MANTLE PETROLOGY AS KEY TO THE UNDERSTANDING OF OCEANIC LITHOSPHERE FORMATION: INFERENCES FROM THE LIGURIAN OPHIOLITES

The Ligurian ophiolites (Erro-Tobbio (ET) Units of the Voltri Massif, External (EL) and Internal (IL) Liguride Units of the Northern Apennine) constitute a representative sampling of the diversity of the oceanic lithosphere flooring the Ligurian Tethys (a small oceanic basin separating the Europe and Adria plates during Late Jurassic – Cretaceous times). Striking features of these ophiolites are: i) the overall abundance of mantle rocks, sporadically intruded by gabbroic bodies, ii) the presence of volcanic and sedimentary rocks directly overling the peridotite-gabbro basement.

Mantle peridotites show heterogeneous compositions, from fertile to depleted spinel lherzolites: Nd model ages indicate Proterozoic time of accretion to the subcontinental lithosphere for the EL lherzolites and Permian time (275 Ma) of depletion for the IL peridotites.

Peridotites record a subsequent subsolidus evolution (i.e. recrystallization to plagioclase- and amphibole-facies conditions and development of km-scale extensional shear zones) which led to their progressive upwelling and seafloor emplacement. Based on Sm-Nd mineral isochron ages, the plagioclase-facies reequilibration occurred at 273-313 Ma in the ET peridotites, and at 165 Ma in the EL peridotites. Thus, decompressional evolution of lithospheric mantle of the Europe-Adria system was already active since Late Carboniferous – Permian times and continued till the Late Jurassic opening of the Ligurian Tethys.

Gabbroic rocks and basaltic volcanites display overall MORB affinity (initial e_Nd in the range 8.5-8.9 for gabbros, 8.4-9.0 for basalts). Available geochronological data yield 185-160 Ma ages for the gabbroic intrusions, whereas relatively younger ages (160-150 Ma) are recorded by basaltic lavas.

The Ligurian ophiolites thus represent the association of: - Proterozoic and Permian subcontinental lithospheric mantle peridotites; - Early Jurassic gabbroic rocks; - Late Jurassic basaltic volcanites. This peculiar lithologic association cannot be reconciled with present-day mature oceanic lithosphere, where peridotites and associated crustal rocks are linked by cogenetic relationships. Rather, it strikingly resembles the peculiar oceanic lithosphere typically formed in embryonic oceans (e.g. the Red Sea) and at passive continental margins.