Summary and Conclusions
Tertiary to present magmatism in Italy is basically related to the convergence between the African and European plates, a process which has been going on since late Cretaceous and is still presently active. Cretaceous-Oligocene subduction of the European plate beneath the northern African margin and the following continental collision, have been the cause of Alpine orogenesis and associated magmatism. Tertiary orogenic magmatism along the Alpine chain is variable but overall indicates an origin within upper mantle sources that were contaminated by upper crustal material during subduction. Potassic rocks in the Western Alps show the strongest crustal-like geochemical and isotopic signatures, indicating a very important contribution by upper crustal material in the origin of magmatism. Note that deep subduction of continental crust (Dora Maira Massif) is documented in this region.
Mantle-originated melts along the Alps, underwent strong modification during emplacement, forming derivative magmas by combined processes of fractional crystallisation and crustal assimilation. Therefore, significant mantle-crust interaction in the Alpine magmatism occurred both in the source (mantle contamination), and during magma emplacement (magma contamination). The two processes superimposed to each other, making their respective effects difficult to recognise, in most cases. Amount of crustal melting seems stronger in the Central and Eastern Alps than in the west, suggesting different thermal regimes for various Alpine sectors.
Younger orogenic magmatism in Italy developed in and around the Tyrrhenian Sea basin and is essentially related to the subduction of the Adriatic-Ionian plate beneath the southern European margin. Ages range from Oligocene to present and become younger from west to south-east. This is the effect of migration of subduction processes.
Magmatic compositions vary very strongly and range from arc-tholeiitic and calcalkaline to shoshonitic and potassic-ultrapotassic alkaline. Overall, there is an increase in potassium with time and from west to east, the most potassium-rich rocks being found among the youngest products of central Italy. This time-related regional modification of magma compositions results from modification of the nature of the undergoing lithosphere from Oligocene to present during eastward migration of subduction. Subduction of an oceanic-type lithosphere is believed to be responsible for tholeiitic to calcalkaline magmatism in Sardinia, the Tyrrhenian Sea basin and the Aeolian Arc. Subduction of a continental-type lithosphere is believed to be the cause of extensive mantle contamination and generation of potassic and ultrapotassic volcanism in central Italy (Peccerillo, 2005a and references therein).
Some sectors of the upper mantle beneath the Italian peninsula also shows geochemical evidence of a previous contamination event, with compositional characteristics similar to the one which occurred in the Western Alps. This is particularly evident in Tuscany, where mafic rocks have major, trace element and radiogenic isotope compositions similar to those of the Western Alps. This is in agreement with hypotheses that the lithospheric sectors of the Tuscany-northern Latium area were part of the Oligocene Alpine overriding margin, which was successively dissected by extensional processes behind the subducting Adriatic plate, and shifted eastward to its present position.
Compositions of pre-metasomatic mantle sources are still poorly understood. Mantle components of both asthenospheric and lithospheric origin were subjected to contamination by subduction-related material. The role of deep mantle plume components is very hypothetical and poorly constrained and conflicts with a number of geochemical, geophysical and field evidence, as discussed by Peccerillo & Lustrino (2005).