Alpine subduction imprint in Apennine volcaniclastic rocks. Geochemical–petrographic constraints and geodynamic implications from Early Oligocene Aveto-Petrignacola Formation (N Italy)

The present-day northern Apennines represent an area where two different orogenic cycles took place. The most ancient is the Alpine Orogeny, initiated with a south- to south-eastward subduction of oceanic lithosphere (Early Cretaceous to Late Eocene phase), followed by continental collision. The younger is referred to the Apennines Orogeny, characterized by a west- to north-westward oceanic lithosphere subduction started from Late Eocene and still active in the southernmost sectors of Italy (Calabrian Arc). In this framework, during the Early Oligocene, an ~800 mthick conglomeratic succession extremely rich in volcanic material (up to ~90% in volume) was deposited in the Northern Apennines, forming the Aveto-Petrignacola Formation (APF). The volcanic fraction of this succession is made up of basaltic andesites, andesites and dacites, with minor rhyolites, basalts and gabbros found as pebbles ~0.001–0.5 m3 in size. Petrographic, mineral chemical, major and trace element analyses, as well as Sr–Nd–Pb isotopic ratios are compatible with calc-alkaline magmatism generated as a consequence of metasomatic modifications related to the subduction of oceanic lithosphere. The large spectrum of lithologies is compatible with processes of fractional crystallization of the main phases observed in thin sections. However, the large range of isotopic ratios (87Sr/86Sr=0.7044–0.71013; 143Nd/144Nd=0.51278– 0.51223; 206Pb/204Pb=18.44–18.83; 207Pb/204Pb=15.55–15.67; 208Pb/204Pb=38.05–38.85) suggests the presence of heterogeneous mantle sources, possibly coupled with interaction of melts with crustal rocks at shallow depth. On the basis of field geology constraints, we propose that the subduction-related geochemical and mineralogical characteristics of the APF volcanic rocks reflect ancient modifications of their mantle sources, lacking any evidence for a genetic link with the Apennine subduction system. Subduction-related metasomatism of APF mantle sources likely occurred during the older south-east dipping subduction of the Alpine Tethys under Northern Adria (present day Northern Italy)