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![-Garnet zonations and mineral chemical data. a, b, c, d, e, f, g: garnet zonation profiles (c = core, r = rim) in almandine (Alm-50%), pyrope (Prp), grossular (Grs) and spessartine (Sps) components (in %). Data in parts by Schulz et al. [2001], and further completions. Trace element zonations of Y and sum of HREE as analysed by LA-ICP-MS. h, i, k, l, m: Garnet compositions in grossular-pyropespessartine (Grs-Prp-Sps) coordinates; c = core; r = rim; rr = retrograded rim. Arrows indicate coreto-rim zonation trends compiled from several single garnet profiles in each sample. Numbers are representative garnet analyses used for thermobarometry, see fig. 3. UGU-Upper Gneiss Unit; LGU-Lower Gneiss Unit; PAU-Parautochthonous Unit (micaschists). -Zonation des grenats et données chimiques des minéraux.. a, b, c, d, e, f, g. Profil de zonation des grenats (c = coeur, r = bordure) dans les composantes almandin (Alm-50 %), pyrope (Prp), grossulaire (Grs) et spessartine (Sps), (en %). Données en partie dans Schulz et al. [2001], et données complémentaires. Les zonations des élément en trace Y and total des HREE analysées par LA-ICP-MS. h, i, k, l, m. Compositions du grenat en grossulairepyrope-spessartine (Grs-Prp-Sps) ; c = coeur ; r = bordure ; rr = bordure retrograde. Les flèches indiquent les zonations du coeur vers les bordures compilées à partir de plusieurs profils de grenats pour chaque échantillon. Les nombres sont représentatifs des analyses de grenat utilisées pour la thermobarométrie (cf. fig. 3). UGU-Unité supérieure des Gneiss ; LGU-Unite inférieure des Gneiss ; PAU-Unité para-autochthone (micaschistes).](profile/Bernhard-Schulz-7/publication/250085818/figure/fig3/AS:669626480480267@1536662802524/Garnet-zonations-and-mineral-chemical-data-a-b-c-d-e-f-g-garnet-zonation.png)
-Garnet zonations and mineral chemical data. a, b, c, d, e, f, g: garnet zonation profiles (c = core, r = rim) in almandine (Alm-50%), pyrope (Prp), grossular (Grs) and spessartine (Sps) components (in %). Data in parts by Schulz et al. [2001], and further completions. Trace element zonations of Y and sum of HREE as analysed by LA-ICP-MS. h, i, k, l, m: Garnet compositions in grossular-pyropespessartine (Grs-Prp-Sps) coordinates; c = core; r = rim; rr = retrograded rim. Arrows indicate coreto-rim zonation trends compiled from several single garnet profiles in each sample. Numbers are representative garnet analyses used for thermobarometry, see fig. 3. UGU-Upper Gneiss Unit; LGU-Lower Gneiss Unit; PAU-Parautochthonous Unit (micaschists). -Zonation des grenats et données chimiques des minéraux.. a, b, c, d, e, f, g. Profil de zonation des grenats (c = coeur, r = bordure) dans les composantes almandin (Alm-50 %), pyrope (Prp), grossulaire (Grs) et spessartine (Sps), (en %). Données en partie dans Schulz et al. [2001], et données complémentaires. Les zonations des élément en trace Y and total des HREE analysées par LA-ICP-MS. h, i, k, l, m. Compositions du grenat en grossulairepyrope-spessartine (Grs-Prp-Sps) ; c = coeur ; r = bordure ; rr = bordure retrograde. Les flèches indiquent les zonations du coeur vers les bordures compilées à partir de plusieurs profils de grenats pour chaque échantillon. Les nombres sont représentatifs des analyses de grenat utilisées pour la thermobarométrie (cf. fig. 3). UGU-Unité supérieure des Gneiss ; LGU-Unite inférieure des Gneiss ; PAU-Unité para-autochthone (micaschistes).
Source publication
In the La Sioule region (northern French Massif Central), a Variscan inverted metamorphic sequence is made up by high-grade
Upper Gneiss (UGU) and Lower Gneiss Units (LGU) which overlie amphibolite-facies micaschists of a Parautochthonous Unit (PAU).
Growth-zoned garnets crystallized in gneisses and micaschists and display different Mg-Fe-Mn-Ca evo...
Contexts in source publication
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... analyses, enclosing detailed garnet zonation traverses. Single traverses may not have passed the entire garnet core region; porphyroblasts may display zonation gaps, and some garnets may show only a part of the complete garnet chemical evolution in a sample. These complications are not necessarily evident from the zonation profiles alone ( fig. 2a-g). Therefore, gar- net compositions were checked in grossular -pyrope - spessartine coordinates ( fig. 2h-m). As the Mn-component is controlled by Rayleigh fractionation during crystalliza- tion [Hollister, 1966], this allows recognizing the relative temporal chemical growth evolution when garnet is the main Mn fractionating phase. For ...
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... garnet core region; porphyroblasts may display zonation gaps, and some garnets may show only a part of the complete garnet chemical evolution in a sample. These complications are not necessarily evident from the zonation profiles alone ( fig. 2a-g). Therefore, gar- net compositions were checked in grossular -pyrope - spessartine coordinates ( fig. 2h-m). As the Mn-component is controlled by Rayleigh fractionation during crystalliza- tion [Hollister, 1966], this allows recognizing the relative temporal chemical growth evolution when garnet is the main Mn fractionating phase. For each sample, a characte- ristic garnet mineral chemical evolution trend can be deri- ved from a compilation ...
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... chemical growth evolution when garnet is the main Mn fractionating phase. For each sample, a characte- ristic garnet mineral chemical evolution trend can be deri- ved from a compilation of the single core-rim zonation profiles. Such a garnet chemical evolution trend can be defi- ned by selected analyses, as labelled in the ternary diagrams ( fig. 2h-m) and can be used for thermobarometry (see be- ...
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... of kyanite, plagioclase, K-feldspar, biotite, quartz and opaques. The inclusion trails in garnet signalize a top-to-WNW sense of shear. The assemblage during gar- net crystallization is garnet-biotite-plagioclase-K-feldspar- kyanite-quartz. Garnets are strongly zoned with high Mn and Ca in the large cores and decreasing Mn toward the rims ( fig. 2a). Broad rims with decreasing Mg and increas- ing Ca and Mn can occur. In sample Dou (Doussat, R 1494 375, H 5118 925), fibrolitic sillimanite, prismatic kyanite and biotite underline the foliation S 2 . Biotite, kyanite, quartz, plagioclase and opaques are enclosed in the garnets. Crystallization of fibrolitic sillimanite posterior to ...
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... samples PdM (Pont de Menat, R 1494 550, H 5105 350), garnets display complex zonations which can be subdivided into two trends ( fig. 2c, d, k). In the large cores and inner rims of the porphyroblasts, an increase of Ca is accompa- nied by decrease of Mn and Mg ( fig. 2c). This is matched by decreasing Ca in the enclosed plagioclase. Toward the outer rim and in other large porphyroblasts, decreasing Ca and Mn are accompanied by increase of Mg ( fig. 2d). Both trends superpose ...
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... samples PdM (Pont de Menat, R 1494 550, H 5105 350), garnets display complex zonations which can be subdivided into two trends ( fig. 2c, d, k). In the large cores and inner rims of the porphyroblasts, an increase of Ca is accompa- nied by decrease of Mn and Mg ( fig. 2c). This is matched by decreasing Ca in the enclosed plagioclase. Toward the outer rim and in other large porphyroblasts, decreasing Ca and Mn are accompanied by increase of Mg ( fig. 2d). Both trends superpose in Grs-Prp-Sps-coordinates ( fig. 2k), but can be clearly distinguished by the growth direction of the garnet porphyroblasts. ...
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... subdivided into two trends ( fig. 2c, d, k). In the large cores and inner rims of the porphyroblasts, an increase of Ca is accompa- nied by decrease of Mn and Mg ( fig. 2c). This is matched by decreasing Ca in the enclosed plagioclase. Toward the outer rim and in other large porphyroblasts, decreasing Ca and Mn are accompanied by increase of Mg ( fig. 2d). Both trends superpose in Grs-Prp-Sps-coordinates ( fig. 2k), but can be clearly distinguished by the growth direction of the garnet porphyroblasts. The two types of garnets display also different trace element zonations: The first trend is characterized by increasing Y and HREE toward the garnet inner rim. The second trend displays ...
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... and inner rims of the porphyroblasts, an increase of Ca is accompa- nied by decrease of Mn and Mg ( fig. 2c). This is matched by decreasing Ca in the enclosed plagioclase. Toward the outer rim and in other large porphyroblasts, decreasing Ca and Mn are accompanied by increase of Mg ( fig. 2d). Both trends superpose in Grs-Prp-Sps-coordinates ( fig. 2k), but can be clearly distinguished by the growth direction of the garnet porphyroblasts. The two types of garnets display also different trace element zonations: The first trend is characterized by increasing Y and HREE toward the garnet inner rim. The second trend displays decreasing Y and HREE toward the outer rims ( fig. 2c, ...
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... ( fig. 2k), but can be clearly distinguished by the growth direction of the garnet porphyroblasts. The two types of garnets display also different trace element zonations: The first trend is characterized by increasing Y and HREE toward the garnet inner rim. The second trend displays decreasing Y and HREE toward the outer rims ( fig. 2c, ...
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... layers curves around microlithons with garnet and staurolite. In garnet and staurolite, a S-type internal foliation S 1 i by mica, plagioclase, quartz and opaques is mostly discordant and not lining up with the external S 2 . The S-shaped S 1 i trails signal a noncoaxial deformation component with a top-to- WNW shear sense [Schulz et al., 2001] fig. 2g, m). Flat trace element profiles with decreas- ing Y and HREE toward the rims are ...
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... figure 1 for sample locations. Numbers on P-T data refer to garnet analyses in figure 2. Re-calculation of data and mineral pairs [repor- ted in Schulz et al., 2001] and from new mineral-chemical analyses; see text for applied thermo-barometers. ...
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... analyses; see text for applied thermo-barometers. Bold crosses mark results and uncer- tainty of ± 50 o C/1.0 kbar for a combination of a geothermometer and a geobarometer to defined mineral pairs; greytone of bold crosses refers to different samples. Arrows are P-T paths according to the core-to-rim zona- tion trends of garnets in fig. 2h-m. Thermobarometric data from the SLoup and Dou samples from the UGU reveal a clockwise P-T path, characterized by increasing pressure and temperature from 550 o C/9 kbar to 760 o C/11.5 kbars, which is followed by nearly isothermal decompression to 750 o C/6 kbar and subsequent cooling/de- compression to 650 o C/5 kbar ( fig. 3a). ...
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Citations
... Such P-T-time paths from various lithotectonic units are necessary for reconstruction and modelling of subduction-accretion environment followed by continental collision. Our study from the Saxothuringian Zone furthermore appends to comparable data and monazite age distribution pattern from Odenwald and Spessart, Armorican Massif, French Massif Central, and basement of the Alps (Schulz 2009;2014;2021b;Will et al. 2017). In this frame, the data contributes to a more detailed view on the multistage temporal and spatial evolution of an orogen, as exemplified by compiling studies and models of the A C C E P T E D M A N U S C R I P T and rarely in the SC. ...
In the Saxothuringian Zone an unique assemblage of high to ultra-high pressure and ultra-high temperature metamorphic units is associated to medium-to-low pressure and temperature rocks. The units were studied in a campaign with garnet and monazite petrochronology of gneisses, micaschists and phyllites, and monazite dating in granites. P-T path segments of garnet crystallisation were reconstructed by geothermobarometry and interpreted in terms of monazite stability field, EPMA-Th-U-Pb monazite ages, and garnet Y+HREE zonations. One can recognise (1) Cambrian plutonism (512-503 Ma) with contact metamorphism in the Münchberg Massif. Subordinate monazite populations may indicate a (2) widespread but weak Silurian (444-418 Ma) thermal event. A (3) Devonian (389-360 Ma) high pressure metamorphism prevails in the Münchberg and Frankenberg Massifs. In the ultra-high pressure and high pressure units of the Erzgebirge the predominant (4) Carboniferous (336-327 Ma) monazites crystallised at the decompression paths. In the Saxonian Granulite Massif, prograde-retrograde P-T paths of cordierite-garnet gneisses can be related to monazite ages from 339 to 317 Ma. A (5) local hydrothermal overprint at 313-302 Ma coincides partly with post-tectonic (345-307 Ma) granite intrusions. Such diverse monazite age pattern and P-T-time paths characterise the tectono-metamorphic evolution of each crustal segment involved in the Variscan Orogeny.
Supplementary material at https://doi.org/10.6084/m9.figshare.c.6793959
... This can be interpreted as an age of metamorphic monazite crystallisation. As has been exemplified for Moldanubian metapsammopelites in NE Bavaria at Weiding (Schulz 2010), and in other parts of the internal zones of the Variscan Orogen (Schulz 2009;, 2014Rode et al. 2012), monazite crystallisation can be expected when the P-T path of metamorphism passes the lower temperature limit of the monazite stability field at increasing temperatures. The lower temperature limit of the monazite stability field is dependent on the bulk rock Al and Ca contents and is shifted to higher temperatures with increasing Ca (Janots et al. 2007;Spear 2010;Spear & Pyle 2010). ...
Monazite in lithoclasts of suevite impact breccia in the Nördlinger Ries (Bavaria, Germany) and its Th-U-Pb dating by electron probe microanalysis
Bernhard Schulz, Jan-Michael Lange, Joachim Krause, Dana Czygan
Abstract
In the Lehenberg (Lehberg or Limberg North) quarry in the NW part of the Megablock Zone of the Nördlinger Ries impact crater, granite and micaschist lithoclasts occur in a polymict suevite impact breccia. The lithoclasts display the petrographic characteristics of the shock metamorphism scale, as cavities filled with diaplectic glass, decorated planar elements in quartz grains, and severly kinked mica. In petrographic thin sections, monazite grains were detected by scanning electron microscope based automated mineralogy methods of spectral mapping. In backscattered electron imaging (BSE), monazite revealed the typical crystal shapes, and internal Th zoning and distribution pattern as known from igneous and metamorphic crystallization. Intragrain signs of shock metamorphism in a minority of monazite grains are strictly straight and parallel crack pattern resembling lamellae structures. Monazite mineral chemistry and bulk chemical Th-U-Pb ages were investigated by electron probe microanalyser (EPMA). The igneous and metamorphic monazites display contrasting and typical mineral-chemical properties. Metamorphic monazites follow strictly the cheralite substitution trend in Th + U vs Ca coordinates. Igneous monazite in an alkalifeldspar granite has the highest Y2O3 contents (~2 wt%) among all studied samples. In ThO2* vs PbO coordinates the monazite data define isochrones. Micaschist lithoclasts yielded 328 ± 3 Ma, 326 ± 6 Ma and 324 ± 5 Ma, interpreted to represent the thermal peak and post-peak age of metamorphic monazite crystallization. The 328 ± 5 Ma age of igneous monazite in the alkalifeldspar granite in contact to micaschist is interpreted to date the crystallisation of a synmetamorphic anatectic melt. This contrasts the 313 ± 3 Ma monazite crystallization age in a post-tectonic monzogranite. No indications of bulk Pb loss in monazite by shock metamorphism have been observed. The EPMA Th-U-Pb monazite ages from the lithoclasts match data from granites and meta-psammopelites in the outcropping pre-Mesozoic basement in the Western Bohemian Massif and the Black Forest. They confirm that the bottom of the Nördlinger Ries impact crater is situated in crystalline basement rocks belonging to the Moldanubian Zone.
Kurzfassung
Im Steinbruch am Lehenberg (auch Lehberg oder Limberg-Nord) im NW-Teil der Megablock-Zone des Nördlinger Rieskraters kommen Granit- und Glimmerschiefer-Lithoklasten in einer polymikten Suevit-Impaktbrekzie vor. Die Lithoklasten zeigen die petrographischen Merkmale der Stoßwellenmetamorphose-Skala, wie Einschlüsse mit diaplektischem Glas, Quarzkörner mit dekorierten planaren Elementen und stark geknickte Glimmer. In petrographischen Dünnschliffen wurde Monazit durch Spektralkartierung mit einem automatisierten Rasterelektronenmikroskop detektiert. Im Rückstreuelektronenbild (BSE) zeigt Monazit die für magmatische und metamorphe Kristallisation typischen Kristallformen sowie die internen Zonierungen und Verteilungsmuster von Th. Nur wenige Monazitkörner zeigen die für Stoßwellenmetamorphose typischen Internstrukturen wie scharf parallel angeordnete Risse die Lamellen bilden. Mineralchemie und Th-U-Pb-Alter der Monazite wurden mit der Elektronenstrahlmikrosonde bestimmt. Magmatische und metamorphe Monazite zeigen unterschiedliche Zusammensetzungen. In Th + U vs Ca Koordinaten folgen die metamorphen Monazite dem Cheralith-Substitutions-Trend. Magmatischer Monazit im Alkalifeldspat-Granit hat die höchsten Y2O3-Gehalte (~2 wt%) der untersuchten Proben. In ThO2* vs PbO Koordinaten definieren die Monazitanalysen Isochronen. In den Glimmerschiefer-Lithoklasten liegen die Th-U-Pb-Monazitalter bei 328 ± 3 Ma, 326 ± 6 Ma und 324 ± 5 Ma und werden als Alter der Metamorphose-Maximaltemperatur und nachfolgender Abkühlung interpretiert. Die Monazit-Isochrone von 328 ± 5 Ma im Alkalifeldspat-Granit im Kontakt zum Glimmerschiefer wird als Kristallisationsalter einer synmetamorphen anatektischen Schmelze interpretiert. Sie unterscheidet sich deutlich vom jüngeren Kristallisationsalter des posttektonischen Monzogranits mit seinen 313 ± 3 Ma alten Monaziten. Es ergeben sich keine Anzeichen für Verlust von Gesamt-Pb in Monazit durch die Stosswellenmetamorphose. Die Elektronenstrahlmikrosonden-Th-U-Pb-Alter der Monazite in den Lithoklasten gleichen denen von Graniten und Metapsammopeliten im prä-mesozoischen Basement der westlichen Böhmischen Masse und des Schwarzwalds. Dies belegt dass im Untergrund des Rieskraters die kristallinen Basementgesteine der Moldanubischen Zone anzutreffen sind.
Keywords: suevite impact breccia, micaschist, granite, monazite, microstructures, Th-U-Pb age dating, Moldanubian Zone
Schlüsselwörter: Suevit-Impaktbrekzie, Glimmerschiefer, Granit, Monazit, Mikrostrukturen, Th-U-Pb-Altersbestimmung, Moldanubisches Basement
... B). Ore deposition took place during different stages of the Variscan belt evolution, likely ranging from subduction and arc magmatism (>360 Ma) to phases of orogenic collapse (310 Ma; Monnier et al., 2021a). The peak of the Barrovian metamorphism in the Sioule nappe occurred at 350-360 Ma (Do Couto et al., 2016), with temperature and pressure estimated at ca. 600 • C and 7 kb for the lower unit of the antiform (Schulz, 2009). ...
In the Echassières district of the French Massif Central, occur several outstanding magmatic/hydrothermal systems enriched in strategic metals, such as the Beauvoir rare-metal granite. In this contribution we propose a systematic approach, based on mica trace chemistry, to decipher the different events leading to mineralization. Twelve groups of micas were defined by their specific petrographic features and/or location in the district. Their trace element composition, obtained by LA-ICP-MS, varies widely from one mica group to another, although homogeneous signatures within groups could be distinguished. Some of the trace elements are remarkably enriched, such as W in igneous lepidolite and Sn in greisen muscovite, both of which occur in the Beauvoir granite. A statistical approach based on a set of multivariate analyses highlights that the trace chemistry of micas is inherited from their source, whether hydrothermal or igneous, thus providing a signature for their origin. This approach also shows that differences in major element composition (i.e., different mica species) impact only slightly the trace-element signature. For instance, muscovite and zinnwaldite from one granite have a coherent signature, but they contrast with same mica species in another granite or hydrothermal veins. It is thus possible to genetically link two different mica species from remote locations, or inversely, to recognize different origins for a same mica species in the same sample (e.g., superposed alterations). A second, important implication is that trace-element signatures of micas provide a record of metal remobilization and transport. In the Echassières district, greisen alteration of the Beauvoir granite caused dissolution of W-rich (ca. 290 ppm in average) lepidolite and cassiterite (SnO2). Newly-formed greisen muscovite incorporated most remobilized Sn (ca. 1000 ppm in average), while W precipitated in distal quartz veins as wolframite. As a consequence, Sn is concentrated in the granite, while W occurs outside of it. This is also indicated by the gradual Sn decrease and W increase recorded in micas from distal veins. Finally, wolframite-bearing veins do not contain cassiterite, validating mica trace-element chemistry as a powerful tool to decipher SnW ore forming hydrothermal processes.
... Considering the metamorphic evolution, the age and the structural position (thrusting over the Lower Gneiss Unit) of the high-pressure Najac unit, we thus propose that it represents the lateral equivalent of the Groix and Bois de Céné unit in the Armorican massif and the Limousin eclogites of Berger et al. (2010a) and assign this unit to the Middle Allochthon Further exploration of phengitic, biotite-poor garnet micaschists and associated eclogites is needed in the French Massif central to better characterize a potential Middle Allochthon and to correlate it with well-known occurrences of the Armorican massif. The Laguépie unit records amphibolite facies metamorphism peaking at 710-730 °C, 10 kbar, corresponding to metamorphic conditions obtained in the Upper Gneiss Unit elsewhere in the French Massif central (Burg et al., 1984;Bellot and Roig, 2007;Schulz, 2009;Lardeaux et al., 2014;Do Couto et al., 2016; Figure III-9b). The UGU hosts retrogressed eclogites enclosed into migmatitic paragneisses, formed during the D0 and D1 events, respectively Melleton et al., 2009;Do Couto et al., 2016). ...
... Proposed P-T path (orange arrows) for the Najac and the Laguépie units. Grey P-T paths are from the literature: Groix blueschists fromBallèvre et al. (2003), Cellier éclogites fromBallèvre et al. (2014); Najac éclogite fromLotout et al. (2018); UGU eclogite fromBellot and Roig (2007); UGU paragneiss fromBellot and Roig (2007) andSchulz et al. (2009), Champtoceaux migmatites afterPitra et al. (2010). Age of the HP metamorphism in the Groix Island fromBosse et al. (2005), age of pressure peak in the Najac eclogite fromLotout et al. (2018). ...
A travers une étude pétrochronologique des roches de haute-pression (HP), cette thèse a pour objectif de contribuer à la compréhension de la dynamique varisque dans le Massif Central français (MCF) via la réponse à différentes questions : i) Quelles unités ont enregistré le métamorphisme de HP dans le MCF ? ii) Quelle est l'origine de la diversité du métamorphisme de HP dans le MCF ? iii) Quel est l'âge du métamorphisme de HP ? iv) Quel est le nombre de zones de subduction impliquées dans la structuration du MCF ? i) Il est généralement admis que les éclogites témoignant du métamorphisme de HP dans le MCF affleurent dans l'Unité Supérieure des Gneiss (UGU), une unité formée par des paragneiss migmatitiques contenant des éclogites rétromorphosées. Cependant, des investigations pression-température (P-T) dans les terrains sud du massif (unités de Najac-Laguépie) ont permis d'identifier une seconde unité de HP. La présence de micaschistes éclogitiques (570°C, 16 kbar) renfermant des lentilles d'éclogites fraiches dans le Massif de Najac a été attribuée à l'existence d'une Unité Intermédiaire (IU), déjà reconnue dans le Limousin. Les amphibolites de Laguépie ayant enregistré des conditions granulitiques (710°C, 10 kbar) vers 363 Ma ont été associées à l'UGU. Ainsi, il est proposé dans ce travail que le métamorphisme de HP dans le MCF se localise dans deux unités structurales distinctes. ii) La subdivision des éclogites dans le MCF est classiquement basée sur les températures de cristallisation au pic de pression. On distingue ainsi des éclogites de HP/BT ayant cristallisé en dessous de 700°C et des éclogites de HP/HT ayant cristallisé au-delà de 700°C. Dans le Limousin, une étude P-T comparative entre les deux types d'éclogites situées dans les deux unités tectono-métamorphiques différentes (éclogites fraiches de l'IU et éclogites rétromorphosées de l'UGU) montre des gradients géothermiques différents au pic de pression, de 7- 8°C/ km pour l'IU et 8-12°C/ km dans l'UGU. La différence dans les gradients calculés reflète différentes pressions pour des températures comparables (globalement entre 650-700°C). Les conditions d'exhumation sont nettement différentes entre les deux unités : les éclogites de l'IU caractérisent une exhumation accompagnée d'une diminution de température tandis que les éclogites de l'UGU témoignent d'une exhumation à température constante voire avec un léger réchauffement. Ainsi, la diversité du métamorphisme de HP dans le MCF est le résultat d'une évolution post-éclogitique contrastée plus qu'à des températures de cristallisation différentes au pic de pression. Par comparaison avec la dynamique égéenne, deux processus d'exhumation différents sont proposés. Les éclogites de l'UGU seraient liées à une exhumation par accrétion à la plaque supérieure via un mécanisme de retrait du panneau plongeant. Ce mécanisme explique l'exhumation " chaude " à l'instar de certaines roches des Cyclades (Naxos). Les éclogites de l'IU auraient été exhumées dans le prisme d'accrétion, à l'image des unités de Crète. iii) L'étude géochronologique des deux éclogites a permis de contraindre un âge U-Pb du métamorphisme de HP par LA-ICP-MS sur zircons de 377± 1 Ma dans les éclogites de l'UGU et de 364 ± 3 Ma dans celles de l'IU. Ces âges Dévoniens supérieurs plus jeunes que l'âge siluro-dévonien connu pour le métamorphisme de HP dans le MCF sont compatibles avec les données récentes acquises dans les terrains sud du massif. Ils s'inscrivent également dans l'âge de la HP reconnu dans le reste de la chaine varisque. iv) Le modèle polycyclique d'évolution de la convergence varisque, impliquant deux zones de subduction dont une, d'âge Silurien à Dévonien inférieur, ne rend pas compte des données acquises dans ce travail. Par conséquent, un scénario géodynamique monocyclique, impliquant une seule subduction au Dévonien moyen à supérieur, est finalement proposé.
... Ore deposition took place during different stages of the Variscan belt evolution, ranging likely from subduction and arc magmatism (>360 Ma) to phases of orogenic collapse (310 Ma; Monnier et al.,202 1a). The peak of the Barrovian metamorphism in the Sioule nappe occurred at 350-360 Ma (Do Couto et al., 2016), with temperature and pressure estimated at ca. 600 °C and 7 kb for the lower unit of the antiform (Schulz, 2009). [B]. ...
In the Echassières district from French Massif Central (FMC), occur several outstanding magmatic/hydrothermal systems related to a variety of mineralizations, such as the Beauvoir rare-metal granite. Among these, crystallization of three generations of wolframite has recently been unraveled. In this contribution, we recognize and analyze by LA-ICP-MS twelve groups of micas displaying specific petrographic features and/or location in the district. Most of the trace-element content varies widely from one mica group to another, however, homogeneous signatures within groups could be distinguished. Some trace elements are remarkably enriched, as is the case for W in igneous lepidolite or Sn in greisen muscovite, both of which occur in the Beauvoir granite. A statistical approach based on a set of multivariate analyses highlights that the trace-element concentration in micas is inherited from the hydrothermal or igneous source, thus providing a signature for their origin. Also, it could be shown that differences in major element composition (i.e., different mica species) only slightly impact the trace-element signature. It is thus possible to genetically link two different mica mineralogical species from remote locations, or inversely, recognizing different origins (e.g., two superposed alterations) for similar mica species in the same sample. A second application is the possibility to use mica trace-element signatures as record of ore metal remobilization and transport. In the Echassières district, greisen alteration of the Beauvoir granite caused dissolution of W-rich (ca. 290 ppm in average) lepidolite and cassiterite (SnO2). Newly-formed greisen muscovite incorporated most remobilized Sn (ca. 1000 ppm in average), while W formed wolframite in distal quartz veins. As a consequence, Sn is mostly concentrated in the granite, while W occurs mostly outside of it, consistent with the gradual decrease in Sn and increase in W observed in micas from distal veins. Wolframite-bearing veins do not contain cassiterite, validating mica trace-element chemistry as a powerful tool to decipher Sn-W ore forming hydrothermal processes.
... Located in the northern part of the FMC, the Echassières district (Sn, W, Li, Nb-Ta, Sb [35]; Figure 1B) is hosted by the Sioule nappe, metamorphosed to 600 • C, 7-8 kbar [36] at 360-350 Ma (U-Th-Pb on monazite [23]). This nappe is composed of three metasedimentary units, forming antiforms that were cored by several intrusions. ...
... In the study area, the units hosting the main mineralization were affected by a regional metamorphic episode that peaked at~600 • C and 7 kbar [36], at 350-360 Ma [23]. Metamorphic wolframite was not observed. ...
... In the study area, the units hosting the main mineralization were affected by a regional metamorphic episode that peaked at ~600 °C and 7 kbar [36], at 350-360 Ma [23]. Metamorphic wolframite was not observed. ...
Monazite and rutile occurring in hydrothermally altered W mineralizations, in the Echassières district of the French Massif Central (FMC), were dated by U-Pb isotopic systematics using in-situ Laser ablation-inductively coupled plasma–quadrupole mass spectrometry (LA-ICP-MS).The resulting dates record superimposed evidence for multiple percolation of mineralizing fluids in the same area. Cross-referencing these ages with cross-cutting relationships and published geochronological data reveals a long history of more than 50 Ma of W mineralization in the district. These data, integrated in the context of the Variscan belt evolution and compared to other major W provinces in the world, point to an original geodynamic-metallogenic scenario. The formation, probably during the Devonian, of a quartz-vein stockwork (1st generation of wolframite, called wolframite
“a”; >360 Ma) of porphyry magmatic arc affinity is analogous to the Sn-W belts of the Andes and the Nanling range in China. This stockwork was affected by Barrovian metamorphism, induced by tectonic accretion and crustal thickening, during the middle Carboniferous (360 to 350 Ma). Intrusion of a concealed post-collisional peraluminous Visean granite, at 333 Ma, was closely followed by precipitation of a second generation of wolframite (termed “b”), from greisen fluids in the stockwork and host schist. This W-fertile magmatic episode has been widely recorded in the Variscan belt of central Europe, e.g. in the Erzgebirge, but with a time lag of 10–15 Ma. During orogenic collapse, a third magmatic episode was characterized by the intrusion of numerous rare-metal granites (RMG), which crystallized at ~310 Ma in the FMC and in Iberia. One of these, the Beauvoir granite in the Echassières district, led to the formation of the wolframite “c” generation during greisen alteration
... The Sioule series records mostly the peak of barrovian metamorphism (ca. 600°C and 7 kbar for the para-autochtonous unit; Schulz et al., 2001;Schulz, 2009) occurring at ca. 360 Ma (Do Couto et al., 2016). It is intruded by the Pouzol-Servant laccolith (ca. ...
... 1): -The Upper Gneiss Unit (UGU): It is made of garnet/ cordierite-bearing diatexites associated with metatexites derived from orthogneisses and paragneisses with relics of granulite facies mineral parageneses. It has recorded a typical maximum pressure of 10 kbar for a temperature up to 900°C retrogressed into amphibolite facies and greenschist facies pointing to decompression and cooling (Audren et al., 1987;Lardeaux et al., 2001;Schulz et al., 2001;Bellot and Roig, 2007;Schulz, 2009) (Fig. 4). Geochronological data are consistent with a Devonian to early Carboniferous age for these HP granulite facies migmatites (Duthou et al., 1981(Duthou et al., , 1994Lafon, 1986;Schulz, 2014;Do Couto et al., 2016). ...
... The top of the nappe pile is made of cordieritebearing diatexites and migmatitic orthogneisses and paragneisses that have recorded isothermal decompression from 12-13 kbar to 2-3 kbar at 650-700°C. Both lithologies display a composite foliation bearing a NE-SW trending lineation (Audren et al., 1987;Schulz et al., 2001;Schulz, 2009). Serpentinite boudins and granulitic relics allow to assign these rocks to the UGU (Ravier and Chenevoy, 1979). ...
We present here a tectonic-geodynamic model for the generation and flow of partially molten rocks and magmatism during the Variscan orogenic evolution from the Silurian to the late Carboniferous based on a synthesis of geological data from the French Massif Central. Eclogite facies metamorphism of mafic and ultramafic rocks records the subduction of the Gondwana hyperextended margin. Part of these eclogites are forming boudins-enclaves in felsic HP granulite facies migmatites partly retrogressed into amphibolite facies attesting for continental subduction followed by thermal relaxation and decompression. We propose that HP partial melting has triggered mechanical decoupling of the partially molten continental rocks from the subducting slab. This would have allowed buoyancy-driven exhumation and entrainment of pieces of oceanic lithosphere and subcontinental mantle. Geochronological data of the eclogite-bearing HP migmatites points to diachronous emplacement of distinct nappes from middle to late Devonian. These nappes were thrusted onto metapelites and orthogneisses affected by MP/MT greenschist to amphibolite facies metamorphism reaching partial melting attributed to the late Devonian to early Carboniferous thickening of the crust. The emplacement of laccoliths rooted into strike-slip transcurrent shear zones capped by low-angle detachments from c. 345 to c. 310 Ma is concomitant with the southward propagation of the Variscan deformation front marked by deposition of clastic sediments in foreland basins. These features reflect the horizontal growth of the Variscan belt and the formation of an orogenic plateau by gravity-driven lateral flow of the partially molten orogenic root. The diversity of the magmatic rocks points to various crustal sources with modest, but systematic mantle-derived input. In the eastern French Massif Central, the southward decrease in age of the mantle- and crustal-derived plutonic rocks from c. 345 Ma to c. 310 Ma suggests southward retreat of a northward subducting slab toward the Paleothethys free boundary. Late Carboniferous destruction of the Variscan belt is dominantly achieved by gravitational collapse accommodated by the activation of low-angle detachments and the exhumation-crystallization of the partially molten orogenic root forming crustal-scale LP migmatite domes from c. 305 Ma to c. 295 Ma, coeval with orogen-parallel flow in the external zone. Laccoliths emplaced along low-angle detachments and intrusive dykes with sharp contacts correspond to the segregation of the last melt fraction leaving behind a thick accumulation of refractory LP felsic and mafic granulites in the lower crust. This model points to the primordial role of partial melting and magmatism in the tectonic-geodynamic evolution of the Variscan orogenic belt. In particular, partial melting and magma transfer (i) triggers mechanical decoupling of subducted units from the downgoing slab and their syn-orogenic exhumation; (ii) the development of an orogenic plateau by lateral flow of the low-viscosity partially molten crust; and, (iii) the formation of metamorphic core complexes and domes that correspond to post-orogenic exhumation during gravitational collapse. All these processes contribute to differentiation and stabilisation of the orogenic crust.
... The Sioule series display Barrovian metamorphism, with maximal recorded temperatures and pressures of ca. 600 and 700 • C, 7 and 10 kbar, for LGU and PAU, respectively [34,35]. The age of metamorphism is controversial, with two dates, obtained by electron probe U-Th-Pb analysis on monazite, of ca. ...
... The age of metamorphism is controversial, with two dates, obtained by electron probe U-Th-Pb analysis on monazite, of ca. 335 Ma and 360 Ma by Schulz [35] and Do Couto et al. [36], respectively, although the latter is probably more valid, as it is obtained using a better statistical treatment of the data. The series consists of two major antiforms; the central parts of each structure, which consist of PAU units, are intruded by peraluminous granites. ...
The Echassières district in central France contains complex rare-element ore deposits, whose formation is related to exotic igneous events and several hydrothermal episodes that are not entirely understood to date. Tungsten mineralization consists of three generations of wolframite, characterized by distinct Fe/Mn ratios (8.4; 3.5 and 0.3, for wolframite a, b and c, respectively), formed during three separate hydrothermal episodes related to the Variscan orogeny. Wolframite a occurs in quartz veins of the La Bosse stockwork where it crystallized before the Barrovian metamorphism that affected these veins and the host rock. After metamorphism, before intrusion of the Beauvoir and Colettes granites, wolframite b crystallized in the stockwork during massive topazification. High concentrations of wolframite c occur in the proximal quartz veins in the Mazet area, while only scant amounts are found in the La Bosse stockwork. In both settings, wolframite c precipitated from the fluid responsible for greisen alteration that massively affected the Beauvoir granite. In the La Bosse stockwork, greisen alteration is characterized by hydrothermal topaz that is texturally and chemically distinct from that precipitated during topazification. Supergene alteration responsible for kaolinization of Beauvoir and Colettes granites caused remobilization of a non-negligible amount of tungsten (W) during replacement of wolframite by W-rich goethite in all units of the Echassières district. This model for multiple W mineralizing events is novel and can prove essential in distinguishing potential economic deposits worldwide.
... Le complexe intrusif granite des Colettes -granite de Beauvoir est intrusif dans les micaschistes du para-autochtone ici représentés par la série de la Sioule (Grolier, 1971). Ces micaschistes sont chevauchés par l'unité inférieure de gneiss qui produit un métamorphisme inverse dans les micaschistes du para-autochone (Grolier, 1971;Faure et al., 1993;Schulz, 2009;Do Couto et al., 2016). Ces micaschistes sont métamorphisés dans le faciès des amphibolites et montrent de bas en haut une paragenèse à biotite-muscovite-staurotidegrenat-andalousite±sillimanite puis une paragenèse à biotite-muscovite-grenat±sillimanite (Feybesse et Tegyey, 1987). ...
... Ces micaschistes sont métamorphisés dans le faciès des amphibolites et montrent de bas en haut une paragenèse à biotite-muscovite-staurotidegrenat-andalousite±sillimanite puis une paragenèse à biotite-muscovite-grenat±sillimanite (Feybesse et Tegyey, 1987). Les conditions du métamorphisme qui ont été déterminées sont de 450 °C et 7 kbar puis 600 °C et 8 kbar indiquant un métamorphisme prograde (Schulz, 2009). Les âges du métamorphisme sont discutés et proposés entre 327 ± 12 Ma -333 ± 18 Ma (Th-Pb sur monazite à la microsonde électronique, Schulz, 2009) et 363 ± 8 Ma (Do Couto et al., 2016. ...
... Les conditions du métamorphisme qui ont été déterminées sont de 450 °C et 7 kbar puis 600 °C et 8 kbar indiquant un métamorphisme prograde (Schulz, 2009). Les âges du métamorphisme sont discutés et proposés entre 327 ± 12 Ma -333 ± 18 Ma (Th-Pb sur monazite à la microsonde électronique, Schulz, 2009) et 363 ± 8 Ma (Do Couto et al., 2016. L'empilement d'unités métamorphiques et la foliation principale sont plissés par des plis droits d'échelle kilométrique et d'axe NNW-SSE. ...
Le rapport " Ressources métropolitaines en lithium et analyse du potentiel par méthodes de prédictivité" vient d’être publié. Ce rapport a été réalisé dans le cadre des opérations d'appui aux politiques publiques du BRGM avec le soutien de la direction de l’eau et de la biodiversité (DGALN/DEB) du ministère de la Transition écologique et solidaire.
Un minimum de 41 ressources minérales de lithium sous la forme de roches dures ont été répertoriées en France métropolitaine. Elles sont liées à des roches magmatiques acides de type granite et rhyolite à métaux rares, de type greisen et de type pegmatites lithinifère. L’étude de prédictivité réalisée met en évidence les districts connus dans lesquels d’autres découvertes sont probables. Le Massif central apparaît clairement comme le domaine le plus prospectif mais d’autres zones montrent un potentiel, notamment la Montagne Noire, les Maures-Tanneron, les Vosges et les massifs externes des Alpes, le nord-ouest et le sud du Massif Armoricain. La comparaison avec les gisements européens, les gisements exploités et le développement des procédés de traitement des différents minéraux lithinifères montre que, sur un plan géologique, le territoire métropolitain possède un net potentiel. Cependant, un travail important d’exploration et de quantification reste à réaliser. A noter que la production de lithium, pour le marché des batteries, en provenance des ressources répertoriées en France métropolitaine nécessiterait le développement de procédés d’extraction du lithium d’échelle industrielle, à partir de minéraux lithinifères à ce jour non valorisés à l’échelle mondiale pour ce marché.
