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Strain accommodation in upper crustal rocks is often accompanied by fluid-mediated crystallization of phyllosilicates, which influence rock strength and shear zone formation. The composition of these phyllosilicates is frequently used for pressure-temperature-time constraints of deformation events, although it is often highly heterogeneous, even in mylonites. This study investigates the reactions producing a phyllosilicate, chlorite, in and below greenschist-facies conditions and the variations in chlorite composition along a strain gradient in the Variscan Bielsa granitoid (axial zone, Pyrenees). Compositional maps of chlorite including iron speciation are compared to nanostructures observed by transmission electron microscopy in increasingly-strained samples and related to mechanisms of fluid percolation. In the Bielsa granitoid, altered at the late-Variscan, Alpine-age shear zones are found with high strain gradients. The undeformed granitoid exhibits local equilibria, pseudomorphic replacement, and high compositional heterogeneities in chlorite. This is attributed to variable reaction mechanisms at nanoscale and element supply, little interconnected intra- and inter-grain nanoporosity, and isolation of fluid evolving in local reservoirs. In samples with discrete and mm-sized fractures, channelized fluid triggered the precipitation of homogeneous Alpine chlorite in fractures, preserving late-Variscan chlorite within the matrix. In low-grade mylonites, where brittle-ductile deformation is observed, micro- and nanocracks and defects allow the fluid percolating into the matrix at the scale of hundreds of [micro]m. This results in a more pervasive but incomplete replacement of late-Variscan chlorite by Alpine chlorite, despite the high strain. In studied granitoids deformed under greenschist-facies conditions, local equilibria and high compositional heterogeneities in phyllosilicates as chlorite are therefore preserved according to reaction mechanisms and element mobility controlled by (1) matrix-fracture porosity contrasts at nanoscale and (2) the location and interconnection of nanoporosity between crystallites of phyllosilicates. This preservation influences our ability to reconstruct the pre- and syn-kinematic metamorphic history of granitic rocks in low-grade units of orogens., Author(s): Laura Airaghi [sup.1] [sup.2], Benoit Dubacq [sup.1], Anne Verlaguet [sup.1], Franck Bourdelle [sup.3], Nicolas Bellahsen [sup.1], Alexandre Gloter [sup.4] Author Affiliations: (1) grid.462844.8, 0000 0001 2308 1657, CNRS-INSU, Institut [...]