Cooling and exhumation history of deep-seated and shallow level, late Hercynian granitoids from Calabria

The thermal and exhumation history of late Hercynian granitoids from Calabria (Sila and Serre massifs) has been studied using thermobarometry and radiometric age determinations. The uplift and erosion which followed contractional tectonics of Tertiary age exposed in Calabria a nearly complete section of the Hercynian crust. Field data, constrained by igneous thermobarometrical data, have enabled us to draw simplified crustal profiles. In both the Sila and Serre massifs, granitoids make up the intermediate portions of the crustal sections and are stacked as tabular intrusions for up to 13 km cumulative thickness. Shallow granitoids are characterized by a weak fabric, mostly developed in the magmatic stage, whereas deep-seated granitoids display a strong fabric developed in the magmatic state and, with decreasing temperatures, in the subsolidus state. The intrusive bodies were emplaced at 300-290 Ma, at a time when the Calabrian crust was undergoing extensional tectonics and crustal thinning. The subsequent post-Hercynian evolution is recorded by Rb-Sr dates of micas and fission track ages of zircon and apatite obtained from granitoids emplaced at different depths. A decrease in Rb-Sr and fission track ages is observed as depth of emplacement increases. Data on the post-Hercynian geological evolution of Calabria were used to model in three stages the cooling and exhumation history of deep-seated and shallow granitoids. The first stage, in Permian to Triassic times, was characterized by slow erosion. It was followed by a second stage of extensional tectonics in Jurassic times. The third stage was exhumation during the Apenninic Orogeny. The model has generated two P-T-t arrays, one for deep-seated and the other for shallow granitoids of the Serre massif. The T-t paths. suggest that the dates of micas, zircon and apatite are cooling ages. They also show that deep-seated granitoids remained at temperatures above the brittle-plastic transition for a long time, whereas shallow granitoids cooled rapidly. Distinct P-T-t paths explain why deep-seated and shallow granitoids display different fabric and microstructural features. Copyright (C) 2000 John Wiley & Sons, Ltd.