Abstract. Complex zoning in garnet from micaschists of the Sesia Zone (Western Italian Alps) preserves evidence of two orogenic cycles and provides detailed insights into resorption, growth and diffusion processes induced by fluid pulses at high pressure. Data on local textures and mineral chemistry are combined with data derived from thermodynamic modelling to understand and quantify these processes. Garnet shows low-Ca porphyroclastic cores that are stable at (Permian) granulite 10 facies conditions. In one sample, the first garnet rim that surrounds the pre-Alpine granulite facies core indicates that preAlpine amphibolite facies metamorphism followed the granulite facies event. The cores show lobate edges and preserve inner fractures, which are sealed by high pressure Alpine garnet. This observation suggests that during the first stages of subduction, before hydration of these high temperature rocks, brittle failure of the garnet occurred, implying high strain

Mode: quartz ~ 50%, micas (2/3 phengite, 1/3 paragonite) ~30%, garnet ~15%, remaining ~5% epidote, chlorite, albite, rutile, graphite and zircon.