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Origine et développement de la thermicité dans les Pyrénées varisques

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.
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Baptiste Lemirre. Origine et développement de la thermicité dans les Pyrénées varisques. Géologie appliquée. Université Paul Sabatier - Toulouse III, 2018. Français. ⟨NNT : 2018TOU30350⟩. ⟨tel-02926169⟩

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