U-Pb zircon and Rb-Sr whole-rock dating of low-grade metasediments example: Montagne Noire (Southern France)

Three detrital, Proterozoic zircon suites extracted from siltstones progressively metamorphosed between chlorite- and staurolite-grade independently date the major Caledonian metamorphism within the gneiss dome of the Montagne Noire (Southern France). From this, the following conclusions concerning U-Pb systematics of zircons in low-, medium- and highgrade metamorphic rocks can be drawn:

1. Temperatures of at most 350–400 °C are sufficient to open U-Pb systems of metamict zircons or domains within zircons.
2. The observed open U-Pb system behaviour during metamorphism of the host rocks was found to be due to a low-temperature recrystallisation of highly radiation damaged zircon lattices, probably enhanced by high concentrations of fluid phases in the dehydrating rock volumes.
3. Recrystallisation of metamict zircons under low temperatures causes maximum U-Pb ages for the thermal climax of metamorphism of medium-and high-grade metamorphic rocks, as annealing and accompaning closing of U-Pb systems took place before the maximum temperatures of metamorphism were reached.
4. Low-temperature recrystallisation of old — generally Proterozoic—zircons can readily help to explain the fact that the numerous zircon suites from ancient shield areas yield “lower intercept ages” which are not correlated to any known geological event. Thus, either a weak thermal pulse, not necessarily registered by the mineral assemblage of the host rock, and/or elevated temperatures during burial in the crust might supply enough energy for a structural reordering and simultaneous lead loss of at least the most disordered lattice domains.

In contrast to the U-Pb zircon method, no unambiguous dating of the Caledonian main metamorphism was possible using the Rb-Sr whole-rock technique for phyllites and mica schists sampled in the same metamorphic profile from which the zircon samples were taken. The scatter of data points can best be explained by their rotation around a probable Caledonian isochron. This rotation very probably took place during the later Hercynian orogeny, not significantly affecting the slope of the least square regression line through the scattered data points.     

Multi-stage fluid incursion in the Palaeozoic basement-hosted Saint-Salvy ore deposit (NW Montagne Noire,southern France)

The Saint-Salvy vein-hosted Zn (+Ge) deposit occurs in an E–W fault system which flanks the southern margin of the late Variscan Sidobre granite, and cross-cuts Cambrian black shales of the Palaeozoic basement. Comprehensive mineralogical and geochemical studies of vein samples have revealed four mineralizing events (M1–M4) related to late and post-Variscan tectonic events. A further late-stage event may be related to weathering.M1 (=skarn deposits) and M2 (=patchily mineralized quartz veinlets) are associated with granite emplacement. Quartz contains low salinity, H₂OCO₂(NaCl)-dominated fluids(⩽6wt% NaCl equiv.) of relatively high temperature (300–580°C), trapped under moderate to high pressure. Estimated M1 fluid δD and calculated fluidδ¹⁸O plot within the metamorphic water field. There appears to be no involvement of magmatic fluids.By contrast, M3 (= barren quartz) and M4 (= zinciferous economic mineralization) stages have H₂OCO₂NaClCaCl₂ fluid inclusions with high salinities (23–25 wt% NaCl equiv.) and low temperatures(∼ 80–140°C), which were trapped under low-pressure conditions. The high salinity and NaCl + CaCl₂ content of both M3 and M4 indicates that their parent fluids leached evaporitic salts. M3 fluids are meteoric water dominated, falling close to the meteoric water line (δD andδ¹⁸O averaging −64 and −8‰, respectively). M4 fluids have highly distinctive δD averaging −101‰, and calculated fluidδ¹⁸O varying from−1.2to+7.1‰. The unusually low δD composition of M4 suggests the involvement of “organic” fluids, in which H is derived directly or indirectly from organic matter. The relatively highδ¹⁸O of M4 fluids indicates that considerable isotopic exchange with sedimentary material took place, displacing theδ¹⁸O from the meteoric water line. The data imply interaction of meteoric waters with evaporite and hydrocarbon-bearing sedimentary sequences, most probably the adjacent Aquitain Basin.The main economic mineralization (M4 stage) took place during a tensional event, probably coincident with the Lias-Dogger transition.Calculatedδ³⁴S_H2S of M4 sulphide(+5.4to+8.2‰) is almost identical toδ³⁴S of local Cambrian sulphides(+4.7to+9.4‰) suggesting a genetic link. Abundant siderite associated with M4 sphalerite hasδ¹³C ranging from−2.6to−4.4‰ indicating that carbon was sourced from sedimentary carbonate mobilized by, or equilibrated with the hydrothermal fluid.Late-stage sulphides exhibit extraordinary and highly distinctiveδ³⁴S. Sphalerite has extremely low δ³⁴S(−42.5to−50.5‰), whereas pyrite has an extraordinary large range from−33.2‰to+74.3‰. Closed system sulphate reduction is held to be responsible for the extremely highδ³⁴S: whereas more open system reduction produces the very low values. The coincidence of isotopically lowδ¹³C(−7.6to−11.9‰) for co-genetic calcite suggests the involvement of organic matter in the reduction process.     

Two-stage partial melting during the Variscan extensional tectonics (Montagne Noire,France)

International Journal of Earth Sciences - One of the striking features that characterise the late stages of the Variscan orogeny is the development of gneiss and migmatite domes, as well as...     

Barite deposits of the “Montagne Noire”, Southern France

Several small barite deposits of Devonian age are known in the Monts de Cabrières region, Montagne Noire (southern France). A field and laboratory investigation of these stratabound deposits showed their possible diagenetic origin and a limited economic value.

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Zusammenfassung De petits gisements de barytine du Dévonien dans les Monts de Cabrières (Montagne Noire, France) ont été étudié en détail sur le terrain et au laboratoire. Les observations ont apporté des critères pour une explication génétique de ces gisements «stratiformes». Ils indiquent que les concentrations minérales se sont effectuées par un processus de sécrétion au cours de la diagenèse. L'importance économique de ces gisements est limitée.

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...es ist zwar nicht sinnlos, aber doch etwas unlogisch, wenn man von vornherein nicht versucht, eine Lagerstätte in ihre Umgebung auf die wahrscheinlichste Art einzugliedern, sondern sie unbedingt einem ganz fremden, zunächst nicht von selbst verständlichen Bildungsvorgang zuordnen will. H. Schneiderhöhn (1954)     

Environmental controls on the development of Mississippian microbial carbonate mounds and platform limestones in southern Montagne Noire (France)

Late Mississippian carbonates in southern Montagne Noire are dominantly domical to laterally‐accreted microbial mounds in some formations, as well as stratiform microbial limestones occurring in hundreds of olistoliths within a flysch basin, constituting pieces of a giant puzzle that are used to help reconstruct a platform in a region that is no longer preserved. Petrographic data of limestone samples from 14 continuous long sections of olistoliths have been analyzed statistically, using multivariate clustering (Q‐mode) of the components/matrix/cement and canonical correspondence analysis that allow the reconstruction of the environmental parameters of carbonate microbial communities in space and time. Clustering analysis separated microbial and non‐microbial facies. The calculation of indices along the various axes from canonical correspondence analysis allows recognition of the controlling factors of the mounds and microbial growth as being turbidity, light penetration, bathymetry and storms. Turbidity and light penetration are the primary factors controlling the morphology of the microbial limestones. Representation of the light penetration and bathymetry indices on the stratigraphical sections defines two vertical environmental gradients. Light penetration can be subdivided into euphotic, euphotic–dysphotic and dysphotic‐aphotic conditions. The representation of the bathymetry allows the subdivision of samples into a deeper outer ramp, external mid‐ramp and internal mid‐ramp. The curve distance from the section base = f (index) suggests a cyclicity for the platform that cannot be compared with the onlap curve defined from other cratonic areas (Moscow Basin), and thus the cyclic succession of the Montagne Noire is interpreted to have been mostly tectonically‐controlled. Integration of the data allowed the reconstruction of the original Mississippian carbonate platform, where, up to the Mikhailovian, it appears to correspond to a platform morphology, with narrow shallow water facies and wide turbiditic systems, whereas the width of shallow‐water settings expanded during the Venevian to the Protvian, forming a ramp or distally‐steepened ramp with widespread microbial limestones.     

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.     

Late variscan tectonic evolution by thinning of earlier thickened crust. An Ar---Ar 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.     

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 H₂WO ₀ ⁴ , HWO _- ⁴ and WO _2- ⁴ are 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 H₂WO ₀ ⁴ and HWO _- ⁴ concentrations relative to WO _2- ⁴ and 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.     

Deep crustal source of gneiss dome revealed by eclogite in migmatite (Montagne Noire,French Massif Central)

In orogens worldwide and throughout geologic time, large volumes of deep continental crust have been exhumed in domal structures. Extension-driven ascent of bodies of deep, hot crust is a very efficient mechanism for rapid heat and mass transfer from deep to shallow crustal levels and is therefore an important mechanism in the evolution of continents. The dominant rock type in exhumed domes is quartzofeldspathic gneiss (typically migmatitic) that does not record its former high-pressure (HP) conditions in its equilibrium mineral assemblage; rather, it records the conditions of emplacement and cooling in the mid/shallow crust. Mafic rocks included in gneiss may, however, contain a fragmentary record of a HP history, and are evidence that their host rocks were also deeply sourced. An excellent example of exhumed deep crust that retains a partial HP record is in the Montagne Noire dome, French Massif Central, which contains well-preserved eclogite (garnet+omphacite+rutile+quartz) in migmatite in two locations: one in the dome core and the other at the dome margin. Both eclogites record P ~ 1.5 ± 0.2 GPa at T  ~  700 ± 20°C, but differ from each other in whole-rock and mineral composition, deformation features (shape and crystallographic preferred orientation, CPO), extent of record of prograde metamorphism in garnet and zircon, and degree of preservation of inherited zircon. Rim ages of zircon in both eclogites overlap with the oldest crystallization ages of host gneiss at c. 310 Ma, interpreted based on zircon rare earth element abundance in eclogite zircon as the age of HP metamorphism. Dome-margin eclogite zircon retains a widespread record of protolith age (c. 470–450 Ma, the same as host gneiss protolith age), whereas dome-core eclogite zircon has more scarce preservation of inherited zircon. Possible explanations for differences in the two eclogites relate to differences in the protolith mafic magma composition and history and/or the duration of metamorphic heating and extent of interaction with aqueous fluid, affecting zircon crystallization. Differences in HP deformation fabrics may relate to the position of the eclogite facies rocks relative to zones of transpression and transtension at an early stage of dome development. Regardless of differences, both eclogites experienced HP metamorphism and deformation in the deep crust at c. 310 Ma and were exhumed by lithospheric extension—with their host migmatite—near the end of the Variscan orogeny. The deep crust in this region was rapidly exhumed from ~50 to <10 km, where it equilibrated under low-P/high-T conditions, leaving a sparse but compelling record of the deep origin of most of the crust now exposed in the dome.     

Âge des orthogneiss de la zone axiale de la Montagne Noire (France) par la méthode 87Rb-87Sr

The basement of Montagne Noire (South France), as represented by the orthogneisses of the axial zone, has been dated by means of whole rocks isochron at 530 m.y. This age is discussed with the assistance of petrographic observations and compared to similar results obtained by other authors in the Massif Armoricain and eastern Pyrénées ranges. The high initial ratio ⁸⁷Sr/⁸⁶Sr allows one to hypothesize that these igneous masses were originated from a segment of a preexisting crust.

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Contribution I.P.G.N.S. No 8.     

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

Abstract

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.     

Timing and duration of partial melting and magmatism in the Variscan Montagne Noire gneiss dome (French Massif Central)

International Journal of Earth Sciences - Unravelling the detailed pressure–temperature–time-deformation (P–T–t-D) evolution of magmatic and metamorphic rocks provides...     

The tectono‐metamorphic evolution of the very low‐grade hangingwall constrains two‐stage gneiss dome formation in the Montagne Noire (Southern France)

The Montagne Noire in the southernmost French Massif Central is made of an ENE‐elongated gneiss dome flanked by Palaeozoic sedimentary rocks. The tectonic evolution of the gneiss dome has generated controversy for more than half a century. As a result, a multitude of models have been proposed that invoke various tectonic regimes and exhumation mechanisms. Most of these models are based on data from the gneiss dome itself. Here, new constraints on the dome evolution are provided based on a combination of very low‐grade petrology, K–Ar geochronology, field mapping and structural analysis of the Palaeozoic western Mont Peyroux and Faugères units, which constitute part of the southern hangingwall of the dome. It is shown that southward‐directed Variscan nappe‐thrusting (D₁) and a related medium‐P metamorphism (M₁) are only preserved in the area furthest away from the gneiss dome. The regionally dominant pervasive tectono‐metamorphic event D₂/M₂ largely transposes D₁ structures, comprises a higher metamorphic thermal gradient than M₁ (transition low‐P and medium‐P metamorphic facies series) and affected the rocks between c. 309 and 300 Ma, post‐dating D₁/M₁ by more than 20 Ma. D₂‐related fabrics are refolded by D₃, which in its turn, is followed by dextral‐normal shearing along the basal shear zone of both units at c. 297 Ma. In the western Mont Peyroux and Faugères units, D₂/M₂ is largely synchronous with shearing along the southern dome margin between c. 311 and 303 Ma, facilitating the emplacement of the gneiss dome into the upper crust. D₂/M₂ also overlaps in time with granitic magmatism and migmatization in the Zone Axiale between c. 314 and 306 Ma, and a related low‐P/high‐T metamorphism at c. 308 Ma. The shearing that accompanied the exhumation of the dome therefore was synchronous with a peak in temperature expressed by migmatization and intrusion of melts within the dome, and also with the peak of metamorphism in the hangingwall. Both, the intensity of D₂ fabrics and the M₂ metamorphic grade within the hangingwall, decrease away from the gneiss dome, with grades ranging from the anchizone–epizone boundary to the diagenetic zone. The related zonation of the pre‐D₃ metamorphic field gradients paralleled the dome. These observations indicate that D₂/M₂ is controlled by the exhumation of the Zone Axiale, and suggest a coherent kinematic between the different crustal levels at some time during D₂/M₂. Based on integration of these findings with regional geological constraints, a two‐stage exhumation of the gneiss dome is proposed: during a first stage between c. 316 and 300 Ma dome emplacement into the upper crust was controlled by dextral shear zones arranged in a pull‐apart‐like geometry. The second stage from 300 Ma onwards was characterized by northeast to northward extension, with exhumation accommodated by north‐dipping detachments and hangingwall basin formation along the northeastern dome margin.     

Faunal dating of the late Cenozoic of southern Africa,with special reference to the Carnivora

Since chronometric data comparable to that available from the late Cenozoic succession of East Africa have not, and probably cannot be obtained in southern Africa, faunal dating methods retain their traditional significance in the latter area. Five successive late Cenozoic mammal ages, the Namibian, Langebaanian, Makapanian, Cornelian, and Florisian have been proposed as a framework for discussions relating to the chronology of southern African mammalian faunas. The Namibian fauna is poorly known, but is evidently of Miocene age. It is not discussed in this paper. The Langebaanian fauna is well known only from the prolific occurrence at the type site of Langebaanweg and is Pliocene in age. Makapanian faunas are best represented at the Transvaal Australopithecine sites and probably overlap the Plio-Pleistocene boundary. The Cornelian fauna is not as well known as others, the largest assemblage having been recorded from Elandsfontein, although this assemblage is one which is unfortunately temporally heterogeneous. Florisian faunas have been recovered from a relatively large number of localities, including several for which there are radiometric dates. The recorded southern African fossil Carnivora are listed and the local evolution of this group is discussed in terms of the mammal age chronology. It is concluded that secure faunal dating of individual fossil occurrences is enhanced by an appreciation of the nature of changes undergone in evolutionary lineages, while an uncritical knowledge of recoreded taxa is less useful.     

Early formation of K-feldspar in shallow-marine sediments at near-surface temperatures (southern Israel): evidence from K-Ar dating

Authigenic K‐feldspar was investigated in two Albian to Turonian sections in Israel using K‐Ar and Ar‐Ar dating, X‐ray diffraction, scanning electron microscopy and chemical analysis. Both sections comprise a similar succession of shallow‐marine limestones, dolomites and marls, with some sandstone and shale beds of continental origin. The HCl‐insoluble residue fraction of the studied samples consists of clays, quartz, feldspars, goethite and trace amounts of heavy minerals. Most of the insoluble residues have a relatively high K‐feldspar content that has an adularia habit and is concentrated in the 4–7 µm size fraction. The authigenic origin of the K‐feldspar in the fine silt fraction is indicated by its high content relative to quartz, the uniform and idiomorphic shape of the crystals and its limited size range. Of the fine silt (4–7 or 4–10 µm) separates, 40% have ages that are similar to stratigraphic ages within the analytical and biostratigraphic errors. Ar‐Ar dating of a fine silt fraction with 94% K‐feldspar (4–10 µm, sample GYP 7) yields a plateau age identical to the total gas age and similar to the stratigraphic age. These results indicate that the K‐Ar age is not a mixture between detrital and late diagenetic K‐feldspar ages, but is rather an age of formation within a few million years after deposition. It is suggested that the early formation of the K‐feldspar was associated with dolomitization and was induced by residual brines as part of a reflux process.     

Uranium/thorium dating of ferricretes from mid- to late pleistocene glacial sediments,western Tasmania,Australia

The uranium/thorium (U/Th) dating method was applied to pedogenic/diagenic ferricretes developed within glacial drifts from the northern West Coast Ranges of Tasmania. The dates derived from the method are, by definition, minima for the sediments, with the ferricretes being secondary mineral deposits which appear in the form of bands, horizons or nodules comprised of indurated masses of iron oxy(hydr)oxides developed post-depositionally within the host sediment or soil. The absence of such deposits in well-identified drift sediments of the last glaciation suggests the development of ferricrete horizons in glacial drifts occurred during earlier interstadials and interglacials. This has been confirmed by the uranium/thorium dating of ferricrete bands within glacigenic sediments obtained from three drill cores from the Boco Valley. The ages derived were found to be consistent with estimates for the timing of glaciation derived from palaeomagnetic analyses, radiocarbon dating and relative dating techniques. The results show that the uranium/thorium method can be used to successfully date ferricretes obtained from within glacial sediments of ages <350000 yr and provides valuable information regarding the minimum age of the host sediment. © 1997 John Wiley & Sons, Ltd.     

Thermal interaction of middle and upper crust during gneiss dome formation: example from the Montagne Noire (French Massif Central)

This paper aims to decipher the thermal evolution of the Montagne Noire Axial Zone (MNAZ, southern French Massif Central) gneiss core and its metasedimentary cover through determination of P–T paths and temperature gradients. Migmatitic gneiss from the core of the dome record a clockwise evolution culminating at 725 ± 25 °C and 0.8 ± 0.1 GPa with partial melting, followed by a decompression path with only minor cooling to 690 ± 25° C and 0.4 ± 0.1 GPa. Field structural analyses as well as detailed petrological observations indicate that the cover sequence experienced LP‐HT metamorphism. Apparent thermal gradients within the cover were determined with garnet–biotite thermometry and Raman Spectroscopy on Carbonaceous Matter. High‐temperature apparent gradients (e.g. ～530 °C km^?1 along one transect) are explained by late brittle–ductile extensional shearing evidenced by phyllonites that post‐date peak metamorphism. In areas where normal faults are less abundant and closely spaced, gradients of ～20 to 50 °C km^?1 are calculated. These gradients can be accounted for by a combination of dome emplacement and ductile shearing (collapse of isotherms), without additional heat input. Finally, the thermal evolution of the MNAZ is typical for many gneiss domes worldwide as well as with other LP‐HT terranes in the Variscides.     

Early Permian extensional shearing of an Ordovician granite: The Saint-Eutrope “C/S-like” orthogneiss (Montagne Noire,French Massif Central)

Dating the magmatic events in the Montagne Noire gneiss dome is a key point to arbitrate between the different interpretations of the Late Carboniferous–Early Permian tectonics in this southern part of the Variscan belt. The Saint-Eutrope orthogneiss crops out along the northern flank of the dome. We show that the protolith of this orthogneiss is an Ordovician granite dated at 455 ± 2 Ma (LA-ICP-MS U-Pb dating on zircon). This age is identical to that previously obtained on the augen orthogneiss of the southern flank, strongly suggesting that both orthogneiss occurrences have the same Ordovician protolith. The Saint-Eutrope orthogneiss experienced intense shearing along the Espinouse extensional detachment at ca. 295 Ma (LA-ICP-MS U-Pb-Th on monazite), an age close to that determined previously on mica by the ³⁹Ar-⁴⁰Ar method and contemporaneous with the emplacement age of the syntectonic Montalet granite farther to the west. This normal sense shearing reworked previous fabrics related to Variscan thrusting that can be still observed in the augen orthogneiss of the southern flank, and is responsible for the spectacular “C/S-like” pattern of the Saint-Eutrope orthogneiss. This work also shows that care is needed when dealing with C/S-type structures, since they can develop not only in syntectonic intrusions, but also in orthogneisses affected by an intense secondary deformation, at decreasing temperature.