Thermal evolution of the north-Pyrenean granulitic crust: a combinedpetrological, geochronological and thermal study of the Saint-BarthelemyMassif

International audienceThe Variscan orogeny is the result of an oceanic subduction followed by a continental collision that ended witha late-collisional high temperature – low pressure event at the scale of the whole orogen. The singularity of thePyrenean segment is the predominance of this late high temperature event which is characterized by an intensedeformation synchronous with the high temperature metamorphism and an abundant and varied magmatism.The aim of this study is to determine pressure-temperature-time trajectories in the Saint Barthelemy Massif, andto discuss the origin of the geothermal gradients.The Saint Barthelemy Massif is one of the granulitic north-Pyrenean massifs constituting the deepest relics of theVariscan crust. It is composed of two main units separated by a low angle detachment. The upper unit is composedof Paleozoic metasediments, micaschists, migmatites and small plutonic bodies and represents the Variscan uppercrust. The basal unit, made of granulitic to amphibolitic gneisses, corresponds to the intermediate to lower crust.Pressure-temperature-time data indicate a homogeneous temperature of 800 C at a depth between 10 and 20km around 300 Ma (U-Pb dating on zircons and monazites from granulites and migmatites). This temperatureof 800 C corresponds to the dehydration melting of biotite as constrained by thermodynamic modelling. In theupper part of the crust, we observe a maximal gradient higher than 80 C/km above 10 km depth. This hightemperature metamorphic event, characteristic of the north-Pyrenean massifs, followed a magmatic episode atca. 305 Ma in the whole Variscan Pyrenees. The magmatic-metamorphic succession, as well as the absence ofsignificant crustal thickening in the Pyrenean segment of the Variscan belt, suggests a mantellic origin for thelate-Variscan thermal anomaly. A one-dimensional thermal model of the crust is used to investigate the effectof buffering by latent heat of fusion and the effect of advection of melts in response to an abnormal basal heatsource. The buffering of temperature at 800 C in the lower to intermediate crust and a minimum advection rateof 2.5 km/Ma explain the shape of the geotherm with the extremally high gradient in the upper crust. We thusinterpret the geotherm at 300 Ma as induced by crustal melting and melt migration from the lower to the upper crust