Tertiary to Present Evolution of Orogenic Magmatism in Italy

Tertiary to present evolution of magmatism in Italy is mostly related to the overall process of convergence between the African and European plates. This has been going on since late Cretaceous and has been responsible for the formation Alps and Apennines mountain chains and associated backarc basins. The Alpine orogeny and magmatism are the result of eastward and southward subduction of the Tethys ocean basin beneath the Adriatic continental plate (African promontory), and of the continental collision, and post-collisional relaxation along the Alpine arc. Deep subduction of continental crust is documented in the Western Alps (Dora Maira Massif). The Apennine chain and associated magmatism developed as a result of west-directed subduction of the Adriatic plate beneath the southern European margin. The bulk of the magmatism during this complex geodynamic evolution is mostly related to melting of mantle sources that were modified by subduction processes. These magmas, which are referred to as “orogenic”, show island arc geochemical signatures, namely high enrichments in Large Ion Lithophile Elements (LILE: Rb, K, Ba, LREE) and relative depletion in High Filed Strength Elements (HFSE: Ta, Nb, Zr, Ti). A significant amount of magmas, however, are related to melting of mantle sources that were not contaminated by subduction. These magmas are referred to as “anorogenic” and have low LILE/HFSE ratios. They occur mainly in backarc position, or also on the border of the foreland. There is no reliable evidence of Paleocene orogenic magmatism in Italy, whereas anorogenic rocks are found in some places of the Adriatic promontory of the African plate (Punta delle Pietre Nere, Veneto and Trentino regions). In the Eocene, magmatism was concentrated in the Alpine area, with emplacement of both orogenic (e.g., andesitic clasts from the Champsaur Sandstone, dikes and stocks in the Orobic Alps, and the Southern Adamello batholith) and anorogenic magmas (Veneto and Trentino regions). According to recent Ar/Ar dating, orogenic igneous activity may have started also in the Sardinia region, with eruption of calcalkaline andesites. In the Oligocene (~ 32-30 Ma) the climax of Alpine orogenic magmatism was reached. This ranges from calcalkaline to ultra-potassic (lamproitic), the latter being restricted to the Western Alps, i.e. where subduction of upper continental crust is documented. Calcalkaline to shoshonitic activity in the Eastern Alps continues up to Late Oligocene (~ 24 Ma) along the Periadriatic Fault Lineament. Minor anorogenic magmatism occurred in the Veneto area, ending in Early Miocene. Younger activity in Italy is essentially related to the Apennine subduction zone. In Sardinia, the bulk of calcalkaline and high-K calcalkaline activity started around ~ 28 Ma, and reached its climax at about 21-18 Ma, probably continuing until ~ 12 Ma. Successively, orogenic magmatism shifted eastward and southeastward forming several centres in the Tyrrhenian basin and the calcalkaline to ultrapotassic magmatism of the Italian Peninsula and the Southern Tyrrhenian Sea. At the same time anorogenic magmatism (tholeiites to Na-alkaline) developed in several places behind the Apennine compression front (Sardinia and Tyrrhenian Sea basin), and along the northern margin of the Africa foreland (Eastern Sicily and Sicily Channel). Relationships between magmatism and geodynamics in Italy are complex. Orogenic magmatism is sometimes coeval to subduction (e.g. eastern Aeolian arc), suggesting mantle wedge melting under the effect of water-rich fluids released by the active undergoing slab. In other cases, it is much younger than subduction (e.g. Western Alps lamproites, Tuscany Province), resulting from decompression melting of lithospheric mantle that was contaminated during older subduction. In these cases, melting events are related to extensional processes that affected the various areas at different times after cessation of subduction. Petrological and geochemical compositions of orogenic magmatism suggests involvement of variable amounts of upper crustal material in their genesis. This was added to the mantle sources during subduction, and was particularly strong during contamination of mantle sources in the Western Alps and the Tuscany and Roman provinces. Mantle components of deep origin (asthenosphere) were also involved in the orogenic magmatism especially along transversal lithospheric faults cutting the subduction front, or in backarc volcanoes. Asthenospheric and deep-mantle-plume origin has been suggested as possible sources for these components, but the rosles of different reservoirs are poorly understood and still subject of debate.