Constraining the pre-Mesozoic crustal structure in the North Pyrenean realm (Aulus Basin, France): implications for the mode and style of Cretaceous rifting

International audienceThe North Pyrenean Zone recorded lithospheric mantle exhumation and its local exposure to the seafloor duringmid-Cretaceous rifting in response to the eastward drift of the Iberian plate. However, the pre-Cretaceous crustalconfiguration and the role of pre-Mesozoic (Variscan) thinning events remain unconstrained. With the aim toconstrain the pre-Mesozoic crustal structure in the region, we carried out a comprehensive petrochronologicalstudy on the granitoid intrusions emplaced in the Hercynian granulitic envelope of the Lherz peridotite (AulusBasin, France). Field work, meso- and micro-scale investigation, multiequilibrium thermobarometry and zirconand titanite U-(Th)-Pb geochronology were combined to define the P-T-t-deformation history of the granitoids,in the transition from magma emplacement to solid-state deformation. Magmatic mineralogy consists of Kfs +Pl1 + Qz + Amp1 + Bt and Ttn1 + Zrc as accessory minerals. The application of Al-in-Hbl barometry and Zrin-Ttn1 thermometry models constrains magma emplacement at 0.5 GPa and 780C. The tectonic fabric isattested by distributed and narrow (up to 25 cm wide), sub-vertical bands of S-(L) tectonites, developed during thesub-solidus mylonitisation of the granitoid. Ductile shearing was accompanied by Amp-Ttn-Pl recrystallization,which occurred at shallower crustal levels and temperatures compatible with the greenschist- facies metamorphicconditions (450-500C and P 0.2 GPa). The U-(Th)-Pb dating of the magmatic zircon clusters around 280Ma, indicating that the Pyrenean continental crust was already significantly thinned during the Permian (Variscan)post-orogenic collapse. The U-(Th)-Pb dating of the REE-poor metamorphic (syn-shearing) titanite (Ttn2) grainsconstrains the activation of ductile shearing on the envelope of the Lherz peridotites around the Albian-Cenomanianboundary (100 Ma). The major implication of this study is that the Pyrenean continental crust was alreadyattenuated and the Hercynian metamorphic envelope of the Lherz peridotites resided at upper crustal levels inpre-Mesozoic times. We infer that the pre-Cretaceous crustal configuration was likely the result of a polyphasethinning, resulted from the cumulated effects of the Permian post-orogenic collapse and of the Triassic and Liassicaborted rifting events. The polyphase evolution unraveled by this study has major implications for the amount ofstretching required during the Cretaceous Pyrenean rifting to exhume the lithospheric mantle and, more in general,for the long-term post-orogenic evolution of the Variscan domain around the Iberian plat