Montagne Noire Research Articles

A microbial model for the Lower Devonian stromatactis mud mounds of the Montagne Noire (France)

Lower Devonian mud mounds and stromatactis fabrics are exceptionally well exposed in quarry walls and industrially sawed blocks in the Montagne Noire in southern France. Interlayered red biomicrites and white to grey sparitic calcites form mounds up to 70 m high. The red biomicrites contain predominantly bryozoans, sponges and echinoderms. The sparitic layers show typical features of stromatactis fabrics, as outlined byBathurst (1982). We recognize two types of stromatactis fabrics: (1) Stromatactis type A: exentsive cavity systems filled by multiple cement generations, which are interpreted to be related to microbial mats, and (2) Stromatactis type B: smaller patches of blocky spar which are mainly diagenetic in origin, but show characteristic features of stromatactis. Type A is far more important in terms of rock volume. The cyclic interlayering of red biomicrites and sparitic layers is supposed to result from frequent changes in the composition of the mound biota. The bryozoan/sponge community was displaced by short term propagations of microbial mats during times of extremely low sedimentation. Sedimentation and thus the biotic community was probably determined by high frequency (6th order) sea level changes. Despite these changes, mound growth continued, because once established the ecological advantage over the surroundings was maintained by both communities alternating with each other. The microbial mats and the cavities they left after their decay were important for the stabilization of the mounds, the latter allowing for enormous quantities of dissolved carbonate to be transported and precipitated. We anticipate a close interrelation between mound formation and stromatactis formation, and we believe that it is not incidential that both, mud mounds and stromatactis, are mainly restricted to the same interval, namely the Paleozoic.

Upper Devonian iridium anomalies, conodont zonation and the Frasnian-Famennian boundary in the Canning Basin, Western Australia

Two iridium anomalies have been identified near the Frasnian-Famennian boundary (Upper Devonian) in the Canning Basin of Western Australia. Both anomalies are associated with the cyanobacterium Frutexites and are located in marginal slope facies. The first was identified in the Virgin Hills Formation on the west flank of McWhae Ridge near the southeastern end of the Devonian outcrop belt. This anomaly, initially identified as being from the Famennian Upper Palmatolepis triangularis Zone, is now known to be from the Early Palmatolepis crepida Zone, on the basis of the presence of Palmatolepsis crepida in the bed. The second anomaly, found in drill core from the Napier Formation just south of the Napier Range, is older than the first and is from the Frasnian Montagne Noire Conodont Zone 12 or 13 of Klapper (1989) = the Palmatolepis rhenana Zone of Ziegler and Sandberg (1990). These two iridium anomalies are thus significantly below and above the Frasnian-Famennian boundary and are not associated with the extinction event in the Palmatolepis linguiformis Zone. In the Canning Basin the association of two iridium anomalies with beds containing abundant Frutexites microstromatolites indicates that the concentrations of iridium are probably associated with a process of organic concentration. There is no evidence that either example is directly associated with an impact event.

Variscan crustal thickening, extension and late overstacking during the Namurian-Westphalian in the western Montagne Noire (France)

Study of the structural relationships between major lithotectonic units; that is, the Axial Zone, X Schists and southern thrust sheets, on the southwestern slopes of the Montagne Noire in southern France, has cast new light on the main processes and timing for this segment of the Variscan mountain belt. The contact between X Schists and the gneissic Axial Zone is a major shear zone; early movements to the southwest along this contact appear to have been contemporary with the emplacement of nappes on the south slope. Subsequent anatexis of the Axial Zone was synchronous with regional dextral strike-slip shearing and granitization at the end of anatexis sealed the transcurrent structures. Extension commenced during the final stage of granite crystallization. Finally, crustal shortening, with north-northeast vergence, completed the deformation and controlled the emplacement of gold mineralization. We propose a new model for the geodynamic evolution of the Montagne Noire during Namurian-Westphalian times: crustal thickening took place around 330-325 Ma and was followed by uplift, anatexis and dextral transcurrent shearing between 325 and 319 Ma. The subsequent unroofing of the thickened domain continued between 319 and 315 Ma, during a stage of doming and extensional detachment; this crustal thinning was succeeded in the Westphalian-early Stephanian (315-300 Ma) by compressive tectonism during which the lithological stacking of the southern thrust sheets was achieved.

Fluid pressure variations in relation to multistage deformation and uplift: a fluid inclusion study of Au quartz veins

Au-bearing quartz veins result generally from a complex succession of stages of quartz crystallization in relation with long lived deformation. These stages may be related to dynamic rock-fluid interaction, and especially to the pressure variations of a suprahydrostatic fluid. Using a multidisciplinary approach including a detailed characterization of quartz microdomains and related fluid inclusions, and the determination of composition-molar volumes for each stage of fluid migration, a reconstruction of the changes in fluid pressure throughout the crack healing or filling has been carried out on the example of Hercynian quartz veins and more especially of Montagne Noire quartz veins (French Massif Central). The trapped fluids in specific networks of healed microcracks frequently show extreme fluctuation in their density although some chemical variables (salinities, CH 4 /CO 2 ratio) could display little changes. Such features result primarily from changes in fluid pressure in relation with tectonic activity or changes in the structural levels. However, some associated or superimposed processes could result in additional changes in the fluid density, complicating the reconstruction of the P-T paths: i) the local fluid immiscibility which could be accompanied or not by heterogeneous fluid trapping, ii) superimposed fluid trapping in the same quartz grains due to intense and repeated microfissuring which could cause locally post-trapping changes, iii) water leakage linked to ductile deformation of quartz. The use of selected fluid inclusion data obtained on quartz domains apparently preserved from any of these disturbances thanks to the use of SEM cathodoluminescence, appears especially useful i) to give quantitative constraints on pressure changes occurring during quartz vein deformation, ii) to test models depicting the control of fluids on deformation of rocks, especially the necessary fluid pressure (Pf) variations to get fluid-pressure-activated systems, iii) to test the models of gold deposition, involving fluid phase separation. The limitations and advantages to the use of fluid inclusions to reconstruct P changes is discussed

Syn-extension rotations in the Permian St Affrique basin (Massif Central, France): paleomagnetic constraints

We present the results of a paleomagnetic study of the Permian sedimentary basin of St Affrique (southern Massif Central, France). This study was devised to test for tectonic rotations in the Montagne Noire area during the Stephano-Permian extension in the Hecynian chain and to complete a preliminary study [1] in which we suggested an counterclockwise rotation of St Affrique basin with respect to the neighbouring Lodève basin. This rotation was attributed to the domino-like rotation of the basement of St Affrique (i.e., the Paleozoic Monts de Lacaune imbricate units) as expected from the tectonic interpretation of the Montagne Noire area made by Van Den Driessche and Brun [2,3]. However, the rotation of the basin during Permian should induce declination differences between the basal and the upper part of the stratigraphic sequence. We therefore conducted vertical sampling of the basin throughout the Permian sequence. Two sites of Stephanian age gave no results, but twelve sites from the lower to the upper Permian have been sampled in the St Affrique basin and did provide results. Paleomagnetic analysis using thermal procedures confirms our previous analysis and allowed us to demonstrate three magnetization components. The low temperature A component is unblocked by 300°C and conforms to the present-day dipole field direction. An intermediate temperature component (B) has also been isolated, with unblocking temperatures of 300° to 550°–600°C, and this component was possibly carried by magnetite. Between 600° and 680°C a third component (C) may be suspected at some sites, but it has not been isolated with confidence. At a site showing a tilted-block geometry produced by syn-sedimentary normal faulting (site p7), it is shown that the B component is an early magnetization acquired during faulting of the succession, and we are therefore able to give it a stratigraphic age. The mean directions of the B component, which are computed at the stratigraphic stage level, show the following counterclockwise deviations from the reference magnetic field direction for stable Europe: 21° ± 4.5° for the upper Autunian, 10° ± 7° for the Saxonian, 10° ± 9° for the lower Thuringian and 4° ± 6° and 3.5° ± 4.5° for two successive stages of middle/upper Thuringian age. We thus consider that (1) the comparison of the lowest units of St Affrique and Lodève basins [1], and (2) the evolution in paleomagnetic declination during the Permian within the St Affrique basin itself provide evidence, in space and time, of a rotation of the basin during its formation. We attribute this rotation to the domino-like rotation of its basement (the Monts de Lacaune imbricate units) during the Stephano-Permian extension in the Hercynian chain [2,3].

Global iridium anomaly, mass extinction, and redox change at the Devonian-Carboniferous boundary

Iridium abundance anomalies have been found on a global scale in the Devonian-Carboniferous (D-C) boundary interval, which records one of the largest Phanerozoic mass-extinction events, an event that devastated many groups of living organisms, such as plants, ammonoids, trilobites, conodonts, fish, foraminiferans, brachiopods, and ostracodes. At or very close to the D-C boundary, there exists a geographically widespread black-shale interval, and Ir abundances reach anomalous maxima of 0.148 ppb (Montagne Noire, France), 0.138 ppb (Alberta, Canada) 0.140 ppb (Carnic Alps, Austria), 0.156 ppb (Guangxi, China), 0.258 ppb (Guizhou, China), and 0.250 ppb (Oklahoma). The discovery of global D-C Ir anomalies argues for an impact-extinction model. However, nonchondritic ratios of Ir to other important elements and a lack of physical evidence (shocked quartz, microtektites) do not support such a scenario. The fact that all Ir abundance maxima are at sharp redox boundaries in these sections leads us to conclude that the Ir anomalies likely resulted from a sudden change in paleo-redox conditions during deposition and/or early diagenesis. 36 refs., 2 figs., 1 tab.

Upper Silurian Trilobitesof Bohemian affinities from the West Asturian-Leonese zone (NW Spain)

Fossiliferous localities within chloritoid slates of Upper Ludlow age from the West Asturian-Leonese Zone (N.W. Spain) in the Peñalba and Sil synclines are here reviewed. On a regional scale, the occurrence of similar facies and faunas within both synclines indicates that the limit between the West Asturian-Leonese Zone and the Central-Iberian Zone most probably runs within the Sil syncline. Trilobite faunas with representatives of the genera Crotalocephalus, Cerauroides, Cromus, Denckmannites? and Lioharpes (Fritchaspis) have clear affinities with the Prionopeltis archiaci Assemblage of the Upper Ludlow in Bohemia. From a palaeogeographical point of view, the occurrence of Silurian trilobites with Bohemian affinities in N.W. Spain as well as in Pyrenees and Catalonia may suggest the possible existence of a North-Iberian. Domain including these regions and southern France (Aquitaine, Montagne Noire).

Spessartites in the Montagne Noire, France: mineralogical and geochemical data

Spessartites occur as sills in the Lower Cambrian formations on the northern slope of the Montagne Noire, France. They show an unusual mineralogy with phenocrysts of amphibole (magnesio-hastingsite to pargasite) and phlogopite, both including chromian rutiles (up to 3.5 wt.% Cr 2 O 3 ), zincian chromites and Ni-Co-Fe sulfarsenides with pyrrhotite. The lack of magnetite and ilmenite together with the presence of sulphides and sulfarsenides would suggest high fS 2 for fO 2 values slightly above the NNO buffer. The magmatic evolution is firstly marked by a partial cumulation process of early magmatic phases (olivine and associated chromite and sulfarsenides) as suggested by the very high concentrations of MgO (15.5-16 wt%), Cr (1400-1500 ppm), Ni (400 ppm) and Co (75-110 ppm) measured in these rocks. As crystallization proceeds, amphibole and phlogopite become more and more Mg-rich; olivine is destabilized to phlogopite. The latest phases to crystallize are plagioclases and quartz. These rocks are calc-alkaline, SiO 2 saturated with a K/Na ratio higher than 1. They display a strong enrichment of LFS (low-field-strength) and light rare earth elements, with very low concentrations of heavy REE, Ta and Ti. Their chemical and mineralogical features suggest that the primary magma can be generated by relatively small degrees of partial melting from a metasomatically enriched phlogopite + gamet ± amphibole lherzolite. The occurrence of these sills seems to indicate transcrustal fractures allowing a rapid ascent without magma differentiation, consistent with an extensional continental zone. Their magmatic sulfarsenides suggest the possible occurrence of deep-level gold-bearing As-S mineralizations such as found in the Salsigne district

Modeling of the transport and deposition of tungsten in the scheelite-bearing calc-silicate gneisses of the Montagne Noire, France

Scheelite-bearing calc-silicate gneisses (CSG) oceur in the Montagne Noire within a series of dominant micaschists. Detailed petrographical and mineralogical studies reveal three successive stages of metamorphism and hydrothermal alteration: (1) stage 1, a regional metamorphism at 550°C and 4.5 kb where no mineralization is formed; (2) stage 2a, a hydrothermal alteration at 500 to 450°C and 4 to 3 kb which is characterized by an intense sericitization of feldpars and deposition of Sn in Sn-bearing cale-silicates; and (3) stage 2b, a hydrothermal alteration characterized by the crystallization of idocrasegrossular in CSG with concomittant precipitation of scheelite. Tungsten was transported through the micaschist environment and deposited as scheelite only in the CSG of stage 2b at relatively low pressures. To characterize the mechanism of tungsten transport, tungsten speciation at high P-T and scheelite solubility in aqueous solations buffered by the CSG and by the micaschists assemblages were calculated. It was found that H2WO04, HWO-4and WO2-4are the dominant tungsten aqueous species in H2O−NaCl (one molal) solutions at 500°C and 2–4 kb. Calculations also indicate that scheelite deposition is controlled by decreasing pressure and increasing activity of aqueous calcium in this system. This is consistent with the petrographical and mineralogical observations. The consequences of the presence of volatiles (N2, CH4, CO2) in the regional fluids were examined by determining the effect of N2 on tungsten speciation and scheelite solubility. The addition of N2 (up to 10 mol%) to the mineralizing fluids results in a marked increase in H2WO04and HWO-4concentrations relative to WO2-4and in a large decrease of scheelite solubility. This mechanism favours scheelite precipitation and accounts for the commonly observed association of W (and Sn) deposits with graphitic series generating mixed volatiles fluids.

Genesis and significance of the silico-aluminous nodules in the Ordovician of the Montagne Noire and the Massif Armoricain (France)

Siliceous nodules of the Arenigian of the Montagne Noire (southern France) and the Massif Armoricain (western France) are made up of quartz, illite and chlorite with secondary apatite and, less frequently, dolomite. Diffractometrical and geochemical data indicate that the chlorite is a chamosite (trioctahedral 2:1 chlorite), whose development may be accompanied by that of apatite and diminution of illite. Three types of nodules are defined: Type 1, siliceous nodules, with more than 85% of SiO 2, very frequent in the Montagne Noire; Type II, nodules made of a micaceous siltstone with chlorite matrix, abundant in the Massif Armoricain but present also in the Montagne Noire; Type III, chloritic nodules with a finely globular structure, especially developed in the Massif armoricain but observed in the Montagne Noire as well. The nodules are always poorer in illite than their host sediments and often show traces of the initial sediments. Their formation occurs after the deposition and prior to the development of foliation. The frequent presence of undistorted fossil remains suggests an early formation linked to the physico-chemical modifications occurring in an unconsolidated sediment around a decaying organism. The post-depositional mineralogical evolution should have started with an illite/dolomite assemblage and end up with a chamosite-dominant one, under cold climatic conditions. The simultaneous presence of the three types of nodules in the same formation, or even in the same beds, shows that this mineralogical differentiation is not strictly related to the diagenetic and metamorphic transformation of this material.

Biostratigraphy and biogeography of Frasnian, Upper Devonian conodonts

Analysis of Frasnian conodont sequences in the Montagne Noire, southern France, results in the recognition of a conventional zonation consisting of thirteen zones. Graphic correlation of eight Montagne Noire sections with a total of 11 sections in the Alberta Rockies, the Hay River-Trout River areas of the southern Northwest Territories, Canada, the Midcontinent and New York sequences in the United States, and the Canning Basin, Western Australia divides the Frasnian into 34 composite standard units. Most of these sections are dominated by conodonts of the outer-shelf to basinal Palmatolepis biofacies, but those of Hay River-Trout River are occupied by the quite contrasting inner-shelf Polygnathus biofacies. Correlation of the latter with the Montagne Noire zonation is effected only through graphic correlation. The taxonomy of many of the critical species involved in the correlations is based on shape analysis and multielement taxonomy of Palmatolepis.Prevailing opinion is that the Frasnian was a time of cosmopolitanism as exemplified both by benthic organisms, especially corals and brachiopods, as well as conodonts. This contrasts with the Early and Middle Devonian which was a time of significant endemism in benthic fossils and in conodonts. The high point in conodont endemism according to the zone-by-zone analysis of the Early and Middle Devonian (Klapper and Johnson, 1980) was the early Eifelian when there were about 70% endemic species. Although endemism did not reach this high in the Frasnian, analysis of a limited number of intensively collected and studied sections in the Montagne Noire, U.S. and Canada, Western Australia, and the Russian Platform demonstrates levels of endemism ranging from about 30 to 45% in three composites of two zones each. Endemic species occur in both the Palmatolepis and Polygnathus biofacies. Using the Probabilistic Index of Similarity of Raup and Crick (1979), the null hypothesis of randomness is rejected for some paired comparisons between localities representing different biofacies, as would be expected, but it is also rejected between some localities representing the same biofacies on different continents. Although the explanation of significantly dissimilar faunas can be accepted at 95% confidence levels in only a limited number of instances, there are dissimilarities of somewhat lesser magnitude among various paired comparisons. This seems to indicate that the Frasnian was not a time of conodont cosmopolitanism even within the Palmatolepis biofacies.

Tectonic evolution of the Montagne Noire (french Massif Central): a model of extensional gneiss dome

The tertonic interpretation of the Montagne Noire Gneiss Dome (southern French II Massif (Central) has been controversial for a long time. Several models have been proposed : diapirie uplift, wreneching and diapirism, compressive anticline, and metamorphie core complex. Evidence for extensional tectonics in the French Varisean Belt favours the latter interpretation. Strain and metamorphism patterns in the eastern part of the Montagne Noire result from two successive extensional deformations during Late Carboniferous to Permian times. The occurrence of a major detachment zone along the northern edge of the Montagne Noire Gneiss Dome as well the presence of sedimentary) basias to the north point to the asymmetry) of the Stephanian-Permian extensional system. We propose a new model of gneiss dome involving isostatie uprising and consecutive tectonic denudation of the duetile lower crust. This process results in asymmetrical extensional systems characterized by roll-under folding of the footwall and development of basins in the hangingwall as in the Montagne Noire. The model is finally discussed in comparison to previous interpretations.

Aocaspis nov. gen., nouveau Trilobite Nileidae de l'Ordovicien inférieur de Montagne Noire (France)

A new nileid trilobite genus from the lower Ordovician of the Montagne Noire (France), closely related to the worldwide distributed genus Nileus Dalman, 1827, is erected and described under the name Aocaspis nov. gen. A brief discussion on the classification of nileid trilobites is given and leads to assume that two “groups” are present within the family. On the basis of trilobite associations two assemblage zones are defined within the Schistes de Saint-Chinian Formation.

Lead isotope and Pb-Pb model age determinations of ores from Central Europe and their metallogenetic interpretation

The paper presents lead isotope data from 211 samples from Phanerozoic lead-bearing ore occurrences in Central Europe, particularly from the Southern part of the former German Democratic Republic. The data are interpreted in terms of Amov's dynamic model of continuous lead isotope evolution. The relationships between thoro-genic and urano-genic model ages and the source of lead in different regional units are discussed. We observed differences in lead isotope evolution in the Hercynian internides and externides. Within the Moldanubian and Saxothuringian zones we distinguish five main lead-bearing ore associations: (1) Cambrian, stratiform base metal (Hermsdorf-Waldsassen; 206Pb/ 204Pb=17.50–17.70), (2) Devonian, vein type Sb-bearing, metamorphogene (neumuhle-Hartmannsdorf; 17.80–18.00), (3) Upper Carboniferous-Permian, polymetallic, including tin, vein type (Kutna hora-Freiberg-Altenberg; 18.00–18.20), (4) Triassic (-Jurassic), Pb−Ba, vein type (Střibro-Halsbrucke; 18.20–18.60), (5) Cenozoic, polymetallic, vein type, riftogene (Roztoky-Banska Stiavnica; 18.80–19.10). Pb isotope characteristics from ores of the Montagne Noire and the Brioude-Massiac district correspond to this subdivision. Ore associations from the Rheno-Hercynian zone display higher 207Pb/204Pb ratios which can be explained by more evolved and less metamorphosed source rocks. Mineralizations of the eastern Harz (Strasberg-Neudorf) belong to the Permian association, those from the western Harz (Clausthal-Bad Grund) to the Triassic-Jurassic. Because of Pb isotope agreement the stratabound Rammelsberg and the vein bound Ramsbeck-I mineralization are presumed to be isogenetic. Pb isotope identity of distinct mineralizations in the basement zone (Halsbrucke-Bad Grund) and in the Triassic sediments (Gorny Slask-Mechernich-Bleiglanzbanke) suggests a strong genetic coherence. Pb isotope conformity between the Upper Carboniferous-Permian-Triassic ore associations and Hercynian postkinematic granitoids, and lamprophyric rocks, also favours a close relationship. Pb isotope and other data indicate crustal sources. As the age of the ore associations decreases, crustal influences generally increase, apart from the Roztoky mineralization.

Ostracodes, stratigraphie et bathymétrie du passage Dévonien-Carbonifère au Viséen inférieur en Montagne Noire (France)

The Devonian-Carboniferous transitional beds and strata of the Dinantian from Puech de la Suque, near MontPeyroux (Montagne Noire), are reviewed after discoveries of ostracods and conodonts. The biostratigraphical resolution of the succession has been increased by the application of the standard zonal subdivision based on conodonts. Three levels contain remarkable ostracode assemblages which are presented for the first time and related to the “Thuringe ecotype”. The first assemblage is situated just under the Devonian-Carboniferous boundary but above the “Hangenbergschiefer” event also known in the Rhenish Massiv. The second assemblage is or Lower Tournaisian age (Upper part = Upper Cu 1) under the “Lydiennes” of Lower Visean age. The families Tricorninidae, Berounellidae, Bythocytheridae, Healdiidae, Rectonariidae, Acratiidae and Bairdiidae appear to be especially present. Among the 65 species and subspecies, which have been recognized and illustrated, 12 are newly described. They belong to the genera Amphissites, Spinella, Healdianella, Aurigerites, Orthonaria, Necrateria?, “Anahuacia”, Acratia, Famenella and Bairdiolites. Numerous arguments show that these assemblages characterize bathyal environments, colder than the upper marine layer, less oxygenated and with a high amount of dissolved silica. A model using the percentages of a/ typical thuringian species, b/ species of the genus Bairdia and c/ the other ostracode species, allows to estimate the environments locations from the platform to the deeper situations. In the studied area we can conclude to a deepening from the uppermost strunian (do VI) to the Lower Visean. The stratigraphic ranges of several ostracode species are defined. This study corroborates the cospopolitism and the longevity of Thuringian species during the Famennian and Dinantian time.

The perforated trilobiteLaethoprusiagen. nov., and the phylogeny ofKoneprusiaandIsoprusia(Odontopleuridae, Koneprusiinae)

ABSTRACTThe subfamily Koneprusiinae is revised as part of a larger study of the Odontopleuridae, andLaethoprusiagen. nov. is described. The type speciesL. salaxsp. nov. is from the Wenlock of Gotland, Sweden, and the genus is known also from the Wenlock of the Montagne Noire in south France, and from the Lower Devonian of Australia. The type species is most unusual among trilobites in possessing large fenestrae perforating the exoskeleton on segmental boundaries from the cephalic posterior margin to within the pygidium. The fenestrae were probably covered with soft tissues. Their function is enigmatic.Laethoprusiagen. nov. is the sister taxon toIsoprusiaand/orKoneprusia. These genera are revised, and a review of all species assigned to them is given. A cladistic analysis of the best known taxa ofLaethoprusia, Isoprusia, andKoneprusiais presented.Laethoprusiais the most derived genus of the subfamily, although stratigraphically it is the earliest.

Late variscan tectonic evolution by thinning of earlier thickened crust. An 40Ar 39Ar study of the Montagne Noire, southern Massif Central, France

40Ar 39Ar age determinations on samples selected on deformational criteria form the basis of a proposal of a new detailed tectonometamorphic history of the Montagne Noire (the southern part of the Massif Central, France). This range is classically divided into a cover and an axial gneissic and migmatitic core which was metamorphosed under high T medium P conditions. The basement is structurally overlain by epimetamorphic Paleozoic sediments in the north. In the south, equivalent rocks form a well-known pile of recumbent nappes. The Variscan history of this area comprises early shortening and thickening of the continental crust. The climax of this event took place around 320 Ma ago, as is shown by previous RbSr data and by 40Ar 39Ar measurements. New developments in structural analyses have led to a better understanding of the late evolution of the area in terms of diapiric uplift of the core. South-southwestwards verging recumbent nappes have been emplaced and were weakly metamorphosed in their lower parts. This movement was accompanied and followed by dextral extensional wrenching of the Paleozoic cover along the northern and southern margins of the axial zone. Attendantly, ductile shear zones were formed. In order to decipher the complex history, three structural levels have been dated with the 40Ar 39Ar method. These levels are: (1) The migmatitic and gneissic series in the axial core, where micas yield ages of c. 315 Ma; (2) Mylonites from the northern and southern shear zones where biotites and muscovites yield ages of c. 310 Ma; (3) Muscovites and biotites from the epimetamorphic nappe structures yielding ages of c. 297 Ma. The later ages may represent a younger thermal pulse. These data are compared and discussed in the light of previous radiometric studies of the area and precise biostratigraphic constraints. A detailed geodynamic evolution of the studied segment is proposed.

Paleomagnetic evidence for rotation during Stephano-Permian extension in southern Massif Central (France)

We present results of a preliminary paleomagnetic study devised as a test of a tectonic model proposed by Van Den Driessche and Brun [1] to interpret the main structural features of the Montagne Noire (South Massif Central, France). This model suggests that crustal extension during Stephanian to Permian times is responsible for a major flat-lying detachment. The inferred geometrical relationships between fault azimuth and extension direction is believed to induce a counter-clockwise rotation of the Paleozoic cover during its northward motion. In order to test this model, we have sampled Autunian (Lower Permian) grey and red sandstone beds within two basins opened during the extension, the Lodève and the St-Affrique basins. The paleomagnetic study of six sites (three in each basin) using thermal demagnetization procedures, reveals the presence of 3 magnetization components. A first one (A), which is unblocked below 400°C, is interpreted as a recent overprint due to weathering of the sandstones. The B component is unblocked between 400° and 575°C. This B component is assumed to be primary, although the gentle dip of the beds does not allow a fold test. It fits the Lower Permian field direction in the Lodève basin ( in situ: D m = 194° , I m = 6° , k = 50, tilt corrected: D m = 194° , I m = −10° , k = 53). In contrast, it shows a systematic difference in declination in the St-Affrique basin ( in situ: D m = 173° , I m = 2.5° , k = 101, tilt corrected: D m = 172° , I m = 6° , k = 37). This is interpreted as a post-Autunian counter-clockwise rotation of at least 20° of the St-Affrique basin, thus supporting one of the main implications of the tectonic model. Finally, red sediments from two sites of the Lodève basin yield a third component (C) with unblocking temperatures up to 670°C and carried by hematite. Its direction is also consistent with the Permian field direction, showing that hematite crystallisation occurred early in the diagenesis history of the sediments.

Frasnian species of the Late Devonian conodont genus Ancyrognathus

Detailed qualitative analysis of the Pa element and multielement analysis emphasizing the associated Pb element have led to the present revision of the Frasnian species of Ancyrognathus. Diagnostic characters of the Pa element, such as the platform outline, carinal pattern, and shape of the pit, as well as salient features of the Pb element, are the basis of this revision. As a consequence, the restriction in the definition of A. triangularis, formerly a heterogeneous concept including several discrete species, has in turn led to the recognition of a substantially increased number of species of Ancyrognathus.The lowest occurrence of Ancyrognathus primus is used here to delimit the base of Zone 6 of the Montagne Noire Frasnian sequence. The exact relationship of the Montagne Noire zonation to the published standard zonation is problematical. For example, the base of the range of A. triangularis is possibly above the zone for which it is the nominal species in the standard zonation. Two new species are named herein, A. coeni and A. seddoni, both of which have been widely illustrated in the literature, and three new species are recognized in open nomenclature.

Fluids in Hercynian Au veins from the French Variscan belt

AbstractFluids, together with alteration and ore mineral assemblages, were studied in representative hydrothermal gold-bearing quartz veins from the western part of the Variscan belt in France (La Bellière, Montagne Noire district, Villeranges-Le Châtelet district, and Limousin province). Petrographic studies of the relationships between ores, fluid inclusions, microfracturing and quartz textures show that chronological and genetic relationships between gold deposition and fluid trapping may be very complex and difficult to establish for veins which show multi-stage fracturing and shearing. Systematic studies of secondary fluid inclusions in microcracks and recrystallized zones of the early quartz veins indicate two contrasting physical-chemical conditions: 1 relatively high temperature (250–400°C) and pressure (>1 kbar) event with CO2-CH4-H2S-N2 (±H2O-NaCl)-rich fluids related to the early sulphide deposition; 2 lower temperature (150–250°C) and pressure with aqueous fluids related to the late native-gold-sulphide (or sulphosalt) assemblage, which constitutes the economic ores in some deposits.In deposits where gold occurs predominantly in a combined state within arsenopyrite and pyrite (Châtelet and Villeranges), primary fluid inclusions in authigenic quartz combs cogenetic with arsenopyrite are almost purely aqueous (H2O-NaCl) and have a low salinity (1–4 wt. % NaCl). P-T conditions (150–250°C), nearly hydrostatic pressures) are similar to those of the second stage in the multi-stage quartz veins.Consideration of chemical equilibria in the C-O-H-N-S system using microthermometric and Raman spectrometric analysis for the fluids, together with data obtained from mineralogical studies, show that during gold deposition, fO2 was below hematite-magnetite buffer at Villeranges and around the Ni-NiO buffer at La Bellière and Montagne Noire. fS2 calculations based on H2S analyses are in good agreement with mineral assemblage estimates and close to that fixed by the pyrite-pyrrhotite boundary at high temperature. Ore fluid pH was significantly lower than in the host rocks as shown by the complete alteration of the host rocks into a quartz-K-mica assemblage. The data illustrate that during the late Hercynian, fluid circulation evolved from high P-T conditions, in some cases linked to late magma intrusions, towards conditions typical of later hydrothermal systems of the geothermal type.