The Alpine Corsica ophiolite field trip: The primary features, the tectono-metamorphic orogenic imprint and the relationships with the continental units

The Alpine Corsica and the Northern Apennines are characterized by well preserved examples of Jurassic ophiolite sequences and their related sedimentary cover. These ophiolite sequences are considered remnants of the Ligure-Piemontese domain, i.e. part of the western Tethys oceanic basin, developed in the Jurassic time between the Europe/Corsica and the Adria continental margins. These ophiolite sequences have been studied in detail since the 70's owing their features. Their petrological features and the geochemical signature of the magmatic sequences suggest that the Ligure-Piemontese basin was as a narrow ocean, with a width no longer than 400-500 km. Moreover, the ophiolites from Corsica and the Northern Apennines, as well as those from Calabria and the Alps, are characterized by thin, reduced sequences, where the whole section is lower than 700-800 meters thick. Consequently, these sequences are characterized by lithostratigraphic features that are quite different from those of a "typical" ophiolite sequence as defined by Penrose's ophiolite field conference (1972). This is also suggested by the evidences of exposure of mantle Iherzolites before the emplacement of basaltic flows at the seafloor, the lacking of a sheeted dyke complex, and the occurrence of ophiolitic breccias over or below the basaltic flows. An other interesting feature is represented by the association of the ophiolites with granulites and granitoids in the sequences interpreted as representative of the ocean-continent transition at the Adria continental margin and the occurrence of continental-derived debris in the ophiolite sequence derived from the area close to the Corsica continental margin. All these features make these ophiolites unusual and representative of an oceanic basin, whose origin, architecture and development is still under debate. In addition, most of these ophiolite sequences are characterized by a deformation and metamorphic imprint that testified their involvement in the processes connected with the development of an intraoceanic subduction zone. The high pressure metamorphism, ranging from low blueschists to eclogite fades, is probably the most impressive signal of these processes. In addition, the ophiolite sequences involved in the subduction-related processes display complex relationships with slices of continental crust, probably acquired during the first stage of the collisional events. This field trip focuses on the the ophiolite sequences from the Alpine Corsica with the aim to providing a complete picture of their oceanic and orogenic features and their relationships with the continental margin. The first day is devoted to the high-pressure metamorphic ophiolites of the Inzecca area while in the second day the features of the very low-grade metamorphic ophiolite sequence from the Balagne Nappe, representative of the domain of the oceanic basin close to the continental margin will be examined. The third day is devoted to the tectonic relationships of the high-pressure metamorphic ophiolites with the Tenda Massif, a continental slice involved in the subduction processes. During the fourth day, the features of the oceanic peridotites of Monte Maggiore massif will be examined

Trace element chemistry and U-Pb dating of zircons from oceanic gabbros and their relationship with whole rock composition (Lanzo, Italian Alps)

The U-Pb ages and the trace element content of zircon U-Pb along with major and trace element whole rock data on gabbroic dikes from the Lanzo lherzolitic massif, N-Italy, have been determined to constrain crustal accretion in ocean-continent transition zones. Three Fe-Ti gabbros were dated from the central and the southern part of the massif providing middle Jurassic ages of 161 +/- 2, 158 +/- 2 and 163 +/- 1 Ma, which argue for magmatic activity over few millions of years. Zircon crystals are characterized by high but variable Th/U ratios, rare earth element patterns enriched in heavy rare earths, pronounced positive Ce and negative Eu-anomalies consistent with crystallization after substantial plagioclase fractionation. The zircon trace element composition coupled with whole rock chemistry was used to reconstruct the crystallization history of the gabbros. A number of gabbros crystallized in situ, and zircon precipitated from trapped, intercumulus liquid, while other gabbros represent residual liquids that were extracted from a cumulus pile and crystallized along syn-magmatic shear zones. We propose a model in which the emplacement mechanism of gabbroic rocks in ocean-continent transition zones evolves from in situ crystallization to stratified crystallization with efficient extraction of residual liquid along syn-magmatic shear zones. Such an evolution of the crystallization history is probably related to the thermal evolution of the underlying mantle lithosphere