The Messinian volcaniclastic layers of the Northern Apennines:evidence for the initial phases of the Southern Tyrrhenian spreading?

A petrological investigation has been carried out on the Late Miocene (around 5.5 Ma) volcaniclastic layer interbedded in the uppermost Messinian deep-water sediments of the Northern Apennines foredeep basin. The studied samples derive from five sections located on a 200 km long area and reveal that the volcaniclastic layer corresponds to centimetric- to metric-thick turbiditic sequence, essentially composed of lapilli to ash-sized glass shards and scarce (<5%) mineral fragments. The homogeneous chemical composition of the glass shards (calc-alkaline rhyolite) and crystals (plagioclase, sanidine, biotite) suggests a cogenetic origin for the studied samples. In these samples non-volcanic detritus is absent and the morphology of the glass shards is indicative of an origin by an explosive magmatic eruption. Morphological, geochemical and sedimentological features suggest that the volcaniclastic layer can be considered as «primary monomagmatic turbidite» resulting directly from the eruptive vent of a calc-alkaline rhyolitic volcanic arc system. None of the coeval outcropping volcanoes, up to a distance of 1500 km from the studied sections, can be considered as the possible source of this layer. Nevertheless, tectono-magmatic evidence suggests that it may represent the witness of the activity of a calc-alkaline submarine volcanic arc located northwestward of the Aeolian Arc, and related to the first rifting-spreading episode of the Southern Tyrrhenian region.