High-pressure metamorphism in Southern Calabria, Italy: the Cardeto chlorite-garnet metapelites

The Aspromonte Massif (Southern Calabria, Italy) consists of an Alpine crystalline nappe pile, belonging to the Southern Sector of the Calabrian Arc. The Aspromonte Unit, forming the bulk of the Massif, is overlain by the Stilo Unit and overlies, near Cardeto and Africo, phyllites and micaschists. Structural analysis on the Cardeto metapelites emphasized three deformation phases. Abundant “pin-prick” garnets characterize the fine-grained phyllites, porphyroblasts and “pin-prick” crystals are present in the micaschists. Garnets are almandine-rich with a low Mg content and variable amount of spessartine and grossular, depending on the rock composition. The porphyroblastic garnets are strongly zoned and exhibit distinctive bell-shaped Mn profiles typical of a prograde growth. The pin-prick garnets show the same composition as the rims of the porphyroblastic crystals of the same sample, suggesting a late stage growth with respect to the porphyroblasts. Phengitic white mica and chlorite composition suggest crystallization under relatively-high pressure conditions. Physical conditions, inferred using chlorite-garnet geothermometer and modelled P-T pseudosections in the MnNCKFMASH system for chemically similar metapelites, suggest P in the range of 7.5-10 kbar, due to the absence of glaucophane in the coexisting amphibolic rocks, in the T range of 500-550°C. The widespread biotite-free chlorite+ almandine assemblage suggests a crystallization of almandine prior to biotite, as happens in the Sanbagawa metamorphic region of Japan, where the chlorite, garnet, biotite+albite and biotite+oligoclase assemblages at increasing temperature, indicate a P/T ratio intermediate between the blueschist facies and the Barrovian greenschist facies conditions. High pressure conditions have never been estimated in the Variscan metamorphism of the Southern Sector of the Calabrian Arc, which, instead, is characterized by medium-low P/T ratio. Only the Aspromonte Unit, tectonically overlying the Cardeto metamorphics, shows a pervasive Alpine overprint, which is characterized by a higher P/T ratio than that of the Variscan metamorphism. Consequently, we assume that the Cardeto metapelites were probably affected by an Alpine metamorphic event. Owing to the similar structural position, the Cardeto rocks could represent the Calabrian part of the Mandanici Unit, which extensively outcrops in the nearby Peloritani Mountains, that in Calabria has been overprinted by Alpine effects, but no structural or mineralogical relicts which support this interpretation have been observed. More probably, they represent a distinct Alpine tectonic unit which have experienced a relatively-high pressure Alpine (?) metamorphism and that in the Peloritani Mountains was removed from within the tectonic pile during a syn-orogenic extension episode.