Petrology and tectonic setting of Corsica ophiolitic basalt and comparison with those of the Northern Apennine: Implication for the geological evolution of the Corsica-Apennine orogenic system

Both Alpine Corsica and Northern Apennine represent a key area for the study of the ophiolites originated in the Piedmont-Ligurian oceanic basin. In fact, the coexistence in the Alpine Corsica of Ligurian-type ophiolitic units (characterized by absent or modest metamorphic imprinting) and Piedmont-type units (with HP/LT metamorphism) is of particular interest for the reconstruction of the genesis of this oceanic sector, as well as of its consumption during the early orogenic phases. Many of the Corsica ophiolitic units have been object of recent structural-stratigraphic and petrologic studies. The Balagne, Nebbio, Pineto, and Rio Magno units display typical Ligurian-type features, whereas the Schistes Lustrès, the Inzecca units, and several units of the Castagniccia have typical Piedmont-type features. The S. Angelo di Tenda unit singularly display greenschistes facies metamorphism, thus differing from all the other Corsica ophiolites. This unit could be comparable to the less metamorphosed Inzecca unit. The petrological data of Corsica ophiolitic basalts indicate a T-MORB affinity for the Balagne-Nebbio unit and N-MORB affinity for all other ophiolitic units. Ophiolitic basalts of both the Tyrrhenian area (e.g. Elba I.) and Internal Ligurides of the Northern Apennine (e.g. Vara unit) show N-MORB affinity. Ophiolites represented in olistoliths and olistostromes in turbidites from the Northern Apennine display both N-MORB (mainly in Tuscany) and T-MORB (mainly in the Liguria-Parmesan Apennine) affinities. Recent petrological studies have shown that Corsica ophiolitic basalts show geochemical differences, probably related to differences in their mantle sources. The T-MORBs derived from mantle sources variably enriched by plume components probably related to the magmatic activity associated with the continental break-up. The S. Angelo di Tenda and some Pineto basalts display a little but definite enrichment by a plume component. The Schistes Lustrès, Rio Magno, and some Pineto basalts derived from pure sub-oceanic lithosphere without any influence of plume components. The ophiolitic successions of the Balagne are characterized by the occurrence of siliciclastic sediments and breccias, which are interlayered in all the rocks of the succession, from basalts to Cretaceous deposits (Toccone breccias), whereas the ophiolitic successions of the Schistes Lustrès (e.g., Inzecca unit) display general features, which are very similar to those of the Internal Liguride ophiolites. These characteristics led many authors to interpret the Balagne ophiolites (in the western side) and the Liguria-Parmesan ophiolites (in the eastern side) as the initial oceanic products, and therefore as the two opposite margins of the Ligurian ocean. By contrast, the other ophiolitic units of the Corsica and those of the Vara unit (Northern Apennine) represent products formed during the steady-state oceanic phase. The distribution of the ages of the basalts (obtained from biostratigraphical data on radiolarites), as well as of associated gabbroic and plagiogranitic rocks (obtained from absolute datings) show that the older ages of basalts are found in the eastern Ligurian ophiolites. The absolute datings on gabbros of the External Ligurides, as well as of the Voltri Group are estimated at about 180 Ma. By contrast, in the Balagne biostratigraphic data and absolute datings on plagiogranites indicate relatively younger ages (160-150 Ma). The integration of the various stratigraphic, tectonic, and petrological data obtained from Corsica and Northern Apennine ophiolites, as well as their significance for the dynamics of oceanic opening, the spreading phases, and the spatial-temporal evolution of the Ligurian ocean will be discussed. Finally, the several geodynamic models of oceanic consumption up to day proposed in literature will be discussed in order to examine which model can better satisfies the new data from Corsica ophiolites.