New geochemical and petrological data on Corsica ophiolitic basalts: Possible geodynamic implications for the Alpine Corsica-Northern Apennine system

Two types of ophiolitic sequences were classically distinguished in Corsica on the bases of contrasting styles of deformation, metamorphism, and geochemical characteristics of basalts: 1) the Balagne Nappe, characterized by lack of deformation, lack of high-pressure/low-temperature (HP/LT) metamorphism, and T-MORB affinity of basalts; 2) the Schistes Lustrés (SL) of the Inzecca Unit, with HP/LT metamorphism and N-MORB geochemistry. Subsequently, the geochemistry of most of the Corsica ophiolitic units was often supposed only on the bases of their metamorphic style; i.e.: lack of HP/LT metamorphism = T-MORBs (e.g. Nebbio and Pineto Units), presence of HP/LT metamorphism = N-MORBs (e.g. other SL units). Consequently, many tectonic models for Alpine Corsica have essentially been based on the assumption that T-MORBs represent the early-formed oceanic portion close to the European continent, whereas N-MORBs represent deeply subducted inner portions of oceanic crust, as well as on the different evolution of the N-MORB-type Piedmont-Ligurian lithosphere recorded by SL (deep subduction) and by Northern Apennine Internal Liguride ophiolites (lack of deep subduction). However, recent works have demonstrated that there is no relation between metamorphic style and geochemistry; thus these tectonic models should be reconsidered. For this reason, field and chemical works on a number of Corsica ophiolitic basalts have been made for identifying their petrological features in relation to their formation within the oceanic setting, in order to provide a base for a further reconstruction of their emplacement. The Balagne-Nebbio (BN) basalts display T-MORB geochemistry; however, Balagne basalts have a more pronounced transitional character, typified by a marked light LREE enrichment with respect to HREE when compared with Nebbio basalts. The Pineto basalts have REE patterns generally similar to those of N-MORBs; however, two chemically different groups of rocks can be recognized. One group has LREE/HREE depletion, whereas the other displays higher LREE/HREE ratios. The high LREE/HREE ratios of the first group probably reflect a slight enrichment of the mantle source, as also suggested by Th/Ta and Th/Tb ratios, whereas the other group originated from a depleted MORB-type source. Basalts of the Sant’Angelo di Tenda Unit display intense deformation and N-MORB geochemistry, but a metamorphism which differs from both BN and SL types, as it likely developed under medium-temperature greenschist facies conditions. They are characterized by a LREE/HREE enrichment very similar to those of the high LREE/HREE group of Pineto, suggesting a slight influence of an enriched component in their mantle source. The Rio Magno Unit basalts display a clear N-MORB affinity with LREE depletion and low Th/Ta and Th/Tb ratios, which suggest a genesis from a depleted MORB source. Both Upper and Lower SL units have N-MORB geochemistry, with typical LREE depletion, and highly uniform composition. In summary, T-MORBs of BN derived from a mantle source variably enriched by plume-derived components, probably related to the magmatic activity that occurred during the continental break-up, suggesting that they represent fragments of oceanic crust generated in the early stages of the Piedmont-Ligurian basin oceanization. The slight enrichment by a plume component of Sant’Angelo and high LREE/HREE Pineto basalts suggest that they can be referred to an oceanization stage immediately following that of BN generation, whereas the SL, Rio Magno and low LREE/HREE Pineto basalts represent a mature stage of oceanization without any influence of plume-derived components. In addition, the absence of HP/LT metamorphism and the chemistry of Rio Magno basalts suggest that Rio Magno and Internal Liguride ophiolites represent two internal, oceanic paleo-domains, which underwent a similar geodynamic evolution.