Textural features and strain of granitoids emplaced at different depths: the example of the Hercynian tonalites and granodiorites from Calabria

Late Hercynian calc-alkaline granitoids occur at shallow to deep structural levels in the crustal sections exposed in Calabria. The granitoids may be either weakly or strongly foliated and the anistrophy is defined by a preferred orientation of feldspars, biotite, amphibole and elongated quartz aggregates. The fabric developed in the final stages of crystallisation and in the solid-state. In this study we compare P-T estimates obtained by igneous thermobarometry with deformation styles experienced by the late granitoids from the magmatic stage down to subsolidus conditions. P-T estimates for the emplacement of the Calabrian granitoids were obtained by hornblende-plagioclase thermometry and Al-in-hornblende barometry. We use the pressure data to discriminate three categories of granitoids emplaced at different structural levels: a) upper crust granitoids emplaced at depths lower than 12 km: b) intermediate crust granitoids emplaced at depths varying from 17 to 20 km; c) lower crust granitoids emplaced at depths greater than 20 km. A magmatic foliation is recognisable in all the granitoids and is defined by the preferred orientation of feldspars. High T solid-state deformation is present in all three categories but with different intensity and frequency. In the upper crust granitoids solid-state deformation is revealed locally by grain boundary migration in quartz aggregates, whereas in the deep crust granitoids completely recrystallised quartz grains display a form texture, and plagioclase or K-feldspar show evidence of dynamic recrystallisation. In addition, the intermediate and deep crust granitoids experienced deformation under decreasing temperatures during coolin. Our information suggest that the upper crust granitoids have experienced deformation at temperatures higher than 410°C for a short period of time, typically less than 5 Ma, explaining why plastic deformation is not commonly found. In contrast, intermediate and lower crust granitoids have experienced deformation at high T for longer periods, up to 100 Ma, and these granitoids underwent a protracted strain history before the final exhumation of the Calabrian intermediate to lower crust.