Alpine peridotites from the Ligurian Tethys: an updated critical review

Abstract

We present an updated overview of the petrogenetic evolution of upper mantle peridotites exposed in the Alpine–Apennine ophiolites; the latter represent fragments of Jurassic oceanic lithosphere derived from the Ligurian–Piedmontese basin. This synthesis is based on the results of recent multidisciplinary studies of these peridotite massifs in the Eastern, Central and Western Alps, Ligurian Alps, Northern Apennines and Alpine Corsica, and has been developed within the framework of the geodynamic evolution of the Ligurian–Piedmontese basin between Europe and Adria. The Ligurian–Piedmontese basin began forming through continental rifting during the Triassic. As the continental lithosphere was progressively stretched and thinned, the underlying asthenosphere underwent adiabatic upwelling and decompressional partial melting. The subsequent crustal break-up during the Jurassic led to the development of the non-volcanic passive margins of Europe and Adria, and to the exhumation and exposure of the sub-continental lithospheric mantle on the seafloor. The rifting stage of the basin was characterized by percolation through diffuse porous flow of asthenospheric mid-ocean ridge basalt (MORB) melts via the continental lithospheric mantle. Melt–rock interaction caused depletion/re-fertilization of the lithospheric mantle and generated the extreme compositional heterogeneity of the upper mantle peridotites, which were later exposed on the seafloor at distal settings of the basin. Depleted spinel peridotites, which were previously interpreted as refractory residua after asthenosphere partial melting, are now recognized as reactive peridotites formed by melt–rock interaction. MORB melts migrating through the sub-continental lithospheric mantle along the axial zone of the extending system modified the upper mantle peridotites significantly and were entrapped by interstitial crystallization.