Arnaud Villaros
email: arnaudvillaros@gmail.com

Michel Pichavant

the granite-pegmatite connection: The St-Sylvestre complex (Western French Massif Central)

Keywords: granite-pegmatite connection, granitic melt composition, LCT pegmatite, mica crystallization, trace elements

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1 Introduction 4 thus the evolution from the primary (least fractionated) to the residual (most fractionated) melt is unknown. In the same way, due to pegmatite zonation and crystal sizes, representative sampling and whole rock analysis of pegmatite is not straightforward and, so, comparing granite and pegmatite whole-rock compositions can be fraught with difficulties. Finally, hydrothermal reworking below the solidus can lead to whole-rock compositions unrepresentative of their magmatic evolution (e.g., [START_REF] Pollard | Contrasting evolution of fluorine-rich and boron-rich tin systems[END_REF][START_REF] Kontak | Nature and origin of an LCT-suite pegmatite with late-stage sodium enrichment, Brazil Lake, Yarmouth County, Nova Scotia. I. Geological setting and petrology[END_REF], Muller et al., 2006). Alternatives to the use of whole-rock compositions include melt inclusion and obsidian studies (Pichavant et al., 1987a;[START_REF] Webster | Melt inclusions in quartz from an evolved peraluminous pegmatite: Geochemical evidence for strong tin enrichment in fluorine-rich and phosphorus-rich residual liquids[END_REF][START_REF] Thomas | Water in granite and pegmatite-forming melts[END_REF], but these can be applied only to a few systems, thus limiting their practical use. A more general approach, yet rarely applied to granitic systems, consists in using minerals to access the liquid composition [START_REF] Hanson | The application of trace elements to the petrogenesis of igneous rocks of granitic composition[END_REF][START_REF] Bea | Differentiation of strongly peraluminous, perphosphorus granites: The Pedrobernardo pluton, central Spain[END_REF][START_REF] London | Reply to Thomas and Davidson on "A petrologic assessment of internal zonation in granitic pegmatites[END_REF]. Assuming that no post-magmatic re-equilibration occurred, provided that mineral/melt partition coefficients are available and appropriate for the system being considered, the composition of the melt at equilibrium with a mineral (n.b., melt and liquid are used interchangeably below) can be theoretically determined if the composition of the mineral is known. The obtained compositional trends can thus be used to constrain genetic relations between the different melts. In particular, incompatible trace elements allow to distinguish between (1) cogenetic liquids related through fractional crystallization, which show a single linear trend from parental (C 0 ) to residual (C f ) compositions and (2) non-cogenetic liquids evolving by fractional crystallization independently from each other, which show parallel trends from C 0 to C f (figure 1; [START_REF] Treuil | Utilisation des éléments hygromagmatophiles pour la simplification de la modélisation quantitative des processus magmatiques. Exemples de l'Afar et de la dorsale médioatlantique[END_REF][START_REF] Raimbault | Comportements différenciés de W, Sn, U, Ta, Nb dans quelques complexes granitiques du Massif Central français[END_REF][START_REF] Shaw | Trace Elements in Magmas: A Theoretical Treatment[END_REF].

In this paper, we apply this approach by using the trace element compositions of micas to follow the geochemical evolution from two-micas leucogranite to pegmatite melts. The Variscan Saint Sylvestre leucogranite and pegmatite complex in the NW part of the French Massif Central (FMC) is selected as a test site. Results show that leucogranite and pegmatite melts are compositionally close and have undergone a similar evolution. However, the incompatible trace element contents of the respective liquids, reconstructed from compositions of micas, do not follow a single fractionation trend.

Therefore, pegmatites cannot be considered as representing fractionation products from granitic J o u r n a l P r e -p r o o f 10/12/2018

Mica-liquid trace elements partitioning Chem Geol 6 part of the FMC hosts large massifs where leucogranites and pegmatites are found in close spatial association. One is the Saint Sylvestre Leucogranite Complex (SSLC) which hosts the Mont d'Ambazac Pegmatite Field (MAPF), constituted of numerous pegmatitic bodies and which includes the rare metal-and lepidolite-bearing Chèdeville LCT pegmatite [START_REF] Raimbault | Composition of complex lepidolite-type granitic pegmatites and of constituent columbite-tantalite, Chedeville, Massif Central, France[END_REF][START_REF] Deveaud | The genesis of LCT-type granitic pegmatites, as illustrated by Lithium isotopes in micas[END_REF]. This massif satisfies to the main criteria (geometry, distance, relative chronologies, see above and [START_REF] Deveaud | The genesis of LCT-type granitic pegmatites, as illustrated by Lithium isotopes in micas[END_REF] for suggesting a genetic connection between leucogranites and pegmatites.

Consequently, it was selected as a test site for geochemical modelling.

In the following, mica compositions from leucogranites of the SSLC and associated pegmatites of the MAPF will be used to determine the composition of their equilibrium melts. In granitic rocks sl, micas offer better perspectives than feldspars (easily re-equilibrated at low temperatures, [START_REF] Pollard | Contrasting evolution of fluorine-rich and boron-rich tin systems[END_REF]Stemprok, 1987) and quartz (generally very pure except for only a few trace elements, Breiter et al., 2013) to achieve this goal. Focusing on a single phase (i.e., mica) rather than on multiphase assemblages (i.e., whole rock) greatly limits the complexity of the modelling. Compared to this latter approach, the former appears as a first necessary step. It is also more robust given the uncertainties in the mineral/melt Kd database (see below). Last, it has the advantage to eliminate issues about the significance of whole rock assemblages (e.g., [START_REF] Mccarthy | The Blue Tier batholith, northeastern Tasmania -A cumulate-like product of fractional crystallization[END_REF]. Implicit in the approach is the assumption that micas preserve magmatic compositions. For muscovite, this assumption can be tested relatively easily from major elements [START_REF] Miller | Composition of plutonic muscovite: genetic implications[END_REF]. For biotite, distinguishing between magmatic and hydrothermal compositions requires a case-by-case evaluation. In the SSLC, previous studies including Li isotope data have indicated that biotite (and also muscovite) compositions globally reflect the magmatic evolution [START_REF] Deveaud | The genesis of LCT-type granitic pegmatites, as illustrated by Lithium isotopes in micas[END_REF]. However, Li-rich micas such as zinnwaldite and lepidolite (which are rare but present in some of our pegmatites) most probably have a dual (both magmatic and hydrothermal) origin (e.g., [START_REF] Raimbault | Composition of complex lepidolite-type granitic pegmatites and of constituent columbite-tantalite, Chedeville, Massif Central, France[END_REF] and these compositions will be distinguished from the others below.

Evaluation of the Kd database and limitations

Journal Pre-proof J o u r n a l P r e -p r o o f During crystallization, trace elements partition between liquid and minerals. The incompatibility (or compatibility) of a trace element is expressed by the partition coefficient between a given mineral and the melt (Kd = concentration of trace element i in mineral / concentration of trace element i in melt). In principle, Kd varies with temperature, the nature and composition of the mineral, the liquid composition and certain compositional and intensive parameters such as fO 2 (e.g. [START_REF] Icenhower | An experimental study of element partitioning among biotite, muscovite, and coexisting peraluminous silicic melt at 200 MPa (H 2 O)[END_REF][START_REF] Nash | Partition coefficients for trace elements in silicic magmas Geochim[END_REF][START_REF] Mcintire | Trace element partition coefficientsa review of theory and applications to geology[END_REF][START_REF] Pichavant | Influence of redox state on mica crystallization in leucogranitic and pegmatitic liquids[END_REF]. experiments, (2) at temperatures between 750 and 620 °C and (3) at fO 2 between NNO-2 and NNO+3 [START_REF] Icenhower | An experimental study of element partitioning among biotite, muscovite, and coexisting peraluminous silicic melt at 200 MPa (H 2 O)[END_REF]Pichavant et al., 1996). In the following, we use the entire range of Kd (minimum and maximum values selected using the three criteria above, table 1; figure 2) instead of a single value to derive compositions of equilibrium liquids. This has the disadvantage of increasing the dispersion of the calculated liquid concentrations. Despite being less sensitive, this approach is considered as more robust than the choice of a single value because it better accounts for the uncertainties in the available Kd database.

While the temperature range above is adequate for leucogranite, crystallization of pegmatitic melts commonly extends to lower than 620 °C (e.g., Pichavant et al., 1987b;[START_REF] Sirbescu | Crustal melts below 400 degrees C[END_REF][START_REF] London | Reply to Thomas and Davidson on "A petrologic assessment of internal zonation in granitic pegmatites[END_REF]. Owing to their different crystallization temperatures, partition coefficients may thus differ between pegmatite and leucogranite melts. Indeed, values in the Kd database vary with temperature (table 1; figure 2). For example, the Kd biotite/liquid for F and Li both decrease when lowering temperature from 720-750°C to 620°C. The Kd muscovite/liquid for Li drops by a factor > 5 between 650 and 620°C, but the lowest values correspond to charges more oxidized than the highest [START_REF] Pichavant | Influence of redox state on mica crystallization in leucogranitic and pegmatitic liquids[END_REF][START_REF] Icenhower | An experimental study of element partitioning among biotite, muscovite, and coexisting peraluminous silicic melt at 200 MPa (H 2 O)[END_REF]. For Rb, Ba and Cs, the data at 650°C show significant dispersions (figure 2). Given the variability in the Kd database, it is difficult to extract the Journal Pre-proof J o u r n a l P r e -p r o o f 10/12/2018 Mica-liquid trace elements partitioning Chem Geol 8 effect of temperature as would be necessary if leucogranite and pegmatite crystallization are to be modelled at different temperatures. Therefore, below, the same Kd range will be assumed to be equally applicable to leucogranite and pegmatite liquids.

Another factor to be considered for pegmatites is that they commonly crystallise under disequilibrium, i.e., from undercooled melts (e.g., [START_REF] London | Reply to Thomas and Davidson on "A petrologic assessment of internal zonation in granitic pegmatites[END_REF]. This necessarily impacts the use of equilibrium partition coefficients, as done in this study. However, [START_REF] Morgan | Trace-element partitioning at conditions far from equilibrium: Ba and Cs distributions between alkali feldspar and undercooled hydrous granitic liquid at 200 MPa[END_REF] found no influence of the degree of undercooling on the partitioning of incompatible elements (Cs) between alkali feldspar and melt. For a strongly compatible element (Ba), the alkali feldspar/melt Kd was found to increase relative to the reference equilibrium value, although by < 1 order of magnitude [START_REF] Morgan | Trace-element partitioning at conditions far from equilibrium: Ba and Cs distributions between alkali feldspar and undercooled hydrous granitic liquid at 200 MPa[END_REF]. Therefore, disequilibrium crystallization effects are expected for compatible (Ba) but not for incompatible elements which are the focus of this study. Overall, disequilibrium crystallization effects should be of minor importance in this study. We stress that this conclusion applies only to crystallization and not necessarily to other processes such as diffusive exchange (between minerals or minerals and fluid) that can affect trace element concentrations in micas and are fully considered below.

Several elements were tested as a differentiation index, especially Nb. However, the mica/liquid partition coefficient for Nb (and also for Ta), determined at temperatures outside the range defined above [START_REF] Stepanov | Fractionation of Nb and Ta by biotite and phengite: implications for "missing Nb paradox[END_REF], yields highly enriched, irrealistic liquid Nb (and Ta) concentrations. Consequently, below, the liquid concentration results for these two elements will not be discussed and only mica Nb and Ta concentrations will be used. Finally, Cs will be used below as a differentiation index.

Geological context

The SSLC is part of the Variscan orogen which is the consequence of a nappe stacking event between 360 to 320 Ma. This resulted in the superposition of 4 units, from top to bottom, the Thiviers-Payzac Unit (TPU), the Upper Gneiss Unit (UGU), the Lower Gneiss Unit (LGU) and finally the Autochthonous Unit (PAU) (e.g. [START_REF] Faure | A review of the pre-Permian geology of the Variscan French Massif Central[END_REF]. This nappe stacking episode was followed by a collisional stage and the collapse of the belt from 330 Ma to about 300 Ma, during which numerous peraluminous granitic bodies intruded the upper nappes. The Saint Sylvestre Leucogranite Complex (figure 3) was emplaced during the mid-carboniferous (324 ± 4 Ma, [START_REF] Holliger | Age carbonifère de l'unité de Brame du complexe granitique peralumineux de Saint-Sylvestre (N.O. Massif Central) définit par les données isotopiques U-Pb sur zircon et monazite[END_REF]) at the end of the collisional stage of the Variscan orogen [START_REF] Faure | Crustal thinning recorded by the shape of the Namurian-Westphalian leucogranite in the Variscan belt of the northwest Massif Central, France[END_REF][START_REF] Carlier | A two-stage exhumation in Western French Massif Central: New geochronological evidences of syncollisional extension[END_REF]. The SSLC is a thin laccolith (2-3 km in thickness, [START_REF] Audrain | Modèle gravimétrique et mise en place du complexe granitique hyperalumineux de Saint-Sylvestre (Massif Central français)[END_REF]) intrusive in the PAU and the LGU [START_REF] Audrain | Modèle gravimétrique et mise en place du complexe granitique hyperalumineux de Saint-Sylvestre (Massif Central français)[END_REF]; figure 3).

The SSLC is strictly composed of peraluminous leucogranites exclusively derived from the melting of continental crustal components (e.g. [START_REF] Vidal | Geochemical comparison between Himalayan and Hercynian leucogranites[END_REF][START_REF] Turpin | Meta-igneous origin of Hercynian peraluminous granites in N-W French Massif Central : implications for crustal history reconstructions[END_REF]. It is divided in three units from west to east: the Brâme, the St Sylvestre and the St Goussaud Unit. The Brâme unit is mostly composed of the Brâme granite ( Br ), a coarse to medium grained porphyritic biotite ± cordierite ± garnet granite.  Br is characterised by a flat lying foliation and the presence of migmatitic septa and xenolithic gneisses [START_REF] Mollier | Structuration magmatique du complexe granitique de Brame-St Sylvestre-St Goussaud (Limousin, Massif Central Français)[END_REF][START_REF] Mollier | Le Granite de Brame-St Sylvestre-St Goussaud: ses structures magmatiques; une étude de la distribution de l'uranium à l'échelle[END_REF][START_REF] Cuney | Metallogenesis in the French part of the Variscan Orgogen. Part I: U préconcentrations in the pre-Variscan and Variscan formations -A comparison with Sn, W and Au[END_REF]. U-Pb data on monazite and zircon provide crystallization ages for  Br of 324 ± 4 Ma [START_REF] Holliger | Age carbonifère de l'unité de Brame du complexe granitique peralumineux de Saint-Sylvestre (N.O. Massif Central) définit par les données isotopiques U-Pb sur zircon et monazite[END_REF]. The undeformed fine grained two-mica Châteauponsac granite ( Ch ) intrudes  Br as N020 striking dykes and sills. The central St Sylvestre unit comprises three petrographical facies: a biotite ± muscovite facies  1 ) a fine grained muscovite + biotite facies  2 ) and an albite, muscovite ± sillimanite facies Sagne,  sa ). From  1 to  sa , the amounts of biotite and accessory phases decrease while muscovite and albite both increase; whole-rock contents of rare metals (Sn, W, Li, F, Be, U) increase and the Th/U ratio decreases. The contact between  1 and  2 suggest that both facies are nearly synchronous [START_REF] Cuney | Metallogenesis in the French part of the Variscan Orgogen. Part I: U préconcentrations in the pre-Variscan and Variscan formations -A comparison with Sn, W and Au[END_REF]Scaillet et al 2006). In the following,  1 and  2 will be grouped together as the St Sylvestre granite (Sy). Finally, the St Goussaud Unit is mostly composed of an apparently isotropic, large grained, two micas albitic leucogranite ( Go ).  Go only contains very rare angular enclaves [START_REF] Aréne | Feuille de Bourganeuf[END_REF].

Within the SSLC, a rare-element pegmatite field (the Mont d'Ambazac pegmatitic field, MAPF) mostly intrudes the coarse-grained  1 granite. Over one hundred pegmatitic bodies have been identified [START_REF] Raimbault | Composition of complex lepidolite-type granitic pegmatites and of constituent columbite-tantalite, Chedeville, Massif Central, France[END_REF]. According to the classification of [START_REF] Černý | The classification of granitic pegmatites revisited[END_REF], pegmatites from the Journal Pre-proof J o u r n a l P r e -p r o o f 10/12/2018 Mica-liquid trace elements partitioning Chem Geol 10 MAPF belong to the beryl type of the Li -Cs -Ta (LCT) group. Ar-Ar ages of lepidolite from a highly differentiated pegmatite have yielded an age of 309 ± 0.9 Ma ( 40 Ar/ 39 Ar, [START_REF] Cheilletz | 40 Ar/ 39 Ar dating of topaz-lepidolite Beauvoir leucogranite and Chèdeville sodium-lithium pegmatites (North Massif Central, France); Petrological and geodynamical meaning[END_REF].

Although the available geochronological data indicate that the pegmatites are younger than the leucogranites, we stress the difference between dating methods (U/Pb and Ar-Ar respectively) and the composite nature of the SSLC. [START_REF] Deveaud | Spatial statistical analysis applied to rare-element LCT-type pegmatite fields: an original approach to constrain faults-pegmatitesgranites relationships[END_REF][START_REF] London | Reply to Thomas and Davidson on "A petrologic assessment of internal zonation in granitic pegmatites[END_REF] has proposed a classification of MAPF pegmatites based on the observations of Patureau (1987) and the zoning criteria of London (2014). Six types, with mineral characteristics summarised in table 2, have been recognized.

Representative samples of the SSLC were collected and their whole-rock composition determined.

Micas from the different granites were analysed in thin sections. Micas from pegmatite were collected from different samples representative of each type. Sampling was directed at intermediate units to make comparison possible between the different pegmatite types and also because most intermediate units contain a two-mica phase assemblage in addition to quartz and K-feldspar [START_REF] Deveaud | The genesis of LCT-type granitic pegmatites, as illustrated by Lithium isotopes in micas[END_REF].

Due to heterogeneous mica repartition and minimum crystal size requirements analysis, biotites and muscovites were commonly sampled up to metres away from each other and the analyses performed on mineral separates.

Analytical techniques

Whole rocks were analysed at SARM (CRPG, Vandoeuvre-les-Nancy). Major element chemical compositions of micas were obtained using a SX-50 CAMECA electron microprobe (ISTO, Orléans).

Analyses were performed at 15 kV, with a mean electron beam of 10 nA, a spot diameter between 3 and 5 μm, a counting time of 20 s on peak and of 20 s on backgrounds (lower and upper). Standards for calibration were albite (Si, Na, K), MgO (Mg), Al 2 O 3 (Al), topaz (F), apatite (P), andradite (Ca), orthoclase (K), Fe 2 O 3 (Fe), vanadinite (Cl), MnTiO 2 (Mn, Ti).

Trace elements were determined using a Agilent 7500 series laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at LMV (Clermont-Ferrand) using a Resonetics M-50E 193 nm ArF

Excimer laser with a standard 30 cm² cell and 100 % He flushing (mixed with Ar + N 2 after the cell).

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The laser beam was focused to yield either a 44 or a 58 μm spot, with energy of 5 J/cm² and 3 Hz repetition rate. Data were calibrated against the NIST 612 (Pearce et al 1997) glass which was analysed every 20 spots. A BCR ® glass was used as a secondary standard. Accuracy is commonly better than 3 % except 7 Li and 181 Ta for NIST 612 (3.19% and 5.91 respectively). Precisions are strictly below 2 %. Details for sampling and analytical accuracy and precisions are provided in Electronic supplementary material 1.

Results

Whole-rock compositions

Granitic rocks (table 3) range from moderately ferromagnesian and aluminous ( Br ) toward more aluminous and felsic compositions ( Sa ,  Go and  Ch ). The  Sy granite compositions are intermediate between  Br and  Go (figure 4). The intrusive granites ( Ch and  Sa ) are the most differentiated with  Sa being the more felsic and  Ch the more aluminous (figure 4, electronic supplementary material 1).

Trace element contents of selected samples (figure 4) show a continuous enrichment in Li, Cs, Rb and F, in agreement with the observed differentiation from  Br to  Sa . Thus, the whole-rock data suggest that these elements behave incompatibly. On the contrary, Ba concentrations decrease with Cs. The whole-rock Nb/Ta ratio diminishes with granite differentiation, with values from 10-20 in  Br down to 1 in  Sa , most samples ranging between 10 and 4. For comparison with granites, whole-rock compositions of aplitic and lepidolite units of one representative MAPF type VI pegmatite are given in table 3.

Major element composition of micas

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12

Textural relationships in granite and pegmatite of the SSLC and MAPF suggest that micas, dioctahedral and trioctahedral, are both magmatic (Scaillet et al 2006;[START_REF] Deveaud | The genesis of LCT-type granitic pegmatites, as illustrated by Lithium isotopes in micas[END_REF].

Therefore, in  Sy ,  Go ,  Ch as well as in most pegmatite types, two mica phases coexisted during crystallization. In leucogranites, apart from  Br which lacks muscovite and  Sa which lacks biotite, biotite and muscovite occur together either as isolated individual phases or as inclusions in quartz and feldspars. Biotite inclusions in muscovite can be also found. In pegmatites, muscovite is present in all types and the amount of biotite significantly decreases with differentiation from type II to VI. In type II, III and IV, biotite and muscovite form intergrowths and muscovite displays two main different textures [START_REF] Deveaud | The genesis of LCT-type granitic pegmatites, as illustrated by Lithium isotopes in micas[END_REF]. Zinnwaldite compositions appear from types IV and are present in type V and VI, the latter containing also lepidolite [START_REF] Raimbault | Composition of complex lepidolite-type granitic pegmatites and of constituent columbite-tantalite, Chedeville, Massif Central, France[END_REF][START_REF] Deveaud | The genesis of LCT-type granitic pegmatites, as illustrated by Lithium isotopes in micas[END_REF].

Mica structural formulae were calculated on a 22 O basis with Li 2 O contents determined from LA-ICP-MS analysis. No chemical zonation was found within the analysed micas. Therefore, the compositional ranges observed for a given sample and mica (see below) reflect inter-crystal variability in the same thin section and not intra-grain zonation. Our major element data for muscovite and biotite in SSLC leucogranites (table 4) are similar to the results of [START_REF] Monier | Muscovite solid solutions in the system K 2 O-MgO-FeO-Al 2 O 3 -SiO 2 -H 2 O: an experimental study at 2 kbar PH 2 O and comparison with natural Li-free white micas[END_REF] andMonier (1987). In general, biotite composition in leucogranites ranges from Fe-biotite to siderophyllite, overlapping with biotite in pegmatites (figure 5). Fe-biotites occur in less differentiated leucogranites ( Sy and  Br ) and in type II pegmatite, while the most differentiated facies  Ch and  Go and type III pegmatite host Fe-biotite to siderophyllite. Trioctahedral micas in type IV and V pegmatites extend to more evolved compositions (protolithionite and zinnwaldite, figure 5) and lepidolite compositions are found in type VI. In comparison, compositions of white micas are more homogeneous, covering a small range from muscovite to Li-phengite. Again, the data for leucogranites and pegmatite overlap but the most evolved white micas are found in granite ( Sa , figure 5). Compared to granites, muscovites Mica-liquid trace elements partitioning Chem Geol 13 evolved pegmatite type. [START_REF] Deveaud | The genesis of LCT-type granitic pegmatites, as illustrated by Lithium isotopes in micas[END_REF] pointed out that such an evolution takes place under constant 7 Li/ 6 Li in muscovite and biotite. 6; Table 4). In  Go , Li contents are homogeneous in both biotite and muscovite, but Cs, Ba, Ta and Nb/Ta vary considerably.  Ch records different behaviours: the white mica data are tightly grouped but the biotite data show substantial variability in Ba and Nb/Ta. Finally, muscovites in  Sa are the phases that record a significant and systematic enrichment in all incompatible elements (figure 6) and also in Ta (table 4), correlated with a Ba depletion (figure 6). The evolution of trace element concentrations from biotite to muscovite within a given sample is variable. For F, Li, Cs, Rb and Ba, concentrations in biotite are mostly equal ( Sy ), and sometimes higher ( Ch ) than in coexisting muscovite.

Trace element composition of micas

In pegmatites, the trace elements contents (table 4) globally agree with the evolution proposed by [START_REF] Deveaud | Spatial statistical analysis applied to rare-element LCT-type pegmatite fields: an original approach to constrain faults-pegmatitesgranites relationships[END_REF][START_REF] Deveaud | The genesis of LCT-type granitic pegmatites, as illustrated by Lithium isotopes in micas[END_REF]. Contents in incompatible elements (Li, Cs and Ta) progressively increase with differentiation (i.e., from type II to VI), while contents in compatible elements (Ba) decrease.

More in detail, biotite in type II pegmatite contains homogeneous Li and more variable Cs, Ba and Ta.

Type III and IV pegmatites have micas with rather homogeneous trace element contents. Li, Cs, Rb and Ta are higher or equal (Ba) in biotite than in muscovite. Micas from Type V show the same type of behaviour for Li, Cs, Rb and Ta; however, for Ba, some concentrations are higher in muscovite than in biotite. It is worth stressing that the database for type V pegmatites includes one point for biotite and three different (and grouped) points for zinnwaldite (figure 6). Muscovite is also quite variable in Mica Nb/Ta ratios behave in two different ways, being either homogeneous ( Br ,  Sy ,  Ch, type II) or highly variable ( Go ,  Sa , type III, IV, VI) in both muscovite and biotite. The range of Nb/Ta values is similar in micas in granites and pegmatites. Homogeneous values are between 10 and 20, whereas variable values range either from > 2 to ~40 in granites and from < 1 to < 20 in pegmatites (figure 6).

Samples containing lepidolite and zinnwaldite have low Nb/Ta values (< 5). Interestingly, biotite and muscovite can record similar Nb/Ta as for example in  Sy and  Ch . In  Go , higher Nb/Ta are found in biotite than in muscovite whereas the opposite appears in pegmatites, i.e., muscovite in type V records higher Nb/Ta values than biotite and zinnwaldite.

Discussion

6.1 Micas as recorders of differentiation processes

Leucogranites

Trace element compositions of granitic liquids calculated using mica/liquid partition coefficients are plotted as a function of the Nb/Ta ratio of micas (figure 7). In figure 8, their variations with [Cs] liq used as a differentiation index are detailed. The data are also detailed for each granite separately (electronic supplementary material 2). Several differentiation mechanisms can be identified.

( 7). Thus, Li shows a specific behaviour, attributed to its particularly high mobility. In silicate liquids at 1000 K, the diffusivity of Li is ~10 2.8 times higher than Rb and ~10 5.6 times higher than Cs [START_REF] Zhang | Diffusion Data in Silicate Melts[END_REF].

Diffusion data for micas are scarce (Sr: 10 -21 m².s-1 at 700°C, Hammouda and Cherniak, 2000; Rb: 10 - 19 m².s-1 at 650 °C, [START_REF] Hoffman | Diffusion of geochronologically important nuclides in minerals under hydrothermal conditions[END_REF] but, in albite at 1000 K, the diffusivity of Li is ~10 9 times higher than Rb [START_REF] Cherniak | Cation Diffusion in Feldspars[END_REF]. This suggests the possibility to homogenise Li in micas by diffusive equilibration with either a Li-rich residual liquid or an intergranular fluid, alternatively.

Large cations such as Rb, Cs and Ba generally have lower diffusivities. This limits equilibration between crystals for these elements and, so, micas in the same thin section can record a range of melt (or fluid) compositions, as illustrated by the  Go and  Sa data (figure 7). This indicates that a mechanism of selective equilibration of micas with the last melt (or fluid) in contact is operative.

(3) For biotites and muscovites in  Br ,  Sy and  Ch , and biotites in  Go , grouped and "normal"

unfractionated Nb/Ta are found (figure 7). Li being excepted for reasons explained above, no systematic evolution appears between trace element liquid compositions calculated from biotite ( ) and from muscovite ( ). For  Sy , apart from an outlier which is the consequence of an anomalously low biotite Cs concentration (see figure 6), are respectively higher (Cs, Rb) and lower (Ba) than the (figure 7; figure 8). For  Ch , and are both in the same range (figure 8; electronic supplementary material 2). Thus, muscovite in two-mica leucogranites records either less evolved ( Sy ) supplementary material 2). According to leucogranite phase relations [START_REF] Scaillet | r o o f 10/12/2018 Mica-liquid trace elements partitioning[END_REF] we should expect that biotite record a higher temperature equilibration stage.

The lack of systematic differentiation from liquids at equilibrium with biotite to liquids at equilibrium with muscovite could find an explanation if the analysed biotites are late in the magmatic history and contemporaneous of muscovite crystallization (e.g. [START_REF] Monier | Evolution spatiale et temporelle de la composition des micas du granite albitique à topaze-lépidolite de Beauvoir[END_REF]Scaillet et al 2006a). If so, both micas would record identical equilibrium liquids. However, the possibility that biotite is more susceptible than muscovite to late-magmatic trace element reequilibration also needs consideration [START_REF] Dahl | The crystal-chemical basis for Ar retention in micas: inferences from interlayer partitioning and implications for geochronology[END_REF]. Systematic diffusion data to compare the kinetics of trace element reequilibration in biotite and muscovite are presently lacking. Yet, our interpretations of the trace element data suggest that micas in SSLC leucogranites rarely preserve the trace element concentrations of early magmatic liquids.

Pegmatites

Micas in pegmatites exhibit variable Nb/Ta, from > 15 to < 1 (figure 9), the less variable Nb/Ta being for the less fractionated pegmatite (type II). As noted previously for leucogranites, [Li] (Chèdeville) has been interpreted as partly magmatic and partly hydrothermal [START_REF] Raimbault | Composition of complex lepidolite-type granitic pegmatites and of constituent columbite-tantalite, Chedeville, Massif Central, France[END_REF]. If the lepidolite and zinnwaldite data are excluded (see above), micas record [Li] liq between 0.5 and 1.0 wt%, in good agreement with concentrations inferred for pegmatitic melts [START_REF] Stewart | Petrogenesis of lithium-rich pegmatites[END_REF]Pichavant et al., 1987a;[START_REF] London | Reply to Thomas and Davidson on "A petrologic assessment of internal zonation in granitic pegmatites[END_REF].

[F] liq concentrations range from ~1 to ~2 wt% (figure 10), also in agreement with estimates for pegmatitic melts [START_REF] Pichavant | Petrogenesis of tourmaline granites and topaz granites; the contribution of experimental data[END_REF]Pichavant et al., 1987a;[START_REF] London | Reply to Thomas and Davidson on "A petrologic assessment of internal zonation in granitic pegmatites[END_REF]. For both Li and F, the maximum concentrations are those calculated from muscovite in type V. Compared to Li, Rb, Ba and Cs demonstrate a larger variability.

[Rb] liq and [Ba] liq define correlation respectively positive and negative with [Cs] liq , as expected for pegmatite differentiation.

However, liquids at equilibrium with biotite are not systematically less evolved than liquids at equilibrium with muscovite. The former are more enriched in Rb, Cs and less enriched in Li, F than the latter. In comparison, muscovites in type V yield the lowest [Ba] liq (the lepidolite and zinnwaldite data being excluded, figure 10).

The pegmatite data strengthen the importance of late-magmatic processes as factors controlling micas compositions. As noted for leucogranites (6.1.1), pegmatites display large variations in Nb/Ta (figure 10). These highly variable ratios are the result of fractionation processes that, for granites, involve biotite and muscovite together ( Go ) or muscovite alone ( Sa ) and, for pegmatites, generally biotite and muscovite together. Based on preliminary mica/melt partition coefficients (constrained from natural occurrences), [START_REF] Stepanov | The key role of mica during igneous concentration of tantalum[END_REF] suggested that biotite and muscovite crystallization can fractionate Nb and Ta. However, in most granites and pegmatites from this study, the fractionating assemblage is the same (biotite plus muscovite), yet only certain samples (the most evolved) strongly fractionate Nb/Ta. This indicates that the Nb/Ta evolution is not governed by the crystallization of micas (e.g. [START_REF] Stepanov | The key role of mica during igneous concentration of tantalum[END_REF]. [START_REF] Linnen | Columbite solubility in granitic melts: consequences for the enrichment and fractionation of Nb and Ta in the Earth's crust[END_REF] would include muscovite [START_REF] Raimbault | The Richemont rhyolite dyke, Massif Central France: a subvolcanic equivalent of rare-metal granites[END_REF][START_REF] Stepanov | The key role of mica during igneous concentration of tantalum[END_REF] and columbo-tantalite [START_REF] Linnen | Columbite solubility in granitic melts: consequences for the enrichment and fractionation of Nb and Ta in the Earth's crust[END_REF][START_REF] Raimbault | Composition of complex lepidolite-type granitic pegmatites and of constituent columbite-tantalite, Chedeville, Massif Central, France[END_REF][START_REF] Linnen | Granite-related rare-element deposits and experimental constraints on Ta-Nb-W-Sn-Zr-Hf mineralization[END_REF][START_REF] Deveaud | The genesis of LCT-type granitic pegmatites, as illustrated by Lithium isotopes in micas[END_REF], which occur respectively in SSLC leucogranites and MAPF pegmatites. The late-magmatic evolution (since both muscovite and columbo-tantalite are late-crystallizing phases) would thus be characterized by the accumulation of Ta relative to Nb as illustrated by muscovite compositions from this study and by the strong Ta/(Ta+Nb) enrichment in columbo-tantalites from the Chèdeville pegmatites [START_REF] Raimbault | Composition of complex lepidolite-type granitic pegmatites and of constituent columbite-tantalite, Chedeville, Massif Central, France[END_REF]. This magmatic model (compare with the magmatic-hydrothermal model of [START_REF] Ballouard | Nb-Ta fractionation in peraluminous granites: A marker of the magmatic hydrothermal transition[END_REF] satisfactorily accounts for the strong Nb/Ta variations observed in both the leucogranites and pegmatites.

Trace element modelling

Leucogranitic liquids

Given the importance of late-magmatic processes in controlling micas trace element compositions, crystallization models, both reverse and forward, have been constructed to assist in the interpretation of the data. Using Rayleigh's fractional crystallization law, the trace element (i) content of a derivative liquid ( ) can be related to its content in the primary liquid ( ):

(1)

(2)

(3

)
where F is the fraction of remaining liquid and D i the distribution coefficient between liquid and crystallised solids. D i is determined by the sum of the products of partition coefficients ( , table 1)

weighted by the corresponding phase proportions ( ) according to:

(4) Partition coefficients used in the calculations are summarized in table 1 for major phases crystallizing from leucogranitic liquids, which include quartz, alkali feldspar and plagioclase in addition to biotite and muscovite [START_REF] Scaillet | r o o f 10/12/2018 Mica-liquid trace elements partitioning[END_REF]. Phase diagrams for peraluminous leucogranites similar to those of the SSLC enable phase assemblages and proportions to be determined in the crystallization interval from ~800 to ~650°C at 400 MPa [START_REF] Scaillet | r o o f 10/12/2018 Mica-liquid trace elements partitioning[END_REF]. For biotite to crystallize as the sole mica, a residual liquid fraction (F) of 45 wt% minimum in needed. Therefore, the calculations assume that biotite in  Br last equilibrated with a liquid derived from 55 wt% crystallization (F = 45 wt%) of the parental melt. Two-mica (biotite-muscovite) phase assemblages require lower F, in the range 45 wt% to 0 wt%. In the following, we will assume that biotite and muscovite in two-mica leucogranites last equilibrated with a liquid derived from 80 wt% crystallization (F = 20 wt%). For muscovite granite, a F value of 10 wt% has been assumed. Note that this assumes that micas compositions are not affected by post-magmatic re-equilibration (i.e. F = 0). In another words the modelling aims at testing a reasonable case of late-magmatic crystallisation and trace element reequilibration in micas.

For each leucogranite, the content an element i of the original liquid can be calculated according to:

(7) (8) ( 9
)
where F is determined as explained above and C f is the most chemically evolved derivative liquid, obtained from the and data. Once C 0 is determined, it is then possible, using a forward approach, to establish the evolution of liquid composition from C 0 to C f by calculating intermediate compositions (C L ) with phase assemblages and proportions from [START_REF] Scaillet | r o o f 10/12/2018 Mica-liquid trace elements partitioning[END_REF].

Results of calculations for selected leucogranite types are shown in figure 11 would make C 0 concentrations still higher. Lastly, the apparently incompatible behaviour shown by Ba in  Go and  Sa (figure 7) is difficult to explain for magmas crystallizing K-feldspars and suggests hydrothermal mobilisation of Ba, similar to observed for Sr in the Beauvoir granite [START_REF] Charoy | Evidence of Sr mobility in and around the albite-lepidolite-topaz granite of Beauvoir (France): an in-situ ion and electron probe study of secondary Sr-rich phosphates[END_REF]. However, since this study is mainly concerned with incompatible elements, a full understanding of the behaviour of Ba is unwarranted (figure 11).

Whole rock data for leucogranites are compared with simulated trace element evolutions (figure 13a).

Whole rocks all plot left (i.e., have lower [Cs]) than their corresponding liquid evolution trends. The whole-rock data for  Sy and  Ch plot relatively close to their calculated respective primary melts (C 0 ) but, for the other granites, the whole-rocks have lower [Rb] and [Li] and, for [Ba], the  Sy and  Ch whole-rocks are lower than their corresponding C 0 (figure 13a; electronic supplementary material 4).

Therefore, whole-rocks hardly represent liquid compositions. As a consequence, the trends revealed by the whole rock trace element contents (figure 13a) are not the expression of leucogranitic magma differentiation and the use of whole rock trace element contents is not directly relevant to the geochemical evolution of leucogranitic liquids. Liquid compositions exhibit a trace element concentration range considerably wider than whole rocks. Whole-rocks trace element compositions do not reflect the evolution of liquid composition.

Pegmatitic liquids

Journal Pre-proof J o u r n a l P r e -p r o o f 10/12/2018 Mica-liquid trace elements partitioning Chem Geol

21

Although no phase diagram is available for MAPF pegmatites, biotites and muscovites come from intermediate zones mostly composed of K-feldspar and quartz. These zones are viewed as relatively early products of crystallization of the pegmatite leaving a significant volume of residual liquid.

Accordingly, we have assumed that intermediate zones derive from 50 wt% crystallization (F = 50 wt%, see above) of the parental liquid. Knowing the mineral assemblage in intermediate zones, the composition of the original liquid can be calculated using equations and partition coefficients as above. Results of calculations for selected pegmatite types are detailed in figure 12 and the entire results are shown in the electronic supplementary material 5.

In type II pegmatite where only biotite was analysed, the calculated Rb and Ba evolutions match reasonably well the . Similarly to granites, the liquid evolution calculated for Li yields values much lower than the determined [Li] liq (figure 12), because of equilibration of micas with late Li-rich liquids or fluids. For the other pegmatite types, the and data points cannot be reconstructed starting from a single most chemically evolved derivative liquid C f (figure 12; electronic supplementary material 5). For example, the evolution for the type III pegmatite calculated from the most evolved biotite generates liquids with Rb concentrations still higher than those derived from muscovite (figure 12). As noted previously, in type III pegmatite, muscovites record higher [Rb] liq and [Cs] liq and equal or lower [Li] liq , [Ba] liq and [F] liq than biotites. The difference between liquids derived from biotite and muscovite is especially marked for Cs, and biotite can record liquid Cs concentrations up 10 times higher than muscovite (see for example type III, figure 12). Consequently, for most pegmatites, the calculations are divided in two parts, one starting from C f constrained from biotite and the other from C f constrained from muscovite (figure 12; electronic supplementary material 5). These differences between muscovite and biotite are attributed to specific crystallization environments for biotite and muscovite in the same pegmatite. Studies of the Chèdeville pegmatites have demonstrated that they crystallized from several separate melt pulses instead of from a single batch [START_REF] Raimbault | Composition of complex lepidolite-type granitic pegmatites and of constituent columbite-tantalite, Chedeville, Massif Central, France[END_REF].

Consequently, the biotites and muscovites analysed are not necessarily comagmatic. Nevertheless, their compositions can inform on the range of primary melts as well as on trace element evolutions during crystallization. Results of the modelling generate primary melt compositions (C 0 ) that stay For Rb in type III, IV and V, the calculated evolutions reproduce quite well the biotite and muscovite data. For Ba and Li, the fits are less good and variable between mica and pegmatite types. It is possible that some pegmatites bear the mark of late-magmatic fluid-dominated processes as indicated by the crystallization of lepidolite and zinnwaldite instead of biotite in type VI pegmatite [START_REF] Raimbault | Composition of complex lepidolite-type granitic pegmatites and of constituent columbite-tantalite, Chedeville, Massif Central, France[END_REF].

Whole rocks compositions for lepidolite and aplite units in type VI (Chèdeville) are compared to calculated liquid contents in figure 13b.

[Cs] contents of lepidolite and aplite units from type VI (table 3) match the lowest [Cs] liq (figure 13b, electronic supplementary material 6).

[Li] and [Ba] from whole rocks are similar to that of liquids, and [Rb] are significantly higher in whole rocks than in liquids from other pegmatite types (figure 13b, electronic supplementary material 6).

The granite-pegmatite connection

The mutual relations between leucogranites, pegmatites and leucogranites and pegmatites are explored on figure 13. No derivative liquid (C f ) from the crystallization of one leucogranite matches the composition of the primary liquid (C 0 ) for another leucogranite. Thus, no geochemical continuity exists from one leucogranite to the other.  Br ,  Go and  Sa plot on the same evolution line for [Li] liq (electronic supplementary material 6) but this is not the case for [Rb] liq (figure 13c) and, so, these various leucogranite phases cannot be related to each other by fractional crystallization [START_REF] Deniel | Isotopic study of the Manaslu granite (Himalaya, Nepal): inferences on the age and source of Himalayan leucogranites[END_REF][START_REF] Guillot | Geochemical constraints on the bimodal origin of High Himalayan leucogranites[END_REF]. Similarly, compositions of primary liquids in pegmatites and trace element evolutions do not plot in continuity with each other (figure 13c). Differences also exist between liquids calculated from biotite and from muscovite in the same pegmatite (electronic supplementary material 6). Compositions of primary granitic and pegmatitic liquids overlap, the less evolved of the range being represented by  Sy and the most evolved by liquids for the type V pegmatite. Primary liquids for leucogranites can be more evolved than for pegmatites (e.g.,  Sa ) and the reverse is also true. Several primary pegmatitic liquids are more evolved than residual leucogranitic liquids (C f ). Therefore, despite uncertainties in the modelling (partition coefficients and F estimations, (such as, for example, pegmatitic being systematically more evolved than granitic melts). The data and the modelling are consistent with each liquid evolving independently from the others. This implies that both pegmatite and granite melts are differentiated from their origin (i.e., different C 0 ). Differentiation is thus inherited from source processes rather than being the consequence of fractional crystallization of a single parental magma.

In addition, the model above, which emphasizes similarities between granitic and pegmatitic melts, raises the question of the origin of textural differences between granites and pegmatites in the SSLC and the MAPF. Previous studies have suggested that these textural differences could be the consequence of:

1. Differences in volatile concentrations (H 2 O, F, B) between melts at the origin of granites and pegmatites. In particular, the possibility of very high H 2 O contents (> 10 wt.%) in pegmatitic melts has been suggested by Thomas et al. (2010).

2. Different emplacement dynamics and volumes for granitic and pegmatitic melts leading to contrasted cooling and crystallization regimes promoting the development of specific textures [START_REF] Lofgren | An experimental study of plagioclase crystal morphology: isothermal crystallization[END_REF][START_REF] Lofgren | Experimental studies on the dynamic crystallization of silicate melts[END_REF][START_REF] London | The application of experimental petrology to the genesis and crystallization of granitic pegmatites[END_REF].

On the first point, our data show that the [F] liq and [Li] liq concentrations do not largely differ between leucogranitic and pegmatitic liquids. For example, [F] liq at equilibrium with muscovite in  Sa (1 -2 wt.% figure 7) are in the same range than [F] liq for most pegmatites (except in type V and also type VI if the lepidolite and zinnwaldite data are considered, table 4). Thus, neither [F] liq nor [Li] liq contents can account for the textural differences observed between granite and pegmatite. In addition, experimental H 2 O contents in pegmatitic melts containing F, B, Li and P are higher than in melts devoid of these components but solubilities never exceed 10 wt.% H 2 O below 3 kbar [START_REF] Holtz | Effects of F, B 2 O 3 and P 2 0 5 on the solubility of water in haplogranite melts compared to natural silicate melts[END_REF][START_REF] Bartels | Viscosity of flux-rich pegmatitic melts[END_REF]. Therefore, differences in volatile concentrations between respective melts do not appear sufficient to explain the textural differences between granite and pegmatite. We suggest that pegmatites in the MAPF originate from similar processes than SSLC granites but, as melt

Journal Pre-proof J o u r n a l P r e -p r o o f 10/12/2018 Mica-liquid trace elements partitioning Chem Geol 24 compositions were essentially similar, pegmatitic textures developed as a consequence of specific magma volumes; emplacement dynamics; crystallisation and cooling regimes.

Processes controlling magmatic differentiation

The similarities documented above between granitic and pegmatitic melts suggest that they have undergone a similar evolution and so that pegmatites from the MAPF can hardly be considered as fractionation products from the SSLC granitic magmas. According to our reconstructed liquid compositions, the differentiation observed among SSLC granites and MAPF pegmatites does not occur "on site", but prior to emplacement. In this case, magmatic differentiation could alternatively take place either in the source region or during magma transport on its way to the emplacement site.

Previous studies of the SSLC have pointed out the proximity of the granite to its probable source in the Lower Gneiss Unit (e.g. [START_REF] Turpin | Meta-igneous origin of Hercynian peraluminous granites in N-W French Massif Central : implications for crustal history reconstructions[END_REF]. This implies that magma transport would take place over relatively short distances. There is also no evidence for the presence of intermediate granitic bodies

between the source rock level and the SSLC. In that sense, the geochemical variability within SSLC granites and MAPF pegmatites would be most probably related to source processes. The relative geochronological relationships between each granite facies (Friedrich, 1984;[START_REF] Cuney | The timing of W-Sn rare metals mineral deposit formation in the Western Variscan chain in their orogenic setting : the case of the Limousin area (Massif Central, France)[END_REF] support assembly of the SSLC from different but compositionally similar magma batches generated from the same source area in the crust. Equilibrium melting of a single source coupled to discrete episodes of melt extraction could lead to elemental fractionations within the source region. The observed differentiation between the different magma batches could thus be the result of various processes including:

1. Variable degrees of melting. For example, decreasing melting rates would enrich incompatible element concentrations in the melt. It is possible that early-formed magmas are produced through larger melt fractions and are thus less enriched than late formed magmas originating from residual source [START_REF] Wolf | Tin in granitic melts: The role of melting temperature and protolith composition[END_REF] Combinations of all the above processes would account for the differentiation observed in granite but also in pegmatite melts and, in particular, for the differential initial rare metal enrichments observed in the granitic and pegmatitic bodies.

Conclusions

The approach used here appears to be fruitful for reconstituting the evolution of parental liquids.

Despite (partial) equilibrations of micas and uncertainties over partition coefficients, micas record, at least partly, the evolution of granitic and pegmatitic liquid composition. The reconstruction of trace element contents of granitic and pegmatitic liquids from the analysis of mica compositions demonstrate that 1) the SSLC granites and the MAPF pegmatite do not simulate a liquid line of Finally, we stress that whole rock compositions can hardly be used directly to reconstruct the magmatic evolution that leads to granite. These merely provide a qualitative record of the entire process. The use of mineral composition and melt/mineral partition as presented here appears considerably more promising than whole rock compositions studies. It provides a semi-quantitative estimate of melt trace elements contents during crystallization. Further use of this approach requires caution as mineral/melt partitions uncertainties and the effect of equilibration over each element must be considered. Ultimately, the determination of melt composition would benefit additional studies of mineral/melt partitioning and trace element diffusion in micas (and other minerals). and incompatible element enrichments. In the case of fractional crystallization of cogenetic liquids, a single linear trend is observed between liquids and, in principle, between magmas. On the contrary, in the case of fractional crystallization of non-cogenetic liquids, independent liquid and magma evolutionary trends are obtained. See also [START_REF] Treuil | Utilisation des éléments hygromagmatophiles pour la simplification de la modélisation quantitative des processus magmatiques. Exemples de l'Afar et de la dorsale médioatlantique[END_REF], [START_REF] Raimbault | Comportements différenciés de W, Sn, U, Ta, Nb dans quelques complexes granitiques du Massif Central français[END_REF] and [START_REF] Shaw | Trace Elements in Magmas: A Theoretical Treatment[END_REF]. 3) and Friedrich (1984). For data from this study, samples for which micas have been analysed are distinguished with larger diamonds.

Figure captions

Journal

  from pegmatites (irrespective of the two textures noted above) are intermediate between compositions in  Sy,  Go and  Ch on the one hand and in  Sa on the other hand. The evolution towards increasing F and Li concentration in micas is progressively marked from  Br to  Sa and from the less toward the more Journal Pre-proof J o u r n a l P r e -p r o o f 10/12/2018

  and Ta. Finally, lepidolite from type VI pegmatite records the most extreme concentrations with Ba contents < 5 ppm, Ta from < 50 to > 200 ppm and Cs in the 1000-200 ppm range (figure6).

  have demonstrated that, during the evolution of low temperature peraluminous melts, activity coefficients of Ta 2 O 5 and Nb 2 O 5 are progressively modified. These modifications impact all mineral/melt partition coefficients for Nb and Ta in the same way, always leading to an enrichment of Ta relative to Nb in residual liquids. During the evolution of low temperature peraluminous melts, phases most likely to fractionate Nb from Ta

  no systematic relation emerges between compositions of granitic and pegmatitic melts

  descent and, thus, that granitic magmas are not parental to pegmatitic melts. Both granites and pegmatites have a common origin and exhibit different levels of fractionation as a consequence of mainly source processes. 2) Liquid fractionation recorded by biotite and muscovite occurs during the each igneous body, which includes the effects of crystallization of major and accessory mineral phases and also the involvement of late-magmatic fluids.

Figure 1 .

 1 Figure 1. Schematic theoretical evolution of two highly incompatible elements (i and j) in liquids in

Figure 2 .

 2 Figure 2. Experimentally determined mica/melt partition coefficients (Kd = concentration of trace

Figure 3 .

 3 Figure 3. Geological map of the St Sylvestre leucogranite Complex (SSLC) and location of known

Figure 4 .

 4 Figure 4. Whole rock compositions of SSLC leucogranites. (a) Debon and Le Fort (1983) A-B major

Figure 5 .

 5 Figure 5. Compositions of micas in leucogranites and pegmatites from the SSLC in the classification

Figure 6 .

 6 Figure 6. Trace elements contents (F, Li, Cs, Rb, Ba in ppm) and Nb/Ta ratio (in ppm) in micas from

Figure 7 .

 7 Figure 7. Calculated trace elements contents (Li, F, Ba, Rb, Cs in ppm) of leucogranitic liquids at

Figure 8 .

 8 Figure 8. Calculated trace element contents (Li, F, Rb, Ba in ppm) of leucogranitic liquids at

Figure 9 .

 9 Figure 9. Calculated trace elements contents (Li, F, Ba, Rb, Cs in ppm) of liquids at equilibrium with

Figure 10 .

 10 Figure 10. Calculated trace element contents (Li, F, Rb, Ba in ppm) of pegmatitic melts at equilibrium

Figure 11 .Figure 12 .

 1112 Figure 11. Calculated trace element contents of liquid at equilibrium with micas in granites

Figure 13 .

 13 Figure 13. Incompatible element evolution of leucogranites and pegmatites from the SSLC and

  Trace and minor elements considered in this study include Li, Rb, Nb, Ta, Ba, Cs and F, this choice being largely dictated by the availability of suitable partition coefficients. Focus is placed on the incompatible elements, Ba serving to illustrate the behaviour of compatible elements. Experimental

mica/liquid partition coefficients used in this study are summarized in table 1, together with partition coefficients for other major phases. The mica/liquid Kd values were selected to come only from (1)

  Trace elements in micas are highly variable in both granite and pegmatite (table4). In granites, concentrations are clustered: biotite from  Br and  Sy and both biotite and muscovite from  Ch and  Go form well-defined clusters (figure6). Alike for whole rocks, incompatible element (Li, Cs or Rb) concentrations progressively increase with differentiation (i.e., with lowering Mg-Li in mica) while compatible element (e.g. Ba) and Nb/Ta concentrations decrease. The highest Li, Cs and Nb/Ta and the lowest Ba concentrations are recorded by white micas in  Sa (figure

  1) White micas in  Go and  Sa are characterized by low (< 10) and highly variable Nb/Ta ratios correlated with either constant ([Li] liq ) or increasing([Rb] liq , [Cs] liq , [Ba] liq ) concentrations (figure7). ., on late-crystallising phases) and are specific of the two most fractionated leucogranites. Since these variations are also encountered in most pegmatites (figure6), differentiation processes responsible for these Nb/Ta variations are discussed in the pegmatite section (6.1.2). What is important for the present purpose is that these trace element patterns result from mechanisms late in granite crystallization histories.(2) The  Go and  Sa data indicate a more "equilibrated" behaviour for [Li] liq than for [Rb] liq , [Cs] liq and[Ba] liq (figure7). At the crystal scale, trace element concentrations are homogeneous. However, in  Sa , at the thin section scale, the different muscovite crystals analysed record homogeneous[Li] liq (and, in  Go , the different biotite and muscovite crystals record identical [Li] liq ) whereas Nb/Ta, [Rb] liq , [Cs] liq and [Ba] liq all vary to different extents depending on sample and mica (figure

	When plotted as a function of [Cs] liq , incompatible elements ([Rb] liq ) increase while [Li] liq stays almost J o u r n a l P r e -p r o o f 10/12/2018 Mica-liquid trace elements partitioning Chem Geol constant and [F] liq (n.b., F is compatible in micas, table 1) show little variations (figure 8). [Ba] Journal Pre-proof (i.e
	15

liq show an unexpected behaviour (e.g., slight positive correlation with [Cs] liq , figure

8

). We note that the variations in Nb/Ta (and the associated trace element evolutions) are marked mostly on white micas

  Ch ) trace element concentrations than biotite.  Go is the only granite with indications for liquid compositions at equilibrium with biotite being less evolved than at equilibrium with muscovite ( respectively equal or lower than for [Cs] liq , [Rb] liq and higher for [Ba]liq, electronic

	10/12/2018	Journal Pre-proof Mica-liquid trace elements partitioning	Chem Geol
	or similar (
		J o u r n a l P r e -p r o o f
			16

  liq are very homogeneous with Nb/Ta, while [Cs] liq , [Rb] liq and [Ba] liq are more variable with Nb/Ta. Liquid compositions for type VI and, to a lesser extent, type V, stand out of the main pegmatite trend on most

	10/12/2018	Journal Pre-proof Mica-liquid trace elements partitioning	Chem Geol
	of mica compositions with a late-magmatic residual melt or fluid. Lepidolite in type VI pegmatite
		J o u r n a l P r e -p r o o f
	diagrams (figure 9 and 10). These data points must be considered with caution since they are
	calculated using partition coefficients for biotite, yet the analysed micas are lepidolites and
	zinnwaldites.
	The evolution of liquid compositions is illustrated in figure 10 and detailed for each pegmatite type
			17

separately (electronic Supplementary Material 3). [Li] liq are very similar in most pegmatites, whether calculated from biotite or muscovite. The homogeneous

[Li]

liq concentrations suggest reequilibration

  Li] liq values, especially for  Br and  Sa (figure11). As suggested above, this behaviour can be explained by micas having equilibrated with late residual Li-rich liquids or fluids. Significant differences between measured and modelled liquid concentrations were also noted for Ba. Most C 0 and C L are higher than [Ba] liq especially for  Br ,  Go and  Sa (electronic Supplementary Material 4). C 0 sometimes reach very high concentrations (> 1000 ppm for  Sa , electronic Supplementary Material 4) which casts doubt on the Kd values used in the calculations (table1). Note that increasing D for Ba (to account for the possibility of disequilibrium crystallization, see above)

	, the entire set of calculations being compiled in the electronic Supplementary Material 4. The same principle is applied to pegmatites and presented in figure 12 and electronic Supplementary Material 5. In all cases, calculated C 0 are less evolved i.e., have lower incompatible and higher compatible element J o u r n a l P r e -p r o o f 10/12/2018 Mica-liquid trace elements partitioning Chem Geol concentrations than C Journal Pre-proof systematically lower than the [
	20

f . For Rb, calculated C 0 -C L trends reasonably reproduce the [Rb] liq vs. [Cs] liq evolutions in particular for  Sy and  Sa . In comparison, the calculated C 0 -C L values and trends are

  . A progressive decrease in melting rate with time could be the consequence of previous melt extraction reducing source fertility. Near-source prograde crystallization of quartz and feldspars (e.g.[START_REF] Nicoli | Insights into the complexity of crustal differentiation: K 2 O-poor leucosomes within metasedimentary migmatites from the Southern Marginal Zone of the Limpopo Belt[END_REF]. Such processes would have geochemical implications (i.e., increased fractionation) similar to those proposed by[START_REF] Tartese | Differentiation of peraluminous leucogranites "en route" to the surface[END_REF] although, in this case, these would occur within or near the Melting affecting progressively different layers of a heterogeneously enriched source.[START_REF] Romer | Lithium and boron in late-orogenic granites. Isotopic fingerprints for the source of crustal melts ?[END_REF] have suggested that the source of rare metal granite magmas can be anomalously and heterogeneously enriched as a consequence of previous sedimentary processes. In the Variscan Orogen, such a model is supported by the location of rare metals magmas.

	10/12/2018	Journal Pre-proof Mica-liquid trace elements partitioning	Chem Geol
	2. migmatitic area.
	3. Disequilibrium melting (Nabelek and Glascock, 1995; Villaros et al 2009; McLeod et al 2012)
	allowing the retention of incompatible elements during the earlier stages of melting. In this
	case, rare metals would remain "trapped" in the source and only progressively "released" in
	the melt.
	4.	J o u r n a l P r e -p r o o f
			25

Acknowledgements

The authors acknowledge funding from Labex VOLTAIRE (ANR-10-LABX-100-01) and the ANR project VARPEG. Sarah Deveaud is thanked for her participation to the analysis of micas from pegmatites. Thanks to Don Dingwell for handling of the manuscript and to anonymous reviewers for their suggestions that helped improving this manuscript.