Amphibole-Veins Formation Within a Syn-Tectonic Contact Aureole (Elba Island, Italy)

The eastern part of Elba Island (northern Tyrrhenian Sea, Italy) is an exhumed sector of thinned continental crust, intruded by a monzogranitic pluton during the Messinian. Pluton emplacement occurred at a depth of c. 6 km as a consequence of permeability enhancement in the upper crust due to extensional tectonics. The contact aureole records coeval intense circulation of geothermal fluids and deformation. Here, we describe syn-tectonic amphibole + quartz veins that cross the contact aureole and are associated with extensional structures that formed during contact metamorphism. These veins (shear and extensional veins) are perpendicular or sub-parallel to the main foliation of the host rock and are either arranged in en-echelon arrays or developed in extensional jogs. Calcite, chlorite and titanite occur as accessory minerals in the veins. The interaction between hot fluids and host rocks produced localized metasomatism indicated by white haloes symmetrically developed adjacent to the veins. Fluid inclusions in quartz from amphibole + quartz veins are of two types: (i) two-phase liquid-rich and (ii) two-phase vapor-rich, in which the vapor phase includes mostly CO2 with minor amounts of CH4 and N2, the latter detected by Raman spectroscopy. Microthermometry indicates maximum temperatures of * 500 °C, assuming a maximum P of about 0.8 kbar and salinity up to 6.4 wt% NaCl equivalent, suggesting a metamorphic origin.