FORMATION AND COMPOSITION OF THE OCEANIC LITHOSPHERE OF THE LIGURIAN TETHYS: INFERENCES FROM THE LIGURIAN OPHIOLITES

Abstract

Ophiolites exposed along the Western Alpine - Northern Apennine (WA-NA) orogenic chain represent the oceanic lithosphere of the Ligurian Tethys which separated, during Late Jurassic - Cretaceous times, the Europe and Adria plates. WA-NA ophiolites show peculiar compositional, structural and stratigraphic characteristics: 1) mantle peridotites are both fertile, cpx-rich lherzolites, and depleted, cpx-poor peridotites; 2) gabbroic intrusives and basaltic volcanites have a MORB affinity and show a high-Ti character of primary magmas; 3) gabbroic rocks were intruded into mantle peridotites; 4) mantle rocks underwent decompressional, not adiabatic, subsolidus evolution, starting from subcontinental lithospheric mantle depths (spinel-facies conditions) towards the sea-floor. The Jurassic Ligurian Tethys was floored by an older peridotite-gabbro basement, subsequently covered by extrusion of discontinuous basaltic flows and by sedimentation of radiolarian cherts. The Ligurian ophiolites represent the spatial association of: 1) old (Proterozoic and Permian) subcontinental lithospheric mantle peridotites; 2) Lower - Middle Jurassic MORB-type gabbroic rocks, intruded in the peridotites; 3) Upper Jurassic MORB-type basaltic volcanics, interlayered with radiolarian cherts, i.e. the first oceanic sediments. Present knowledge on the Western Alps ophiolites suggest that this association is well representative of the oceanic lithosphere of the Ligurian Tethys. This peculiar ophiolitic association, which shows no cogenetic relationships between the different lithological components, cannot be reconciled with a mature oceanic lithosphere formed at present-day mid-oceanic ridges, where the mantle peridotites and the associated gabbroic-basaltic crust are linked by a direct cogenetic relationship. The large exposure of subcontinental mantle peridotites to the sea-floor, and the long history of subsolidus decompressional upwelling recorded by peridotites, are in favour of a geodynamic evolution driven by the passive extension of the Europe-Adria continental lithosphere. Passive extension caused: 1) the progressive exhumation and the tectonic unroofing at the sea-floor of the lithospheric subcontinental mantle, and 2) the passive upwelling of the asthenospheric mantle, which underwent decompressional partial melting and produced MORB-type parental melts for the gabbroic intrusions and for the basaltic extrusions. During the late stages of lithosphere extension, most probably in Jurassic times and prior to complete oceanization, the conductive lithospheric mantle was permeated and impregnated by strongly depleted single melt increments and, later on, it was intruded by aggregated MORB melts. Melt impregnation is mainly confined to the more depleted, more “internal” peridotites massifs (i.e. Internal Ligurides and Lanzo South) which were, most probably, closer to the transition to more typical oceanic lithosphere (i.e. cogenetic mantle and crustal rocks), which has not, so far, been found in the ophiolites derived from the Ligurian Tethys Ocean.