Permo-Triassic thermal events in the lower Variscan continental crust section of the Northern Calabrian Arc, Southern Italy: Insights from petrological data and in situ U-Pb zircon geochronology on gabbros

摘要

The Calabride nappe is the uppermost structural element of the Calabria-Peloritani Arc, it consists of a Variscan continental crust section intruded by late-Variscan granitoids, on which a Mesozoic syn-rift sedimentary cover was deposited. The deepest part of the Variscan continental section crops out in Northern Catena Costiera. The transition from crustal to mantle-derived rocks is often marked by gabbros. P-T estimates for gabbro emplacement have been calculated applying conventional thermobarometry and a mean crystallization temperature of about 800 ℃ at pressures between -0.46 and 0.65 GPa has been determined. Gabbro emplacement occurs at the base of the crust, during a low-pressure granulitic stage that affected the country rocks at the end of an isothermal decompression. The gabbros can be classified as tholeiites with MORB affinity. In situ LA-ICP-MS U-Pb analyses have been performed on zircons from representative samples of gabbro. U-Pb concordia ages range from 224 ± 4 Ma to 296 ± 6 Ma. Two age populations of middle-early Permian and middle Triassic age, respectively, seem to document the magmatic activity responsible for gabbros intrusion. According to cathodoluminescent images and REE pattern of zircon, the early Permian age refers to a metamorphic inherited core.Lithospheric thinning and a positive thermal anomaly, related to the exhumation of the deepest portion of the continental lithosphere, can explain the post-Variscan metamorphic evolution of the Calabride lithosphere and the underplating gabbroic magmatism. Since the major episode of gabbro emplacement is Triassic, this event could be related to the continental rifting that led to the opening of the Jurassic Tethys and Alpine cycle.