Séminaire Laura Airaghi

Deformation of the continental crust near and below the brittle-ductile transition is strongly influenced by metamorphic reactions: dissolution of hard mineral phases such as feldspar and crystallization of weaker, hydrous phases such as micas soften rocks by orders of magnitude. In the Bielsa massif (Axial zone, Pyrenees), the originally granitic pluton dated of 310-320 Ma was pervasively altered in greenschist-facies conditions (250-350°C) and spectacularly deformed in places. Interestingly, alteration took place regardless of the degree of deformation. In undeformed granite, metamorphic reactions are pseudomorphic and localized along microcracks at grain boundaries. Minerals formed during alteration are zoned at the µm-scale, showing several alteration events with transient fluid interconnection. Titanite and rutile associated to alteration yield U-Pb ages of c. 250-300 Ma, attesting of a metamorphic overprint shortly after the pluton emplacement. A second generation of metamorphic minerals is observed interstratified with the first one in mylonites or in secondary fractures in granites deformed under brittle conditions. Rutile and monazite grains which re-crystallized in deformed domains yield U-Pb ages of c. 40 Ma, relating this second metamorphic overprint to the Alpine deformation. Increasing deformation leads to greater homogeneity, showing the interplay between deformation, metamorphic reactions and fluid interconnectivity controlling equilibrium length scales. Hence, at the scale of the Bielsa massif, the basement appears softened by pre-orogenic alteration 200 Ma before the Alpine compressional phase: the diffuse vs localized character of Alpine deformation was likely controlled by the crustal weakening caused by the Late to post Variscan greenschist-facies metamorphism.