The tectonic significance of Italian magmatism: an alternative view to the popular interpretation

The Tyrrhenian rift zone has been the site of widespread magmatism since late Tortonian times. A pronounced asymmetrical distribution, reflecting the tectonic structure, characterizes Italian magmatism. Sodic basalts occur on the western Tyrrhenian flank and transitional-MORB basalts occur in the Tyrhenian Sea. The eastern flank, however, is characterized by K-alkaline and HK- to ultra-alkaline (e.g. carbonatites and melilitites) rocks. Major trace elements and isotopic compositions allow two major magmatic lineages to be identified: one relating to a non-radiogenic basaltic end-member and the other to a mantle end-member enriched in Ca, with high LILE/HFSE ratio and high Sr isotopic ratios. Their mantle sources are located within the lithosphere thermal boundary layer (TBL) and the metasomatized phlogopite-carbonate asthenosphere at the base of the TBL, respectively. The composition and spatial distribution of volcanism and relative mantle sources tend to map the geometry of the lithospheric mantle and to define a pronounced increase in depth from less than 60 km to about 100 km across the boundary between the thinned Tyrrhenian lithosphere and the Adriatic lithosphere. A mechanism of intra-continental passive rifting, which drives mantle upwelling, is sufficient to satisfy the petrological and geochemical constraints and the observed tectonic environment without requiring a subduction plane.