Tectonometamorphic evolution of the Massa Unit in Punta Bianca area (Northern Apennines, Italy)

The Northern Apennines (Italy) is characterized by the occurrence of low-grade metamorphic units derived from the Adria plate margin, which are the lowermost tectonic units of the apenninic nappe-pile. They are commonly referred to as the Tuscan Metamorphic Units and comprise the Apuane and Massa units. These units, including basement rocks of Hercynian age and a Mesozoic-Tertiary sequence, were affected by a polyphase tectonic history, during which they reached peak metamorphic conditions and were subsequently exhumed to the surface. New quantitative data about the structural and metamorphic evolution of the Massa Unit cropping out in the Punta Bianca area will be presented. Three main ductile tectonic phases, followed by two brittle and brittle-ductile tectonic events have been recognized. The first tectonic phase (D1) was related to continental collision and northeastward thrusting. The second tectonic phase (D2) was linked to the main exhumation stage of the metamorphic rocks, achieved during syn-orogenic extension. The later D3 phase was related to a renewal of the compressive tectonics. In addition an episode of ductile to brittle NE-thrusting (D4) is documented before the main event of high-angle normal faulting (D5). Finite strain analyses were performed on metabreccias and metabasalts with deformed strain-markers (clasts and vesicles), supporting that the D2 phase occurred under general flattening. A study of progressive deformation was performed on vein sets developed in different moments during the tectonic history, allowing to reconstruct a more detailed tectonic evolution and to understand the complex relationships between structural elements formed during different moments of the tectonometamorphic evolution of the area. Microstructural analyses were combined with petrographic studies to reconstruct e P-T path. Synkinematic recrystallization of white micas + chlorite + quartz + albite ± epidote in metapelites has been recognized along both S1 and S2 foliations developed parallel to F1 and F2 folds respectively. Electron microprobe X-Ray maps and chemical points highlighted different compositions of white micas and chlorite recristallized along both S1 and S2 foliations. Temperature estimations constrained D1 phase at 330 ± 10 °C and D2 phase at 350 ± 5 °C while decreasing pressure values have been detected from D1 to D2 tectonic phases