Montagne Noire Research Articles

A lower carboniferous plant assemblage from La Serre (Montagne Noire, France). Part II. Gymnosperms

The Tournaisian plant assemblage from La Serre, Montagne Noire, France contains 21 elements which are assigned to the gymnosperms. They include stems of Calamopitys, petioles of Kalymma and small branched axes referable to Lyginorachis. Five types of foliage are identified of which two are preserved both as permineralization and compression. These are most comparable with species of “ Cyclopteris” Unger and represent some of the most common elements of the assemblage. The anatomy of this foliage is well preserved and is compared with previously described, permineralized foliage from the Montagne Noire. Six types of presumed reproductive organs are described from compression material. Four of these are suggested as having affinities with gymnosperms whilst two others are problematical. The flora from La Serre represents a different assemblage of plants from those seen elsewhere in the Montagne Noire although there are some elements which are undoubtedly common to all. This may be explained to some extent by the fact that the depositional sequence at La Serre represents a more near-shore depositional environment than outcrops of lydite found elsewhere in the Montagne Noire. The new data from La Serre confirms the floral similarities between the Montagne Noire in Southern France, the Cypridina Schiefer of Saalfeld, East Germany and the New Albany Shale in the U.S.A.

Extensional tectonics, basement uplift and Stephano-Permian collapse basin in a late Variscan metamorphic core complex (Montagne Noire, Southern Massif Central)

The Montagne Noire is interpreted as a Variscan metamorphic core complex composed of: 1. (1) an uplifted core of assumed Proterozoic gneisses and migmatitic rocks associated with anatectic granites related to a late Hercynian LP/HT metamorphism and 2. (2) an upper plate composed of low-grade or non-metamorphic folded Palaeozoic sediments. These two units are separated by normal and wrench/normal ductile shear zones and detachment faults. These fault zones are characterized by synmetamorphic S- C type mylonites and stretching lineations superimposed on the earlier compressional deformation features. Early orogenic shortening is marked by polyphase, large-scale folding in the core rocks (Axial Zone) and in the thrust nappes of the upper plate. The late Variscan evolution of this massif is in general characterized by extensional tectonics. Kinematic analysis of the extensional deformation shows that displacement on the fault zones is parallel to retrogressive stretching lineations. Shear sense criteria indicate an upper plate movement towards the northeast in the northeastern part of the massif, and towards the south-southwest in the southwestern part. At the same time, transcurrent right-lateral movements occurred along the subvertical ENE-trending boundaries of the core zone. The late orogenic extensional tectonism in the Montagne Noire is attributed to uplift of the previously thickened core zone controlled by an extensional stage between two right-lateral wrench fault zones. Associated with the late Hercynian metamorphic core complex, a Stephano-Permian continental sedimentary basin was formed along the active detachment to the northeast of the uplifted core. The tectonic fabric of the basal coarse clastic Stephanian sediments shows an extension parallel to the ductile stretching in the underlying extensional shear zones. The geometry of unconformably layered clastic sequences, synsedimentary brittle faults, and the various deformation features are consistent with emplacement in a detachment-controlled basin with a NE-oriented extension direction. Together with regional radiometric data the occurrence of very low-grade metamorphism and intense carbonatization within the basal sediments in the vicinity of the core suggests that the basin was formed during the extensional tectonism related to the uplift of the metamorphic core. The Montagne Noire is a well-defined example of late orogenic extensional tectonics on the southern edge of the Variscan Massif Central. Crustal extension and Stephano-Permian basin formation is related to collapse of the thick, wide Hercynian chain during late Palaeozoic times.

Geometry and kinematics of recumbent folding and low-angle detachment in the Pardailhan nappe (Montagne Noire, Southern French Massif Central)

The Pardailhan nappe is the uppermost unit of the superimposed Variscan recumbent fold nappes in the Southern Montagne Noire thrust belt. Detailed mapping and structural investigation demonstrates the presence of a welldevel-oped upper normal limb, a frontal ENE-WSW oriented major hinge and a predominant overturned limb many kilometres in size in lower Palaeozoic non-metamorphic sediments. Finite strain and microstructures related to recumbent folding show maximum elongation and stretching perpendicular to the fold axes consistent with tangential shortening and general south-southwestward overthrust tectonics. Minor internal deformation throughout the fold structure suggests a rigid overturning mechanism and only small amounts of progressive shear during the Variscan recumbent folding (late Early Carboniferous). Post-shortening deformation in the nappe is accounted for by subsequent heterogeneous deformation in extensional shear and fault zones. The earlier folded and schistose structures were displaced along E-W trending low-angle normal faults. Microstructures of the superimposed brittle and ductile deformation in gently S-dipping detachment zones indicate southwest to southward hanging wall movements in the uppermost units. Throughout the lower limb, the extensional features show a progressive rotation into a W-SW direction and a dextral wrench component along the tectonic and metamorphic discontinuity with underlying metamorphic polyphase fold structures. Mechanisms and timing of recumbent fold development and subsequent detachment tectonics are discussed in relation to Variscan orogenic convergence and late orogenic (Late Carboniferous) extension induced by the uplift of metamorphic basement. The compressional and subsequent extensional tectonics in the Southern Montagne Noire nappe belt are associated with the evolution of a late Variscan metamorphic core complex.

A lower carboniferous plant assemblage from La Serre (Montagne Noire, France). Part I.

The Tournaisian plant assemblage from La Serre, Montagne Noire, France, contains floral elements preserved as compressions, impressions and as cellular permineralizations. Over thirty elements of the flora have been recognized with representatives of lycophytes, sphenophytes, ferns or pre-ferns and gymnosperms. Thirteen taxa referable to the first three groups are described in this paper. While some of the plants from La Serre may be compared with currently known plants from other localities in the Montagne Noire such as Sphenophyllum, Cladoxylon and several Clepsydropsis, others are undoubtedly new to the flora. The latter include Serripteris gen. nov. which is a possible younger representative of the Devonian Iridopteridales as well as some foliage preserved as compression which is possibly referable to the cladoxylaleans. The lycopod stem compressions differ from the permineralized Trabicaulis and Landeyrodendron previously described from the Montagne Noire and Paurodendron is recorded from a Tournaisian deposit for the first time. The discovery of new plant elements in the Montagne Noire flora allows a renewed comparison of this flora as a whole with those of similar age and facies elsewhere in Europe and North America.

Environmentally controlled phyletic evolution, blindness and extinction in Late Devonian tropidocoryphine trilobites

The Middle and Upper Devonian carbonate succession of the Montagne Noire, Southern France has been precisely zoned by an unbroken sequence of conodont zones. Stratigraphic control is excellent, and has allowed evolutionary changes in tropidocoryphine trilobites, which occur throughout the succession, to be directly established. The tropidocoryphine had been a stable group for some 40 million years prior to the Middle Givetian. In their last few million years, however, they underwent rapid evolution and exhibit some striking transformations of the cephalon and the regression and virtual disappearance of the eye within a relatively short space of time. They also show a marked diminution in size, and lose their original relief so that the glabella becomes virtually flush with the surface. There are two separate lineages, both of which show eye-reduction and subsequent blindness. In the earlier lineage Tropidocoryphe (Longicoryphe)-Erbenicoryphe, the main features of the ancestral rootstock are conserved and the stable, strongly divergent anterior sutural pattern of the cephalon remains the same. The eye, however, became reduced to a slightly convex surface lacking lenses, only indistinctly defined. Erbenicorphe is confined to well-oxygenated facies, and probably lived as a shallow burrower within the sediment; it became extinct in the early Frasnian. The second lineage T. (Longicoryphe)-Pterocoryphe- Pteroparia shows a remarkable backward migration of the suture, which progressively swings posteriorly in successive species spanning four Frasnian conodont zones (about three million years duration). At the same time the eye progressively degenerates so that the last forms are blind. Sutural migration and eye reduction are not genetically linked, however; the unusual form of the cephalon and suture probably resulted from an adaptation to the euxinic environment in which Pterocoryphe originated. The loss of the eye resulted from the adoption of an endobenthic habit in Pteroparia which descended from the ancestral Pterocoryphe but which had migrated to an oxygenated facies. Eye-reduction is therefore parallel in the two lineages, but superimposed upon a different original cephalic configuration. The last Pteroparia became extinct when the late Frasnian fauna became overwhelmed by the first pulse of the ‘Kellwasser Event’ (probably an anoxic overturn). The evolving characters, through virtually all observed steps, show progressive unidirectional change without sudden breaks or saltations. Such unidirectional evolution is an adaptive response to constant long-lasting environmental influences. □Evolution, Devonian trilobites, France, gradualism, eye reduction.

A primitive seed-like structure and its implications for early gymnosperm evolution

THE ecological diversity and success of seed plants is generally attributed to their reproductive system. But the origin of the seed habit or gymnospermous reproduction is still in discussion1, and the question of seed plant monophyly or polyphyly is a matter of considerable debate2-9. The unique anatomically preserved seed-like structure found in the early Carboniferous, Montagne Noire, France, shows the unequivocal absence of a pollen chamber indicating a more primitive organization than that known from any other late Devonian or early Carboniferous seed. We suggest that the unmodified megasporangial apex may have persisted in this plant under wet ecological conditions, as indicated by anatomical features of the integument lobes. The plant organ represents new fossil evidence of an evolutionary stage between pteridophytic and gymnospermous reproduction and could be interpreted as a very primitive gymnosperm which contradicts current theories of monophyletic gymnosperm origins3,4,6,7,9.

The development of the Central European continental crust since the Early Archaean based on conventional and ion-microprobe dating of up to 3.84 b.y. old detrital zircons

Archaean components up to 3.84 b.y. old have been detected by ion-microprobe dating of zircon from various sediments and metasediments of the European Hercynides (Moldanubian of northeastern Bavaria and the Massif Central). The Early Archaean age of 3.84 b.y. was obtained within the euhedral core of a detrital grain from a Moldanubian paragneiss. The same detrital zircon grain reveals at least three metamorphic overprints at 2.59 b.y. B.P., 1.94 b.y. B.P. and 460 m.y. B.P., and at least two cycles of mechanical abrasion during surface recycling in the Early Proterozoic and after the Pan-African cycle. It is, thus, the crystal with the most complex geological history ever detected. Further Archaean ages for the crystallization of primary magmatic zircons, probably in the course of continental crust formation in the first cycle provenances of the two studied areas, were found at 3.15, 2.9, 2.76 and 2.65 b.y. B.P., and in a few cases between 2.6 and 2.5 b.y. B.P. Early Proterozoic crust-forming events between about 1.75 and 2.15 b.y. B.P. play an important role, especially for the sediments and metasediments of the Montagne Noire at the southern rim of the Massif Central. Middle and Late Proterozoic ages cluster around 1.0 b.y. B.P. and ≈ 600 m.y. B.P. for all analysed samples. Therefore, the deposition of the metasedimentary precursors took place after the Pan-African cycle, which is in line with the Rb-Sr whole-rock systematics of Central European metasediments. The ages of metamorphism in the different source areas could be inferred in some cases by lower intercept ages obtained on single detrital grains. The following sequence coinciding with times of magmatic events has been so far established: 2.59,1.94 and 1.0 b.y. B.P., and 600 m.y. B.P. Thus, ion-probe dating is also capable of defining secondary effects in the respective provenances which are typical for orogenic cycles. The ubiquitous presence of Pan-African detrital zircons in European metasediments is a strong indication that the detrital material was derived from Gondwana. However, from the presence of metamorphic units including eclogitefacies rocks which probably formed at different continental margins and at various times between about 500 m.y. and 330 m.y. ago it is probable that various Gondwana-derived microcontinental plates contributed to the different basin fillings. The Gondwana-derived detritus is also supported by the observation of two periods of relative quiescence or non-activity around 2.3 b.y. ago, and especially around 1.5 b.y. ago, which is a typical feature observed in Gondwana (Cahen et al., 1984) but not in Laurasia, the other possible source of the detritus. The lack of zircons crystallized between 3.84 and 3.15 b.y. ago might have different causes. Thus, the continental crust of the European Hercynides probably developed since the Early Archaean and grew via many crustal accretion events to its present state.

A revision of the Ordovician agnostid trilobiteLeiagnostusJaekel 1909

Abstract The holotype and only known specimen of Leiagnostus erraticus Jaekel 1909 is redescribed. It is a well-preserved, enrolled, and highly effaced specimen from a glacial erratic found in Berlin, probably originating from the upper Lower Ordovician (upper Arenig-lower Llanvirn) of Baltoscandia, and not from the Middle Ordovician as previously suggested. A closely comparable form, Leiagnostus sp., is described from the upper Arenig of Västergötland, south-central Sweden. L. erraticus is the type species of Leiagnostus Jaekel 1909, a widespread genus of agnostid trilobites in the Lower Ordovician. The generic concept of Leiagnostus is revised. Phoidagnostoides Capera, Courtessole & Pillet 1978, from the Tremadoc of Montagne Noire, France, is considered to be a junior synonym of Leiagnostus.

Repartition and origin of N 2 in the fluid inclusions of the dome de montredon, montagne noire, France

Repartition and origin of N 2 in the fluid inclusions of the dome de montredon, montagne noire, France

Ramseyocrinus (Crinoidea) from the Arenig of Morocco

The discovery of the arms of the disparid inadunate crinoid Ramseyocrinus Bates from the Arenig (Lower Ordovician) of Morocco extends the geographic range of this genus, previously only known from south Wales, United Kingdom, and the Montagne Noire, France. It is probable that the Moroccan specimen represents a new species. This is the first Lower Ordovician crinoid to be described from Africa.

Late Palaeozoic metamorphism in the northwestern Iberian Peninsula, Brittany and related areas in SW Europe

Summary In the NW of the Iberian Peninsula elongated and coalescent thermal domes are centred in P 3 antiforms and associated with granite plutonism. After the Emsian and before the Namurian, assemblages with garnet, staurolite and kyanite changed into others with andalusite-sillimanite, biotite and cordierite giving evidence of heating and/or decompression related to syn-metamorphic granite intrusion. The Namurian is roughly the metamorphic peak of low-pressure conditions in the inner zones. In the central Armorican domain in Brittany the main metamorphic event has an age between 330 and 345 Ma. There is also a pressure decrease with time and the earlier zones are telescoped in the W by subautochthonous leucogranites associated with high-grade zones. Contact aureoles affecting the Dinantian basins in Brittany must be late Carboniferous. In the other areas, i.e. Catalonia, the Pyrenees, Montagne Noire and the French Massif Central, the history is essentially the same, with the low-pressure metamorphic peak being Namuro-Westphalian.

Distribution de l'uranium, du thorium, du zirconium, du hafnium et des terres rares (TR) dans des grains de phosphates sédimentaires

U, Th, Zr, Hf and REE contents have been determined in phosphate grains from central Tunisia, southern France (Montagne Noire) and the southeastern Pacific, and in organic fractions (humic acids, kerogens and stable residues) separated in Paleocene-Eocene phosphates of the Gafsa-Metlaoui Basin (central Tunisia). Due to adsorption, phosphatic pellets concentrate high Zr, REE and Th contents, and fish debris takes up considerable amounts of Zr but not of Hf. A separation of Zr from Hf, related to the adsorption of Zr preferred to Hf in apatite, and a clear positive correlation between Zr and REE contents are found. We also note a more or less pronounced fractionation between light and heavy REE, which is influenced mainly by organic complexes, as shown from the clear enrichment of the heavy REE in each of all of the organic fractions.

Trilobites Calmoniidae de l'Ordovicien supérieur du Maroc et les origines de la Province Malvino‐Cafre

A re-examination of the trilobite Baniaspis globosa Destombes (Phacopina) from the Ashgill of the Anti-Atlas (Morocco) shows that it has several derived characters which allow us to classify it in the family Calmoniidae. Calmoniids were characteristic members of the Malvino-Kaffric province during the Lower Devonian. Hence we regard the origins of this palaeogeographic province as located on the northern Gondwanan margin as early as Ashgill, or even Caradoc, times. We agree with those authors who have suggested that the trilobite associations present in the Anti-Atlas, the Montagne Noire (France) and Bohemia during the earlier Ordovician argue against the existence of a ‘proto-Tethys’ ocean.

A magnetotectonic study of the Hercynian Montagne Noire (France)

A paleomagnetic study of eleven sites of Devonian limestones sampled along the flanks and hinge of a polyphase Hercynian recumbent fold in the Montagne Noire reveals well defined directions of characteristic remanent magnetization (ChRM). The directions have good within‐site precision but a large between‐site scatter, which fails to be reduced by a classical tilt correction. This is readily understood if widespread remagnetization occurred between the two major Hercynian phases of folding (A and B), which are known to have shaped the Montagne Noire. This remagnetization appears to be an instance of a continent‐wide chemical event that wiped out much of the magnetic memory of Devonian and lower Carboniferous rocks in Laurasia, Baltica and northern Gondwana, possibly due to groundwater circulation following the major Hercynian tectonic phases. Comparison of observed and predicted Hercynian magnetic directions allows one to determine, through a fairly simple geometric construction, the tectonic elements of the later phase B. It is found that different rotations affected the flanks and hinge zone of the folds generated by phase A, thus demonstrating how a pre‐existing structure can control the development of deformation in a later tectonic phase. The effects of the non cylindrical phase B can next be removed to reveal the broad tectonic features of phase A deformations. The reversed (Kiaman) polarity of the ChRM allows a determination of the age of phase B, which is found to be about 300–310 Ma and of the age of the remagnetization itself at about 310–320 Ma. The search for the age of remagnetization included paleomagnetic measurements at two lower Permian sites from the nearby Lodève basin. These provide new data for the Permian pole of Europe and point out the limitations of some earlier studies, due to insufficient magnetic cleaning. Finally, the principal shortening directions of the two phases can be determined from the magnetic data and are found to have rotated counterclockwise by 30° between phase A and phase B. This feature, already observed in the collision of India against Eurasia, may be related to the progressive suturing of former subduction zones during the Hercynian collision of Gondwana against Laurasia. This study demonstrates the potential of remagnetizations for the solution of rather complex tectonic problems. We propose the term “magnetotectonics” for this rapidly expanding field of studies, in which the methods of paleomagnetism and structural analysis are closely associated.

Quantification of outlines in Frasnian (Upper Devonian) platform conodonts

The outline of the platform in Pa elements of conodont genera such as Palmatolepis is commonly one of the most heavily weighted taxonomic characters. But a taxonomy based only on visual discrimination of Pa elements of Frasnian species of Palmatolepis has resulted in widespread misidentification. Although their Pa elements can be confused, Palmatolepis subrecta, Palmatolepis hassi, and related species from the Frasnian of the Montagne Noire and Iowa can be separated consistently by use of features displayed by other elements of the apparatus, especially the Pb and M elements. To determine if significant differences exist among the Pa elements of these multielement species, digitized outlines of the Pa elements were analyzed using an orientation function similar to the tangent-angle function of F. L. Bookstein. A multielement classification based on the Pb and M elements was tested by a discriminant analysis of the outlines of the Pa elements. The results show that there is close agreement between the two procedures and that systematic differences exist among the Pa elements of groupings based on the Pb and M elements. Unquestionably, quantification of the outlines of platform conodonts will substantially improve taxonomic methods, with consequent enhancement of biostratigraphy.

Tectonic interpretation of paleomagnetic results from the Upper Devonian griotte of the Montagne Noire, southern France

The Devonian (Famennian) griotte acquired its magnetization during and subsequent to emplacement during the Late Carboniferous-Early Permian. The absence of normal secondary components suggests that tectonic emplacement postdates the period of frequent Carboniferous reversals, that is post-Westphalian A and presumably after the onset of the Kiamian quiet period. A remanence attributed to the Permian, as found in the south of France, has been isolated in a few samples; the remainder of the samples show a stable magnetization with a great circle distribution which passes through the zone of Permian directions.

Étude des Radiolaires d'un nodule phosphatédu Carbonifère inférieur de Bareilles, Hautes-Pyrénées, France

The discovery of well-preserved Radiolaria yielded into a single phosphatic nodule from Carboniferous lydites of Bareilles (Hautes-Pyrénées, France) allowed the identification of ten genera and nineteen species. Four of these species are new. The separation of the fauna from the phosphatic matrix has permitted it to be studied with the scanning electron microscope and to be correlated with faunas recently described by the same process. At least, seven species are identical with middle and upper Tournaisian phosphatic nodules from the Montagne Noire (Deflandre, 1944 to 1973b) and from Istanbul, Turkey (Holdsworth, 1973), and eight others with Visean from the «Rheinische Schiefergebirge, West Germany (Won, 1983). The author questions the age of lydites from Hautes-Pyrénées, attributed to middle to upper Tournaisian (early Mississippian).

The eclogite-facies rocks of the Montagne Noire, France

Eclogite-facies rocks are very rare in the Montagne Noire, the local typical metamorphism being a low-pressure type with extensive development of migmatites. The Cabardès and Le Jounié eclogites (garnet + omphacite + quartz + rutile, 700–800°C, 9 ± 2 kbar) derive from tholeiitic basalts interbedded in paragneisses very close to the orthogneissic basement. Both are strongly retromorphosed and re-equilibrated in the conditions of the amphibolite-facies metamorphism of the surrounding gneisses. The eclogite-facies garnet ultramafite of the Airette pod derives from a ferromagnesian calc-alkalic cumulate; its reactional mineral associations show a complex polyphase evolution: a primary spinel websterite (Cpx + Opx + pleonaste + chromite) is reset in the eclogite facies, where garnet develops as coronas on the spinel. A further retromorphic evolution leads to a paragenesis (hornblende + tremolite + picotite + olivine (Fo 70) ± phlogopite) and finally to the paragenesis chlorite + amphibole. Those rocks may be interpreted either as tectonically emplaced slabs or as the only remnants of a medium- to high-pressure regional metamorphism. But it must be pointed out that all these rocks occur only in the southern part of the axial zone of the Montagne Noire, i.e. in the “root zone” of the large Penninic nappes which characterize this part of the Hercynian belt.

Facies patterns and depositional environments of Palaeozoic cephalopod limestones

In the eastern Anti-Atlas (Morocco) a platform and basin topography was established during the late Devonian, probably as a result of early Variscan tensional tectonics. Cephalopod limestones were deposited on shallow pelagic platforms, platform slopes and shallow, slowly subsiding basins. On the platform a transition from land areas into nearshore quartzose brachiopod coquinas, crinoidal limestones, condensed cephalopod limestones and finally into nodular limestones is observed. The latter often become disintegrated into incipient debris flows which pass into nodular limestone/marl alternations of a shallow basin. Deeper basins with shale sedimentation lack cephalopod limestones. Similar facies types also occur in the late Devonian of the Montagne Noire (France), Rheinisches Schiefergebirge (West Germany), Moravian Karst (Czechoslovakia), Holy Cross Mountains (Poland) and in the early Carboniferous of the Cantabrian Mountains (Spain). Due to strong late Variscan compressional tectonics and limited outcrops, detailed facies patterns could not be mapped in these regions, but the same facies types as in the eastern Anti-Atlas suggest similar coast/platform, slope and shallow basin topographies. During cephalopod limestone deposition water depth on the platforms was in the order of several tens to about one hundred metres, as is inferred from repeated subaerial exposures and distinctive depositional and faunal/floral features. Water depth in the adjacent shallow basins might have reached several hundreds of metres. Cephalopod limestones represent a typical stage in the evolution of geosynclines, characterized by extremely low sedimentation rates (1–5 m m.y. −1). This stage is preceded by deposition of thick neritic clastics and/or carbonates and is succeeded by deposition of deep-water clastics or flysch.

Stenomyelon from the Upper Tournaisian of the Montagne Noire (France)

Stenomyelon bifasciculare nov. sp. is described from the Tournaisian of the Montagne Noire. Represented by six fragments of stems measuring 25 to 40 mm in diameter, these fossil axes bear swollen petiole bases separated by short internodes. Anatomically, they are characterized by a mixed stele with eccentrically mesarch protoxylem poles, secondary xylem with tracheids pitted on all walls, petiolar traces of Kalymma type, and few sclerotic nests in the cortex. The divergence of a pair of leaf traces from each rib of the stele has given the taxon its specific epithet. This feature is new for the genus. Stenomyelon bifasciculare is more distinctive than the Scottish and American taxa described to date. Except for Stenomyelon heterangioides, in which this character is doubtful, no sympodia from which leaf traces diverge are recognized in any Stenomyelon, including Stenomyelon bifasciculare; protoxylem maturation has no cauline expression. This character is suggested as the most important one to define the genus.