The biotite isograd, Central Pyrenees: a deformation-controlled reaction

The biotite isograd reaction in Cambro–Ordovician pelites from the Garonne dome in the Central Pyrenees involves the production of biotite at the expense of chlorite, and the gradual reduction in the celadonite (Si) content of individual white micas. The mineral assemblages in the biotite-bearing rocks in the Melles area retain abundant evidence of chemical disequilibrium, due to the sluggish nature of major element diffusion within the white micas. The progress of the isograd reaction is strongly controlled by the progressive development of regional Variscan fabrics, and chemical exchange in the white micas in these metamorphic conditions is only possible during active deformation. Chemical resetting of the white micas takes place via the development of compositional zoning in the deforming micas; this probably occurs as a consequence of the introduction of defects and dislocations, causing more efficient diffusion within parts of individual grains. In the absence of deformation, this biotite isograd reaction would take place at significantly higher temperature and be controlled by the relatively high closure temperature of major element diffusion in white micas. Thus, assigning thermal significance to such continuous isograd reactions is impossible without independent constraints: kinetic factors such as deformation may be the dominant influence in many cases, not the thermodynamic controls.