Element mobility in low-grade shear zones and strain accommodation: insights from geochemistry and microstructures of granitoids of central Pyrenees (Axial zone)

International audienceShear zones developed in greenschist-facies conditions are often considered as open geochemical systems and preferential pathways for fluid percolation that favours the synkinematic growth of weak phases (i.e. phyllosilicates) and strain localisation. However, differences in the geochemical behaviour of low-grade shear zones are observed, with potential implications on strain accommodation from a mineral aggregate to an outcrop scale. In the Axial zone of the Pyrenees, the Bielsa and Maladeta granitoids are altered and deformed in greenschist-facies conditions at 250-350°C (chlorite thermometry). While shear zones of Bielsa are spaced of ~ 100-200 m, strain is localised in Maladeta, with shear zones spaced of ~ 1.5 km, leaving large parts of the granite undeformed. At microscopic scale, quartz shows in Bielsa little recrystallisation by bulging, intra-grain misorientation up to 40° but pervasive recrystallisation by sub-grain rotation in Maladeta mylonites. The similarity of temperature of 250-350°C estimated by chlorite thermometry for shearing in both massifs indicates that quartz recrystallisation regimes mainly depend on the amount of weak (phyllosilicates) mineral phases rather than temperature. The Bielsa granitoid is pervasively altered with more than ~ 50 % volume of mylonites made of white mica while in Maladeta high strain samples, the magmatic mineral assemblage is largely preserved (white mica representing less than 30 % volume). Trace and rare earth elements variations uncorrelated to strain gradients indicate a more pervasive pre-kinematic granite alteration in Bielsa than in Maladeta. Major elemental geochemistry showsvolume variations, systematic loss up to 50 % of Na and K and variations in Ca due to feldspar breakdown and Ca-bearing fluid circulation across Bielsa shear zones. In contrast, no systematic volume and element loss or gain is observed across Maladeta shear zone, but only lateral element migration. This points to an open shear zone during shearing in Bielsa and a “closer” shear zone in Maladeta. Hence, for these massifs, different effective opening of the shear zones - likely due to variations in the extent of pre-kinematic alteration - leads to 30 % vol. difference in white mica abundance. This controls the frequency of quartz-quartz grains boundaries, quartz recrystallisation regimes and distributed vs localised deformation at an outcrop scale, despite similar greenschist-facies conditions