STRUCTURAL SIGNATURE AND EXHUMATION P-T-T PATH OF THE GORGONA BLUESCHIST SEQUENCE (TUSCAN ARCHIPELAGO, ITALY)

Abstract

In this paper we present a study on the metamorphic and structural evolution of the Gorgona Island, located in the Northern Tyrrhenian Sea. Based on their contrasting P-T histories, two major tectonic units bounded by a mylonitic contact are recognised. The lower one (CS Unit) shows typical HP/LT parageneses (Fe-carpholite and glaucophane); peak metamorphic conditions are estimated around 1.5 GPa at temperature lower than 350°C. A Late Oligocene (25±0.3 Ma) age is proposed for this blueschist event on the basis of 40Ar/39Ar geochronology on phengites. The upper unit (Oph Unit) contains parageneses representative of the blueschist-greenschist transition; peak metamorphic conditions are estimated around 0.6-0.8 GPa and at least 300°C. Therefore, a normal- sense metamorphic gap is observed across the mylonitic contact (in the order of 0.7 GPa), with pressure increasing towards the lower CS Unit. Eastward asymmetric ductile shear, syn-kinematic relative to the retrograde greenschist stage, accompanied exhumation of the CS Unit, controlling the contact with the upper Oph Unit. On the basis of both structures and metamorphic history, the contact between the two units may be thus considered as an extensional detachment. The good preservation of carpholite and the absence of retrograde chloritoid below the shear zone show that exhumation of the CS Unit occurred along a cooling, nearly isothermal path. We link this retrogressive evolution to a syn-orogenic extensional unroofing mechanism connected to the dynamics of the Northern Apennine-Alpine Corsica orogenic wedge.