Timescale of relaxation of garnet growth zoning via multi-component diffusion modeling: the example of Mammola Paragneiss Complex (Serre Massif-Southern Calabria)

Diffusion modeling of the major elemental compositional zonings is a valuable tool to know how long a zonedmineral can resist in chemical disequilibrium with the surrounding matrix at high temperature before the pre-existingcomposition is modified by diffusion. It allows to derive the timescales of metamorphic processes such as heating andcooling rates, providing information about the burial and exhumation history if mineral diffusion properties, crystal sizeand temperature are known.In this study we have investigated millimeter almandine-rich garnet crystals from garnet-micaschists of theMammola Paragneiss Complex (Serre Massif) (Angì et al., 2010), in order to estimate the timescales of metamorphicevents registered in the upper crust section of the Variscan European Belt, using the extent of relaxation of garnetgrowth zoning by means of a finite-difference diffusion model (Crank, 1975; Storm & Spear, 2005). According to Angìet al. (2010), these rocks highlight a multistage metamorphic evolution consisting of an orogenic cycle partlyoverprinted by a thermal one, both of them ascribable to the Variscan orogenesis. In particular, the authors recognize aprograde low amphibolite facies evolution (P = 5.9-7.5 Kbar; T = 500-550 °C) followed by a retrograde quasi-adiabaticdecompression (P = 4 Kbar; T = 500 °C), evolving toward a retrograde deep seated shearing stage (P = 3 Kbar; T = 470°C). The subsequent post-tectonic progressive emplacement of huge masses of granitoid bodies gave rise then to agradually distributed thermal metamorphic overprint (P = 3 Kbar; T = 685 °C), followed by a low pressure cooling path(P = 1.5 Kbar; T = 500 °C) consistent with the final unroofing stage of the former crystalline basement complex.In this tangled scenario, relaxation of garnet zoning profiles was modelled taking into account two distinct set ofdiffusion data (Chakraborty & Ganguly, 1992; Carlson, 2006). Preliminary results obtained with the two distinct dataset highlight a timescale of relaxation of the growth zoning around 1-3 My, suggesting prograde and exhumation eventsto be relatively rapid in this portion of the dismembered southern Hercynian European Belt, presently involved withinthe internal Alpine sector of the western Mediterranean geodynamics.