Asynchronous extension of the late-Hercynian crust in Calabria

Abstract: Quartz-monzodioritic dykes have intruded the Hercynian continental lower crust of Calabria at 323±5 Ma. Pseudosection and average PT calculations have been performed using THERMOCALC. The obtained P (average PT) and T (pseudosection and average PT) values indicate that the quartzmonzodioritic dykes have intruded the 750 °C hot metapelitic-migmatitic country-rocks at a depth of about 32 km, with a temperature of ca. 850 °C. Microstructural analysis suggests that deformation of the dykes started at hypersolidus conditions during emplacement and continued after cooling at subsolidus granulite facies conditions together with the country rocks. During this deformation event, which started not later than 323±5 Ma ago, the main Sn+1 schistosity evolved. Vorticity analysis of the quartz-metadioritic dykes and of the metapelitic country rocks shows that the Sn+1 schistosity evolved during a general shear deformation characterized by 60 to 70 percent of pure shear. This high percentage of pure shear suggests that the main Sn+1 schistosity evolved in the hot lower crust undergoing extension during vertical shortening. A horizontal attitude of this extensional schistosity is confirmed by retro-deformation of the today obliquely SE-ward dipping Sn+1 schistosity. Extension continued with time and propagated upwards within the crust, affecting the middle crust in a time interval of 306±1 to ca. 300 Ma when huge masses of granitoid rocks intruded it. The deepest batches of these granitoid bodies show a gneissic schistosity which developed under ductile conditions, while granitoids at higher levels became deformed in a brittle fashion. Our whole data set is consistent with a late Hercynian evolution in Calabria characterized by a progressively crust overlying a mantle that gradually rose during lithospheric extension. This extension was asynchronous, starting at 323±5 Ma in the lower crust and affecting higher crustal levels at progressively later times.