Transpressional tectonics and Carboniferous magmatism in the Limousin, Massif Central, France: Structural and ⁴⁰Ar/³⁹Ar investigations

New structural, microstructural, and 40Ar/39 Ar data from the NW Massif Central (France) provide additional constraints on the timing and tectonic setting of late Variscan granite magmatism. Previous studies had emphasized the role of late orogenic extension in the emplacement of granite plutons in the Limousin region. In contrast, the new data set is consistent with syntectonic emplacement of magma in a dextral simple shear active from 350 to 300 Ma in a transpressional regime. As an alternative hypothesis to late orogenic extension, we propose that magmas migrated into tensional bridges between active P shears associated with a lithospheric shear zone comparable to a pop-up structure. The Galician region, in the western end of the Ibero-Armorican tectonic arc, exhibits major left-lateral ductile shear zones which can be interpreted as conjugate structures to the Limousin and Armorican shear zones. Copyright 2007 by the American Geophysical Union

A multidisciplinary study of a syntectonic pluton close to a major lithospheric-scale fault: relationships between the Montmarault granitic massif and the Sillon Houiller Fault in the Variscan French Massif Central. Part II: Gravity, aeromagnetic investigations and 3D geologic modeling.

International audienceNew gravity and aeromagnetic investigations have been carried out to understand the emplacement mechanisms of a granitic pluton and the relationships with a nearby lithospheric-scale fault. This paper concerns the second part of a methodological multidisciplinary study and complements previous geochronologic and Anisotropy of Magnetic Susceptibility (AMS) studies on the same pluton. In the northern part of the Variscan French Massif Central (FMC), the Montmarault massif crops out along the Sillon Houiller Fault (SHF). Bouguer and aeromagnetic anomaly maps imply thickening of the pluton along the SHF and suggest laccolitic spreading northwestwards. Based on petrophysical measurements, direct 2D joint gravity and magnetic modeling has been performed along 10 cross-sections. In order to quantitatively constrain the 3D pluton geometry and its relationships with surrounding geologic units, these geophysical cross-sections, new structural information (field and AMS measurements) and petrophysical data have been integrated into a regional 3D geological and geophysical model. Altogether, the results obtained from geochronology, petro-magnetic fabrics (Part I), gravity and aeromagnetic investigations as well as 3D modeling (Part II), demonstrate that the Montmarault pluton was emplaced during the Namurian period along the SHF. Our results further show that, at that time, in response to a NW-SE regional extension, if the SHF existed, it behaved as a normal fault. Mylonites attesting for synmagmatic normal motion on the northeastern part of the Montmarault pluton strengthen this tectonic scheme. During the Late Carboniferous, the FMC experienced NE-SW extension along the SHF by 80 km of brittle left lateral wrench offset. This second tectonic event is well recorded in the Stephanian coal basins which were formed along NW- SE listric brittle faults and constrain the present-day shape of the Montmarault pluton

Late orogenic carboniferous extensions in the Variscan French Massif Central

International audienceThe Variscan French Massif Central experienced two successive stages of extension from Middle Carboniferous to Early Permian. In the northern Massif Central, the first stage began in the late Visean, immediately after nappe stacking, and is well recorded by Namurian-Westphalian synkinematic plutonism. The Middle Carboniferous leucogranites widespread in the NW Massif Central (Limousin and Sioule area) were emplaced within a crust extending along a NE-SW direction. At the same time, the hanging wall or "Guéret extensional allochton" moved toward the SE. Several examples of the synextensional plutonism are also recognized in central Limousin: Saint Mathieu dome, La Porcherie, and Cornil leucogranites. These examples illustrate the relationship between granite emplacement and crustal scale deformation characterized by NW-SE stretching and NE-SW shortening. In the central and southern Massif Central (Cévennes, Châtaigneraie, and Margeride areas), plutonism is dominantly granodioritic and exhibits the same structural features: NW-SE maximum stretching and overturning to the SE. Middle Carboniferous (Namurian-Westphalian) extension was parallel to the Variscan belt both in the Massif Central and southern Armorican area. This extensional regime was active from the late Visean in the north, while compression dominated in the southernmost domains (Montagne Noire and Pyrenées). The second extensional stage occurred from Late Carboniferous to Early Permian. This event was responsible for the opening of intramontane coal basins, brittle deformation in the upper crust, and ductile normal faulting localized on the margin of cordierite granite-migmatite domes. Data from the coal basins show that the half-graben is the dominant structural style, except for basins located along submeridianal left-lateral faults which have pull-apart geometries. Late Carboniferous extension occurred along the NE-SW direction. The NE-SW maximum stretching direction can be found in the whole Massif Central but is more developed in the eastern part. The extensional direction is transverse to the general trend of the belt, and top-to-the-NE shearing is dominant. Correlations of these two extension directions with neighboring Variscan massifs are discussed

Les éclogites du Bas-Limousin, Massif Central français. Comportement des clino-pyroxènes et des plagioclases antérieurement à l'amphibolitisation

During Devonian, the Bas-Limousin series has been affected by two successive main deformation events associated with a plurifacial barrovian metamorphism grading from epizone up to Sillimanite + Muscovite zone (Santallier et al., 1978). The eclogites found in the series crystallized during the high pressure early phases of the Hercynian s.l. orogenic cycle, prior to Devonian (Santallier, 1981). They were amphibolitized during the first barrovian metamorphic phase, as a consequence of their tectonic emplacement in their present greywackous mesozonal country-rocks. They can be distributed into two geographically distinct groups (Figure 1). — The eclogites of the eastern area have a tholeiitic composition (Figure 3) either undifferenciate or enriched with plagioclase cumulates. In the first case, the eclogitic paragenesis comprised mostly garnet (Alm 39 to 57 and Py 16 to 39 : Figure 2) and omphacite (Jd + Ac 35 to 45). In the second case, kyanite and zoïsite were part of the eclogitic assemblage. After their tectonic emplacement in their present staurolite-, kyanite- or sillimanite-bearing country-rocks, the eastern eclogites have undergone a retrograde evolution with two separate episods. The first one predated the hydration process and developped an anhydrous paragenesis comprising mostly diopside (Figure 14) and oligoclase, which result from the omphacite breakdown (Figure 12). In the "bimineralic" eclogites (devoided of kyanite and/or zoïsite), the first so-called retromorphic assemblage is mostly composed of secondary diopside and oligoclase associated with undisturbed primary garnet. In the kyanite and/or zoïsite eclogites, the two mineral phases are retrogressed into anorthite with minor other constituents (spinel + corundum ± staurolite where primary kyanite was concerned). The connected breakdown of kyanite and omphacite induced the crystallization of a Mg-rich secondary garnet (Figure 10). The second assemblage is a hydrous one and is connected with amphibolitization. It comprises hornblende and andesine. — In the western area, the eclogites also present a tholeiitic composition. The primaiy, high pressure metamorphic mineral assemblage and its retrograde evolution represent a continual process occurring under regularly decreasing pressure conditions with the following characteristics : The jadeite content in the Cpx decreased progressively (Figures 13 and 14) from an hypothetical Jdx omphacite 1 (now disappeared and replaced by a Cpx + plagioclase intergrowth in the core of omphacite 2), through a massive Jd₃₅ to ₄₅ omphacite 2, a symplectitic Jd₂₃ omphacite 3, till a Jd₄ to ₁₂ Cpx in the reaction rims between quartz and garnet. Plagioclase was present in the western "eclogites" from the very beginning of the primary stage (garnets frequently have plagioclase inclusions), and its Ca content increases progressively as the jadeite content in the Cpx decreases (Figure 13) : Plagioclase 1 An₁₀ to ₁₅ is included in garnets ; plagioclase 2 An₂₀ is associated with massive and symplectitic omphacites 2 and 3 ; plagioclase 3 An₂₀ to ₃₀ appears in reaction rims with Cpx 4. Amphibolitization occurred a short time latterly : Cpx 2, 3 and 4 can be direcdy replaced by green hornblende. The P/T eclogitization conditions have been estimated (Santallier, 1981) at at least 15 kbar and 650 to 750 °C (Figure 19). These are just indicative values because of the scarcity of the generally poorly preserved available mineral relicts. But the results obtained in both western and eastern areas seem to be very comparable. The two eastern and western eclogites groups have a slightly different retromorphic history. But they all have the same chemical composition. They were tholeiites with alkaline affinities (Cabanis et al., in press) originally emplaced in a rift zone ; which suggests that they all belong to the same magmatic unit generated in a distensional environment. They all had very comparable primary eclogitic assemblages. They are all found as lense-shaped boudins in an homogeneous greywackous gneisses series. This gneisses series shows everywhere in Bas-Limousin the same number of deformation phases associated with the same number of metamorphic phases which occurred in the same P/T conditions. The eclogites boudins are everywhere moulded by the same F1 cleavage expressed both in the gneisses and in the external rim of the amphibolitized eclogite bodies. All these facts strongly suggest that all the Bas-Limousin eclogites were generated during the same high pressure event, that they were emplaced at the same time, during the first Devonian deformation phase, which induced two slightly different retrogressive processes occurring in the same barrovian P/T conditions.Les éclogites du Bas-Limousin sont des témoins des phases précoces de haute pression du cycle Hercynien s.l. Elles ont été amphibolitisées au Dévonien à la suite de leur mise en place tectonique dans leur encaissant greywackeux actuel. Elles se répartissent en deux groupes. A l'Est, elles ont une composition de tholéiites soit indifférenciées (et sont alors essentiellement composées de grenat et d'omphacite), soit enrichies en plagioclases cumulatifs (le disthène et/ou la zoïsite font alors partie de la paragenèse éclogitique primaire). Les éclogites orientales ont subi une évolution rétrograde en deux stades distincts avec développement d'une première paragenèse anhydre, à diopside et oligoclase, dite de rétromorphose, puis d'une deuxième paragenèse hydratée qui correspond à l'amphibolitisation. A l'Ouest, la cristallisation de la paragenèse éclogitique primaire et sa déstabilisation au profit d'un assemblage rétrograde (à Cpx sodique et oligoclase) s'enchaînent en un processus continu à pression régulièrement décroissante avant l'amphibolitisation proprement dite.Santallier Danielle. Les éclogites du Bas-Limousin, Massif Central français. Comportement des clino-pyroxènes et des plagioclases antérieurement à l'amphibolitisation. In: Bulletin de Minéralogie, volume 106, 6, 1983. Eclogites et ultrabasites à grenat - First International Eclogite Conference

Les haches polies de Charente-Maritime (France) : étude des provenances des roches utilisées

We were able to study 320 polished axes what have been collected on the top of the ground in The major part (87 %) is shaped in flints, often from Ecoyeux's area in the same département. But one finds also axes shaped in allochtones rocks : metadolerite, eclogite, fibrolite, jadéitite, cornfels. The exact origin of these axes remains to be determined, except for pieces in jadéitite which come from the Italian Alps. This study shows the importance of the flows of the traffic of possessions during the Neolithic.Nous avons pu étudier un ensemble de 320 haches polies provenant surtout de ramassages de surface effectués en Charente-Maritime. La majeure partie (87 %) est façonnée dans des silex, souvent originaires du secteur d'Ecoyeux, dans le même département. Mais l'on trouve aussi des pièces façonnées dans des roches allochtones : métadolérite, éclo- gite, fibrolite, jadéitite, coméenne. La provenance exacte de ces haches reste à déterminer, sauf pour les pièces en jadéitite qui sont issues des Alpes italiennes. Cette étude montre la vigueur des courants de circulation des biens au cours du Néolithique.Surmely Frédéric, Santallier Danielle. Les haches polies de Charente-Maritime (France) : étude des provenances des roches utilisées. In: Revue d'Archéométrie, n°25, 2001. pp. 141-143

Etude du matériel néolithique dans la région de Bellegarde (Ain)

The Michaille plateau, which is part of the Haut-Bugey and located in the southern Jura foothills, is a Mesozoic limestone formation overlain in places by the Cenozoic Burdigalian molasse. This very heterogeneous rock was heavily dismantled in the vicinity of Villes and Ochiaz (Chatillon en Michaille - Ain), where the scores of flint pebbles it contains have been ploughed up and are strewn all over the fields. These raw material sources were intensively exploited during the Neolithic ; the morphology of the pebbles made it necessary for the prehistoric knappers to apply a particular débitage technique - bipolar percussion on an anvil -, which can be identified on pebbles and pitted blocks collected on two plots of land near Villes. The experimentation carried out on the basis of our observations have confirmed that part of the surface material was knapped using bipolar percussion. However, the numerous splintered pieces recovered on the site could best be interpreted as «a posteriori tools» answering specific requirements in the socio-economic context of the regional Bell Beaker Culture. This type of artefact is very well documented in the neighbouring site of Chatanay, as well as in Géovreissiat (Ain) and in Noir-Bois (Switzerland), two regional settlements corresponding to a beaker occupation. The study is complemented by pétrographie analyses of the flints and pitted stones collected in situ, thus furthering our knowledge of the raw materials available in this part of the Bugey.Le plateau de la Michaille, situé dans le Haut-Bugey, au pied du Jura méridional, est constitué d'un substrat calcaire secondaire, recouvert par endroit par la molasse burdigalienne tertiaire. Cette roche très hétérogène s'est près des villages de Villes et d'Ochiaz (commune de Chatillon en Michaille - Ain) en abandonnant de nombreux galets de silex que l'on retrouve en surface des terres labourées. Ces gîtes à silex ont été intensivement mis à profit pendant le Néolithique ; la morphologie particulière des galets a contraint les préhistoriques à mettre en pratique des techniques de débitage par percussion bipolaire sur enclume, reconnaissables sur des galets et blocs cupules recueillis sur 2 parcelles proches de Villes. Ces observations nous ont permis de rassembler les éléments nécessaires à des expérimentations. Les résultats obtenus confirment la méthode de percussion bipolaire pour le débitage d'une partie du matériel. Cependant, des pièces esquillées récoltées en nombre sur le gisement pourraient se rapporter plutôt à des "outils à posteriori" liés à des besoins spécifiques dans un contexte socio-économique du Campaniforme régional. Ce type de pièce est très présent dans le site voisin de Chatanay, ainsi qu'à Géovreissiat (Ain) et Noir-Bois (Suisse), deux gisements régionaux occupés pendant le Campaniforme. L'étude sera complétée par des analyses pétrographiques des silex et roches cupulées recueillis "in situ" qui permettent d'enrichir notre connaissance des matériaux de cette région bugiste.Fillion Jean-Pierre, Féblot-Augustins Jehanne, Santallier D. Etude du matériel néolithique dans la région de Bellegarde (Ain). In: Revue d'Archéométrie, n°24, 2000. pp. 129-147

Les haches polies en roche dure du Limousin. Premières réflexions sur la base de leur étude pétrographique / Petrographie analysis of stone axes from Limousin

The petrographie analysis of one hundred and forty six stone axes from Limousin permit today a better knowledge of the neolithic industry. Eight pétrographie groups have been defined. Their equivalents have been researched in the different geological formations recognized in the field. The precise descriptions of these groups allow to compare with the taxonomy in use in Brittany for thirty years.L'analyse pétrographique de cent-quarante-six haches polies du Limousin apporte aujourd'hui une meilleure connaissance de l'industrie des roches dures au Néolithique. Huit groupes pétrographiques ont été définis. Leurs équivalents ont été recherchés dans les différentes formations géologiques reconnues sur le terrain. Les descriptions précises de ces groupes permettent la comparaison avec la taxinomie en usage en Bretagne depuis trente ans.Santallier Danièle, Tardiveau Denis, Vuaillat Dominique. Les haches polies en roche dure du Limousin. Premières réflexions sur la base de leur étude pétrographique / Petrographie analysis of stone axes from Limousin. In: Revue archéologique du Centre de la France, tome 25, fascicule 1, 1986. pp. 7-20

Quelques aspects du métamorphisme dévonien en Bas-Limousin (Massif central, France)

The Bas-Limousin terranes are affected by a plurifacial metamorphism of devonian age. The metamorphic main phase belongs to the barrovian type, with a geothermal gradient of about 25° C/km. The grade of metamorphism increases north-estwards from chlorite to sillimanite zone. In the mesozonal area, there are abundant relicts of early parageneses, essentially eclogitic and ultrabasic rocks. Their cartographic distribution indicates that they represent high-pressure early stages of the metamorphism. The low-pressure final stage is essentially anatexitic with a higher geothermal gradient.La série du Bas-Limousin a été affectée par un métamorphisme polyphasé d'âge dévonien. La culmination du métamorphisme est de type barrovien, avec un gradient géothermique voisin de 25° C/km. L'intensité croît du Sud-Ouest vers le Nord-Est depuis la zone à chlorite jusqu'à la zone à silli-manite. Les formations mésozonales montrent d'abondantes reliques de paragenèses précoces essentiellement éclogitiques et ultrabasiques. D'après leur répartition cartographique, ces reliques témoignent de l'existence d'une phase précoce à haute pression. Les dernières manifestations du métamorphisme sont de type basse pression ; elles sont le plus souvent de caractère anatectique et correspondent à un gradient géothermique plus élevé que celui de la phase principale.Santallier Danielle, Floc'h Jean-Pierre, Guillot Pierre-Louis. Quelques aspects du métamorphisme dévonien en Bas-Limousin (Massif central, France). In: Bulletin de Minéralogie, volume 101, 1, 1978. pp. 77-88

Éléments de parure du Néolithique syrien ; identification et provenances des matériaux

International audience85 neolithic stone ornaments from Mureybet (Middle Euphrates, Syria) have been studied by X-Raydiffractometer method. The mineralogical compositions were determined ; die question of thegeographic origin of the materials utilised is discussed by reference with the geological environmentFrom the quantitative point of view, three main mineralogical groups were thus identified : (1) Mg-richchlorites of opbiolitic origin which most probably come from Turkey ; (2) Phosphates and associatedminerals come from the syrian or turkish economic deposits ; (3) Carbonates, associated with gypsumand halite are typical of the surrounding epicontinental series on the Arabian microplate. Some othermineral groups are less abundant such as : (4) talc, which is also of ophiolitic origin ; (5) kaolinite whichwas of local origin as well as the carbonates ; and a few other ones.Among the choice criteria, besides the availability of the various materials, there could be the coloursand the hardness (most of the minerals utilised are no more than degree 4 compared with the Mohsscale). It also appears that, depending upon the different kinds of ornaments and the successivechronological periods, the selected minerals were not the same. Generally speaking, the carbonateswere first preferred. Then the phosphates were used during the khiamian times. Finally, the chloriteswere mostly selected during the mureybetian final periods.85 objets de parure néolithiques provenant du site de Mureybet (Moyen-Euphrate, Syrie) ont étéétudiés par diffractométrie X. Les compositions minéralogiques ont été déterminées et les provenancespossibles ont été envisagées en liaison avec l'environnement géologique. Par ordre quantitatifdécroisant, trois groupes principaux ont été identifiés : (1) les chlorites magnésiennes d'originetypiquement ophiolitique qui proviennent de Turquie ; (2) les phosphates et minéraux associés issusdes grands gisements économiques de Syrie ou de Turquie méridionale ; (3) les carbonates, souventaccompagnés d'évaporites, typiques de la série épicontinentale sur la plaque arabique. Quelquesgroupes plus mineurs sont (4) le talc également d'origine ophiolitique ; (S) la kaolinite de provenancelocale et quelques autres.A côté de la disponibilité, plusieurs critères de choix sont intervenus tels les couleurs, la faible dureté(on ne dépasse presque jamais le degré 4 sur l'échelle de Mohs) mais aussi la forme des objets et lespériodes d'utilisation. Très schématiquement, les carbonates ont été préférés dans les périodesanciennes, puis les phosphates durant le khiamien, enfin les chlorites durant le mureybetien plusrécen

Les outils sur galet du site magdalénien du Rocher de la Caille.

About 10 cobbles, most of them made of phonolit or granit, bear use stigmata. These tools can be connected to technical and domestic activities as flint knapping (hammerstone and anvils), bone cracking (pounders and anvils), bone and antler working (mallets), pounding and grounding of a substance probably vegetable (grinder-pestle). These remains confirm that the site surely was a settlement, the occupation time of which is not still determined.Une trentaine de galets, pour la plupart en phonolite ou en granite, porte des traces d'utilisation. Ces outils sont à rattacher à des activités de base telles que le débitage et la taille du silex (percuteurs et enclumes), le concassage des os (percuteurs de concassage et enclumes), le travail de matières dures animales (maillets), la mouture et le broyage de matières peut-être végétales (pilons-broyeurs). Ces vestiges confirment que le site était bien un habitat dont la durée d'occupation reste à déterminer