During the Neogene and Quaternary, potassic and ultrapotassic magmas erupted in association with shoshonitic and calc-alkalic magmas across much of the Italian peninsula. On the basis of the temporal and spatial distribution of this volcanism, and its mineralogical and compositional characteristics, three different magmatic provinces have been defined. The northwesternmost province, the Tuscan Magmatic Province, is domi nated by leucite-free ultrapotassic rocks (lamproite), shoshonite, and minor calc-alkalic rocks. The Roman Magmatic Province is dominated by leucite-bearing rocks with variable degrees of silica saturation, from undersaturated (leucitites and plagio-leucitites) to strongly undersaturated (kamafugites), but minor amounts of shoshonitic to high-potassium calc-alkalic rocks are still present. The Lucanian Magmatic Province, located at the southeasternmost edge of the volcanic belt, is dominated by foiditic (haüynites and leucitites) and kamafugitic (melilitites) members, all strongly under-saturated in silica. In spite of these petrologic differences, the Neogene Italian potassic and ultrapotassic rocks display similar trace-element patterns. Depletion in high field strength elements with respect to large ion lithophile elements is a common feature. Sr, Nd, and Pb isotopic compositions of mafic high-MgO rocks range widely, relating mainly to geographic location of eruption and to enrichment in alkalies. The Os isotopic composition of these samples, however, does not clearly correlate with eruption location, but is dependent on the amount of “continental crust component” added to the magmas. Some of the studied samples are compatible with crustal contamination en route to the surface. In most cases, however, there are several lines of evidence suggesting the possibility that crustal components were added directly to the mantle source prior to partial melting. Large amounts (many tens of weight percent) of “crustal component” must be added to the peridotitic mantle in order to obtain the 187 Os/ 188 Os of the lamproites in Tuscany. These large amounts of crustal components have been recycled into the mantle in the form of either melts or fluids. The recycling can be reconciled with a veined mantle in which the crustal component is concentrated. Partial melting of veins would then produce the high-silica and high-magnesium lamproitic magmas from Tuscany. Dilution of the crustal components by increasing partial melting of surrounding mantle peridotite, or alternatively, a reduction of metasomatic veins, could then produce shoshonitic and high-K calc-alkalic mafic magmas. Southeastward geochemical and isotopic variations are reconciled with decreasing direct contributions from crustal components introduced into the mantle by sub-duction, but an increasing role of subducted fluids from dehydration of CO 2 -rich sediments. The coupled isotopic and chemical characteristics of Italian magmas cannot be reconciled with an ocean-island basalt (OIB)–like primary magma composition due to the substantial overprinting by crustal- and or subduction-related components.

The southern Tyrrhenian region represents a rare case of an active backarc basin where island-arc basalt (IAB)–type and ocean-island basalt (OIB)–type magmas coexist. IAB-type magmatism is the most common, whereas OIB-type magmas are restricted to a few areas. Geochemical and isotopic characteristics of southern Tyrrhenian submarine volcanic samples are summarized herein, with special attention to samples recovered during recent seafloor sampling. Samples derived from Marsili volcano, the biggest seamount in the Tyrrhenian Sea, together with those of the Prometeo submarine lava field and Palinuro seamount, give a comprehensive picture of the mantle sources of southern Tyrrhenian magmatism. Most of these samples are relatively high-magnesian basalts that approximate primary magma compositions, thus providing insight into petrogenesis of IAB- and OIB-type magmas of this area. Petrological data, integrated with new bathymetric data, allow the geodynamic evolution of the southern Tyrrhenian backarc basin to be reconstructed. We suggest that the transition from IAB- to OIB-type magmas recorded in the southern Tyrrhenian backarc basin is related to the propagation of asthenospheric mantle from beneath Africa around the edges of the Ionian slab. The data suggest that the inflow of asthenosphere from beneath Africa has been important since the early Neogene evolution of the southern Tyrrhenian region.