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Origine Et Développement De La Thermicité Dans Les Pyrénées Varisques

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Origine Et Développement De La Thermicité Dans Les Pyrénées Varisques 1 Origine et développement de la thermicité dans les Pyrénées varisques Conséquences sur le segment ouest-européen de l’orogène 2 3 Baptiste Lemirre – thèse de doctorat – 2018 Résumé Le cycle varisque est caractérisé par une phase tardi-orogénique de haute température (HT) généralisée dans l’ensemble de la chaîne. Cependant, la nature et la contribution des sources de chaleur potentielles à l’origine de l’anomalie thermique tardi-varisque sont encore débattues. Une évaluation précise (i) de l’évolution spatio-temporelle de l’état thermique de la croûte, (ii) de la chronologie entre le métamorphisme, la production et le transfert de magmas dans la croûte et (iii) de la relation entre l’évolution tectonique tardi-orogénique et l’épisode de HT est donc nécessaire. Le segment pyrénéen est classiquement interprété comme un avant-pays chaud au cours de l’orogenèse varisque. Les Pyrénées sont caractérisées par un épaississement crustal modéré et un épisode métamorphique de haute température – basse pression (HT-BP) associé à du magmatisme tardi- orogénique abondant et une déformation en transpression à l’échelle de la croûte. Ils constituent donc une cible de choix pour l’étude de l’épisode de HT tardi-varisque. Des estimations des conditions pression- température ont été obtenues par modélisation thermodynamique sur 4 massifs de la zone nord- pyrénéenne (massifs du Saint Barthélémy, de Bessède de Sault, du Castillon et d’Ursuya). Les résultats montrent un géotherme en deux parties au paroxysme de l’épisode de HT avec de forts gradients atteignant 80 °C/km dans la partie supérieure et des conditions isothermes dans la croûte moyenne avec des températures de l’ordre de 800 °C. Les données de géochronologie U-Pb sur zircon et monazite dans les roches métamorphiques mettent en évidence que la croûte moyenne, affectée par une déformation extensive, reste chaude et partiellement fondue pendant une 20aine de millions d’années de ca. 300 Ma à ca. 280 Ma. Une coupe nord-sud dans les Pyrénées Centrales, couplée à de la spectroscopie Raman sur matière carbonée permet de quantifier l’évolution de la température dans la couverture déformée en transpression hors de dômes tecto-métamorphiques. Les résultats indiquent que la croûte supérieure est aussi affectée par le métamorphisme régional avec des températures supérieures à 500 °C dans les sédiments paléozoïques. Les études géochimiques et géochronologiques (U-Pb sur zircon) dans les massifs du Chiroulet-Lesponne et de l’Aston soulignent une origine à la fois mantellique et crustale des magmas et montrent que le magmatisme est contemporain du métamorphisme, avec des âges compris entre ca. 305 Ma à ca. 290 Ma. Les données de modélisation numérique confirment la source mantellique de la chaleur avec un transfert de chaleur jusqu’à la croûte supérieure par la migration des magmas produits par la fusion partielle de la croûte inférieure à moyenne. Mots clés : Magmatisme ; Métamorphisme ; Pyrénées ; Varisque ; Datation ; Haute Température 4 Baptiste Lemirre – thèse de doctorat – 2018 Abstract The Variscan cycle is characterized by a late-orogenic phase of high temperature (HT) in the whole belt. However, the nature and relative contribution of potential heat sources for the thermal anomaly are still debated. A careful assessment of (i) the spatial and temporal evolution of the thermal state of the crust, (ii) the chronology between metamorphism, magmas production and magmas transfer in the crust and (iii) the relationships between the late-orogenic tectonic evolution and the HT event is thus required. The Pyrenean segment is usually interpreted as a hot foreland during the Variscan orogeny. The Pyrenees are characterized by a moderate crustal thickening and a high temperature – low pressure (HT-LP) metamorphism associated with abundant late-orogenic magmatism and transpressional deformation of the crust. They are thus a prime target to study the late-variscan HT event. Pressure-temperature estimates were obtained by thermodynamical modelling in 4 massifs of the north-pyrenean zone (Saint Barthelemy, Bessede de Sault, Castillon and Ursuya). Results show a kinded geotherm at the paroxysm of the HT event with gradients higher than 80 °C/km in the upper part of the crust and near-isothermal conditions in the middle crust with temperature of about 800 °C. U-Pb data on zircon and monazite grains in metamorphic rocks highlight that the middle crust, which were affected by extensional deformation, remained hot and partially molten over 20 Myr from ca. 300 Ma to ca. 280 Ma. A north-south cross-section in the Central Pyrenees, coupled with Raman spectroscopy of carbonaceous materials allows to quantify the evolution of the temperature in the sedimentary cover deformed by transpression apart from the tectono-metamorphic domes. Paleozoic sediments recorded temperature higher than 500 °C. This indicates that the upper crust was affected by regional HT-BP metamorphism in the whole belt. Geochemical and geochronological (U-Pb on zircon) studies in the Chiroulet-Lesponne and Aston massifs evidence a mantle and crustal origin of magmas and show that magmatism is coeval to metamorphism with ages comprised between ca. 305 to ca. 290 Ma. Numerical modelling data confirm that HT-LT metamorphism appears as a response to enhanced mantle flux coupled with a heat transfer from the lower to middle crust and up to the upper crust by melt migration. Keywords: Magmatism, Metamorphism, Pyrenees, Variscan, Dating, High Temperature 5 Baptiste Lemirre – thèse de doctorat – 2018 6 Baptiste Lemirre – thèse de doctorat – 2018 7 Baptiste Lemirre – thèse de doctorat – 2018 Remerciements Merci Stéphanie et Michel ; Merci Lukas, Valérie, Julia, Jean-Marc, Yoann, Daniel et Benjamin; Merci Marc, Muriel, Abdeltif, Romain ; Merci Bryan (Variscan Pyrennees for ever), Mathieu et Florent (F104 for ever) ; Merci Fabienne, Jeff, Thierry, Stéphanie, Manu, Jérôme, Philippe et Sophie ; Merci Philippe, Olivier, toute l’équipe TIL, et tous les membres du GET et du RGF ; Merci tous les doctorants du GET, du RGF et d’ailleurs ; Merci toutes les personnes ayant contribué de près ou de loin (géologiquement et géographiquement parlant) à la réussite de cette thèse ; Merci Papa, Maman, Simon, Thibaut et Martin ; Merci Anna (d’avoir attendu une semaine) ; Et « last but not least », MERCI Pauline ! A Mandette et Mamie 8 Baptiste Lemirre – thèse de doctorat – 2018 9 Baptiste Lemirre – thèse de doctorat – 2018 Table des matières Résumé .......................................................................................................................................................... 4 Remerciements ............................................................................................................................................. 8 Chapitre 1 : Introduction générale .............................................................................................................. 12 1.1. Pourquoi s’intéresser à la chaîne des Pyrénées et particulièrement à l’épisode varisque dans les Pyrénées ? ............................................................................................................................................... 13 1.2. Signification et origine du métamorphisme haute température – basse pression (HT-BP) et du magmatisme associé dans les chaînes de montagnes ............................................................................ 14 1.3. La chaîne varisque ouest-européenne, un exemple de métamorphisme HT-BP tardi-orogénique ..................................................................................................................................................... 16 1.4. Les Pyrénées, une cible de choix pour étudier le métamorphisme HT-BP varisque? ................. 19 1.5. Organisation du manuscrit et cadre de l’étude ........................................................................... 20 Chapitre 2 : Contexte géologique ................................................................................................................ 22 2.1. Introduction ................................................................................................................................. 23 2.2. Les Pyrénées, une chaîne issue de la succession de plusieurs cycles orogéniques .................... 23 2.3. Les caractéristiques du cycle Varisque dans les Pyrénées .......................................................... 28 Chapitre 3 : Origine et développement du métamorphisme haute température – basse pression dans les Pyrénées varisques ...................................................................................................................................... 52 3.1. Introduction ................................................................................................................................. 53 3.2. Etude des massifs métamorphiques pyrénéens (article #1) ....................................................... 54 3.3. Etude de la température dans la couverture sédimentaire paléozoïque (article #2) ............... 135 3.4. Conclusion ................................................................................................................................. 160 Chapitre 4 : Origine et durée du magmatisme tardi-orogénique dans les Pyrénées varisques ............... 161 4.1. Introduction ............................................................................................................................... 162 4.2. Article #3: Origin and duration of late orogenic magmatism in the foreland
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