Rb-Sr age constraints on the Alpine metamorphic overprint in the Aspromonte Nappe (Calabria-Peloritani Composite Terrane, southern Italy).

In the pre-Alpine basement of the Aspromonte Nappe (southern Calabria and northeastern Sicily), a Barrovian greenschist to amphibolite facies tectono-metamorphic overprint has been recognized since the end of the last century. It was tentatively ascribed to the Alpine cycle from field and petrological evidence. The results of a systematic isotopic study on Southern Calabria rocks, supporting that interpretation, are reported in the present paper. Rb-Sr isotopic data from biotites and white micas show ages ranging from 314 to 22 Ma, with a maximum frequency around 30-25 Ma. The oldest values (314-308 Ma) are interpreted as Variscan ages, whereas the youngest values (32-22 Ma) support, for the first time isotopically, the inferred occurrence of an Alpine metamorphism in the Aspromonte Nappe. The intermediate values (273-48 Ma) are considered as mixed «ages». The thermal peak of the Alpine metamorphic overprint indicates the actual age of deformation in the Aspromonte Nappe, related to a major tectonic phase responsible for the nappe stacking. The age of the metamorphic overprint is in agreement with the available stratigraphic data and further strengthens the distinction in the Calabria-Peloritani Arc of two terranes, characterized by significant differences in the Alpine tectono-sedimentary and tectono-metamorphic evolution. The occurrence of a late Oligocene-early Miocene metamorphism also in the Tellian Maghrebids and in the Sardinia Channel further supports the interpretation which considers these portions of the circum-Mediterranean mountain belt and the northern and southern sectors of the Calabria-Peloritani Arc as fragments of a former continuous orogenic belt, dismembered by the opening of the Algero-Provençal and Tyrrhenian basins.