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 © 2000 John Wiley & Sons, Ltd.     

Petrology of basic and intermediate orogenic granitoids from the Sila Massif (Calabria,southern Italy)

Hercynian gabbroic, dioritic and tonalitic rocks crop out in the neighbourhood of Rovale (Sila Grande, Calabria). They make up a crude rectangular outcrop with the western part consisting of gabbroic rocks and the eastern of dioritic and tonalitic rocks. They come into contact with medium to high grade metapelites on the western side and with heterogeneous granodiorites on the other sides. In the gabbroic body both opx ± ol bearing cumulates and amphibole differentiates occur and are characterized by the widespread presence of brown pargasite. Sporadic magmatic to subsolidus corona textures between olivine and plagioclase or orthopyroxene and plagioclase can be observed and their preservation clearly suggests a post-tectonic emplacement for the gabbroic magma. Diorites and tonalites display hypidiomorphic textures free of olivine and orthopyroxene and bearing green Mg-hornblende. The granitoids, on the basis of chemical data, display orogenic features of the continent-continent collision type. The gabbroic rocks have high Al tholeiitic composition and fractionation of orthopyroxene and plagioclase played an important part in their evolution. The Rb/Sr isochron method did not give a precise emplacement age for the granitoids as a whole. Initial ⁸⁷Sr/⁸⁶Sr ratios (at 290 Ma) are higher in the gabbroic body (0.7091–0.7095) than in diorites and tonalites (0.7083–0.7092). Thus gabbroic rocks appear more displaced than diorites and tonalites towards crustal isotopic composition. The eNd data seem to confirm this feature, thus suggesting that the gabbroic rocks and diorites derived from distinct mantle magma batches. Interestingly, small isotropic gabbroic masses occur within the diorites and show general features that allow them to be considered as possibly parental with respect to the host diorites. The evolution to the dioritic composition might have occurred through fractionation and minor mixing with a more acidic component such as the northern granodiorites. Geochemical, Sr and Nd isotopic data indicate a scenario of a composite plutonic body formed by distinct magma batches of mixed crust and mantle origin.