During the Mesozoic the break-up of the Pangea supercontinent led to the formation of the Tethys Ocean between the European continent to the north (Eurasia) and the African continent to the south (Frisch, 1981; Platt et al., 1989). The portion of Tethys between the north- eastern margin of the African continent (Adria) and the southern margin of Eurasia was the Ligure-Piemontese Ocean. The Alpine chain formed as a consequence of the Mesozoic subduction of the Ligure-Piemontese Ocean and of the subsequent collision between Adria and Eurasia.

The Western Alpine arc consists essentially of three nappe complexes: the western Austroalpine, the Penninic (upper, middle, lower) and the Helvetic (Ultrahelvetic, Helvetic and Dauphinois units) (Figure 1). All nappe complexes (except the upper Penninic) comprise a crystalline basement covered by Mesozoic and Tertiary sediments. The upper Penninic belongs to the oceanic domain and includes the Piedmont ophiolitic suture. The middle (e.g. Briançonnais domain) and lower Penninic together with the Helvetic belong to the European margin while the Austroalpine domain belongs to the Adria margin, which was separated from Eurasia during the Jurassic by the Ligure-Piemontese Ocean.

In general the Western Alps experienced a complicated poly-metamorphic evolution reflecting continued convergence, hence the importance of establishing the complete sequence of metamorphic events in time. In the following the most important HP-UHP metamorphic phases are summarised (see also Agard et al., 2002 for a review). In sequence, from the top of the nappe stack to the bottom:

-Western Austroalpine: Eo-Alpine (~130?-110?/65 Ma) (Polino et al., 1990; Gebauer, 1999 for a review) eclogitic metamorphism (e.g. Sesia Lanzo and Dent Blanche; Dal Piaz et al., 1983; Inger et al., 1996; Cortiana, et al., 1998; Rubatto et al., 1999; Ruffet et al., 1995) and Meso-Alpine overprint (Late Eocene-Early Oligocene) greenschist metamorphism (Reddy et al., 1996; Dal Piaz, 1999);

-Piedmont ophiolitic suture: Eo-Alpine (~110?-70 Ma) (Polino et al., 1990; Gebauer, 1999; Schwartz et al., 2000) eclogitic and blueschist metamorphism (e.g. Schistes Lustrés; see also Agard et al., 2002; Takeshita et al., 1994) and Meso-Alpine (Late Eocene-Early Oligocene) greenschist and UHP (e.g. coesite-glaucophane assemblages) overprint (e.g. Zermat-Saas zone; Dal Piaz, 1999; Dal Piaz et al., 2001; Reinecke, 1998; Rubatto et al., 1998; Dûchene et al., 1997; and references therein).

-Upper Penninic domain: Eo-Alpine (~110?-60 Ma) (Polino et al., 1990; Gebauer, 1999 and references therein) eclogitic metamorphism and greenschist Meso-Alpine (~35 Ma) overprint (e.g Dal Piaz 2001). Coesite-pyrope assemblages have been recognised in the Dora Maira Massif and interpreted as the result of Meso-Alpine UHP metamorphism followed by fast cooling/exhumation during the Eocene (e.g. Avigad, 1992; Chopin, 1984; Cortiana et al., 1998; Gebauer et al., 1997; Hurford et al., 1991; Monié and Chopin, 1991; Rubatto et al., 1999; Rubatto and Hermann, 2001).

-Lower Penninic domain: Eo-Alpine Meso-Alpine eclogitic to blueschist metamorphism (Bocquet et al., 1984), and Meso-Alpine greenschist to amphibolite facies metamorphic overprint (Dal Piaz, 1999).

The meaning of Eo-Alpine ages detected in the Western Alps, especially in the Internal Western Alpine Massifs (e.g. Dora Maira; Monte Rosa; Chopin, 1984; Monié and Chopin, 1991; Paquette et al., 1989; Scaillet et al., 1992; Scaillet et al., 1990) is still a matter of debate. Many authors have dismissed the geological meaning of these ages by attributing them to the presence of excess ⁴⁰Ar (e.g. Arnaud and Kelley, 1995; Kelley et al., 1994; Ruffet et al., 1995). However, thermochronometers such as U/Pb on zircon, Rb/Sr on whole rock and zircon fission track analysis (Oberhänsli et al., 1985; Paquette et al., 1989; Cortiana et al., 1998; Gebauer et al., 1999; Vance, 1999; Schwartz et al., 2000) have recorded complementary data suggesting that real geological events occurred in the Cretaceous in the Western Alps. In particular, Paquette et al. (1989) proposed an Eo-Alpine eclogite-facies metamorphism phase dated between 95 and 120 Ma in the Monte Rosa, Dora Maira Massifs and Sesia Lanzo zone. Their results, obtained by U-Pb zircon, Rb-Sr on whole rock, apatite and phengite and whole rock Sm-Nd on meta-acid rocks, are concordant with ⁴⁰Ar/³⁹Ar plateau ages on phengite (105-110 Ma) obtained by Monié (1984) and Chopin & Monié (1984) on the same rock samples. Also Cortiana et al. (1998) suggested a Late Cretaceous HP event for the Austroalpine-Piedmont domain. Their interpretation is based on concordant Rb/Sr and ⁴⁰Ar/³⁹Ar plateau ages on phengitic micas. These data already suggest that a possible link between Cretaceous ages to HP metamorphism in the Alps is not unrealistic.

The Ligurian Alps form the south-westernmost segment of the Alpine collisional belt (Figure 3) and comprise different domains (Provençal, Dauphinois, Briançonnais and Piemontese domains) belonging to the paleo-European continent and the oceanic crust of the Ligure-Piemontese domain (Figure 3) (Vanossi et al., 1984; Vanossi, 1991).

The Provençal-Dauphinois domain consists of a crystalline basement and a Permo-Cenozoic cover. The pre-Upper Carboniferous basement (gneisses, amphibolites, granitoids) and cover units crop out at present in the Argentera-Mercantour Massif.

The Briançonnais domain is a tectonic complex, which records the pre-Alpine history of the European crust, related mostly to the Variscan orogenic cycle (Devonian- Carboniferous) and Late Paleozoic-Mesozoic sedimentary evolution (Vanossi et al., 1984; Cortesogno et al., 1993). The Briançonnais domain and the Provençal Dauphinois domain both comprise a crystalline basement(i.e. Variscan Crystalline Massifs) and Permo-Cenozoic covers.

The Piemontese domain consists of either only Meso-Cenozoic covers or crystalline rocks (e.g. Valosio Crystalline Massif). The Ligure-Piemontese domain consists of ophiolitic units (i.e. Montenotte Nappe, Voltri Group, Erro-Tobbio, Sestri Voltaggio) and flyschoid sequences. The Montenotte Nappe is derived from the transitional domain between the ocean and the palaeo-European continental margin and experienced subduction metamorphism mainly under blueschist conditions during Alpine orogenesis (Messiga, 1984).

The Voltri Group is a meta-ophiolite massif, which represents a remnant of Piedmont-Ligurian oceanic crust with its deep-sea cover. The Piedmont-Ligurian Ocean formed as a result of spreading in the Jurassic between Europe and Africa, and closed during Cretaceous-Paleogene Alpine convergence between the two continental blocks. The Voltri Group records a complex retrograde metamorphism from eclogitic (Alpine subduction metamorphism) to greenschist conditions (Messiga, 1984).

Few geochronological data available on the Ligurian Alps concern mainly the Briançonnais domain and the Voltri Group (Del Moro et al., 1981; Hunziker et al., 1992). The Briançonnais domain is characterised by Variscan ages (Del Moro et al., 1981; Hunziker et al., 1992), while the Voltri Group exhibits mainly Eocene- Oligocene ages (Barbieri et al., 2003). Jurassic and Cretaceous ages around 180- 120 Ma have been recorded by zircon fission track (ZFT) analysis in the Briançonnais-Piemontese domain and in the Helmintoid Flysch of the Ligurian Alps (Vance, 1999).