Introduction
The Alpine-Apennine mantle peridotites outcropping within ophiolite sequences in the Central and Western Alps, the Ligurian Alps, the Northern Apennines and Alpine Corsica (Fig. 1) derive from the oceanic lithosphere of the Ligure-Piemontese (Ligurian or Western Tethys) basin. This oceanic basin was formed by continental extension and break-up during late Jurassic times between the Europe and Adria continental margins (Fig. 2).
Figure 1. Location of the major ophiolite massifs
Location of the major ophiolite massifs (in black) in the Central-Western-Ligurian Alps and Northern Apennines (and Corsica) (pink = Tertiary basins; oblique lines = European Units; points = Brianconnais & Valais Units; Green = Liguria-Piemontese Units; horizontal lines = Adriatic Units) (Redrawn and modified after Schaltegger et al., 2002; Piccardo et al., 2009). Central Alps: TO= Totalp, MA=Malenco, PL=Platta; Western Alps: LA=Lanzo, CH=Chenaillet; Liguria: VM=Voltri Massif, EL=External Ligurides, IL=Internal Ligurides; Corsica = CO-Alpine Corsica.
Before the nineties, structural, petrologic and geochemical studies were done on some ophiolitic peridotite massifs from the Western Alps (Lanzo Massif), the Ligurian Alps (Erro-Tobbio Massif) and the Northern Apennines (External and Internal Ligurides) (Bezzi and Piccardo, 1971; Nicolas, 1974; 1986; 1989; Boudier, 1976; 1978; Boudier and Nicolas, 1972; Piccardo, 1976; Ernst, 1978; Ernst and Piccardo, 1979; Ottonello et al., 1979; Beccaluva et al., 1984; Ottonello et al., 1984; Pognante et al., 1985; Bodinier, 1988). The early studies recognized the presence of both fertile spinel lherzolites and depleted spinel peridotites.
Figure 2. Mesozoic evolution of the Central Atlantic and Ligurian Tethys oceans
Mesozoic evolution of the Central Atlantic and Ligurian Tethys oceans, from rifting to ocean formation (redrawn and modified after Lemoine et al., 1987).
The Ligurian ophiolitic peridotites from the Northern Apennine were recognized to derive from more peri-continental settings (the External Ligurides) and more intra-oceanic settings (the Internal Ligurides) of the Jurassic basin. On the basis of their fertile lherzolite composition and their tectonic-metamorphic evolution, the ophiolitic peridotites from the External Liguride Units were interpreted as deriving from the sub-continental lithospheric mantle (e.g. Bezzi and Piccardo, 1971; Piccardo, 1976). On the basis of their depleted compositions, the ophiolitic peridotites from the Internal Liguride Units were considered refractory residua after oceanic partial melting (e.g. Beccaluva et al., 1984; Ottonello et al., 1984).
Accordingly, it was inferred that the more fertile External Liguride lherzolites, that were exposed on the sea-floor at more marginal, pericontinental settings of the basin, derived from the sub-continental lithospheric mantle and were not subjected to MORB-forming partial melting during the Triassic-Jurassic rifting. The more depleted Internal Liguride peridotites, that were exposed at more internal, oceanic settings, were believed to represent refractory residua after Jurassic asthenosphere partial melting and MORB extraction and to be similar to modern abyssal peridotites.
In summary, studies before the nineties evidenced the existence in the Ligurian sector of the Ligure-Piemontese basin of two main groups of mantle peridotites characterized by significantly different modal and chemical compositions, that were originated in the sub-continental lithosphere or in the oceanic asthenosphere.
Knowledge has been considerably improved as a result of the wealth of multidisciplinary studies performed during the last twenty years. The extreme heterogeneity of the ophiolitic peridotites of the basin has been evidenced and their genetic processes have been more carefully investigated. It has been, in particular, recognized the role of melt diffuse percolation through the mantle lithosphere and melt-peridotite interaction during the rifting stage of the basin. The mutual relationships between mantle processes and structural evolution of the lithosphere have been, thus, better recognized and the new results have been tentatively integrated in a composite geodynamic scenario.
In this paper we present and discuss available structural, petrologic and geochronological knowledge on the Alpine-Apennine ophiolitic peridotites deriving from the different palaeogeographic settings of the Ligure-Piemontese basin. We show that the structural and compositional characteristics of the peridotites varied in space and time and that such variability was induced by both tectonic-metamorphic and magmatic processes. Mantle processes were induced in the lithosphere-asthenosphere system by continental extension. The asthenosphere underwent partial melting under decompression and the lithosphere was profoundly modified by the action of percolating asthenospheric melts. Mantle processes were strictly connected to geodynamics and were good indicators of the geodynamic evolution of the extensional continental system.
This work aims at presenting the composite evolution of the lithospheric mantle in relation with the geodynamic evolution the Ligure-Piemontese continental lithosphere.