Âge, durée et enregistrement du métamorphisme de haute pression dans le massif Central

Subduction is one of the key stages of the mountain building processes. It leads to the development of high-pressure (HP) metamorphism in the rocks that typically equilibrate in the blueschist or eclogite-facies conditions. Dating the HP metamorphism and estimating its intensity is therefore a major challenge when reconstructing geodynamics through time. Through a petrological and geochronological study, this PhD dissertation aims to better constrain conditions, durations and timings of HP metamorphism in the southern French Massif Central (European Variscan Belt). The massifs of Najac, Montagne Noire and Lévézou were investigated by a multi-method geochronological approach (zircon, rutile and apatite U-Pb dating, garnet Lu-Hf and Sm-Nd dating, biotite and muscovite 40Ar-39Ar dating) associated with a petrological analysis including numerical modelling of phase equilibria (Theriak-Domino and THERMOCALC). The Najac eclogites reached 560-630 °C at 15-20 kbar and the prograde part of the highpressure metamorphic event lasted for ~ 7 Myr starting at ~ 383 and peaking at ~ 376 Ma. Eclogites hosted in sillimanite-bearing migmatites in the Montagne Noire dome (French Massif Central) reached c. 750°C, 21 kbar before significant decompression at high temperatures. However, none of the obtained geochronological dates could be associated with the HP event. The study of the Najac massif and the Montagne Noire Dome highlight potential decoupling between the REE and the U-Pb isotopic systems. The emplacement of the protoliths of felsic and mafic HP rocks in the Lévézou Massif was estimated at ca. 470 Ma. HP metamorphism peaked at 21-23 kbar and 680-800°C at ~358 Ma. The subsequent fast exhumation reached 8-9.5 kbar and ~600°C at ~352 Ma, highlighting a very fast exhumation followed by a cooling rate of 50°C/Ma. Granites from the Lévézou massif display kyanite-garnet-muscovite-quartz pseudomorphs after cordierite that equilibrated at ca. 15-17 kbar and ~670°C. The major deformation in these granites is associated with the exhumation stage, locally accompanied by partial melting, and seems to occur in different episodes, from ~352 Ma to ~340 Ma. On a larger scale, these results are fully in line with the HP ages described in the Variscan Belt and allow to reconsider the tectonics in the French Massif Central.Les processus de subduction sont une étape clé de la formation des orogènes et induisent un métamorphisme de haute pression, localisé dans les faciès des schistes bleus et éclogites. Caractériser la durée et l’intensité de ce métamorphisme est ainsi une étape cruciale puisque amenant des contraintes quantitatives sur la géodynamique d’un orogène. Par une étude pétrologique et géochronologique, cette thèse a ainsi pour objectif de préciser les conditions et durées du métamorphisme de haute pression dans le sud du Massif Central Français (chaîne Varisque), à travers l’étude des massifs de Najac, de la Montagne Noire et du Lévézou. Cette thèse associe ainsi une géochronologie multiméthodes (U-Pb sur zircon, rutile et apatite, Lu-Hf et Sm-Nd sur grenat, 40Ar-39Ar sur biotite et muscovite) à des analyses pétrologiques impliquant des modélisations numériques d’équilibres de phases (Theriak-Domino et THERMOCALC). L’étude d’une éclogite du massif de Najac a ainsi permis de déterminer des conditions de 15 à 20 kbar et 560 à 630°C pour le métamorphisme de haute pression. Le début du faciès éclogite y est daté à ~383 Ma, tandis que le pic du métamorphisme éclogitique est atteint à 375.7 ± 1.2 Ma. La datation des éclogites de la Montagne Noire n’a pas permis de préciser un âge solide de l’évènement de haute pression. Néanmoins, les conditions de pression et température du faciès éclogite y sont estimées à ~ 21 kbar et ~ 750°C. L’étude des massifs de Najac et de la Montagne Noire a mis en évidence de potentiels découplages entre les systèmes de terres rares et le système isotopique U-Pb. Les protolithes des terrains éclogitiques du massif du Lévézou, tant mafiques que felsiques, se sont mis en place à ca. 470 Ma. Le métamorphisme éclogitique affectant les roches mafiques est estimé à 21-23 kbar pour 680- 800°C et atteint à ~358 Ma. L’exhumation, bien caractérisée, y est rapide : les terrains éclogitiques atteignent 8-9.5 kbar et ~600°C à ~352 Ma, impliquant une exhumation très rapide, suivie d’un refroidissement de plus de 50°C/Ma. Les granites du massif du Lévézou présentent des pseudomorphoses de cordiérite à disthène-grenat-muscovite-quartz, développées lors du métamorphisme de haute pression et équilibrées à ca. 15-17 kbar et ~670°C. La déformation majeure observée dans ces granites peut s’accompagner de fusion localisée, et semble se produire en différentes étapes, depuis ~352 Ma à ~340 Ma. Replacées dans un contexte général, ces données s’inscrivent pleinement dans les gammes d’âges de la haute pression décrites pour la chaîne varisque et permettent de reconsidérer la tectonique du Massif Central

Timing, duration and record of high-pressure metamorphism in the French massif Central

Les processus de subduction sont une étape clé de la formation des orogènes et induisent un métamorphisme de haute pression, localisé dans les faciès des schistes bleus et éclogites. Caractériser la durée et l’intensité de ce métamorphisme est ainsi une étape cruciale puisque amenant des contraintes quantitatives sur la géodynamique d’un orogène. Par une étude pétrologique et géochronologique, cette thèse a ainsi pour objectif de préciser les conditions et durées du métamorphisme de haute pression dans le sud du Massif Central Français (chaîne Varisque), à travers l’étude des massifs de Najac, de la Montagne Noire et du Lévézou. Cette thèse associe ainsi une géochronologie multiméthodes (U-Pb sur zircon, rutile et apatite, Lu-Hf et Sm-Nd sur grenat, 40Ar-39Ar sur biotite et muscovite) à des analyses pétrologiques impliquant des modélisations numériques d’équilibres de phases (Theriak-Domino et THERMOCALC). L’étude d’une éclogite du massif de Najac a ainsi permis de déterminer des conditions de 15 à 20 kbar et 560 à 630°C pour le métamorphisme de haute pression. Le début du faciès éclogite y est daté à ~383 Ma, tandis que le pic du métamorphisme éclogitique est atteint à 375.7 ± 1.2 Ma. La datation des éclogites de la Montagne Noire n’a pas permis de préciser un âge solide de l’évènement de haute pression. Néanmoins, les conditions de pression et température du faciès éclogite y sont estimées à ~ 21 kbar et ~ 750°C. L’étude des massifs de Najac et de la Montagne Noire a mis en évidence de potentiels découplages entre les systèmes de terres rares et le système isotopique U-Pb. Les protolithes des terrains éclogitiques du massif du Lévézou, tant mafiques que felsiques, se sont mis en place à ca. 470 Ma. Le métamorphisme éclogitique affectant les roches mafiques est estimé à 21-23 kbar pour 680- 800°C et atteint à ~358 Ma. L’exhumation, bien caractérisée, y est rapide : les terrains éclogitiques atteignent 8-9.5 kbar et ~600°C à ~352 Ma, impliquant une exhumation très rapide, suivie d’un refroidissement de plus de 50°C/Ma. Les granites du massif du Lévézou présentent des pseudomorphoses de cordiérite à disthène-grenat-muscovite-quartz, développées lors du métamorphisme de haute pression et équilibrées à ca. 15-17 kbar et ~670°C. La déformation majeure observée dans ces granites peut s’accompagner de fusion localisée, et semble se produire en différentes étapes, depuis ~352 Ma à ~340 Ma. Replacées dans un contexte général, ces données s’inscrivent pleinement dans les gammes d’âges de la haute pression décrites pour la chaîne varisque et permettent de reconsidérer la tectonique du Massif Central.Subduction is one of the key stages of the mountain building processes. It leads to the development of high-pressure (HP) metamorphism in the rocks that typically equilibrate in the blueschist or eclogite-facies conditions. Dating the HP metamorphism and estimating its intensity is therefore a major challenge when reconstructing geodynamics through time. Through a petrological and geochronological study, this PhD dissertation aims to better constrain conditions, durations and timings of HP metamorphism in the southern French Massif Central (European Variscan Belt). The massifs of Najac, Montagne Noire and Lévézou were investigated by a multi-method geochronological approach (zircon, rutile and apatite U-Pb dating, garnet Lu-Hf and Sm-Nd dating, biotite and muscovite 40Ar-39Ar dating) associated with a petrological analysis including numerical modelling of phase equilibria (Theriak-Domino and THERMOCALC). The Najac eclogites reached 560-630 °C at 15-20 kbar and the prograde part of the highpressure metamorphic event lasted for ~ 7 Myr starting at ~ 383 and peaking at ~ 376 Ma. Eclogites hosted in sillimanite-bearing migmatites in the Montagne Noire dome (French Massif Central) reached c. 750°C, 21 kbar before significant decompression at high temperatures. However, none of the obtained geochronological dates could be associated with the HP event. The study of the Najac massif and the Montagne Noire Dome highlight potential decoupling between the REE and the U-Pb isotopic systems. The emplacement of the protoliths of felsic and mafic HP rocks in the Lévézou Massif was estimated at ca. 470 Ma. HP metamorphism peaked at 21-23 kbar and 680-800°C at ~358 Ma. The subsequent fast exhumation reached 8-9.5 kbar and ~600°C at ~352 Ma, highlighting a very fast exhumation followed by a cooling rate of 50°C/Ma. Granites from the Lévézou massif display kyanite-garnet-muscovite-quartz pseudomorphs after cordierite that equilibrated at ca. 15-17 kbar and ~670°C. The major deformation in these granites is associated with the exhumation stage, locally accompanied by partial melting, and seems to occur in different episodes, from ~352 Ma to ~340 Ma. On a larger scale, these results are fully in line with the HP ages described in the Variscan Belt and allow to reconsider the tectonics in the French Massif Central

High-P metamorphism in the Mesoproterozoic: petrochronological insights from the Grenville Province

International audienceIn the Grenville Province, high-P rocks that are discontinuously exposed along the margin of the allochthonous belt preserve the record of deep crustal processes and are an essential puzzle piece to understanding Mesoproterozoic geodynamics. The Manicouagan Imbricate zone (MIZ) is one of the three high-P domains from the Grenville Province. It is located in the Central Grenville between the Parautochthonous Belt to the North and the orogenic hinterland to the South, that were metamorphosed during the 1005–980 Ma Rigolet and 1080–1020 Ma Ottawan orogenic phases, respectively. In the western MIZ, the Lelukuau Terrane (LT) mostly consists of Labradorian-age (~1650 Ma) mafic suites with a fringe of aluminous rocks at its southern edge. Metamafic samples from the Western and Eastern parts of the MIZ display a peak assemblage of garnet, clinopyroxene, plagioclase, rare pargasite or edenite, and quartz ± kyanite. Pseudosection modelling suggests high-P granulitepeak conditions at ca. 14 to 16 kbar and 800–900 ◦C, with the scarcity of hydrous phases and quartz explaining the lack of evidence for partial melting. Zircon cores from the Western LT sample show a maximum magmatic age of ca 1.6 Ga. Lu–Hf and Sm–Nd dating on garnet from this sample yield ages of 1020 ± 7 Ma and 1005 ± 13 Ma, respectively, overlapping within error and inferred to represent peak metamorphic conditions followed by fast cooling. In the Eastern LT sample, garnet Lu–Hf dating yields two ages that are consistent with a petrographically preserved two stage growth, at 1033 ± 6 Ma and 1013 ± 6 Ma, while the Sm–Nd age indicates cooling at 1003 ± 8 Ma. The recorded high-P granulite facies conditions highlight a late Ottawan to Rigolet-age localized crustal thickening at the margin of the hinterland during the propagation of the orogen to the NW, with a possible younging of the high-P granulite-facies metamorphism from the Eastern to Western LT. These new results indicate that the high-P belt in the Central Grenville does not represent the exhumed base of an Ottawan age orogenic plateau, as previously proposed, and that no tectonic hiatus exists between the two orogenic phases, as generally thought. Finally, this publication highlights the diversity and diachronicity of the high-P domains in the Grenville Province

Cordierite pseudomorphs in a peraluminous granite: an unexpected evidence of high pressure metamorphism

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Ordovician magmatism in the Lévézou massif (French Massif Central): tectonic and geodynamic implications

International audienceNew U–Pb dating on zircon yielded ca. 470 Ma ages for the granitoids from the Lévézou massif in the southern French Massif Central. These new ages do not support the previous interpretation of these granitoids as syn-tectonic intrusions emplaced during the Late Devonian-Early Carboniferous thrusting. The geochemical and isotopic nature of this magmatism is linked to a major magmatic Ordovician event recorded throughout the European Variscan belt and related to extreme thinning of continental margins during a rifting event or a back-arc extension. The comparable isotopic signatures of these granitoids on each side of the eclogite-bearing leptyno-amphibolitic complex in the Lévézou massif, together with the fact that they were emplaced at the same time, strongly suggest that these granitoids were originally part of a single unit, tectonically duplicated by either isoclinal folding or thrusting during the Variscan tectonics

From burial to exhumation: evolution of eclogitic terranes through multimethod petrochronology, Lévézou Massif, French Massif Central

International audienceTo unravel the evolution of metamorphic rocks, the understanding of the mineral growth through time is required. However, dating early metamorphic stages is a challenge, due to subsequent retrograde overprinting. This is particularly true for HP metamorphism that highlights the early stages of orogenesis.A perfectly fresh eclogite and a former eclogite retrogressed under amphibolite facies from the southern French Massif Central (Lévézou massif, Variscan Belt), were investigated with a large panel of geochronometers (U-Pb on zircon, rutile and apatite, Lu-Hf and Sm-Nd on garnet) in a petrological context tightly constrained by petrographic observation, trace element analyses and phase equilibrium modelling.Both samples recorded similar HP conditions at 18-23 kbar and 680-800°C, while the retrogressed eclogite later equilibrated at 8-9.5 kbar and ca. 620°C. In the retrogressed sample, most of the zircon grains are characterized by negative Eu anomalies and HREE enrichment, and yield an Ordovician U-Pb date of 472.3 ± 1.7 Ma. This is interpreted as the emplacement age of the mafic protoliths and, in agreement with other data available for the Variscan belt, is considered to represent the extreme thinning of continental margins during a rifting event or a back-arc extension. Few zircon rims show a weaker HREE enrichment and yield a date of 378 ± 5.7 Ma, interpreted as a prograde, pre-eclogitic, age. Lu-Hf garnet dating from both samples yields identical dates of 357 ± 13 Ma and 357.5 ± 4 Ma, that approximate the age of the high pressure metamorphic peak. Fresh and retrogressed samples respectively yield 350.4 ± 7.7 Ma and 352 ± 20 Ma dates for Sm-Nd garnet dating, and 367.8 ± 9.1 Ma and 354.9 ± 9.5 Ma for U-Pb rutile dating. Apatite grains from the retrogressed sample give a mean age of 351.8 ± 2.8 Ma. All these ages are identical within error and, when combined with P-T estimations, highlight a major decompression ranging from 8.5 to 15 kbar in less than 6 My, suggesting exhumation rates in excess of 5 mm/yr up to more than 1 cm/yr. Decompression is nearly adiabatic with gradient ranging from 7.8 to 1.3°C/km, followed by cooling rates after major decompression exceeding 50°C/Ma

Timing and duration of Variscan high-pressure metamorphism in the French Massif Central: A multimethod geochronological study from the Najac Massif

International audienceAccurate dating of eclogite-faciesmetamorphismis of paramount importance in order to understand the tectonicevolution of an orogen. An eclogite sample fromtheNajac Massif (FrenchMassif Central, Variscan belt) displays azircon-bearing garnet-omphacite-amphibole-rutile-quartz peak assemblage. Pseudosection modeling suggestspeak pressure conditions of 15–20 kbar, 560–630 °C. Eclogite-facies garnet displays Lu-enriched cores and Smrichrims and yields a Lu-Hf age of 382.8±1.0Ma and a Sm-Nd age of 376.7±3.3Ma. The ages are interpretedas marking the beginning of the prograde garnet growth during the initial stages of the eclogite-facies metamorphism,and the high-pressure (and temperature) peak reached by the rock, respectively. Zircon grains displaychondrite-normalized REE spectra with variably negative, positive or no Eu anomalies and are characterized byeither enriched or flat HREE patterns. However, they yield a well constrained in situ LA-ICP-MS U-Pb age of385.5±2.3 Ma, despite this REE pattern variability. Zr zonation in garnet, Y content in zircon and the diversityof zircon HREE spectra may suggest that zircon crystallized prior to and during incipient garnet growth on theprograde P-T path, recording the initial stages of the eclogite-facies conditions. Consequently, the zircon age of385.5±2.3Ma, comparable within error with the Lu-Hf age obtained on garnet, is interpreted as dating the beginningof the eclogite-faciesmetamorphism. Accordingly, the duration of the prograde part of the eclogite-faciesevent is estimated at 6.1±4.3 Myr. Subsequent exhumation is constrained by an apatite U-Pb age at 369±13Ma

Timing and duration of Variscan high-pressure metamorphism in the French Massif Central: petrochronological insights from the Najac Massif

International audiencePrecise and accurate investigation of geochronometers in the light of a detailed petrologi- cal context is the best way to fully understand the evolution of metamorphic rocks. Eclogite-facies rocks highlight the early stages of orogenesis; constraining their P-T-t evolution is then of paramount importance to any geodynamic interpretation. An eclogite sample from the Najac Massif (south French Massif Central) displays a zircon-bearing garnet-omphacite-amphibole-rutile-quartz peak assemblage. Pseudosection modelling suggests peak pressure conditions of 15–20 kbar, 560–630°C. Eclogite-facies garnet displays Lu-rich cores and Sm-enriched rims and yields a Lu-Hf age of 382.8 ± 1.0 Ma and a Sm-Nd age of 376.7 ± 3.3 Ma. These ages are interpreted as marking the beginning of the prograde garnet growth during the initial stages of the eclogite-facies metamorphism, and the high-pressure (and temperature) peak reached by the rock, respectively. Zircon grains display chondrite-normalized REE spectra with variably negative, positive or no Eu anomalies and are characterized by either enriched or flat HREE patterns. However, they yield a well constrained in situ LA-ICP-MS U-Pb age of 385.5 ± 2.3 Ma, despite this REE pattern variability. Zr zonation in garnet, Y content in zircon and the diversity of zircon HREE spectra may suggest (i) that zircon crystallized prior to and during incipient garnet growth on the prograde P-T path, in numerous pulses, or (ii) that local REE equilibria were recorded and preserved in zircon. The zircon age of 385.5 ± 2.3 Ma, comparable within error with the Lu-Hf age obtained on garnet, is interpreted as dating the beginning of the eclogite-facies metamorphism. Accordingly, the duration of the prograde part of the eclogite-facies event is estimated at 6.1 ± 4.3 Myr. Coarse crystals of apatite, that formed late in the metamorphic history at significantly lower pressure, yield a U-Pb age of 369 ± 13 Ma interpreted as the timing of the subsequent exhumeation. Combined with the HP dating results obtained, via a similar petrochronological approach, in the adjacent Lévézou Massif (Lotout et al., same volume), constrained at ca. 358 Ma, these results allow to discuss not only the age of the HP metamorphism but also the geodynamics of the French Massif Central

From burial to exhumation: emplacement and metamorphism of mafic eclogitic terranes constrained through multimethod petrochronology, case study from the Lévézou massif (French Massif Central, Variscan belt)

International audienceLinking mineral growth and time is required to unravel the evolution of metamorphic rocks. However, dating early metamorphic stages is a challenge, due to subsequent retrograde overprinting. A perfectly fresh eclogite and a former eclogite retrogressed under amphibolite facies from the southern French Massif Central (Lévézou massif, Variscan belt) were investigated with a large panel of geochronometers (U-Pb on zircon, rutile and apatite, Lu-Hf and Sm-Nd on garnet) in a petrological context tightly constrained by petrographic observation, trace element analyses and phase equilibrium modelling. Both samples recorded similar HP conditions at 18-23 kbar and 680-800°C, while the retrogressed eclogite later equilibrated at 8-9.5 kbar and ca. 600°C. In the retrogressed sample, most of the zircon grains are characterized by negative Eu anomalies and HREE enrichment, and yield an Ordovician U-Pb date of 472.3 ± 1.7 Ma, interpreted as the emplacement age of the mafic protolith. In agreement with other data available for the Variscan belt, and based on zircon trace elements record and whole rock geochemistry, this age is considered to represent the magmatism associated with the extreme thinning of the continental margins during the Ordovician. In the same sample, few zircon rims show a weaker HREE enrichment and yield a date of 378 ± 5.7 Ma, interpreted as a prograde pre-eclogitic age. Lu-Hf garnet dating from both samples yields identical dates of 357 ± 13 Ma and 358.0 ± 1.5 Ma inferred to approximate the age of the high-pressure metamorphic peak. Fresh and retrogressed samples yield respectively 350.4 ± 7.7 Ma and 352 ± 20 Ma dates for Sm-Nd garnet dating, and 367.8 ± 9.1 Ma and 354.9 ± 9.5 Ma for U-Pb rutile dating. Apatite grains from the retrogressed sample give a mean age of 351.8 ± 2.8 Ma. The similarity between all recorded ages from distinct chronometers and radiometric methods (U-Pb – rutile, apatite; Lu-Hf – garnet; Sm-Nd – garnet) combined with P–T estimations from high-pressure metamorphic rocks equilibrated under different conditions testifies to very fast processes that occurred during the Variscan orogeny, highlighting a major decompression of 15-8.5 kbar in less than 7 Myr, and suggesting mean exhumation rates in excess of 6.3 mm/yr

Evolution of eclogite terranes through multimethod petrochronology, a case study from the French Massif Central, Variscan belt

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