Syn-tectonic contact aureole and metasomatic reaction zones in carbonate and pelitic host rocks (Elba Island, Italy)

In this paper we describe deformation processes and metasomatic reaction zones developed in a contact aureole formed in the upper crust (c. 6 km depth) due to the emplacement of a felsic pluton (Porto Azzurro monzogranite, Elba Island, Italy). This pluton emplaced in the Northern Apennines tectonic units during late Miocene, in an extensional tectonic setting active since early-middle Miocene, after the end of the Cretaceous-late Oligocene/ early Miocene compressional tectonics. Consequently, a syn-tectonic contact aureole locally developed in a sedimentary succession consisting of shale, marl and siliceous limestone (Argille a Palombini Fm, lower Cretaceous). Integration of detailed structural analysis of ductile and brittle structures with petrologic analysis and modelling of the metasomatic reaction zones, highlights the different rocks' rheology during contact metamorphism. Our results indicate that fluids from devolatilisation reactions remained initially trapped within mechanical discontinuities such as lithological boundaries, where reaction buffering took place. Then, fluids migrated into the opening fractures generated by the boudinage of the metapelite levels, diffusing elements in the hosting rocks. While temperature in the contact aureole was increasing up to peak temperature (T) and pressure (P) conditions of -570-600 degrees C and -0.2 GPa, a rheology inversion occurred, documented by the different ductile-brittle behaviour of metacarbonate and metapelite (hornfels). This process occurred when the contact aureole was subjected to a regional, progressive, top-to-the-NE sense of shear, which determined boudinage, rotation and flattening of the magma-hosting rocks.