Deformation and pressure-temperature-time history of the External Tuscan Units in the Northern Apennines (Italy): The case of the Punta Bianca Unit

In this study we investigated through a multidisciplinary approach the still poorly known tectono-metamorphic evolution of the Punta Bianca Unit in the Northern Apennines. The Punta Bianca Unit is part of the Tuscan Metamorphic Units, a group of units derived from the Adria passive margin, metamorphosed at different conditions, and forming the backbone of the Northern Apennine belt. We combined meso- and microstructural analyses, ⁴⁰Ar/³⁹Ar white-mica geochronology and multi-equilibrium geothermobarometry from high-resolution X-ray chemical maps, to unravel the deformation and metamorphic history of this part of the belt. Meso- and microstructural data indicate that the Punta Bianca Unit recorded two main phases of ductile deformation (here referred to D_p-1 and D_p) associated with syn-kinematic growth of K-white mica, chlorite, calcite, quartz on the related tectonic foliations (S_p-1 and S_p), followed by a later ductile deformation phase (D_p+1) lacking of metamorphic blastesis. P-T estimates complemented by microstructural data suggest that peak metamorphic conditions reached ∼0.8 GPa and ∼350°C and occurred synchronously with the first deformation phase (D_p-1). Temperature values were also confirmed by Raman spectroscopy of carbonaceous material on selected samples. This stage was followed by the exhumation of the Punta Bianca Unit, as testified by decreasing pressure and temperature down to ∼0.4 GPa and ∼300°C respectively, together with the development of the main foliation (S_p). At the regional scale, the Tuscan Metamorphic Units have been mostly affected by HP-LT metamorphic gradients equilibrated under blueschist-facies conditions (up to ∼1.4 GPa). Results from the present work on the contrary, suggest that the Punta Bianca Unit never reached such HP-LT conditions, testifying that it was deformed at relatively upper structural levels, thus highlighting an important variation in the tectono-metamorphic evolution of the Tuscan Metamorphic Units along strike in the Northern Apennines. ⁴⁰Ar/³⁹Ar laserprobe data (using both the in-situ and step-heating techniques) indicate a minimum age for the onset of continental subduction of ∼20 Ma (D_p-1), which was followed in close succession by exhumation at ∼16 Ma. This approach, if applied to different tectonic units building up the nappe pile of the Northern Apennines, could be successful in better unravelling the tectonic history.