly younger than the Serre Chevalier crystalline terrains.

In the Briançonnais domain, different occurrences of intrusive felsic rocks dated between c.

510 and 500 Ma are reported (Table 1). These veins cut accross metasedimentary sequences whose minimum deposit age is therefore of 510-500 Ma but no other geochronological constraints are available.

More to the east, data obtained in the Lepontine Alps and Austroalpine domain indicate the occurrence of a large set of metabasic rocks with ages between c. 540 and 500 Ma (Table 1)

which have been interpreted as scattered relicts of former magmatic arcs of Ediacarian to Cambrian age [START_REF] Von Raumer | Ediacaran to Cambrian oceanic rocks of the Gondwana margin and their tectonic interpretation[END_REF]. Precise ages older than 540 Ma are however rather scarce (Table 1) and obtained on particular samples of local occurrence.

A piece of basement comparable to the one we date in the Serre Chevalier area is therefore unknown in the Alpine context which makes impossible to correlate the Serre Chevalier with any geological unit around. Nevertheless, the occurrence of a Neoproterozoic basement is suspected in many places and consistent with the ages obtained to date on inherited zircons in some metasedimentary sequences [START_REF] Fréville | Structural, metamorphic and geochronological insights on the Variscan evolution of the Alpine basement in the Belledonne Massif (France)[END_REF][START_REF] Chu | Detrital zircon U-Pb ages and Hf isotopic constraints on the terrigeneous sediments of the western Alpes and their paleogeographic implications[END_REF][START_REF] Manzotti | Detrital zircon geochronology in blueschist-facies meta-conglomerates from the Western Alps: implications for the late Carboniferous to early Permian palaeogeography[END_REF][START_REF] Manzotti | Detrital zircon geochronology in the Dora-Maira and Zone Houillère: a record of sediment travel paths in the Carboniferous[END_REF] or on veins cutting accross them (see refs. in Table 1).

Conclusions and perspectives

The new results presented in this article provide the first occurrence of pre-Alpine basementrocks of Ediacarian age in the Western Alps. Ediacarian ages are obtained on a boulder of granite in a coarse-grained conglomerate (582 ± 4 Ma) and in a micaceous quartzite (c. 580

Ma) and leptynitic gneiss (597 ± 4 Ma) where they correspond to maximal deposit ages with no evidence of younger ones. A minimum emplacement age is constrained by the date at 517 ± 3 Ma obtained on a coarse-grained garnet amphibolite intercalated in the metasediments.

The interval of 582-517 Ma is comparable to that obtained for the metasedimentary units of the Brioverian group of the Armorican Massif [START_REF] Chantraine | The Cadomian active margin (North Armorican Massif, France): a segment of the North Atlantic Panafrican belt[END_REF] suggesting a probable « Cadomian » origin for the Serre Chevalier unit.

Finally, an age of 492 ± 4 Ma obtained from zircon rims in a granite block included in a conglomerate is interpreted as the age of a late stage metamorphic event.

Beyond these results, many aspects of the Serre Chevalier basement remain to be studied: (i) a detailed geological mapping at 1/25,000 or even 1/10,000 scale would help to elucidate the exact nature of the original sedimentary series and a possible olistostrome origin suggests by the presence of large granitic boulders; (ii) a quantitative petro-structural investigation would enable the determination of the tectono-metamorphic evolution at the pre-Alpine and Alpine stages including the signification and extent of the upper Cambrian event detected in the granitic block ; (iii) regional comparisons are necessary to search for possible correlations between the Serre Chevalier terrain and other basement domains of the Alps.

Beyond the Alps, the place and signification of the Serre Chevalier unit must be envisaged in the frame of the whole Cadomian belt, that is an arc and back-arc system located at the northern margin of Gondwana during Ediacarian (e.g. Linneman et al., 2007;[START_REF] Stampfli | The formation of Pangea[END_REF][START_REF] Couzinié | Cadomian S-type granites as basement rocks of the Variscan belt (Massif Central, France): Implications for the crustal evolution of the north Gondwana margin[END_REF].

Figure and table captions

Fig. 1 -Simplified geological of the Western Alps (modified fom [START_REF] Chantraine | Carte géologique de la France à l'échelle du millionième, 6 e édition bis[END_REF]after Schmid et al., 2004[START_REF] Bousquet | Metamorphic framework of the Alps[END_REF] with indication of the location of the studied area (rectangle in the center of the figure) Fig. 1 -Carte géologique simplifiée des Alpes occidentales (modifiée de [START_REF] Chantraine | Carte géologique de la France à l'échelle du millionième, 6 e édition bis[END_REF]d'après Schmid et al., 2004[START_REF] Bousquet | Metamorphic framework of the Alps[END_REF] avec indication de la zone d'étude (rectangle dans le centre de la figure)

Fig. 2 -Structural sketch map of the studied area (simplified from Barféty et al., 1995) depicting the geological environment of the Serre Chevalier basement. For simplicity, the Subbriançonnais domain has been represented as part of the External Briançonnais. The A-A' line corresponds to the cross section exposed in fig. 3. Fig. 2 -Schéma structural de la zone d'étude (simplifié d'après Barféty et al., 1995) montrant la localisation du socle de Serre Chevalier dans son environnement géologique. Dans un but de simplification, le Subbriançonnais a été inclus dans le Briançonnais externe. La ligne A-A' correspond au trait de la coupe présentée dans la fig. 3.

Fig. 3 -W-E geological section accross the Alps at the latitude of Briançon (simplified from Barféty et al., 1995). Legend as in fig. 2 except for the Houiller zone were Carboniferous and Permian are not distinguished Fig. 3 -Coupe géologique Ouest-Est au travers des Alpes à la latitude de Briançon (simplifié de Barféty et al., 1995). Légende identique à celle de la fig. 2

Figures and table

Fig Table 1. Tableau S4-1 -Résultats analytiques obtenus sur les zircons de l'échantillon BRI0321. Les analyses dactylographiées en italic ne sont pas prises en compte en raison de teneurs trop élevées en plomb commun ou de degrés de discordance supérieurs à 4% Table S4-1 (continued) Tableau S4-1 (suite) Table S4-1 (continued) Tableau S4-1 (suite) Table S4-2 -Analytical results obtained from the zircons of sample BRI0058. The analyses typed in italic are not taken into account because of high contents of common lead or high degrees of discordance Tableau S4-2 -Résultats analytiques obtenus sur les zircons de l'échantillon BRI0058. Les analyses dactylographiées en italic ne sont pas prises en compte en raison de teneurs en plomb commun ou de degrés de discordance trop élevés Table S4-3 -Analytical results obtained from the zircons of sample BRI0334. The analyses typed in italic are not taken into account because of high contents of common lead or high degrees of discordance Tableau S4-3 -Résultats analytiques obtenus sur les zircons de l'échantillon BRI0334. Les analyses dactylographiées en italic ne sont pas prises en compte en raison de teneurs en plomb commun ou de degrés de discordance trop élevés Table S4-4 -Analytical results obtained from the zircons of sample BRI0057A. The analyses typed in italic are not taken into account because of high contents of common lead or high degrees of discordance Tableau S4-4 -Résultats analytiques obtenus sur les zircons de l'échantillon BRI0057A. Les analyses dactylographiées en italic ne sont pas prises en compte en raison de teneurs en plomb commun ou de degrés de discordance trop élevés

S3 -Zircon cathodoluminescence images for the studied samples with location of all analytical spots S3 -Images en cathodoluminescence des zircons analysés avec localisation de tous les spots analytiques

  and 2), a West-East section through the Alpine nappes stack cut accross the following domains: (i) the external Dauphinois domain which exposes a pre-Alpine basement (External Crystalline Massifs or ECM) covered by weakly to non metamorphosed Mesozoic to Cenozoic sedimentary series; (ii) the narrow Subbriançonnais unit which is bounded to the west by a major thrust (Frontal Penninic Thrust) putting the internal Alps in contact with the external Alps (Dauphinois); (iii) the Briançonnais which is a continental domain made of a stack of nappes with a lithostratigraphical succession clearly distinct from that of the Dauphinois; (iv) the Liguro-Piemontese domain, an ocanic domain including remnants of the original oceanic floor (ophiolites) and associated sediments (Schistes lustrés)

  204 (Pb+Hg) signal intensity. No common Pb correction was applied. The 235 U signal is calculated from 238 U based on the ratio 238 U/ 235 U = 137.88 for the analysis performed with XserieII spectrometer, but are directly measured with the Agilent 8900 spectrometer. Data were corrected for U-Pb and Th-Pb fractionation and for the mass bias by standard bracketing

  Concordance is calculated according to: Conc = (Age 206 Pb/ 238 U) X100/ (Age 207 Pb/ 235 U).

  rims could form in solid state by (re)crystallization or dissolution-precipitation in presence of a metamorphic fluid. Conversely data from[START_REF] Lopez-Sanchez | An example of low-Th/U zircon overgrowths of magmatic origin in a late orogenic Variscan intrusion: the San Ciprián massif (NW Spain)[END_REF] show that such rims may also result from crystallization at a magmatic stage.Kirkland et al. (2015) proposed a method for relating measured zircon Th/U ratio to that expected for zircon crystallized in equilibrium with the host magmatic rock. The objective is to evaluate the consistency between the Th/U ratio of zircon and nature of the magmatic host rock. Calculation of zircon/whole rock fractionation factors of U and Th for a range of magmatic rocks of different temperatures indicates a decrease of the solubility of both elements in zircon at increasing temperature. The method ofKirkland et al. (2015) has been tested for the granite block BRI0334.

Fig. 6 -Fig. 7 -

 67 Fig.3-W-E geological section accross the Alps at the latitude of Briançon (simplified fromBarféty et al., 1995). Legend as in fig.2except for the Houiller zone were Carboniferous and Permian are not distinguished Fig.3-Coupe géologique Ouest-Est au travers des Alpes à la latitude de Briançon (simplifié deBarféty et al., 1995). Légende identique à celle de la fig.2excepté pour la zone houillère où Carbonifère et Permien ne sont pas distingués Fig. 4 -Landscape of the Serre Chevalier ski station showing two of the three outcrops of crystalline rocks in this area. The Eocene olistostrome is intercalated between the two outcrops and the more distant one (Eychauda moutain) passes downward to a coarse-grained breccia mainly composed of crystalline fragments (Eychauda breccia) Fig. 4 -Vue sur la station de ski de Serre Chevalier montrant deux des trois affleurements de roches cristallines de ce secteur. L'olistostrome éocène s'intercale entre les deux affleurements et à l'arrière plan, les terrains cristallins du sommet de l'Eychauda passent vers le bas à une brèche grossière principalement constituée de blocs de roches cristallines (brèche de l'Eychauda) Fig. 5 -Outcrop of massive coarse-grained garnet amphibolite (sample BRI0057A) Fig. 5 -Affleurement en banc massif d'une amphibolite à grenat à grain grossier (échantillon BR0057A) Fig. 6 -Outcrop of metaconglomerate with schist and granite (blue circle) pebbles Fig. 6 -Affleurement d'un métaconglomérat à galets de schiste et granite (rond bleu) Fig. 7 -Zircon population extracted from the BRI0321 sample observed with a binocular magnifier Fig. 7 -Population de zircons extraites de l'échantillon BRI0321 observée à la loupe binoculaire Fig. 8 -Wetherill (1956) normal concordia diagram showing the U-Pb isotopic results of zircons from sample BRI0321 (micaceous quartzite) Fig. 8 -Diagramme concordia de Wetherill (1956) montrant les résultats isotopiques U-Pb obtenus sur les zircons de l'échantillon BRI0321 (quartzite micacé) Fig. 9 -Histograms and probability density plots of 206 Pb/ 238 U and 207 Pb-206 Pb ages in the BRI0321 sample

Fig. 11 -

 11 Fig. 11 -Wetherill (1956) diagram for the analyses performed in sample BRI0058 (a) with emphasis on one concordant age giving an age around 2100 Ma (b) Fig. 11 -Diagramme de Wetherill (1956) pour les analyses réalisées dans l'échantillon BRI0058 (a) avec focus sur une analyse concordante indiquant un âge vers 2100 Ma (b) Fig. 12 -Zircons analyzed from the sample BRI0334 observed with a binocular magnifier Fig. 12 -Zircons analysés dans l'échantillonsBRI0334 vus à la loupe binoculaire Fig. 13 -Cathodoluminescence image of a representative zircon crystal from sample BRI0334, showing the typical core-rim relationship, with distinct 206 Pb/ 238 U ages and Th/U ratios obtained for spots 8 and 9 respectively in the core and rim of the zircon Fig. 13 -Image en cathodoluminescence d'un zircon representatif de l'échantillon BRI0334 montrant des âges 206 Pb/ 238 U et des rapports U/Th distincts entre coeur et périphérie. Les résultats correspondent aux spots 8 et 9 respectivement au coeur et à la périphérie du zircon Fig. 14 -Wetherill (1956) normal concordia diagram showing the entire set of U-Pb isotopic results obtained from sample BRI0334 (a) and focus on the lower part of the diagram (350-750 Ma interval) (b) Fig. 14 -Diagramme concordia de Wetherill (1956) montrant l'ensemble des résultats isotopiques U-Pb obtenus sur l'échantillon BRI0334 (a) avec un focus sur la partie basse du diagramme (intervalle 350-750 Ma) (b) Fig. 15 -Wetherill (1956) normal concordia diagram showing the U-Pb isotopic results obtained in sample BRI0334 in the ranges 380-520 Ma (a) and 500-780 Ma (c) with details of analyses around the two calculated ages at 492 Ma (b) and 582 Ma (d) Fig. 15 -Diagramme concordia de Wetherill (1956) montrant les résultats isotopiques U-Pb obtenus sur l'échantillon BRI0334 avec focus sur les intervalles 380-520 Ma (a) et 500-780 Ma (c) et détail autour des 2 âges obtenus de 492 Ma (b) et 582 Ma (d) Fig. 16 -Plot of Th/U ratio against 206 Pb/ 238 U age for the analyses performed in the zircons of sample BRI0334 with distinction between core and rim analyses Fig. 16 -Diagramme Th/U vs. âge 206 Pb/ 238 U pour les analyses réalisées dans les zircons de l'échantillon BRI0334 avec distinction entre les analyses réalisées au coeur et à la périphérie de zircons Fig. 17 -Zircons (translucent) and rutile (brown to red) extracted from sample BRI0057A. Binocular magnifier view Fig. 17 -Zircons (cristaux incolores et translucides) et rutiles (cristaux rouges à bruns) extraits de l'échantillon BRI0057A. Vue à la loupe binoculaire Fig. 18 -Wetherill (1956) normal concordia diagram showing the U-Pb isotopic results obtained from the 45 zircon analyses performed in sample BRI0057A Fig. 18 -Diagramme concordia de Wetherill (1956) montrant les résultats isotopiques obtenus sur les 45 analyses de zircon réalisées dans l'échantillon BRI0057A

  Fig. 1.

Fig. 3 Fig

 3 Fig. 3

Fig. 5 Fig

 5 Fig. 5

Fig

  Fig. 8.

Fig

  Fig. 11.

Fig

  Fig. 14.

Fig. 15 Fig

 15 Fig. 15

Fig. 17

 17 Fig. 17.

  Fig. 17.

Fig

  Fig. 18.

Fig. S2 -

 S2 Fig. S2 -Results of repeated measurements on the Plešovice zircon standard Fig. S2 -Résultats des répétitions d'analyses sur le zircon de Plešovice (Slama et al., 2008)

Fig

  Fig. S3-1 -Location of analytical spots in the 105 zircons analyzed in sample BRI0321 Fig. S3-1 -Localisation des spots analytiques dans les 105 zircons analysés dans l'échantillon BRI0321

  Pb/ 238 U and 207 Pb-206 Pb ages in the BRI0321 sample Fig. 9 -Histogrammes et diagrammes de densité de probabilité pour les âges 206 Pb/ 238 U et 207 Pb-206 Pb dans l'échantillon BRI0321 Fig. 10 -Cathodoluminescence image of some zircons of the BRI0058 sample. The ages obtained at each spot are listed in tableS4-2 (Supplementary materials) Fig.10-Image en cathodoluminescence de quelques zircons de l'échantillon BRI0058. Les âges obtenus à chaque spot sont reportés dans la table S4-2 (Supplementary materials)

Table 1 -

 1 Review of precise geochronological data for rocks dated at > 500 Ma in different areas of the Alps Tableau 1 -Inventaire de données géochronologiques précises pour des roches datées de > 500 Ma dans différentes zones des Alpes

Table S2 -

 S2

		U (ppm) Pb (ppm) Th (ppm)	727 34 65	1182 57 165	626 30 57	594 5250 756	697 6146 935	776 6844 1080	801 7059 1048	620 30 54	631 30 56	628 30 54	470 22 40	589 28 53	609 29 55	491 24 43	617 30 56	628 30 57	709 33 64	683 32 63	547 26 49	489 23 43
																					11	11
	206 Pb/ 204 Pb U/Th	19 5549	12 1938	19 741	14 21693	13 14289	13 2371	14 50499	11 17077	11 9782	12 2746	12 3161	11 1263	11 91359	11 1149	11 9052	11 153726	11 3232	11 114855	7299	12312
																					98	97
		conc	101	101	100	99	99	98	99	100	99	100	99	98	98	99	99	100	99	99
																					4	4
		Pb/ 238 U ±1σ	336 4	340 4	339 4	337 3	336 3	336 3	336 3	339 4	337 4	338 4	337 4	336 4	337 4	339 4	338 4	339 4	338 4	339 4	345	331
	Ages	Pb/ 235 U ±1σ 206	8 333	7 335	8 338	5 339	5 340	5 342	6 339	5 340	6 341	7 338	5 342	6 343	6 343	7 343	6 340	6 338	7 341	8 341	7 351	7 342
		Pb ±1σ 207	64	58	66	44	41	40	45	32	35	42	35	35	35	42	34	36	41	46	384 39	395 39
		Pb/ 206	312	306	330	356	368	387	360	376	398	337	331	390	361	346	378	391	363	366
		207
		Rho	0.40	0.43	0.39	0.56	0.58	0.59	0.54	0.63	0.57	0.46	0.59	0.57	0.55	0.45	0.58	0.54	0.47	0.42	0.51	0.49
		±1σ	0.00060	0.00059	0.00061	0.00057	0.00056	0.00056	0.00057	0.00059	0.00060	0.00063	0.00060	0.00059	0.00059	0.00061	0.00060	0.00061	0.00063	0.00066	0.00063	0.00059
		206 Pb/ 238 U	0.05352	0.05408	0.05398	0.05368	0.05355	0.05353	0.0535	0.05392	0.05370	0.05385	0.05372	0.05357	0.05367	0.05404	0.05380	0.05397	0.05380	0.05398	0.05489	0.05263
	Isotopic ratios	207 Pb/ 206 Pb ±1σ 207 Pb/ 235 U ±1σ	pleso-1 0.05260 0.00151 0.38811 0.01076	pleso-2 0.05246 0.00137 0.39115 0.00990	pleso-3 0.05303 0.00158 0.39461 0.01141 X serie II pleso-1 0.05365 0.00104 0.39659 0.00752 pleso-2 0.05392 0.001 0.39761 0.00718	pleso-3 0.05438 0.00099 0.40084 0.00711	pleso-4 0.05373 0.00108 0.3959 0.00776	pleso-1 0.05413 0.00079 0.39764 0.00695	pleso-2 0.05464 0.00087 0.39884 0.00778	pleso-3 0.05320 0.00100 0.39460 0.00996	pleso-4 0.05304 0.00083 0.40006 0.00754	pleso-5 0.05445 0.00087 0.40117 0.00778 Agilent 8900 pleso-6 0.05376 0.00085 0.40244 0.00804 pleso-7 0.05341 0.00101 0.40141 0.01015 pleso-8 0.05417 0.00083 0.39712 0.00764	pleso-9 0.05448 0.00089 0.39550 0.00826	pleso-10 0.05380 0.00099 0.39850 0.00983	pleso-11 0.05388 0.00112 0.39878 0.01166	pleso-12 0.05431 0.00096 0.41323 0.00926	pleso-13 0.05457 0.00097 0.40022 0.00921	table : U-Pb analyses of quality contrôle sample Plesovice (Slama et al., 2008)

2 -U-Pb analyses of quality control on Plešovice zircon

(Slama et al., 2008)

Table

S2

-2 -Analyses U-Pb réalisées pour contrôle qualité sur le zircon de Plešovice

(Slama et al., 2008)

Acknowledgments

This work has been realized in the frame of the « Chantier Alpes et bassins périphériques » project as part of the RGF (« Référentiel Géologie de la France ») national program sponsored by BRGM. Oscar Laurent and an anonymous reviewer are kindly thanked for their constructive comments which lead to significant improvements in the final version.

Supplementary materials

S1 -Petrographical descriptions -Descriptions pétrographiques

Amphibole-Garnet-Rutile amphibolite (sample BRI0057A) -X -6.55044 -Y -44.90961 -Coarse-grained (max. 2-3 mm) basic rock composed of: (i) green to pale brown hornblende as large subhedral crystals (1-3 mm) and small crystals associated with feldspar in the matrix; (ii) large (max. 3-4 mm) euhedral garnet grains with inclusions of feldspar and amphibole; (iii) feldspar (plagioclase) as small grains forming the matrix in association with amphibole; (iv) rutile (frequent) as grains and granules sometime associated with titanite. Chlorite is subordinate, and grows either after garnet or as local patches in the matrix. Zircon, apatite and epidote as accessory minerals. Rare blue green hornblende as small crystals. Rare white mica as agregates associated with large feldspar grains.

Leptynitic gneiss (sample BRI0058) -X -6.55148 -Y -44.90877 -Fine-grained leucocratic rock mainly composed of a mosaic of small-sized quartz and plagioclase crystals. White and brown (rare, more or less chloritized) micas occur as narrow elongated crystals underlining foliation. White mica locally abundant as agregates sometimes located in fold hinges. Garnet occurs as euhedral grains (< 1 mm) more or less pseudomorphosed by chlorite. Accessories: oxyde grains, zircon.

Granite boulder in conglomerate (sample BRI0334) -X -6.55093 -Y -44.90956 -Coarsegrained porphyritic granite with large poecilitic K-feldspar phenocrysts. Plagioclase occurs as large euhedral crystals, always sericitized. Biotite occurs as large folded agregates more or less retromorphosed in chlorite, white mica and oxyde grains. Interstitial quartz occurs as a mosaic of small crystals. Zircon mainly present in biotite with dark halos around it. Opaque minerals occur as clusters of oxydized grain.

Micaceous quartzite (sample BRI0321) -X -6.57339 -Y 44.9080 -It is mainly composed of quartz found as small (< 1 mm) imbricated crystals in beds of variable grain size alternating with discontinuous beds of strongly elongated white micas underlining a very intense and regular foliation. Elongated clusters of opaque minerals may be observed. Feldspar (albite) is locally present in association with quartz in discontinuous lenses parallel to the foliation. Scaterred zircons form rounded grains to elongated crystals parallel to the foliation.

S2 -Analytical techniques -Techniques analytiques

Other information

Depth profiling and single-pulse ablation studies used a low-volume ablation cell, sample line of 3 m from ablation cell to torch and a 20 sec wait time between ablations. Cell washout time was ca. 0.8 secs to 1% of peak signal.

S4 -U-Pb analyses -Analyses U-Pb

Table S4-1 -Analytical results obtained from the zircons of sample BRI0321. The analyses typed in italic are not taken into account because of high contents of common lead or degrees of discordance higher than 4% S5 -Geochemical analyse of the BRI0334 granite -Analyse géochimique du granite BRI0334

The analyse has been performed at the OMAC Laboratories, Loughrea, Ireland, https://www.alsglobal.com/en/geochemistry L'analyse a été effectuée dans les laboratoires OMAC, Loughrea, Irelande


M Viprey

I Durieu

H Rouzé

S Poupon-Bourdy

Q Reynaud

S Touzet

Costs of care for 242 adults and children with cystic fibrosis followed up in four reference medical centers in France

Objectives: Costs of care are influenced by evolution of characteristics of cystic fibrosis (CF) patients due to increasing life expectancy and by recent apparition of expensive innovative therapeutics. The aim of our study was to update CF-related costs in France in 2015. Methods: We conducted a retrospective cohort study with non-pulmonary-transplanted patients followed up in 2 adult and 2 pediatric reference medical centers in France and covered by the French National Health Insurance (NHI). Information on sociodemographic data and CF characteristics were collected from the patients' medical records. Consumptions related to CF were extracted from the regional French medical insurance health care funding database. Economic evaluation was performed from the perspective of the NHI with a time horizon of 1 year. Overall annual CFrelated mean cost and annual mean cost per source of expenditure were evaluated. Results: A total of 242 patients have given their consent for extraction of their health-care consumption data and were included. Among them 38.8% were adults and 46.3% were females. Median age was 15.0 (min: 0; max: 65). Children were distributed in 21.1% aged 0-5 years old, 21.9% aged 6-11 years old and 18.2% aged 12-17 years old. The mean overall annual CFrelated cost per patient was €27 315. Hospital care represents 30.5% of costs and home care 69.5% of costs, among them 46.2% for drugs and 8.4% for physiotherapists' consultations. Concerning drug consumption, 93.8% of patients had at last 1 dispensing of oral or intravenous antibiotic, 38.0 of inhaled tobramycin, 26.9% of inhaled colymicin, 54.1% of dornase alpha, 83.9% of pancreatic enzymes. Four patients were on ivacaftor with a mean overall annual CF-related cost per patient of €204 395. Objectives: This ongoing, long-term, postapproval observational safety study uses existing CF registries' data to evaluate outcomes in pts treated with IVA in a real-world setting. Here we present results of disease progression analyses using 2015 data from the UK CF Registry. Methods: Pts with a record of IVA use in the first year of commercial availability in the UK (2013), who continued treatment through 2015 and had no organ transplantation, were compared to comparator (COMP) pts with no IVA use record matched (≈1:5) on age, sex, and genotype severity (eg, G551D mutation IVA pts were primarily matched to pts homozygous for F508del ineligible for IVA therapy). Outcomes included percent predicted FEV 1 ( ppFEV 1 ), pulmonary exacerbations (PEx), PEx hospitalizations, body mass index (BMI), distal intestinal obstruction syndrome (DIOS), and CF-related diabetes (CFRD). Results: IVA (N = 260) and COMP (N = 1,289) pts were similar in age, sex, lung function, and medication use in 2012. From 2012 to 2015, mean ppFEV 1 change was +5.8 percentage points in IVAvs -3.1 percentage points in COMP pts; mean BMI increase was 1.7 kg/m 2 for IVA vs 0.6 kg/m 2 for COMP pts. The proportion of IVA pts experiencing PEx decreased from 54.6% to 35.4%, compared to an increase from 46.3% to 56.4% in COMP pts. Other outcomes are summarized in the Table.

Conclusion:

In addition to maintained improvements in lung function and nutritional status, as well as decreased risk of PEx and PEx hospitalizations, pts treated with IVA had a lower prevalence of DIOS and CFRD relative to COMP pts. These results support disease modification by CFTR modulator therapy. Acknowledgement: Sponsored by Vertex Pharmaceuticals Incorporated

2012, baseline

Objectives: To investigate the clinical impact (so far unknown) of V232D, a valine to aspartate change in exon 6a reported in 1993 [START_REF]www.genet.sickkids.on.ca Poster Sessions[END_REF].

Methods

  Conclusion: CF-related costs are increasing since 2003 where a mean overall annual CF-related cost per patient of €22 725 was evaluated. Major sources of expenditure stay drugs consumptions and hospital care. 411 Disease progression in patients ( pts) with cystic fibrosis (CF) treated with ivacaftor (IVA): analysis of real-world data from the UK CF Registry L. Bessonova 1 , N. Volkova 1 , M. Higgins 2 , L. Bengtsson 1 , S. Tian 1 , C. Simard 1 , S.O. Nyangoma 3 , D. Bilton 3,4 .

  There was no difference between groups in the number of patients with chronic bronchial infection with S aureus. Conclusions: Most V232D patients had classical CF albeit with comparatively lower sweat chloride values. They were nearly always PS, and their average age at diagnosis was higher. Adults had significantly better FEV 1 and lower prevalence of cPa. Still 6/2 (26%) of those with available pulmonary function data, had FEV 1 % pred 50-69%, so close follow-up is warranted as a sizeable minority may develop substantial pulmonary disease.

	: Out of the 2015 national dataset of 2003 patients with CF under
	care at 19 CF centres participating in the ECFSPR, we identified 29 patients
	carrying the V232D allele and: F508del (14), other CF-causing mutations
	(12), or another unknown mutation (3). Clinical variables in patients with
	V232D (V232Dp), and in all 1718 Spanish patients on the 2014 Spanish
	national dataset (allCF) were compared
	Results: V232D accounted for 0.72% of Spanish CF alleles. V232Dp were
	older at diagnosis 9.2 ± 15.3 years (median 1.1) vs 4.5 ± 9.9 (median 1.1),
	p = 0.048. Age at follow-up was not different (20.6 ± 16 vs 18.6 ± 13). Just 4
	V232Dp had been diagnosed by newborn screening. Sweat chloride
	(mmol/l) was lower: 71.7 ± 17.2, range 40-102, vs 90.4 ± 26 ( p < 0.001).
	There was no difference in the proportion of patients with sweat chloride
	≥60 and < 60 mmol/L. One single V232Dp was on pancreatic enzymes vs
	1105 (73%), p < 001. FEV 1 %Pred. was 87.9 ± 20.2 in V232Dp vs 79.3 ± 24.7 in
	allCF ( p = 0.098). It was not different in patients aged < 18 (93.4 ± 16 vs
	92.6 ± 20, but adult V232Dp fared better (83.6 ± 22 vs 67.4 ± 22 ( p = 0.01). No
	V232Dp younger than 18 had chronic bronchial infection with P aeruginosa
	(cPa) vs 121(13.8%) in allCF (NS), while 3/13 (23.1%) adults had cPa vs 626/
	1478 (42.4%), ( p = 0.01).