U-Th-Pb LA-ICPMS (Laser Ablation − Inductively Coupled Plasma Mass Spectrometry) has the power to elucidate the timing of metamorphism, deformation, migmatization, and plutonism. The Montagne Noire Variscan gneiss dome (southern French Massif Central) has been extensively studied, but interpretations of geochronology data remain highly debated. In tribute to Jean-Louis Paquette’s work, we first present the results of the last 10 yr of reflection on our knowledge of the Montagne Noire dome, highlighting the various contributions of LA-ICPMS U-Th-Pb geochronology. Then, based on new structural, petrological and U-Th-Pb data obtained in the southeastern part of the Montagne Noire Axial Zone (MNAZ), in “Les Gorges d’Héric” valley, we provide new insight into the structure and strain partitioning in this region and propose the existence of a crustal-scale dextral shear zone that we name the “Gorges d’Héric Shear Zone” (GHSZ). The LA-ICPMS U-Th-Pb age on monazite, obtained on a pegmatite cross-cutting vertical structures on the southern margin of the GHSZ, constrains the end of ductile deformation at ca. 295 Ma in that part of the shear zone. Further north, a localized deformation corridor is accompanied by significant fluid circulation recorded in retrogressed metamafic lenses. Petrographic observations reveal a complex mineralogical evolution characterised by the formation of garnet, amphibole, orthopyroxene, and secondary spinel, interpreted as the product of fluid-driven mineral reactions. Potassium metasomatism is responsible for the development of syn-kinematic biotite. The final retrograde stage is marked by the development of amphibole + chlorite assemblages and late serpentinization reactions. U-Th-Pb results obtained on zircon grains highlight a long and complex geological history. Three age populations have been identified and associated with: (1) the magmatic protolith emplacement likely to be linked to early Paleozoic event(s), (2) the existence of a ca. 315 Ma event interpreted as a high-grade metamorphic imprint and (3) a last corresponding to most of the zircon U-Pb data gives a concordia age of 280.6 ± 2.5 Ma, interpreted as the age of zircon (re)crystallization during localized deformation and aqueous fluid circulation along the GHSZ, probably related to the well-known early Permian regional volcanic episode. Within the GHSZ, the geochronological dataset spans the period from 320 Ma to 280 Ma, suggesting that this is a significant structure.

Recent work has revealed that the Montagne Noire dome, located in the foreland of the Variscan belt (French Massif Central), contains a record of nearly coeval, late Variscan eclogitization and migmatization. Given these new results, it is important to understand the chronology of events that produced high-grade metamorphism and exhumation. Using U-Th-Pb dating of monazite, this study confirms ages of similar to 315-300 Ma for high-T metamorphism in the augen gneiss that makes up a large fraction of the Montagne Noire dome, and documents for the first time similar to 295 Ma monazite ages in compositionally varied fine-grained gneisses that form folded continuous layers within the core of the dome. The fine-grained gneiss layers are intensely sheared and are interpreted to have localized late, high-T deformation in the core of the dome. These sheared fine-grained gneisses form a network of shear zones that were kinematically linked to the extensional and strike-slip deformation zones that exhumed the Montagne Noire dome in a pull-apart (s.l.) domain. Continued deformation-recrystallization and fluid flow within these shear zones likely drove rejuvenation of monazite for similar to 5 million years after much of the melt had crystallized in the Montagne Noire dome.

In many orogens, high-pressure (HP) metamorphic rocks such as eclogite occur as lenses in quartzofeldspathic gneiss that equilibrated at much lower pressures. The pressure-temperature-time (P-T-t) history of eclogite relative to host gneiss provides information about mechanisms and timescales of exhumation of orogenic crust. The Montagne Noire of the southern Massif Central, France, is an eclogitebearing gneiss (migmatite) dome located at the orogen-foreland transition of the Variscan belt. Results of our study show that it contains the youngest eclogite in the orogen, similar in age to migmatite and granite that crystallized under low-pressure conditions. P-T conditions for an exceptionally unaltered eclogite from the central Montagne Noire were estimated using a pseudosection supplemented by garnet-clinopyroxene and Zr-in-rutile thermometry. Results indicate peak P similar to 1.4 GPa and T similar to 725 degrees C for Mg-rich garnet rim (50 mol% pyrope) + omphacite (36 mol% jadeite) + rutile + quartz. U-Pb geochronology (LA-ICP-MS) of 16 zoned zircon grains yielded similar to 360 Ma (4 cores) and similar to 315 Ma (12 rims and cores). Rare earth element abundances determined by LA-ICP-MS for dated zircon are consistent with crystallization of similar to 315 Ma zircon under garnet-stable, plagioclase-unstable conditions that we interpret to indicate high pressure; in contrast, the similar to 360 Ma zircon core corresponds to crystallization under lower pressure plagioclase-stable conditions. Based on garnet zoning and inclusion suites, rutile textures and Zr zoning, P-T results, and zircon petrochronology, we interpret the similar to 315 Ma date as the age of eclogite-facies metamorphism that only slightly preceded dome formation and crystallization at 315-300 Ma. This age relation indicates that eclogite formation at high pressure and migmatite dome emplacement at low pressure were closely spaced in time. We propose that collapse-driven material transfer from the hot orogen to the cool foreland resulted in thickening of the orogen edge, leading to eclogite facies metamorphism of the deep crust. Soon after, the low-viscosity partially molten crust flowed from the plateau toward the foreland, incorporating and exhuming eclogite. The P-T-t history of the Montagne Noire eclogite shows that some dome material ascended from >40 km depth to shallow crustal levels, likely in a single decompression event, and that migmatite domes are therefore very efficient at exhuming the deep crust. (C) 2015 Elsevier B.V. All rights reserved.

New U–Pb monazite, zircon, and xenotime ages date the Late Carboniferous crystallization of the anatectic Vialais granite in the Montagne Noire Axial Zone and the high-temperature deformation and metamorphism of the augen gneiss (Ordovician granite protolith) from which the granite was likely derived. The U–Pb monazite ages obtained from the augen gneiss (308±3Ma), late kinematic Vialais granite (303±4Ma), and post-kinematic leucogranite (298±2Ma), bracket the high-temperature deformation and metamorphism at ~310–300Ma, clearly postdating regional contraction and nappe emplacement (>320Ma). The planar–linear and locally linear fabrics in the augen gneiss outline a regional-scale double dome structure (Caroux and Espinouse sub-domes) containing smaller (km-scale) upright folds. Compared to the Caroux sub-dome, the larger, migmatitic Espinouse sub-dome contains abundant leucosome, leucogranite bodies and late-kinematic intrusions (Vialais granite). The Vialais granite displays a weak magmatic foliation that defines the main layering of the Espinouse migmatite. Ellipsoidal quartz–sillimanite nodules concentrated above the roof of the granite recorded an increment of strain (vertical shortening, E–NE elongation) during granite emplacement and crystallization, consistent with the extension event that exhumed the domes. These new geochronological and structural data suggest that the Montagne Noire double dome formed in a pull-apart structure within a dextral strike–slip system. Upright folding in the sub-domes has been traditionally assigned to a regional contraction event; alternatively, we propose that local contraction is associated with the convergence of low-viscosity crust beneath the upper crust pull-apart. Dynamic models of extension of hot crust indicate that contraction at depth is generated by flow of low-viscosity orogenic crust converging and rising to fill the gap created by upper crust extension. This interpretation solves the long-standing problem of apparent coeval contraction and extension in the Montagne Noire double dome and establishes migmatite dome emplacement in strike–slip corridors as a style of late Variscan tectonics.

The Dien Bien Phu (DBP) Fault strikes nearly perpendicular to the general tectonic framework, separating northwest Vietnam into two compartments. The DBP and Muong Lay granites are exposed respectively east and west of the fault. The U/Pb age of 277±2Ma obtained for the Muong Lay granite is similar to the age of the Chieng Khuong complex exposed within the Song Ma structure east of the DBP Fault. These two magmatic intrusions are considered as belonging to a same tectonic structure related to the subduction of the Song Ma Ocean under the Indochina Block during the Indosinian orogeny. The DBP Fault with a total post Permian dextral offset of 30–40km shifts the Song Ma Suture. It could then be connected to the Ailao Shan–Shuanggou suture to the north. The undeformed DBP granite gave a U/Pb age of 230±1Ma sealing the Indosinian deformation on the fault. The Ladinian–Carnian activity of the fault is confirmed by the 40Ar/39Ar (biotite) age of 228±4Ma obtained from the paragneiss forming the fault zone. A geodynamic model is proposed to describe the various tectonic phases recorded by on the DBP Fault. It associates field data, stratigraphic observations, and geochronology analyses (U/Pb, Ar/Ar, fission tracks). Following the Late Triassic exhumation of the granite and the end of the Indosinian orogeny marked by the Norian unconformity, the granite was buried under Jurassic and Lower Cretaceous sediments. A strong cooling event during the early Late Cretaceous (100Ma) is associated with the reactivation of the DBP Fault and the accommodation of a southward thrusting of the Song Da terrane onto the Truong Son Belt. This newly recognised tectonic phase could be associated to the subduction of the Paleo-Pacific plate under the Indochina Block. No data are available yet to constrain the Tertiary evolution of the DBP system. However, the Tertiary deformation is likely similar to the Cretaceous one.

During the development of the Variscan orogeny, large amounts of granitic melt were produced, giving rise to the intrusion of granitoids at different structural levels. Despite numerous studies, ages available from previous work on the Cévennes granites remain largely imprecise. In order to better constrain the age and emplacement mode of these granites, we have combined U–Pb dating on monazites and zircons and 40Ar/39Ar dating on biotites with petrological observations, major element chemical analysis and SEM zircon imaging on five samples from the Aigoual–St Guiral–Liron and Mont Lozère granitic massifs. The results revealed that granitic intrusions and cooling in Southern Cévennes occurred in a short time span at ∼306 Ma after the main episode of regional metamorphism. Petrological and chemical data suggest that they result from a mixing between mantle-derived basic magmas (lamprophyres) and lower crust acid magmas. At a regional scale the production of these melts occurred at the end of crustal thickening induced by nappe stacking, at the same time as the late anatectic events recorded further north in the Velay dome and the granulite facies metamorphism recorded in metasedimentary granulite enclaves brought up by Tertiary volcanoes of the Velay area (Bournac).

We present new U-Pb results on felsic Augen orthogneisses from the Axial Zone of the Montagne Noire (French Massif Central). The data indicate Ordovician ages, 456 +/- 3 and 450 +/- 6 Ma for two samples collected at 'Pont-de-Larn' and 'Gorges d'Heric', respectively. These ages are interpreted as the igneous emplacement age of the granitic protolith. (C) 2003 Academie des sciences. Publie par Elsevier SAS. Tous droits reserves.

Zircon UPb dating by SIMS of the Mont-Louis granite yields an age of 305±5 Ma, intrepreted to reflect the igneous emplacement age of the massif. It is in agreement with the Hercynian syntectonic character of Pyrenees granite. 40Ar/39Ar on hornblende, biotite and K-feldspar permit, to estimate the massif cooling. A rapid temperature decrease (≈30 °C/Ma) is revealed from Westphalian to Late Stephanian, coeval with the emplacement of a laccolithe in the upper crust. Then, the cooling rate decreases to ≈1 °C/Ma. This would be consistent with a long time residence for the pluton from the Late Palaeozoic to the Early Cainozoic at 6–8 km depth. To cite this article: O. Maurel et al., C. R. Geoscience 336 (2004). Un âge UPb sur zircon de 305±5 Ma est déterminé sur le granite de Mont-Louis. Il confirme le caractère syntectonique de la mise en place des plutons pyrénéens. L'application de la méthode 40Ar/39Ar sur hornblende, sur biotite et sur feldspath-K permet d'estimer les modalités de refroidissement du massif. On révèle ainsi une diminution rapide de la température d'environ 30 °C/Ma du Westphalien jusqu'à la fin du Stéphanien correspondant à la mise en place d'un laccolithe dans la croûte supérieure. Le taux de refroidissement diminue ensuite jusqu'à environ 1 °C/Ma. Ainsi, le pluton serait resté à 6–8 km de profondeur depuis la fin du Paléozoı̈que jusqu'au Cénozoı̈que sans être affecté par un événement thermo-tectonique. Pour citer cet article : O. Maurel et al., C. R. Geoscience 336 (2004).

The N-S Jacobina foreland basin is a Paleoproterozoic basin (2080 to 1900 Ma) of stacked detrital and schist formations developed along a sinistral wrench - overthrust fault of the Transamazonian orogeny. At least five successive stages are identified in the westward progradation of the basin, wherein bedding-parallel strike-slip thrusts and prograde metamorphism indicate progressive involvement in the Transamazonian collision. Shearing and associated metamorphic recrystallization indicate a high geothermal gradient and a gradual loading of the underthrust units, with those to the east being deeper seated than those to the west. Basic - ultrabasic dykes and granite stocks cross-cut the lithologies and contributed to the thermal evolution. Conglomerates and chaotic breccia layers found in several formations mark periods of tectonic instability accompanied by erosion of the earlier stages of basin development. Multistage hydrothermal activity is indicated by a widespread alteration with gold-bearing quartz lodes., conglomerates showing a sulphide-gold-Cr-mineral association, leaching of ultrabasic rocks, and emerald mineralization related to late-tectonic granite emplacement. The economic gold-bearing elastic sediments (conglomerates and minor quartzites deposited during the fourth unit/stage of basin development) have generally been interpreted according to the "modified paleoplacer" model. Such a model, which implies the existence of a gold preconcentration within the quartz-pebble conglomerates, is supported by the lithological controls, reworked pebbles (e.g. cataclastic quartz, quartz-pyrite mylonite and rare fuchsite-bearing rocks) that had been hydrothermally altered during an earlier hydrothermal stage, and lead isotopic compositions revealing an Archean inheritance that is partly preserved in galena inclusions. New basin-scale structural and metallogenic data obtained by the authors, suggest a basin model characterized by successive cycles of sedimentation-faulting/thrusting-erosion ("cannibalism" model) that favours both a "paleoplacer" model and a "mesothermal shear-zone-related epigenetic" model. Heterochronous shear zones, formed between 1988 and 1912 Ma during the development of the basin, acted as feeder zones for postsedimentary mesothermal quartz-tourmaline muscovite/fuchsite lodes. which constitute a possible source of the reworked hydrothermally altered pebbles. These shear zones controlled (a) an acid leaching of the sediments and the subsequent development of Al-rich assemblages interpreted as advanced argillic alteration zones, and (b) a sulphidation process affecting detrital ilmenite and magnetite and responsible for the high concentration of gold within shear veins containing assemblages of Cr-Ni-bearing minerals (euhedral chromite, Cr-rutile, linnaeite, Cr-tourmaline and fuchsite) and euhedral pyrite that crystallized over andalusite. The conglomerates, with their pore and fissure permeability, acted as a "reservoir and trap" for the mineralizing fluids to give a "hydrothermal shear-reservoir" model. The homogeneous sulphur isotopic composition of the sulphides (between -0.5parts per thousand and 1.3parts per thousand) and the delta(18)O-deltaD values of the hydrothermal fluid (+ 8 +/- 1parts per thousand and - 36 +/- 3parts per thousand, respectively) suggest a magmatic source for the fluids. Moreover, the high thermal regime that characterizes the Jacobina basin could reflect a contribution from both the regional granitic magmatism and the basic-ultrabasic dyke network responsible for the Cr-(Ni-Co) mineralogical signature of the mineralization.Under the oxidation conditions of the Paleoproterozoic, the sulphidation of the gold-bearing conglomerates would depend largely on the extent of the hydrothermal-magmatic processes. (C) 2002 Elsevier Science B.V. All rights reserved.

In most orogenic belts, the age of HP metamorphism and subsequent exhumation events still remain open to debate since geochronology can yield results which appear to conflict with the closure temperature concept [Dodson, M.H., 1973. Contrib. Mineral. Petrol. 40, 259–274], and because the behaviour of daughter radiogenic isotopes under HP to UHP conditions is poorly constrained. To obtain new data on isotope migration under high-pressure conditions, two undeformed HP metagranites with partially preserved magmatic assemblages from the French Variscan belt were investigated using the 40Ar/39Ar laser probe and U–Pb ion probe methods. In both cases, 40Ar/39Ar biotite and U–Pb zircon ages are consistent and could be related to the emplacement of pre-orogenic granites, despite petrological evidence of a strong metamorphic overprint during the Variscan orogeny. Temperatures experienced by the granites during subduction and exhumation events were more than 400 °C above that normally estimated for argon retention in biotite, but failed to cause significant resetting of the mica 40Ar/39Ar chronometer. Only a weak intragrain redistribution of argon is evidenced with the laser probe up to the contact with metamorphic garnet fringing biotite. By contrast, a complete resetting of biotite ages occurs in gneisses enclosing the metagranites. These results suggest that, in these dry undeformed HP metagranites, the thermally activated diffusion was relatively ineffective and that recrystallisation is the main process which controlled isotopic exchanges of Ar and Pb. It is likely that the absence of pervasive deformation and fluid circulation has also exercised some control on the preservation of pre-metamorphic isotopic signature in the studied rocks. The possible influence of several other parameters is also discussed. This example reveals that recovering thermochronological information in high-pressure metamorphic rocks is not straightforward and that great caution must be paid in the use of ages for the reconstruction of P–T–t paths.

The thermal history of the Velay anatectic dome, which extends over 10000 km(2) in the southern part of the French Massif Central, is still poorly constrained, especially as to its duration. We studied by U-Pb chronology the accessory minerals (zircon, monazite, apatite) of three samples collected on the southern edge, which represent the main types recognized in the dome and the best candidates to constrain the age of the two migmatization stages (M3 and M4) already described. The M3 stage is constrained at 314+/-5 Ma by the U-Pb data on abraded monazites from the leucosome part of a paragneiss. The M4 stage has been dated with monazites from the cordierite-bearing Velay granite at 301+/-5 Ma. Monazites from the paragneiss and the Velay granite even after an abrasion process are still discordant in the concordia diagram, and monazites from the third sample, an orthogneiss linked to the M3 stage, reveal an unusual pattern with a reverse discordia. Zircons in each case demonstrate inheritance, presenting any precise age determination. Even in the case of the Velay granite where zircon fractions define a normal discordia pattern, single grain analysis reveal the presence of inherited lead. The apatites in all three samples indicate an age at 289+/-5 Ma, which is interpreted as a cooling age during the uplift of the Velay region.

The Ax-les-Thermes granite is a Variscan leucogranite of anatectic origin, intrusive into the gneisses of the Axial Zone of the Pyrenees. We studied this granite using the U-Pb method on zircon fractions and the direct-evaporation single-zircon-grain method. In the concordia diagram, the experimental points define two arrays, depending on the size of the zircon grains: a reverse discordia for the coarse zircons and a normal discordia for the fine ones. However the ages obtained with these discordias (at 350 and 370 Ma) are inconsistent with previous Rb-Sr dating at 300 Ma and with field evidence for a syn- to late-Variscan intrusion (ca. 300 Ma). This exceptional pattern may be caused by a combination of different processes including the coexistence of relict and newly crystallized zircons and involving inheritance of old radiogenic lead and various stages of lead loss

The Pierres-Plantees uranium mine (S.E. French Massif Central) is located in an alteration pipe which crosscuts the Margeride monzogranite as well as sills of the Grandrieu peraluminous granite. This alteration occurred at temperatures greater than 300-degrees-C and mainly involved quartz leaching which significantly increased the permeability of the altered rocks. During pitchblende deposition, within the pipe, fluid/rock interactions induced the crystallization of illite.U-Pb dating of pitchblende from the first uranium ore stage provides an age of mineralization of 188 +/- 12 Ma. This age is clearly younger than the emplacement of the surrounding granitoids (324 +/- 7 Ma and 305 +/- 9 Ma) or the formation of the alteration pipe (269 +/- 9 Ma). During Liassic times uranium was mobilized from an unknown source, the most probable being the Grandrieu leucogranite, by hydrothermal oxidizing fluids at temperatures between 150-250-degrees-C ; U mineralization resulted from reaction between these fluids and Ca-Mg rich rocks like quartz diorites and to a smaller extent, the Margeride monzogranite. As with other U ore deposits in southern France (Lodeve and Bertholene), the formation of Liassic pitchblende is related to a period of regional hydrothermal activity during a distensive phase i.e. the opening of the Tethys Ocean.A strong alteration of the 188-Ma old U ore occurred at a late stage through large-scale transformation of pitchblende into coffinite, associated with the smectitization of silicates. Pitchblende incorporates up to 11 % SiO2 but contains less Ca and Pb than the original U-ore ; it is transformed into intermediate phases between UO2 and USiO4, which induces leakage of U-238 intermediate daughter elements. This reworking of Liassic pitchblendes along oxidation-reduction fronts is related to low-temperature fluid circulation, and the uranium crystallized as pitchblende nodules about 100 Ma ago.

U-Pb systematics is applied to the study of two leucogranites from the French Massif Central : the "La Porcherie" leucogranite which cross cuts major Variscan thrusts in the Saint-Germain-les-Belles area (Limousin), and the "Grandrieu" leucogranite which is associated with the Margeride calc-alkaline granite (Lozère). It is shown that : 1) The U-Pb system of zircon is unsuitable for the dating of the emplacement. For "La Porcherie" leucogranite, the zircons contain xenocryst core with euhedral overgrowths ; this old inherited component seems to be characteristic for leucogranites associated with major shear zones (Galicia, South Brittany). Such a feature is absent in the "Grandrieu" leucogranite. 2) Monazites appear to be a good tool for the geochemical and geochronological studies of leucogranites. U-Pb data for these monazites yield sub-concordant ages interpreted as emplacement ages : 317 ± 3 Ma for "La Porcherie" leucogranite, 305 ± 4 Ma for "Grandrieu" leucogranite.

Other| January 01, 1986 La faille NW-SE du Pays de Bray, un decrochement ductile dextre hercynien; deformation a 330 Ma d'un granite a 570 Ma dans le sondage Pays de Bray 201 Philippe Matte; Philippe Matte Author U.S.T.L., U.E.R. 9, Montpellier 34060, France Search for other works by this author on: GSW Google Scholar Jean-Patrick Respaut; Jean-Patrick Respaut Author Search for other works by this author on: GSW Google Scholar Henri Maluski; Henri Maluski Author Search for other works by this author on: GSW Google Scholar Joel R. Lancelot; Joel R. Lancelot Author Search for other works by this author on: GSW Google Scholar Maurice Brunel Maurice Brunel Author Search for other works by this author on: GSW Google Scholar Author and Article Information Philippe Matte Author U.S.T.L., U.E.R. 9, Montpellier 34060, France Jean-Patrick Respaut Author Henri Maluski Author Joel R. Lancelot Author Maurice Brunel Author Publisher: Société Géologique de France First Online: 07 Mar 2017 Online Issn: 1777-5817 Print Issn: 0037-9409 GeoRef, Copyright 2012, American Geosciences Institute. Reference includes data from PASCAL, Institute de l'Information Scientifique et Technique, Vandoeuvre-les-Nancy, France Bulletin de la Société Géologique de France (1986) II (1): 69–77. https://doi.org/10.2113/gssgfbull.II.1.69 Article history First Online: 07 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Philippe Matte, Jean-Patrick Respaut, Henri Maluski, Joel R. Lancelot, Maurice Brunel; La faille NW-SE du Pays de Bray, un decrochement ductile dextre hercynien; deformation a 330 Ma d'un granite a 570 Ma dans le sondage Pays de Bray 201. Bulletin de la Société Géologique de France 1986;; II (1): 69–77. doi: https://doi.org/10.2113/gssgfbull.II.1.69 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin de la Société Géologique de France Search Advanced Search This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.