Hydrolytic weakening controls Jurassic to early Cretaceous mylonitisation in the basement of the Pyrenees

ABSTRACT The age of the mylonite belts in the basement rocks of the Pyrenees is a subject of debate in the structural geology and petrology communities because of its potential implication on the regional tectono-thermal history and on the tectonic evolution of SW Europe. Here we address when and how mylonitisation took place in two key areas of the Eastern Pyrenees, where shear zones are associated with Giant Quartz Veins (GQVs). We conducted zircon U–Pb and muscovite 40Ar/39Ar dating coupled with structural, textural, and crystallographic preferred orientation (CPO) analyses of mylonites from the Cap de Creus and Canigó Massifs. U–Pb zircon dating of a dacite porphyry dyke crosscut by GQVs and mylonitic bands yields a maximum shear zone and GQV formation age of ca. 292 ± 3 Ma. 40Ar/39Ar analyses of muscovite within mylonitised GQVs yield initial crystallisation ages between ca. 164 and 188 Ma, as well as younger recrystallisation ages of ca. 110-118 Ma. A qualitative assessment of the GQV history is inferred from step-heating spectra of muscovite and quartz CPOs. The results indicate that GQV formation and mylonitisation were coupled, coeval, and long-lasting processes that took place from early Jurassic to early Cretaceous times. A comparative evaluation of quartz CPOs reveals inconsistencies regarding the strain distribution, quartz slip systems activity, and deformation temperatures depending on the deformed rock type. Quartz mylonites have stronger CPOs dominated by basal , prism or prism slip systems, whilst phyllonites and granite mylonites show weaker fabrics mostly dominated by mixed slip. This apparently suggests higher deformation temperatures in quartz mylonites than those inferred from more reliable proxies, such as mineral assemblages, brittle behaviour of K-feldspar, and fluid inclusion data. We suggest that the water-weakening effect caused by coeval formation and deformation of GQVs enabled easier dislocation glide and creep, allowing strain localisation and transitions between slip systems at lower temperatures than commonly inferred due to enhanced ductility. U-Pb zircon dating further suggests the existence of an early Carboniferous (ca. 332 ± 4 Ma; Visean) magmatic episode in the Pyrenees, in agreement with a cyclic, rather than a progressive, geodynamic history of the region during Variscan times. The present work challenges classical interpretations stating that Pyrenean mylonite belts developed during the retrograde stages of the Variscan Orogeny, highlighting that the structural evolution of this region during Mesozoic times deserves further investigation. Results have implications for interpreting deformation localisation mechanisms and conditions in crustal rocks, for the formation mechanisms of GQVs in worldwide orogenic belts, and for the tectono-thermal history of the Pyrenees since late-Variscan times.