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 facies conditions. In one sample, the first garnet rim that surrounds the pre-Alpine granulite facies core indicates that pre-Alpine 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 rates. Several Alpine rims show different textures indicative of interaction with hydrous fluid: (a) resorption-dominated textures produced lobate edges, at the expense of the outer part of the granulite core; (b) peninsulas and atoll garnets are produced by replacement reactions; (c) spatially limited resorption and enhanced transport of elements due the fluid phase is evident along brittle fractures and their immediate proximity. Thermodynamic modelling shows that all of these Alpine rims formed at eclogite facies conditions. Structurally controlled samples allow these fluid-garnet interaction phenomena to be traced across a portion of the Sesia Zone, with decreasing in fluid-garnet interaction toward external areas (NW). Replacement of the Permian HT assemblages by hydrate-rich Alpine assemblages can reach nearly 100%. However, no clear relationship is visible between deformation structures and fluids that triggered eclogite facies metamorphism; suggesting disperse fluid flow.
Pressure–temperature–time–deformation path of kyanite-bearing migmatitic paragneiss in the Kali Gandaki valley (Central Nepal): Investigation of Late Eocene–Early Oligocene melting processes
