The role of Carbon from recycled carbonated metapelites in the transition from leucite-free to leucite–bearing ultrapotassic rocks: the Central Mediterranean case

The Central Mediterranean region is one of the most important areas on Earth for studying subduction-related potassic and ultrapotassic magmatism, derived from partial melting of the metasomatised lithospheric mantle wedge. In this region, leucite-free (i.e., lamproite) and leucite-bearing (i.e., kamafugite, leucitite, and plagioleucitite) ultrapotassic rocks closely occur, in a time-related progression, linked to the evolution of both the mantle source and the regional tectonic regime. Time- and space-related magmatism migration followed the roll-back of the subducting slab and the anticlockwise drift of the Italian Peninsula. Leucite-free silica-rich lamproites are restricted to the early stage of magmatism and are associated with ultrapotassic shoshonites and high-K calc-alkaline volcanic rocks. Leucite-bearing (i.e., Roman Province) rocks are erupted consistently later than lamproite-like and associated shoshonitic rocks, with post-leucititic volcanism occurring in the late stage of volcanic activity with eruption of alkali-basaltic to latitic and trachytic rocks, often after major caldera-forming events. Present-day ultrapotassic volcanism is restricted to the Neapolitan area. Central Mediterranean potassic and ultrapotassic rocks are extremely enriched in incompatible trace elements with variable fractionation of Ta, Nb, and Ti in comparison to Th and large ion lithophile elements (LILE). They are also variably enriched in radiogenic Sr and Pb and unradiogenic Nd. The main geochemical and isotopic signatures are consistent with sediment recycling within the mantle wedge via subduction. A twofold metasomatism, induced by the recycle of pelitic sediments and dehydration of lawsonite-bearing schists generates the early metasomatic events that enriched the mantle wedge from which leucite-free ultrapotassic rocks (i.e., lamproite) were generated. Recycling of carbonate-rich pelites played an important role in the shift to silica-undersaturated ultrapotassic rocks (kalsilite- and leucite-bearing) of the classic ‘Roman province’.