Tertiary to Present Evolution of Orogenic Magmatism in Italy

Keegan E. Alagna, Angelo Peccerillo, Silvana Martin, and Carmelita Donati

Tertiary to present magmatism in Italy is related to the convergence between the African and European plates. The Alpine orogeny and magmatism are the result of eastward and southward subduction of the Tethys ocean basin beneath the Adriatic continental plate and of the continental collision and post-collisional relaxation along the Alpine arc. 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 related to melting of mantle sources that were modified by subduction processes. These magmas, which are referred to as “orogenic”, show high enrichments in Large Ion Lithophile Elements and relative depletion in High Field Strength Elements. 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.

Eocene magmatism in Italy was concentrated in the Alpine area, with emplacement of both orogenic and anorogenic magmas. Orogenic igneous activity may have started also in Sardinia.

In the Oligocene, the climax of Alpine orogenic magmatism was reached. Compositions range from calcalkaline to ultra-potassic (lamproitic), the latter being restricted to the Western Alps, where subduction of upper continental crust is documented. Calcalkaline to shoshonitic activity in the Eastern Alps continues up to Late Oligocene.

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 in the Italian peninsula. At the same time anorogenic magmatism (tholeiitic 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 with subduction, suggesting mantle wedge melting under the effect of water-rich fluids released by the undergoing slabs. In other cases, magmatism is younger than subduction, resulting from post-collisional decompression melting of contaminated mantle wedge.

Petrological and geochemical compositions of orogenic magmatism suggests involvement of variable types of upper crustal material in its genesis. This was added to the mantle sources during subduction, and was particularly abundandant in the Tuscany and Roman provinces. Mantle components of deep origin were also involved in the orogenic magmatism especially along transversal lithospheric faults cutting the subduction front, or in backarc volcanoes. Asthenosphe and deep-mantle-plume have been suggested as possible sources for these components, but such an issue is poorly understood and still subject of debate.