Journal of the Virtual Explorer

In the field, pyroxenite-bearing lithospheric lherzolites and deformed peridotites of the shear zones locally show primary contact with the reactive spinel harzburgites (Fig. 4). The contact is usually transitional in a few decimetre-wide zones: the deformed textures, the widespread pyroxenite banding and the clinopyroxene-rich compositions of the former rocks are rapidly modified passing to the reactive spinel harzburgites (e.g. Erro-Tobbio Massif, Piccardo and Vissers, 2007; Mt. Maggiore, Piccardo and Guarnieri, 2010b). In fact, the transition to the depleted rock type is marked by development of isotropic coarse granular textures, strong clinopyroxene depletion and olivine enrichment and disappearance of the spinel pyroxenite bands. At Monte Maggiore the spinel pyroxenite bands in the lithospheric lherzolites are progressively replaced by spinel dunite bands in the reactive harzburgites (Piccardo and Guarnieri, 2010b). Clinopyroxene-consuming and olivine-forming reactions, causing progressive clinopyroxene dissolution in peridotites and pyroxenites, are clearly evidenced by the presence of olivine replacement micro-textures on clinopyroxenes. Frequently, the reactive spinel harzburgites and the spinel dunite bands preserve structural relics, i.e. rounded spinel+orthopyroxene clusters, inherited from the pristine lithospheric lherzolites and pyroxenites that they replace.

In the distal peridotite massifs, decametric-hectometric remnants of pyroxenite-veined lithospheric lherzolites are preserved within the km-scale masses of reactive spinel harzburgites (e.g. Piccardo and Vissers, 2007; Piccardo and Guarnieri, 2010b). The replacement relationships of the reactive spinel harzburgites on the pyroxenite-bearing lithospheric lherzolites provide clear field evidence that the pristine subcontinental lithospheric mantle was early diffusely percolated by silica(-pyroxene)-undersaturated melts. It was significantly modified as for its structural and compositional characteristics and it was transformed into clinopyroxene-depleted reactive spinel harzburgites (e.g. Piccardo, 2003).

In the distal peridotite massifs, both pyroxenite-bearing lithospheric lherzolites and pyroxene-depleted reactive harzburgites show primary contacts with the enriched plagioclase peridotites (Fig. 6). Plagioclase-free and plagioclase-rich domains are usually in sharp contact and, locally, plagioclase-rich veins propagate from the plagioclase-rich peridotites inside the plagioclase-free spinel peridotites (e.g. Piccardo et al., 2007a). Field relationships evidence that plagioclase enrichment occurred on km-scale masses of pre-existing reactive spinel harzburgites: this indicate that plagioclase enrichment represents a further process of melt diffuse percolation accompanied by early interstitial crystallization of plagioclase (and micro-gabbroic aggregates).

The Alpine-Apennine ophiolitic peridotites were exposed during Jurassic times at the sea-floor of the Ligure-Piemontese oceanic basin following continental break-up and formation of the paired Europe and Adria non-volcanic passive margins. These peridotites derived from the sub-continental lithosphere of the pre-Triassic Europe-Adria system that was exhumed starting from Triassic times as a consequence of passive extension of the Europe-Adria continental lithosphere (see discussion in Piccardo et al., 2009, and references therein).

Present knowledge indicates that mantle peridotites of the marginal ophiolite sequences deriving from ocean-continent transition (OCT) zones of the Jurassic Ligure-Piemontese basin mostly consist of pyroxenite-bearing lithospheric spinel lherzolites, which show effects of subsolidus evolution and non-adiabatic exhumation from sub-continental spinel-facies mantle depths to the sea-floor (e.g. Hoogerduijn Strating et al., 1993; Montanini et al., 2006; Piccardo and Vissers, 2007). Mantle peridotites of the distal ophiolite sequences deriving from more internal oceanic (MIO) settings of the basin mostly consist of reactive spinel harzburgites and impregnated plagioclase peridotites, which record significant effects of interaction with percolating asthenospheric melts (e.g. Müntener and Piccardo, 2003; Piccardo et al., 2004, 2007a; Piccardo and Vissers, 2007). This indicates that the sub-continental mantle that was exhumed close to the continental margin escaped significant percolation and interaction of asthenospheric melts, whereas the sub-continental mantle that was exhumed at the more internal setting of the basin was profoundly percolated and interacted by the MORB-type melts rising from the melting asthenosphere.