Metamorphism of the mandatoriccio unit in the context of the hercynian and pre-hercynian evolution of the calabrian-peloritan arc (southern italy).

Abstract

The Mandatoriccio Unit represents one of the structural elements of the Hercynian Calabrian-Peloritan Chain and is constituted by unknown age metamorphic rocks considered to be part of the pre-Cambro-Ordovician basement of the Calabrian-Peloritan Arc. In the Sila this unit tectonically overlie the Bocchigliero Paleozoic sequence, which represents the original cover of the pre-Cambro-Ordovician basement, metamorphosed during the Hercynian orogeny (330 Ma). Four metamorphic events are recognized in the rocks of the Mandatoriccio Unit. The two oldest greenschists facies events are marked by penetrative deformations, the third low to medium-low temperature event lacks them, and the fourth greenschists facies event bears weak penetrative deformation characteristics. Geochronological studies demonstrate that the last metamorphic event has an age of 326 ± 6 Ma, which coincides perfectly with the age of the Bocchigliero Unit Paleozoic metamorphic rocks; also the environment in which the metamorphism originated is analogous. The presence of the Alpine metamorphism is excluded; the mica ages ranging from 250 to 43 Ma are interpreted as partial openings of the systems caused mainly by the Alpine tectonics. The thermal metamorphism, which is certainly older than 3 30 Ma, is considered to be Ordovician and linked to the tensional phenomena responsible for the melting of crustal rocks and for volcanism in the Paleozoic basins. The two oldest metamorphic events of dynamic type are related to the pre-Cambro-Ordovician orogeny. It is suggested that the pre-Cambro-Ordovician basement of the Calabrian-Peloritan Arc has a metamorphic history similar to that recorded in the rocks of the Mandatoriccio Unit. In particular, the last greenschists facies metamorphic effects appear in most cases to be of the Hercynian age. The variable and younger than 270 Ma ages may rather represent the effects of Alpine tectonics, fluid circulation or weathering. The presence of Caledonian metamorphism in the Arc is excluded.