Timing of rare-elements (Li-Be-Ta-Sn-Nb) magmatism in the European Variscan belt

International audienceHigh-phosphorus peraluminous rare-elements granites and rare-elements LCT (Lithium, Caesium, Tantalum) pegmatites are the most important sources of raw materials for some critical metals like tantalum (1,2)represent important economic storehouses for industrial minerals like feldspar, quartz, mica or kaolin. They principally emplace in orogenic settings (3). Afast overview of three mainEuropean Variscan districts, i.e. the Moldanubian domain of the Bohemian massif, the French Massif Central (FMC) and the NW Iberia provides a basis for questioning the origin of rare-elements magmatism and the actual classification of rare-elements pegmatites, in particular the LCT pegmatites. Granitic pegmatites are widespread in most of the Bohemian Massifbut LCT pegmatites are most common in the Moldanubian domain. In this area, their emplacements seem mainly controlled by migmatitic domes and shear zones and correspond to two events(4). The older at ~ 333 ± 3 Ma just follow HT-MP event of the end of the Moravo-Moldanubian phase and the younger at ~ 325 ± 4 Ma is contemporaneous with beginning of the Bavarian phase (U-Pb ages on colombite and tantalite). In the FMC, most of the actually known rare-elements magmatic bodies form a province in the North Limousin area, which represents the northwestern part of the FMC.U-Pb dating of columbite-group minerals from Beauvoir, Montebras and Chèdeville rare-elements magmatic bodies leads to emplacement ages at 317 ± 6 Ma, 314 ± 4 Ma and 309 ± 5 Ma respectively. The contemporaneous Marche fault system (5), which crosscuts in a general E-W trend all the northern part of the Limousin, seems to be a key-structure for the rare-elements magmatism of the area

Typology of hard-rock Li-hosted deposits in Europe

International audienceLithium became a strategic metal in the last decade due to its widespread use in electromobility and green technologies. Consequently, demand has increased significantly reviving European interest in lithium mining and leading many countries to assess their own mineral resources/reserves to secure their own supplies. A compilation of European lithium hard-rock occurrences and a systematic assessment of metallogenic processes related to Li-mineralization have been produced. Accordingly, it appears that lithium is well represented through various deposit types related to several orogenies from Precambrian to Miocene ages. Thus, these deposits have been identified as mostly resulting from endogenous processes such as lithium-cesium-tantalum (LCT) pegmatites (e.g. Sepeda in Portugal; Aclare in Ireland; Läntta in Finland), rare-metal granites (RMG; Beauvoir in France; Argemela in Portugal) and greisens (e.g. Tregonning-Godolphin, Meldon in UK; Cinovec in Czech). Local exogenous processes may be related to significant Li-endowments such as jadarite precipitation in the Jadar Basin (Serbia) but are rarely related to economic grade and tonnage of lithium. Moreover, common parameters are identified in the Li endowment processes including: 1) a pre-existing Li-rich source; 2) a lithospheric thickening; and 3) an extensional regime

Precambrian protoliths and Early Paleozoic magmatism in the French Massif Central: U–Pb data and the North Gondwana connection in the west European Variscan belt

International audienceU–Pb geochronological data were collected on zircon by LA-MC-ICP-MS on orthogneiss and paragneiss from the Limousin area in the French Massif Central (FMC), in order to investigate the connection between the west European Variscan belt and the northern margin of Gondwana. Fifteen samples were collected in the four main tectonostratigraphic units of the FMC, namely: the Para-Autochthonous Unit, the Lower Gneiss Unit (LGU), the Upper Gneiss Unit (UGU) and the Thiviers–Payzac Unit. Orthogneiss yield intrusion age between 521 ± 7 and 446 ± 6 Ma. Considering all the results from both magmatic and metasedimentary samples, two peaks at 531 and 473 Ma are recognized. Rifting processes taking place along the North Gondwana margin during the Lower Paleozoic went on until the end of the Ordovician, as suggested by the magmatic event recorded around 450 Ma. Several maximum depositional ages were ascertained in the metasedimentary formations of the FMC, as determined by the youngest detected detrital zircon crystal, ranging from 604 ± 16 Ma for metasediments of the Para-Autochthonous unit, 573 ± 12 Ma for the LGU, 564 ± 9 Ma for the Thiviers–Payzac Unit, and 523 ± 4 Ma in the UGU. Minimum depositional ages are given by magmatic emplacement ages obtained in the crosscutting orthogneiss. There is some evidence for a decrease of this maximum age upwards in the tectonostratigraphy. Detrital zircon in metasedimentary formations and inherited zircon in orthogneiss display a wide spectrum of ages with significant peaks at around 590 Ma and 560 Ma. Archean, Paleoproterozoic and Neoproterozoic detrital zircons suggest a West African craton source of the sedimentation. The large amount of Neoproterozoic and Lower Paleozoic ages obtained in this study suggests that these periods played a significant role in the continental crustal growth history of Western Europe

The South Millevaches Middle Carboniferous crustal melting and its place in the French Variscan Belt

International audienceSeveral episodes of crustal melting are now well identified in the Variscan French Massif Central. Middle Devonian (ca 385-375 Ma) migmatites are recognized in the Upper and Lower Gneiss Units involved in the stack of nappes. Late Carboniferous migmatites (ca 300 Ma) are exposed in the Velay Massif only wilst and Middle Carboniferous migmatites crop out in the Para-Autochthonous Unit and Southern Fold-and-Thrust Belt. In the SW part of the Massif Central, the South Millevaches massif exposes migmatites developed at the expense of ortho- and paragneiss. Theform kilometer-sized septa within the foliated Goulles leucogranitic pluton, which is in turn intruded by the non-foliated Glény two micas granite pluton. Monazite grains extracted from these three rock-types have been dated by the EPMA chemical method. Three samples of migmatite yield a late Visean age (ca 337-328 Ma), the Goulles and Glény granitic plutons yield ages at 324-323 Ma and 324-318 Ma, respectively. These new results enlarge the evidence of a Middle Carboniferous crustal melting imprint that, up to now was only reported in the eastern part of the French Massif Central, in the northern Cévennes and in the Montagne Noire axial zone. At the scale of the French Variscan massifs, the Visean crustal melting event is conspicuously developed since it is recognized from the Massif Armoricain (Vendée and south coast of Brittany) to the Central Vosges. This episode is synchronous with the huge thermal event responsible for the “Tuffs anthracifères” magmatism of the northern Massif Central and Vosges, and took place immediately after the last thickening phase recorded both in Montagne Noire and Ardennes, this is on the southern and northern outer zones of the Variscan Belt, respectively. However, the geodynamic significance of this major event is not fully understood yet

The Zanhuang Massif, the second and eastern suture zone of the Paleoproterozoic Trans-North China Orogen

International audienceThis paper presents a reappraisal of the tectonic evolution of the Zanhuang Massif that lies at the eastern margin of the Trans-North China Orogen, a continent–continent collision belt that marked the amalgamation of the North China Craton in Late Paleoproterozoic. Detailed field work with focus on geometries of structures and kinematics was performed. This was completed with LA-ICP-MS U–Pb analyses on zircon, EPMA U–Th/Pb dating on monazite and 40Ar/39Ar dating on amphibole. These studies led us to propose a new three-fold litho-tectonic subdivision of the massif: The Western Zanhuang Domain (WZD) made of TTG, migmatite and pink anatectic granite is correlated to the Fuping Massif that crops out to the north-west. Both areas represent a continental block, called the Fuping Block, which acquired most of its architecture around 2100 Ma ago. The Eastern Zanhuang Domain (EZD) made of TTG and migmatite represents the western edge of an Eastern Neoarchean Block. In between, the Central Zanhuang Domain (CZD) is a NE–SW trending stack of supracrustal, gneiss and mafic magmatic rocks thrust sheets displaced toward the ESE upon the Eastern Block. The lithological features suggest that the CZD represents the remnant of an oceanic basin, called the Taihang Ocean that closed during the amalgamation of the Eastern Block and the Fuping Block around 1880–1850 Ma. In agreement with recent work done along the western margin of the belt, in the Lüliang Massif, this study documents the amalgamation of the North China Craton in response to the closure of two oceanic basins, namely the Lüliang Ocean and the Taihang Ocean. West-dipping subductions and collisions involving three distinct continental blocks, called the Western, the Fuping and the Eastern Blocks, took place around 1880–1850 Ma

Variscan orogeny in Corsica: new structural and geochronological insights, and its place in the Variscan geodynamic framework

International audienceIn Western Corsica, remnants of pre-batholitic lithological and metamorphic assemblages are preserved as km-scale septa enclosed within Lower Carboniferous to Early Permian plutons. Two groups of septa were recognized: (1) the Argentella and Agriates-Tenda fragments correspond to Neoproterozoic rocks deformed and metamorphosed during the Cadomian-Panafrican orogeny, and (2) the Zicavo, Porto-Vecchio, Solenzara-Fautea, Belgodère, Topiti, and Vignola fragments consist of Variscan metamorphic rocks. The lithological content and the main ductile deformation events for each septum are presented. In the Zicavo, Porto-Vecchio, and Topiti septa, a top-to-the-SW ductile shearing (D1 event) coeval with an amphibolite facies metamorphism is responsible for crustal thickening at ca 360 Ma. This main event was preceded by eclogite and granulite facies metamorphic events preserved as restites within migmatites dated at ca 345-330 Ma. A top-to-the-SE ductile shearing (D2 event) coeval with the crustal melting accommodated the exhumation of the D1 event. In contrast, the Belgodère segment is peculiar as it exhibits a top-to-the-E vergence, although retrogressed high-pressure rocks are also recognized. The pre-Permian fragments are arranged in four NW-SE-striking stripes that define a SW-NE zoning with (1) a Western domain in Topiti, Vignola, Zicavo, Porto-Vecchio, and Solenzara-Fautea; (2) a Neoproterozoic basement with its unconformable Early Paleozoic sedimentary cover in Argentella; (3) an Eastern metamorphic domain in Belgodère; (4) another Neoproterozoic basement with its Upper Paleozoic sedimentary cover in Agriates-Tenda. The Argentella basement is separated from the Western and Eastern domains by two sutures: S1 and S2. The Variscan Corsica represents the Eastern part of the Sardinia-Corsica-Maures segment. The comparison of this segment with other Variscan domains allows us to propose some possible correlations. We argue that the Western domain, Argentella, Belgodère, and Agriates-Tenda domains can be compared with the Southern Variscan belt exposed in French Massif Central-Southern Massif Armoricain, Armorica microblock, Léon block, respectively

Petrogenesis and tectonic-magmatic context of emplacement of lepidolite and petalite pegmatites from the Fregeneda-Almendra field (Variscan Central Iberian Zone): clues from Nb-Ta-Sn oxide U-Pb geochronology and mineral geochemistry

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 H2O/F activity ratio. For the Fregeneda-Almendra Li-mica-rich pegmatites, this could be explained by a decrease of melt H2O solubility due to a relatively low pressure of emplacement

Modalités du recyclage de la croûte continentale dans l'orogène varisque par traçage in situ des zircons hérités<br />(mesures U-Pb/LA-MC-ICPMS).

This work is essentially directed to the investigation of zircons populations and protolith ages of variscan formations performing in situ U-Pb dating using a Laser-MC-ICPMS system. Both metagranites, metasedimentary and late-orogenic formations from French Massif Central and South Armorican area had been investigated. This study highlighted the zircons inheritance, with ages Both obtained ages peaks and Mesoproterozoic (1.7-1.1 Ga) age lacking are in good agreement with the Gondwanan affinity of the studied formations. Maximum deposition ages are proposed for metasedimentary sequences of each main units of the French Massif Central. Moreover, these ages are decreasing following the described nappes stacking. Emplacement ages of ca. 300 Ma are exposed for the Sillon Houiller (French Massif Central) and Morbihan golfe granitoids. Older monazite ages obtained from previous studies are interpreted as inheritance. The presence of inherited Ordovician and Neoproterozoic zircons is an argument for the participation of fertile metagranite and metasedimentary source of these ages. Moreover, Sarzeau leucogranite shows a Silurian protolith. Continental crust recycling during variscan orogeny is characterized by a polycyclic evolution. The first step is constituted by the erosion of Neoproterozoic formations from the West African craton. These metasediments are then reworked during Cambro-ordovician and syn-orogenic magmatism.Ce travail de thèse est essentiellement consacré à l'investigation des populations de zircons et de l'âge des protolithes des formations varisques grâce à une datation U-Pb in situ systématique par le couplage Laser/MC-ICPMS, dans des formations orthodérivées, métasédimentaires et magmatiques tardi-orogéniques du Massif Central français et du domaine Sud armoricain. Cette étude a montré que les populations de zircons sont largement héritées, s'étalant de l'Archéen au Paléozoïque inférieur. L'ensemble des pics d'âges obtenus, ainsi que l'absence d'âge mésoprotérozoïque (1.7-1.1 Ga) accréditent l'affinité gondwanienne de ces formations. Des âges maximum de dépôt sont proposés pour les métasédiments des principales unités définies dans le Massif Central. Ces âges maximum de dépôt sont décroissants suivant l'empilement lithotectonique reconnu. En marge de ce travail, nous avons pu déterminer que les âges de mise en place des granitoïdes du Sillon Houiller (Massif Central) et du golfe du Morbihan se situent aux alentours de 300 Ma. Des âges plus anciens, principalement obtenus sur monazite lors d'études antérieures, ont été réinterprétés comme des âges hérités. L'abondance de zircons hérités d'âge ordovicien et néoprotérozoïque met en évidence la large contribution des métagranites et métasédiments fertiles de ces périodes dans la source des granites tardi-orogéniques varisques. Le granite de Sarzeau expose de plus les traces d'un protolithe silurien. Le recyclage de la croûte continentale est caractérisé par une évolution polycyclique au cours de l'orogénèse varisque, avec tout d'abord l'érosion de formations du craton africain, majoritairement d'âge néoprotérozoïque, puis la superposition des événements magmatiques cambro-ordoviciens et syn-orogéniques varisques

Rare-elements (Li-Be-Ta-Sn-Nb) magmatism in the European Variscan belt, a review

International audienceThe European Variscan belt, which is characterized by a huge amount of granitic intrusions, displays several significant rare-elements pegmatites and/or granites fields which could represent an important source of raw materials. An overview of these occurrences gave the possibility to discuss the processes at their origin and their common classification. Indeed, the spatial and temporal distributions of rare-element pegmatites in the European Variscan belt suggest that rare-elements magmatism would rather be related to local specific conditions (in particular sources, tectonic and thermal regime) than lower crustal processes like rare-elements enrichment subsequent to their release during granulite metamorphism. Actual classification of rare-elements pegmatites, based on their geochemistry and mineral composition, does notallow to discern pegmatites formed in different tectonic regimes and which exhibit various internal structures (e.g. the pegmatites of the lepidolite-subclass from Western Iberia and from the Moldanubian domain of the Bohemian Massif for example)

La recherche de nouvelles ressources en lithium, niobium, tantale en Europe

International audienceLe développement de nouvelles technologies et des pays émergents exerce une forte pression sur le marché d'un certain nombre d'éléments chimiques. Par exemple, la demande actuelle pour le lithium (Li) est en constante augmentation du fait du développement des batteries Li-ion, particulièrement pour les véhicules électriques ou hybrides ; le tantale (Ta) est très utilisé dans l'industrie électronique, notamment les systèmes de télécommunica-tions et informatiques, et le niobium (Nb) est très utilisé dans l'industrie aéronautique. Ces trois métaux sont principalement produits à l'échelle mondiale à partir de gisements de types différents. Le lithium provient pour environ 60 % des salars (Amérique du Sud principalement) et pour 40 % des pegmatites, le tantale est issu des pegmatites géantes à éléments rares de la famille Li-Cs-Ta (Australie, Canada, Brésil, Afrique et Chine) et le niobium provient de carbonatites (Brésil, Canada). Ces types de gisements sont peu communs ou absents en Europe, sauf les pegmatites et granites à éléments rares (PGER à Li-Cs-Ta, Scandinavie et chaîne varisque). Les PGER sont presque toutes exploitées pour l'industrie de la céramique (exemples : Beauvoir, Monts d'Ambazac en France, champs de pegmatites du Portugal, d'Espagne…). La valorisation des sous-produits de ces exploitations (Li, Cs, Ta, Nb, Sn…) pourrait permettre une certaine indépendance de l'Union européenne. De plus, même si ce type de ressource n'est pas aussi rentable que celle des salars, leur exploitation peut être utile du fait de la réactivité de production possible (par exemple, le Li 2 CO 3 peut être produit en 24 heures à partir d'une pegmatite contre un an à partir des salars) et des autres éléments valorisables en sous-produits