Evaluation of the impacts of former mining activities on soil quality requires the determination of pedo-geochemical backgrounds for potentially toxic elements such as arsenic. Difficulties in such a context may arise from high geogenic concentrations related to the presence of mineralization in the mined area. In this study, we investigated the pedo-geochemical background in the former Au-As Salsigne mining district located in the South of the French Massif Central. Sampling of topsoils was designed such as to discard sites with evident influence of mining activities and with the aim to obtain a sufficient amount of data to be able to determine representative statistical parameters. A suitable number of sampling sites (150) was obtained by grouping existing litho-tectonic units into three coherent geological domains: the Axial zone domain, the Minervois nappes domain and the Cenozoic domain. For some of the investigated elements such as arsenic, the distribution of soil concentrations shows areas with higher pedo-geochemical background close to mineralization. Advanced statistical methods such as Minimum-Maximum Autocorrelation Factors (MAFs) allow a distinction of such enriched zones in order to better estimate pedo-geochemical background values. The proposed methodology for determining pedogeochemical backgrounds is applicable in other areas that display complex geology with significant natural variations of soil concentrations such as in mineralized areas where mining activities may have occurred.

The Sioule region in the northern part of the French Massif Central of the Variscan belt hosts the Echassières complex, where a large W(-Sn) quartz vein system was intruded by the Beauvoir rare-metal granite (RMG) hosting disseminated Sn, Nb-Ta and Li mineralization. We combined whole-rock geochemical data, zircon U-Pb geochronology of migmatites and felsic igneous rocks of the Sioule area, along with U-Pb dating of cassiterite, wolframite, and apatite in the Echassières complex to reconstruct the regional magmatic and metallogenic evolution. Results reveal: (i) early W mineralization at 351 ± 9 Ma, coeval with peraluminous granite magmatism (Chantelle and St-Gervais massifs) and a N115-striking dextral shear zone; (ii) a second W mineralization phase at 329 ± 5 Ma, linked to rare-metal-rich rhyolitic dykes and the Colettes granite during NW-SE extension, synchronous with regional biotite microgranites (Pouzol-Servant Massif) and trachy-dacitic tuffs and lavas; and (iii) late-orogenic emplacement of the Beauvoir RMG with minor hydrothermal Sn and W at ca. 320-310 Ma, synchronous with biotite granites hosting quartz-tourmaline orbicules (Champs Massif) and pyroclastic flows. Our study reveals a 40 Myr-long metallogenic evolution with hydrothermal W mineralization preceding hydrothermal Sn and magmatic Li-Sn-Nb-Ta. Each mineralization period coincided with widespread crustal magmatism involving the anatexis of late-Ediacaran metasedimentary rocks and Cambrian-Ordovician metagranites, as shown by the dating of zircon in migmatites and inherited zircon in Variscan igneous rocks. However, rare-metal-enriched magmas remained spatially confined to the Echassières complex suggesting a structural corridor that repeatedly focused magmas and fluids from a localized fertile source.

The Saharan metacraton was assembled during pre-Neoproterozoic to Neoproterozoic times and was strongly remobilised during the Pan African orogeny. The Ouadda & iuml; massif in eastern Chad represents the core of the Saharan metacraton, yet its geological history remains poorly documented. In this study, we combine of field observations, petrological analysis of metamorphic rocks, and geochemical and geochronological constraints to reconstruct the tectono-metamorphic evolution of the Ouadda & iuml; massif. The study area is dominated by collisionrelated plutonic rocks and migmatitic gneisses, within which inliers of older basement occur. Geochemical data indicate a transition from active-margin to collisional magmatism, with a compositional evolution from diorite to granite. This suite is cross-cut by syenogranites and alkaline granites with shoshonitic affinities, interpreted as post-collisional granitoids derived from tonalitic rocks to sedimentary protoliths. Geochronological data (U-Pb on zircon and monazite) point to a ca. 1000 Ma age for orthogneisses inliers. Granitoids of the Ouadda & iuml; massif record emplacement ages from 620 to 590 Ma, coeval with high-temperature metamorphism characterized by peak pressure-temperature conditions of 1.2 GPa and 850 degrees C (mafic granulite) and around 0.8 GPa and 700 degrees C (sillimanite-garnet bearing migmatites). Integrating our new results with regional data, we discuss the existence and geodynamic evolution of the Saharan Metacraton. Our findings emphasize the significance of the Saharan Metacraton as a key region for understanding the extensive reworking of cratonic lithosphere during both a Tonian magmatic phase prior and the assembly of Gondwana.

This paper evaluates the impact of integrating various geological, geophysical, and geochemical datasets into mineral prospectivity mapping for undiscovered tungsten (W) and tin (Sn) deposits, using the Variscan French Massif Central (FMC) as a case study. Previous studies on the prospectivity of W–Sn deposits in Variscan terranes have primarily focused on geological and geochemical datasets, rarely integrating multi-method geophysical data. The FMC, located in the internal zone of the Western European Variscan belt, is a historic mining region known to host numerous granite-related W–Sn deposits. However, a comprehensive assessment is required to identify new prospective areas, including possible targets at depth. We applied a data-driven predictive approach using the disk-based association method coupled with random forest classification (DBA–RF) to predict the likelihood of undiscovered W–Sn deposits in the Puy-les-Vignes/Saint-Goussaud district (approximately 800 km2) located in the northern FMC. Favorable criteria from geological maps (lithostratigraphy, distance to faults, and known mineral occurrences) were analyzed, along with multiple datasets from airborne and ground geophysics (gravimetry, magnetics, electromagnetics, and gamma spectrometry), and stream sediment geochemistry (As, B, Be, Cu, Sn, and W concentration maps). The resulting prospectivity maps highlight several zones with a total area of 67 km2 for W and 33 km2 for Sn, which represent 8.3

Rare-metal granites and pegmatites are enriched in critical metals, such as lithium (>5000 ppm), relative to conventional granites in the crust (<100 ppm). The petrogenesis of lithium-bearing pegmatites has been historically associated with the high-degree fractional crystallisation of parental granites. However, the extent of lithium enrichment during this process remains debated due to uncertainties concerning the partitioning behaviour of lithium between minerals and felsic melts. Alternatively, lithium enrichment in granitic pegmatites may reflect the composition of their crustal source. To test the anatectic origin of rare-metal granites and pegmatites, partial melting experiments were performed on variably enriched metasedimentary rocks (100–800 ppm lithium). The experiments produced felsic melts with a lithium content of 180–2200 ppm and constrained mineral-melt partition coefficients for granitic systems. Trace element modelling demonstrates that high-grade granite-related deposits (>5000 ppm lithium) are sourced from the anatexis of enriched crustal rocks (>300 ppm lithium), followed by the moderate fractional crystallisation of the partial melts. Lithium-rich granites and pegmatites form by the anatexis of enriched crustal rocks, followed by the moderate fractional crystallisation of the extracted melts, as revealed through partial melting experiments and trace element modelling.

This study investigates the P–T–t–D evolution of two metapelitic samples from the middle crust exposed in the Aiguilles Rouges Massif. Garnet compositional mapping, phase equilibrium modelling, zirconium-in-rutile thermometry, trace element geochemistry of garnet and monazite, and U-Pb LA-ICP-MS dating on monazite were used to better understand the tectonic and thermal history of the variscan External Crystalline Massifs. In the sample representing the upper-middle crust (AR736, southwestern part of the massif), using the preserved mineral assemblage in garnet inclusion (Grt + St + Bt + Ms + Qz + Pl + Rt) and garnet compositions, the prograde P–T path was constrained from ∼0.5–0.6 GPa and 550–625 °C to ∼0.76–0.82 GPa and 600–640 °C. The P–T conditions at the onset of this prograde evolution suggest a high geothermal gradient (∼30–35 °C/km) prior to the onset of crustal thickening. In the sample representing the lower-middle crust (AR14, central part of the massif), using the preserved mineral assemblage in garnet inclusion (Grt + Bt + Ms + Qz + Pl + Rt), the occurrence of sillimanite and ilmenite in the matrix and garnet compositions, a β-shaped P–T path characterised by a late temperature increase during exhumation was identified. Both samples recorded a retrograde P–T stage at ∼0.4 GPa and 545 °C, dated at 315–305 Ma. Microstructural analysis indicates dextral transcurrent deformation from the late crustal thickening stage to the exhumation phase. Comparison with previously published P–T paths from eclogitic lenses highlights the juxtaposition of middle and lower crustal domains during dextral transcurrent deformation. We propose a tectonic model in which the formation of supra-subduction volcano-sedimentary basins (∼350 Ma) is followed by crustal thickening between 350 and 340 Ma under a thermal gradient of ∼5–15 °C/km. The exhumation of the lower and middle crust took place in a transcurrent regime between 340 and 305 Ma. This prolonged transcurrent tectonic activity suggests that the numerous transcurrent shear zones in the Variscan belt are not merely late orogenic structures but played a significant role in the geodynamic evolution, particularly in the exhumation of the orogenic crust, from the end of continental collision to the closure of the Variscan orogeny.

In the frame of a 1:200,000 mapping project of Cameroon coordinated by PRECASEM for the Ministry of Mine, Industry and Technological Development (MINMITD), between 2016 and 2021, and carried out by BRGM-GTKBEIG3 Consortium, a dataset of nearly 15,500 stream sediment samples were collected, at an average sampling density of 1 sample per 10 km2, over an area of some 157,000 km2, and analysed for 49 major and traces elements plus gold. The first aim of the geochemical survey was to identify, at the country scale, metallic anomalies and districts to promote the mining potential of Cameroon to mining investors. In order to expand the use of this unique geochemical dataset, multivariate geostatistical processing by Principal Component Analysis (PCA) and Agglomerative Hierarchical Clustering (AHC) was performed for lithogeochemical purpose. This processing enabled the identification of several lithogeochemical signatures which match very well with the main geological domains and formations obtained by conventional geological field survey. Moreover, in areas covered by thick lateritic profile like in the Adamawa-Yade subdomain, the geostatistical processing was able to identify the granitic nature of the bedrock. Secondly, the geochemical results obtained for U, K and Th on stream sediments were handled by inverse distance interpolation (IDW), and represented in a ternary combination in the RGB colour space to generate a pseudo-radiometric map to be compared with the high-resolution regional-scale airborne geophysical survey (magnetic and radiometric data) undertaken prior to the beginning of the mapping project. Similarities between both maps are remarkable. Moreover, the pseudo-radiometric map appears also more discriminant in areas covered by thick forest and thick lateritic profile, in humid and swampy environments allowing the identification of structural features and geochemical contrasts unrevealed by the airborne survey. This contribution shows that stream sediment regional-to country-scale dataset should not only be used to highlight the mining potential of the studied area, but also represent a powerful tool, once derived in lithogeochemical and pseudo-radiometric interpretative maps, which can be efficiently used to decipher the major geological contrasts and structural feature of the country rocks. This information is of primary importance to define and calibrate the targets, logistics and costs of the geological field survey classically associated with these national programs, especially in areas with poor outcrops and/or not covered by airborne geophysical surveys.

In sedimentary basins, unconformity between basement and sediments is the ideal site where fluids from different sources can flow and mix, initiating the formation of ore deposits. In western Europe there are numerous F-Pb-Zn-Ba (+Ag, Ge) basin-hosted deposits located near the unconformity between Mesozoic Basins and the Variscan basement as for example the deposits of the Vendee Coast (France) containing fluorite, baryte, pyrite and quartz. Here, microthermometric data on the primary fluid inclusions of these minerals indicate salinity ranging from 1 to 20 wt% eq. NaCl and homogenization temperatures between 100 and 390 degrees C. We interpret these data as resulting of a fluids circulation in the ore deposit zone, with an early incursion of basin brines expelled by the leaching of Hettangian evaporites buried several tens of kilometers away, followed by an ascent of basement-derived fluids and, finally, a recharge of seawater. In other French deposits, the delta 34S isotopes of baryte are also consistent with a source of fluids from buried evaporites. The 87Sr/86Sr ratio of baryte demonstrates a crustal source of elements associated with brines-leached base metals and F-Ba. The process of buried-derived evaporites brines altering the basement along the unconformity is ubiquitous in all unconformity deposits in France. With our results, we confirm that the basin deposits in the Southeastern Massif Central occur along structures formed during Tethys rifting at around 200 Ma, whereas in the Western part they form at ca. 145 Ma in link with the opening of the Bay of Biscay. This highlights that these basin-hosted deposits are preferentially formed during extensional activity in rifting settings, rather than in compressive settings.

The Fregeneda-Almendra pegmatite field of the Iberian Massif represents a typical expression of peraluminous rare-metal magmatism that occurred over western Europe at the end of the Variscan orogeny. It is the host for two main types of Li-mineralized intrusions, identified at the scale of the Variscan belt, including petalite- or spodumene-rich pegmatites, as well as Li-mica-rich pegmatites, for which the origin of mineralogical-chemical differences is not yet understood. Here, we provide cassiterite and columbite-group mineral (CGM) U-Pb ages along with oxide, mica and phosphate mineral compositions for Li-pegmatites from the Fregeneda-Almendra field in order to assess their petrogenesis and tectonic-magmatic context of emplacement. U-Pb geochronology indicates that petalite-rich and Li-mica-rich pegmatites were mostly emplaced sub-synchronously from 315 ± 6 to 308 ± 6 Ma, during strike-slip deformation and granitic magmatism within an anatectic dome bounding the pegmatite field. U-Pb data and pegmatite geographic zonation suggest that Li-pegmatites were sourced from buried equivalents of leucogranites and migmatites from the dome. Li-pegmatites experienced a complex crystallization including K-feldspar, petalite, topaz, Nb-Ta-Fe-Mn-rich cassiterite, amblygonite-group minerals (AGM) and CGM as early magmatic phases, followed by lepidolite for Li-mica-rich pegmatites. At the magmatic-hydrothermal transition, notably leading to the formation of Nb-Ta-Mn-Fe-poor cassiterite hosting CGM inclusions, earlier minerals were resorbed by muscovite and albite. A later F-rich hydrothermalism is locally reflected by zinnwaldite overgrowths on muscovite. Cassiterite, CGM and micas from petalite-rich pegmatites show lower Mn/Fe ratios and higher Ti contents, along with lower Zr-Ga contents for cassiterite, than that from Li-mica-rich pegmatites. Such behavior is consistent with a magmatic differentiation process whereby Ti content decreased and the degree of Mn-Fe geochemical fractionation and solubilities of Ga and Zr increased in the melts, possibly in relation with high fluorine activity. In Li-mica-rich pegmatites, AGM equilibrated with a melt with up to 2 wt% F, similar to that in equilibrium with lepidolite (1–3 wt%). In petalite-rich pegmatites, the relatively high F concentration of the melts equilibrated with AGM (≤ 1.5 wt% F) contrasts with the liquid equilibrated with muscovite (< 0.5 wt% F). This can be accounted for by muscovite crystallization after the exsolution of a F-rich aqueous phase at the magmatic-hydrothermal transition. Relatively similar F contents in the initial melts of petalite- and Li-mica-rich pegmatites support the hypothesis that the stability of lepidolite does not only involve high F but also a low H 2 O/F activity ratio. For the Fregeneda-Almendra Li-mica-rich pegmatites, this could be explained by a decrease of melt H 2 O solubility due to a relatively low pressure of emplacement.

Variscan granitoids and associated mafic rocks exposed in the External Crystalline Massifs (ECM) of the Western Alps document the Variscan stages from the early Carboniferous collision to the early Permian post-collisional setting. Our study focuses on the Central part of the ECM, synthesizing newly acquired and existing geochronological, whole-rock geochemical and isotopic data. We identified two distinctive magmatic series: (i) high-K calc-alkaline granitoids, which range from magnesian (MgG) to ferro-magnesian (FeMgG) rocks; (ii) ultra-high-K metaluminous (UHKM) rocks (“durbachites”). These series were emplaced roughly simultaneously between ca. 350 and 300 Ma, with two main episodes during the Visean (ca. 348–335 Ma) and the late Carboniferous (305–299 Ma), with a more limited activity in between. A younger Permian event at ca. 280–275 Ma has also been identified in one granitoid pluton. Contemporaneous emplacement of these two series reflects concomitant crustal anatexis and melting of LILE–LREE-rich metasomatized lithospheric mantle. Trace elements and Nd–Sr isotopes reveal significant hybridization between these two magmatic end members, by magma mixing, or assimilation of crystallized mafic ultrapotassic enclaves in the high-K calc-alkaline granitoids. Granitoid composition evolves over time, especially SiO2, Mg#, Sr/Y, La/Yb and Nb/Ta, possibly explained by increasing differentiation of magmas over time, changes in the crust versus mantle sources mass-balance, and decrease in melting pressure due to the orogenic collapse. The εNdi values of both high-K calc-alkaline granitoids and durbachites decreases from [− 3.8; − 2.9] to [− 6.4; − 5.2] between 345 and 320 Ma, possibly indicating an increasing influence of subducted/relaminated crustal material contaminating the lithospheric mantle source. εNdi values then rise to [− 3.7; − 0.5] during the late Carboniferous, possibly due to progressive exhaustion of the enriched mantle source, or advection of the asthenosphere during the post-collisional stage. Possible geodynamic scenario along the central-eastern segment of the Variscan Belt, which may account for the temporal evolution of Variscan magmatism in the External Western Alps.

We present new geochemical and geochronological data for Mesoarchean to Neoarchean plutons of the Chaillu inlier of the Congo Craton that demonstrate an evolution of magma sources in relation with crustal growth. Four distinct plutonic suites were newly identified: (1) sodic low medium/high-pressure tonalite-trondhjemite-granodiorite, (2) Mg–K-rich monzogranite to monzodiorite (sanukitoids), (3) K-rich monzogranite–syenogranite, (4) K-rich granodiorite–monzogranite with medium/high-pressure TTG-like signatures. The TTGs are metaluminous to slightly peraluminous, silica-rich magmatic rocks (SiO2 > 67.5 wt%) with high contents of Na2O (3.5 < Na2O < 6.2 wt%), and low contents of ferromagnesian oxides, and K2O contents (K2O < 2.6 wt%). The sanukitoids (s.l.) are metaluminous to slightly peraluminous, with a large range of silica contents (53 < SiO2 < 69 wt%), high Mg# (42–74) and potassic with variable K2O contents (1.2 < K2O < 5.0 wt%). The high-K granites are slightly peraluminous, with high silica contents (SiO2 > 67.7 wt%), low amounts of ferromagnesian oxides, and a clear potassic signature, with K2O contents (2.5 < K2O < 4.7 wt%), and K2O/Na2O ratios higher than 0.55. The zircon U–Pb dating results indicate that: i) TTGs emplaced between 3012 ± 16 Ma and 2840 ± 9 Ma; ii) sanukitoids between 2853 ± 14 Ma and 2832 ± 9 Ma; and iii) are K-rich biotite granites between 2893 ± 11 Ma and 2847 ± 16 Ma. Two main well-defined periods are identified: i) one of TTG-type magmatism between 3.02 and 2.92 Ga; and ii) one of sanukitoid-TTG-biotite granite-type association between 2.91 and 2.82 Ga. Such temporal evolution from TTG-domination through coexisting sanukitoids (s.l.) – TTGs – biotite granites during the late-Archean in the Congo Craton resembles other Archean cratons, such as the Dharwar, North China, Karelian, Superior Province, Pilbara, Kaapvaal, São Francisco and Amazonian.

Free Access Appendix 1: Lithium Mineralization, Contributions of Paleoclimates and Orogens Éric GLOAGUEN, Éric GLOAGUEN BRGM, Orléans, France ISTO, CNRS, University of Orléans, FranceSearch for more papers by this authorJérémie MELLETON, Jérémie MELLETON BRGM, Orléans, FranceSearch for more papers by this authorBlandine GOURCEROL, Blandine GOURCEROL BRGM, Orléans, FranceSearch for more papers by this authorRomain MILLOT, Romain MILLOT BRGM, Orléans, FranceSearch for more papers by this author Éric GLOAGUEN, Éric GLOAGUEN BRGM, Orléans, France ISTO, CNRS, University of Orléans, FranceSearch for more papers by this authorJérémie MELLETON, Jérémie MELLETON BRGM, Orléans, FranceSearch for more papers by this authorBlandine GOURCEROL, Blandine GOURCEROL BRGM, Orléans, FranceSearch for more papers by this authorRomain MILLOT, Romain MILLOT BRGM, Orléans, FranceSearch for more papers by this author Sophie Decrée, Sophie DecréeSearch for more papers by this author Book Author(s):Sophie Decrée, Sophie DecréeSearch for more papers by this author First published: 26 December 2023 https://doi.org/10.1002/9781394264841.app1 AboutPDFPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShareShare a linkShare onEmailFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Metallic Resources 2: Geodynamic Framework and Remarkable Examples in the World RelatedInformation

Lithium is the third element in the periodic table, containing three protons, hence Z = 3, which defines its place in Mendeleev's periodic table of elements. It is a very light alkaline metal element with a density of 0.53, which is half the density of pure water. Lithium is used in various forms: minerals, metals, carbonates, hydroxides, various chemical compounds. A minimum of 120 lithium-bearing mineral species are identified in the database of the International Mineralogical Association. This number is a minimum, as many mineral species containing low amounts of lithium are not recorded. At the Earth's surface, several processes have an effect on the mobilization or immobilization of lithium, particularly processes of alteration, erosion and evaporation in the presence of variable amounts of water. By comparison, alkaline metals with larger ionic radius, such as rubidium and cesium, form octahedral complexes with water.

Exposure to potentially toxic trace elements (PTTEs) in inhalable particulate matter (PM10) is associated with an increased risk of developing cardiorespiratory diseases. Therefore, in multi-source polluted urban contexts, a spatially-resolved evaluation of health risks associated with exposure to PTTEs in PM is essential to identify critical risk areas. In this study, a very-low volume device for high spatial resolution sampling and analysis of PM10 was employed in Terni (Central Italy) in a wide and dense network (23 sampling sites, about 1 km between each other) during a 15-month monitoring campaign. The soluble and insoluble fraction of 33 elements in PM10 was analysed through a chemical fractionation procedure that increased the selectivity of the elements as source tracers. Total carcinogenic risk (CR) and non-carcinogenic risk (NCR) for adults and children due to concentrations of PTTEs in PM10 were calculated and quantitative source-specific risk apportionment was carried out by applying Positive Matrix Factorization (PMF) to the spatially-resolved concentrations of the chemically fractionated elements. PMF analysis identified 5 factors: steel plant, biomass burning, brake dust, soil dust and road dust. Steel plant showed the greatest risk contribution. Total CR and NCR, and source-specific risk contributions at the 23 sites were interpolated using the ordinary kriging (OK) method and mapped to geo-reference the health risks of the identified sources in the whole study area. This also allowed risk estimation in areas not directly measured and the assessment of the risk contribution of individual sources at each point of the study area. This innovative experimental approach is an effective tool to localize the health risks of spatially disaggregated sources of PTTEs and it may allow for better planning of control strategies and mitigation measures to reduce airborne pollutant concentrations in urban settings polluted by multiple sources.

Based on new structural, petrological and U-Th-Pb geochronological data, a reappraisal of the Variscan tectono-metamorphic history of the Pelvoux Massif (External Crystalline Massif, French Alps) is proposed with the aim to understand the flow pattern and kinematics of the Variscan partially molten crust and the Eastern Variscan Shear Zone. The Pelvoux Massif consists of high-grade metamorphic rocks of middle to lower crust, mostly migmatites, that record a prominent syn-metamorphic deformation event (D2) characterized by a pervasive NE-SW striking, steeply dipping, S2 foliation, and a network of anastomosed NS and NW-SE trending shear zones, the kinematics of which indicates a sinistral transpression. Relics of an early syn-metamorphic event (D1/M1) related to crustal thickening and top-to-the-east nappe stacking are also reported. Both the D1 and D2 features are interpreted as reflecting a NW-SE shortening event, firstly marked by dominant nappe stacking, and secondly overprinted by a sinistral transpression that started at peak metamorphism with the onset of crustal partial melting at ca. 650 °C during the late Visean (ca. 335–330 Ma). Ongoing sinistral D2 transpression in the partially molten middle-lower crust of the Pelvoux involved strain partitioning between C and C’ shear zones and horizontal longitudinal flow in the range 330–300 Ma. Along the anatectic front, vertical shortening and top-to-the-NW shearing (D3) is coeval with D2 and argue for southeastward motion of the partially molten crust. The contemporaneity between NW-SE directed transpressional flow and vertical shortening is supported by our radiometric data of D2 and D3 and attests for strain partitioning between the suprastructure and infrastructure during horizontal crustal flow under transpressive regime. The exhumation of deep-seated rocks during sinistral transpression followed a near isothermal (ca. 700 °C) evolution down to pressure of ca. 0.5 GPa in the period 325–306 Ma. The sinistral transpression recorded in the Pelvoux Massif might corresponds to an antithetic shear zone coeval with the dextral East-Variscan Shear Zone, proposed for this part of the Variscan orogen.

Rare-element granites and pegmatites represent important sources of raw materials for “clean, green and high technologies”, such as lithium and tantalum, for example. However, mechanisms of rare-element granites and pegmatite’s origin are still far from being fully understood. Several rare-element pegmatite fields and a rare-element granite are known in the Variscan realms located in Iberia (Spain and Portugal), enhancing the interest of this area for studying the formation of these extremely fractionated melts. In situ U-Pb dating by LA-SF-ICP-MS of columbite-group minerals from rare-element granites and pegmatites of the Iberian Variscan belt provides new constraints on the generation of rare-element melts. Three events have been recognized: (i) Emplacement of the Argemela rare-element granite, in the Central Iberian Zone (CIZ), with an age of 326 ± 3 Ma; (ii) Emplacement of rare-element pegmatites from the Galicia-Trás-os-Montes Zone (GTOMZ), at an average age of 310 ± 5 Ma; (iii) Emplacement of rare-element pegmatites in the CIZ and in the southern GTOMZ at about 301 ± 3 Ma. These two last events are coeval with the two peaks of ages for the late orogenic magmatism at ca. 308 Ma and 299 Ma, and all dated rare-element pegmatites clearly emplaced during the late-orogenic evolution of the Variscan belt. Contemporaneous fields of rare-element pegmatites are arranged in belts following those formed by similar granitoid suites. Pegmatite fields from both the GTOMZ and the CIZ reveal a southward propagation of ages of emplacement, which matches the observed propagation of deformation, metamorphism and magmatism in the two different geotectonic zones. Existence of three successive rare-element events in the Iberian Massif argues against the involvement of lower crustal HP-HT metamorphism in the generation of rare-element melts. Possible sources of rare-element-enriched melts are more likely located in the middle to upper crust, as are the major components of granitic magmatism. Analyses of U and Pb isotopes from columbite-group minerals are very robust and reproducible, making them good candidates for dating ore deposits related to peraluminous magmatism as well as REE- and Nb-bearing deposits.

The Oisans-Pelvoux massif belongs to the Paleozoic basement of the External western Alps and records high temperature (HT) metamorphism associated with intense migmatization during the syn to post-collisional stages of the Variscan orogeny. Metamorphic assemblages related to the early collision stages have been obliterated making it difficult to constrain the earlier evolution of the unit. In this study, we report the finding of new bodies of high pressure (HP) mafic granulites, which preserve relics of the prograde evolution. Relics of Mn-rich garnet cores containing inclusions of epidote, titanite, chlorite and rare white micas constrain an early prograde stage at 460-550 ?C and 0.4-1.0 GPa. The HP assemblage consists in garnet + clinopyroxene+ quartz + rutile +/- plagioclase +/- amphibole +/- biotite and yield peak-P conditions at 650-730 ?C and 1.5-1.7 GPa. Decompression was associated with heating to HT granulite-facies conditions of 800-870 ?C and 0.6-0.9 GPa, which led to the development of granoblastic polycrystalline mosaics over the resorbing HP assemblage. The U-Pb dating of magmatic zircon cores constrains the emplacement age of the mafic protoliths at 479 +/- 5 Ma. Timing of the prograde to HP evolution is estimated around 345-330 Ma based on rutile U-Pb dating, which is contemporaneous with HP recorded in the other crystalline massifs of the western Alps. Zircon metamorphic rims yields U-Pb dates scattering between 337 and 294 Ma, which are interpreted to record crystallization during decompression and heating to granulite facies. Metamorphic conditions during the early prograde stage precludes scenarios involving subduction of a cold passive margin and are better reconciled with thickening of an orogenic wedge during collision. In contrast with other Variscan HP relics of the external Alps, the HP rocks of the Oisans-Pelvoux massif record heating to HT granulitic conditions during decompression, which may result from longer residence time in the lower crust before exhumation.