FROM THE EARLY SIMS TO RECENT LAM DATA: THE ALPINE-APENNINE OPHIOLITIC PERIDOTITES REVISITED

Early ion-microprobe data on minerals of the Alpine-Apennine (AA) mantle peridotites from the ophiolite sequences exposed in the Central and Western Alps, Ligurian Alps, Northern Apennine and Corsica were obtained about twenty years ago. These data provided basic chemical information that highlighted their evolution, with special emphasis to melt extraction processes and sub-solidus re-equilibration that preceded and accompanied the exposure of mantle peridotites at the sea-floor of the Jurassic Ligurian-Piedmontese (or Western Tethys) oceanic basin. In more recent years a wealth of field, structural-petrographic, petrological and geochemical studies allowed to track a more comprehensive petrogenetic scenario of the Ligurian-Piedmontese upper mantle, in the frame of the geodynamic evolution of the Europe-Adria lithosphere during Mesozoic continental extension and rifting. These investigations evidenced that sectors of the sub-continental lithospheric mantle were exposed on the sea-floor at marginal settings of the basin (e.g. North Lanzo and External Ligurides ophiolites) Isotope data and related model ages revealed that they became isolated by the convective mantle and accreted to the sub-continental lithosphere long time before Mesozoic extension. Conversely, peridotites exposed at more distal settings show extreme compositional heterogeneity recognized as effects of melt-peridotite interaction during reactive percolation of melt though the lithospheric mantle during continental extension and rifting. Isotope data pointed to MORB-type signatures and Jurassic ages for gabbroic intrusions and melt percolated peridotites from both marginal and distal settings. Structural and compositional characteristics suggested that peridotite-melt reactions had a crucial role in modifying texture, mode and chemistry of mantle assemblages and highlighted the need of a re-consideration of the evolutionary processes recorded by the ophiolitic peridotites from the Alpine-Apennine orogenic belt. With this aim, in the last ten years early ion-microprobe data were progressively integrated by extensive texture-related LA-ICPMS studies that provided a huge dataset for a complete series of trace elements in AA ophiolitic peridotite minerals and, particularly, clinopyroxene. The whole dataset confirm that clinopyroxenes from most AA ophiolitic peridotites have REE signatures resulting from other processes than batch or fractional melting. These features result from a complex history of depletion and melt-peridotite reaction events, with the latter largely overprinting pristine signatures recorded by the lithospheric sub-continental mantle before melt percolation. Although old melting episodes under garnet-facies conditions are recorded in places, most REE signatures of AA clinopyroxenes can be explained by reaction of variously depleted melting residua with MORB-like infiltrating melts (either single fractional increments or aggregate melts) and, eventually, later chemical modification due to interaction with trapped melts. The whole dataset is re-considered here with the aim of providing an updated scenario for the evolution of the AA ophiolitic mantle peridotites.