Ophiolitic peridotites from Liguria(NWItaly):implications on rifting and subduction processes

Studies on the petrology and geochemistry of mantle rocks contribute significantly to understanding the petrogenetic and kinematic processes governing the evolution of the fithosphere-asthenosphere system under different geodynamic settings. This paper summarizes present knowledge on mantle peridotites from the Ligurian sector (Voltri Massif and Northern Apennines) of the Alpine-Apennine chain. These upper mantle slices were emplaced during the Jurassic on the ocean-floor of the Ligure-Piemontese basin (LPB) through a pre-oceanic rifting stage under sub-continental conditions. Within the Ligurian sector of the Alpine-Apennine orogenic belt, mantle peridotites occur in various structural units (Beigua and Erro-Tobbio, Voltri Massif; Internal and External Ligurides, Northern Apennine), where they are frequently associated with MORB-type mafic intrusives and volcanics to form ophiolite sequences. Present knowledge highlights some basic features ruling the mantle history of the Ligurian ophiolitic peridotites. The External Liguride Units include large fragments of fertile lithospheric sub-continental mantle representing previous asthenosphere accreted to the lithosphere during Proterozoic times, and recrystallized to spinel-facies assemblages during thermal equilibration to the local geotherm (i.e., transformation into lithospheric mantle). The Erro-Tobbio fertile sub-continental peridotites from the Voltri Massif reveal a composite tectono metamorphic history following an almost subsolidus non-adiabatic evolution during uplift. This decompressional path, related to the early pre-oceanic rifting in the Ligure-Piernontese basin, is consistent with mechanisms of tectonic unroofing following passive and asymmetric extension of the lithosphere. The Internal Liguride ophiolites include bodies of former asthenospheric residual mantle depleted by a Permian MORB-generating melting event. During uplift, they followed a subsolidus history and were intruded by gabbroic rocks. The mafic intrusives in turn record a sequence of deformation and subsolidus retrograde recrystallization, starting from high temperature (granulite/amphibolite) conditions. AD these features are related to the pre-oceanic rifting stage. Fertile and depleted mantle peridotites and associated gabbros were later exposed on the sea-floor where they were overlain by continental and ophiolitic sedimentary breccias and MORB volcanics (i.e., the formation of a discontinuous and incomplete oceanic crust). Structural and petrological evidence prevent relating these ophiolite sequences to active mantle mechanisms as mature mid-ocean - transform fault settings. More properly, the Ligurian ophiolites are believed to represent the lithological associations which are expected to develop after continental breakup due to passive and asymmetric extension of the lithosphere. In fact, they consist of pre-Jurassic sub continental rocks, unroofed and exposed on the sea-floor during the extensional processes, associated with late-Jurassic MORB-type basalts produced by partial melting of upwelling asthenosphere. The Permian age for the partial melting of some Internal Liguride residual peridotites gives indications on the age of rift inception: it is, in fact, almost coeval with the post-Variscan extension recorded by the Austroalpine and Southalpine units, recently interpreted as a signature of the late Paleozoic onset of extension within the Europe-Adria lithosphere. Some peridotite bodies from Liguria (i.e., those of the Voltri Massif) underwent a composite tectonic-metamorphic history during the Alpine evolution, which is mainly marked by antigorite and antigorite + olivine + Ti-clinohumite-bearing associations. Other typical features of the peak metamorphic event are veins filled by the same olivine + Ti-clinohumite association. The developement of these assemblages appears to be roughly coeval with the widepread formation of eclogite facies Na-pyroxene+garnet associations in meta-gabbros, which are enclosed within the peridotite bodies as former MORB-type intrusive bodies and dykes. Typical subduction P-T paths have been estimated for the Voltri Massif units, with different eclogite climax conditions for the different units: 13 kb minimum pressure at 450-500' C has been inferred for the Beigua Unit, 25 kb and 600'C for the Erro-Tobbio Massif. Our investigations indicate that large volunics of peridotites participate in tbc subduction evolution, and that manile ultramafics in high pressure environments develop in large modal proportion hydrated phases which are stable up io 25 kb. As a consequenee, large amounts of water may be retained in the subducted slab. This fact has relevant consequences on rock rheology, subduction-related fluid circulation and magmatism