The meillers Autunian hydrothermal chalcedony : first evidence of a similar to 295 Ma auriferous epithermal sinter in the Frech Massif Central.

The Meillers "quartzite" deposit represents a major hydrothermal siliceous sinter, some 50 m thick and covering an area of some 30 ha within Autunian sandstone of the northern Massif Central. This siliceous sinter comprises three facies of microcrystalline quartz: (i) a dark-colored facies (black chalcedony), locally rich in pyrite, at the base, (ii) a gray-brown fairly massive facies (gray-brown chalcedony) in the middle; and (iii) a white, finely banded, facies (white chalcedony) at the top. Orientation measurements of the banding have revealed the paleo-flow channels of the silica-saturated fluids. The geochemistry of the deposits shows modest metal values, in particular for gold (average of 0.58 g/t Au, giving a gold metal content of some 20 t). The hydrothermal event is stratigraphically well correlated with the basal Autunian (around 295 Ma); an age that has been confirmed through radiochronological determination (300±21 Ma; SHRIMP on hydrothermal zircon). The lead isotopic composition is not very radiogenic (206Pb/204Pb=18.20) and is similar to that of the Late Hercynian gold lodes in the Massif Central. The mineralized sinter appears to have derived from a geyser-type hot springs. This hydrothermal activity coincides with the auriferous metallogenic peak that occurred in the Hercynides around 300 Ma. This is the first known epithermal-type surface manifestation described for the Hercynian gold event

LA-MC-ICPMS and SHRIMP U-Pb dating of complex zircons from Quarternary tephras from the French Massif Central: Magma residence time and geochemical implications

Analyses of zircon grains from the Queureuilh Quaternary tephras (pumice) provide new information about their pre-eruptive history. U-Pb dating was performed in situ using two methods: SHRIMP and LA-MC-ICPMS equipped with a multi-ion counting system. Both methods provided reliable 207Pb/206Pb and 206Pb/238U ratios as well as U and Th abundances required for U-Pb Concordia intercept age determination, after initial 230Th disequilibrium correction. The new LA-MC-ICPMS method was validated by dating a reference zircon (61.308B) and zircons from a phonolitic lava dated independently with the two techniques. A time resolution of about 20 kyr for 1 Ma zircon crystals was achieved for both methods. The clear euhedral zircon population from Queureuilh tephras is quite complex from several points of view: (1) some grains are reddish or yellowish while others are colorless; (2) the U and Th composition changes by more than an order of magnitude and Th/U is generally high (∼1-2); (3) there are three discrete ages recorded at 2.35 ± 0.04, 1.017 ± 0.008 and 0.640 ± 0.010 Ma. From the previously determined 40Ar/39Ar age at 0.571 ± 0.060 Ma [Duffell H. (1999) Contribution géochronologique à la stratigraphie volcanique du Massif des Monts Dore par la méthode 40Ar/39Ar. D.E.A. Univ. Clermont-Ferrand, 56 p.], the discontinuous zircon age populations, the color of the grains and their composition, we favor the following model as explanation: The oldest, less numerous group of reddish zircons represents xenocrystic grains resulting from assimilation of the local material during magma ascent. A primitive magma chamber, perhaps deep in crustal level, was formed at 1.0 Ma. The related magma, previously characterized by high Th/U ratio (2.2 ± 1.1), underwent rejuvenation during ascent to a new chamber at shallow depth and/or during injection of more mafic magmas. During this stage, at 0.64 Ma, the colorless zircon grains of lower Th/U ratio (1.3 ± 0.5) crystallized. This last stage defined the magma residence time of 70 kyr prior to eruption dated by the 40Ar/39Ar method. However, if the primitive magma is considered, the magma residence time as a whole from this first stage reached 446 kyr. In the light of the complex history of such magmas, which commonly involves recycling of zircon grains that precipitated tens to hundreds of kyr earlier than eruptions, the use of Zr concentration in geochemical modeling of whole rock compositional data can be problematic

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

Middle Carboniferous crustal melting in the Variscan Belt: New insights from U-Th-Pbtot. monazite and U-Pb zircon ages of the Montagne Noire Axial Zone (southern French Massif Central)

International audienceIn France, the Devonian-Carboniferous Variscan orogeny developed at the expense of continental crust belonging to the northern margin of Gondwana. A Visean-Serpukhovian crustal melting has been recently documented in several massifs. However, in the Montagne Noire of the Variscan French Massif Central, which is the largest area involved in this partial melting episode, the age of migmatization was not clearly settled. Eleven U-Th-Pbtot. ages on monazite and three U-Pb ages on associated zircon are reported from migmatites (La Salvetat, Ourtigas), anatectic granitoids (Laouzas, Montalet) and post-migmatitic granites (Anglès, Vialais, Soulié) from the Montagne Noire Axial Zone are presented here for the first time. Migmatization and emplacement of anatectic granitoids took place around 333-326Ma (Visean) and late granitoids emplaced around 325-318Ma (Serpukhovian). Inherited zircons and monazite date the orthogneiss source rock of the Late Visean melts between 560Ma and 480Ma. In migmatites and anatectic granites, inherited crystals dominate the zircon populations. The migmatitization is the middle crust expression of a pervasive Visean crustal melting event also represented by the "Tufs anthracifères" volcanism in the northern Massif Central. This crustal melting is widespread in the French Variscan belt, though it is restricted to the upper plate of the collision belt. A mantle input appears as a likely mechanism to release the heat necessary to trigger the melting of the Variscan middle crust at a continental scale

Géochronologie revisitée du dôme du Léon (Massif armoricain, France)

National audienceDe nouvelles datations réalisées sur les principaux ensembles granitiques et métamorphiques du Léon permettent de préciser l'histoire de ce domaine de la chaîne hercynienne. Dans les domaines métamorphiques, les trois âges obtenus sur des zircons des orthogneiss granodioritiques de Brest s'échelonnent de 529 à 504 Ma, confirmant le caractère polyphasé de ce complexe orthodérivé cambrien. Dans l'unité autochtone, l'âge dévonien inférieur (385 - 391 Ma) des orthogneiss de Plounévez-Lochrist est confirmé par une nouvelle datation U-Pb sur zircon réalisée dans l'ensemble oriental. Dans les unités allochtones du Nord, les monazites du complexe migmatitique de Plouguerneau livrent un âge de 331 ± 7 Ma, interprété comme celui de la migmatitisation et conforme à celui proposé récemment par Faure et al. (335 - 327 Ma). L'âge des événements magmatiques hercyniens a également été précisé. Le granite porphyroïde de Kersaint se confirme comme le plus ancien du Léon avec un âge sur monazite de 331 ± 4 Ma. Cependant, les nouvelles datations réalisées montrent que les granites léonards sont essentiellement stéphaniens. Le granite porphyroïde rose de Guissény, satellite du granite de l'Aber-Ildut, est ainsi daté par U-Pb sur zircons à 301,4 ± 4,3 Ma. La monzodiorite de Plounévez-Lochrist, satellite mafique du grand massif de Brignogan-Plouescat, donne un âge de 296 ± 3 Ma, cohérent avec celui de 292 ± 15 Ma obtenu par Rb/Sr sur le monzogranite de Brignogan-Plouescat. Le leucogranite à tourmaline de Sainte-Catherine, considéré comme tardif par son allure en dykes sécants, est en fait également stéphanien ; le faciès le plus répandu est daté à 302,7 ± 0,3 Ma, le faciès sécant plus tardif riche en tourmaline à 301,5 ± 0,3 Ma (Ar-Ar sur muscovites). L'âge obtenu sur la mylonite formée aux dépens du leucogranite de Kernilis (292,5 ± 2,7 Ma par Ar-Ar sur muscovite) correspond vraisemblablement au jeu de l'accident mylonitique de Porspoder. Les granitisations du Léon débutent donc immédiatement après le pic métamorphique hercynien, comme dans le Massif central. La principale période de granitisation du Léon s'effectue cependant au Stéphanien, c'est-à-dire de la fin de l'extension syn-collisionnelle (300-310 Ma) à la fin de l'extension post-collisionnelle (post-300 Ma)

Electron microprobe monazite geochronology : a tool for evaluating magmatic ages with examples from Variscan Massif Central migmatites and granotoids, France.

U–Th–Pb dating of monazite with the electron probe microanalyser (EPMA) is increasingly documented as a reliable geochronological method offering high spatial resolution. This method has been applied on monazite from the Cévennes migmatites and granitoids from the southeast of the French Massif Central. Measurements were performed on separated grains after systematic back-scattered electron (BSE) imaging. Monazites from migmatites record two main ages: (i) a protolith age of about 550–543 Ma obtained on inherited cores, and (ii) a migmatization event between 329 ± 5 and 323 ± 3 Ma recorded by monazite rims and all other monogenetic grains. Monazite from the peraluminous Rocles pluton yields a 318 ± 3 Ma age. Finally, three granite dykes are dated at 333 ± 6, 318 ± 5 and 311 ± 5 Ma; the older dyke is the most deformed of them and is interpreted as linked to the migmatization event; the two other dykes are geochronologically, petrologically and structurally coeval with the Rocles pluton. The data constrain the timing of crustal melting following Variscan thickening in the northern Cévennes area. Migmatization of Ordovician protoliths took place at 329–323 Ma and was shortly followed by intrusion of leucogranite at 318–311 Ma. The study shows that EPMA dating of monazite can be successfully used to resolve a close succession of regional melting events

Structural, AMS and geochronological study of a laccolith emplaced during Late Variscan orogenic extension: the Rocles pluton (SE French Massif Central)

International audienceIn the southern French Massif Central, the Rocles leucogranite of Variscan age consists of three petrographic facies; textural analysis shows that they experienced the same subsolidus deformation. New chemical U-Th-Pb dating on monazite yielded 324 ± 4 Ma and 325 ± 5 Ma ages for muscovite-rich and biotite-rich facies respectively. AMS-study results agree with petrostructural observations. The magnetic planar and linear fabrics, which correspond to the preferred orientation of biotite and muscovite, are consistent with the foliation and lineation defined by the preferred mineral orientation. This fabric developed during pluton emplacement. The accordance of this granite foliation with that observed in the host rock, suggests that the Rocles pluton is a laccolith, but its present geometry resulted from post-emplacement southward tilting due to the uplift of the Late Carboniferous Velay dome. Restoration of the primary geometry of the pluton and its country-rocks to a flat-lying attitude places the granite lineation close to the trend measured in other plutons of the area. This restoration further supports the interpretation of the Rocles laccolith as a pluton emplaced along a tectonic contact reactivated during the late-orogenic collapse of the Variscan Belt

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

A restored section of the “southern Variscan realm” across the Corsica–Sardinia microcontinent

International audienceA complete section of the southern realm of the Variscan orogenic belt can be restored in the Corsica–Sardinia segment. Northern Corsica exposes a nonmetamorphosed Palaeozoic succession lying on Panafrican mica schist related to a microcontinent (most likely Armorica or from a microcontinent from the Hun superterrane) that had drifted away directly from Gondwana. These formations are thrust over the Variscan Internal Zone composed mainly of anatectic high-grade Palaeozoic formations that crop out from central Corsica to northern Sardinia; the metamorphic peak of the eclogite remnants has been dated at c. 420 Ma. The Variscan Internal Zone interpreted here as a collision zone, and also the Eovariscan suture, was intruded in Corsica by Mg–K granite from 345 to 335 Ma. The thrust of this Internal Zone onto the stack of parautochthonous nappes in central Sardinia is cross-cut by the Posada Asinara dextral shear zone. To the south, parautochthonous nappes overthrust the North-Gondwana margin which displays a possible Panafrican basement topped by an Iglesiente–Sulcis nonmetamorphic/anchimetamorphic Palaeozoic succession

Datation U–Pb des deux faciès du granite de Soultz (Fossé rhénan, France)

International audiencePrésenté par Zdenek Johan Résumé Les sondages profonds réalisés dans le socle du fossé rhénan à Soultz-sous-Forêts ont mis en évidence un monzogranite très largement représenté. Le sondage GPK-2 atteignant 5090 m montre, en outre, un faciès plus leucocrate à grain fin et deux micas, présent localement à partir de 4860 m et de manière continue entre 5047 et 5090 m de profondeur. Grâce aux éléments en traces, et plus particulièrement grâce aux terres rares, on montre que le faciès leucocrate à grain fin serait une expression différenciée du magma potassique ayant conduit au pluton monzogranitique. Ce modèle est en accord avec les données isotopiques Sr-Nd, même si une légère contribution du socle préexistant doit aussi être considérée. L'utilisation de la méthode U-Pb, après dissolution, sur zircons du monzogranite (EPS-1) a permis d'en préciser l'âge de mise en place à 334,0 + 3,8/−3,5 Ma (2 σ). Les datations ponctuelles (SHRIMP II) sur les zircons du faciès à grain fin ont montré que cette mise en place se fait dans un socle très hétérogène en âge : depuis le Protérozoïque inférieur jusqu'au Silurien. L'âge estimé de cristallisation de ce dernier granite est de 327 ± 7 Ma (2 σ), c'est-à-dire légèrement postérieur, voire sub-contemporain, à la mise en place du faciès monzogranitique commun, en accord avec les contraintes structurales