The Cainozoic calcalkaline magmatism of the western Mediterranean and its geodynamic significance

Space-time distribution and petrogenetic affinity of the Cainozoic calcalkaline orogenic, magmatism from the western Mediterranean (from Provence to Campania) has been reassessed, on the basis of new petrochemical and Sr-Nd isotopic data, for a better understanding of the geodynamic evolution of the area. The calcalkaline magmatism may be subdivided in two main cycles: the older, Oligo-Miocene in age (34-20 Ma in Provence, and 32-13 Ma in Sardinia); and the younger, Pliocene-Quaternary in age (Eolian area ~ 2 Ma, Campania ca. 2 Ma, and part of the Tyrrhenian oceanic crust 4-1.6 Ma). The tholeiitic/calcalkaline serial affinity of the early magmatism both in Provence and Sardinia and its zonation in Sardinia for the period 21-18 Ma (tholeiitic/calcalkaline in the south, high-K calcalkaline/shoshonitic in the north) imply subduction of N-NW dipping oceanic lithosphere, at least from Upper Eocene to Lower Miocene. The existence at that time of subducting oceanic lithosphere, tentatively assigned to the Sicilide-Canetolo basin, is also required by the chronologically correlated oceanic opening of the Ligure-Balearic basin and anticlockwise rotation of the Sardinia-Corsica microplate. After a period of quiescence, coinciding approximately with the last main compressional phases in the northern Apennines, the second calcalkaline cycle developed during Pliocene Quaternary times along the eastern peri-Tyrrhenian margin, from the Eolian area to Campania, in relation to a rifting phase affecting the Alpine-Apennine chain, east of Sardinia. This orogenic magmatism - as well as che diachronous oceanization of the Tyrrhenian basin and the southeastward migration of the Calabrian arc - can be satisfactorily interpreted in the framework of island are - back-are basin systems where, during the early stages of back-arc, opening, are- related volcanism is absent on both the remnant are (Sardinia) and the migrating fore-are plate (Eolian-Calabrian area) above a seismically active subduction zone (Ionian oceanic: lithosphere). Geochemical and isotopic characteristics of the peri-Tyrrhenian orogenic magmas indicate that their mantle sources were hybridized by continental crust material (mainly upper crust- derived sediments) dragged down during subduction processes to a limited extent in the Eolian area (Stromboli), and more extensively further north in the Campanian and Roman provinces. This confirms the existence of a major lithospheric discontinuity (Ortona-Roccamonfina line and 41N' transform fault) separating a continental subducted slab (Adriatic) under the Central Apennines, and an old oceanic, still seismically active, subducting slab (Ionian) under the Calabrian Eolian area.