Geological setting
Zermatt-Saas and Combin zones (ZSZ and CZ) consist of fragments of mantle and crustal rocks of the Piemonte-Ligurian ocean, marking the suture zone of the Western European Alps (Caron et al., 1984; Reddy et al., 1999; Spalla et al., 2010 and refs therein). These two zones tectonically overlie the continental Monte Rosa and underlie the Dent Blanche – Sesia Lanzo nappes, which belong, respectively, to the Penninic and Austroalpine domains (e.g. Bearth, 1967; Polino et al., 1990). The ZSZ (Fig. 1) is structurally beneath the CZ. Because of the different lithostratigraphic and metamorphic features these two units seem to derive, respectively, from the oceanic lithosphere and from the ocean-continent transition zone. The Pancherot – Cime Bianche unit, consisting of metasediments deriving from Permian to Cretaceous protholits, and slices of pre-Alpine continental rocks separate ZSZ and CZ (Dal Piaz, 1999).
Figure 1. Geological map of the upper Valtournanche
ZSZ = Zermatt - Saas Zone; CZ = Combin Zone. 1 = undifferentiated Quaternary deposits; Austroalpine Domain: 2 = Mesozoic covers; 3 = Kinzigitic complex; 4 = Fine-grained metagranitoids and paraschists; 5 = Permian gabbros; Penninic Domain: 6 = Calcschists; 7 = Prasinites, amphibolites and minor eclogites; 8 = Meta-gabbros; 9 = Serpentinites; 10 = Undifferentiated Permian-Mesozoic covers. In the inset the tectonic sketch map of the Western Alps: HU = Helvetic and Ultrahelvetic Domain; P = Penninic Domain and Pre-Alps; A = Austroalpine Domain; S = Southalpine Domain. Thick red line delimitates the map of Fig. 2. Map redrawn after De Giusti et al. (2003).
Despite the pervasive greenschist facies metamorphism, CZ rocks preserve blueschist facies relicts, whereas remnants of eclogite facies assemblages are not documented even in the less deformed rocks (Cartwright & Barnicoat, 2002; Dal Piaz, 1974; Dal Piaz & Ernst, 1978; Ring, 1995).
The ZSZ consists of serpentinites, metagabbros, and N-MORB metabasalts with minor pelagic metasediments (Beccaluva et al., 1984; Dal Piaz et al., 1981; Pfeiffer et al., 1989; Rebay et al., 2012); metasediments are mainly calcschists and quartzites deriving from the internal part of the oceanic realm (e.g. Bearth, 1967; Ernst & Dal Piaz, 1978).
The Alpine subduction-related eclogite facies imprint is the dominant one in the ZSZ and is partly obliterated by exhumation-related greenschist facies metamorphism (e.g. Cartwright & Barnicoat, 2002; Ernst & Dal Piaz, 1978). The PT evolution of the ZSZ is characterised by a prograde path under blueschist facies predating eclogite facies conditions (e.g. Chinner & Dixon, 1973; Ernst & Dal Piaz, 1978). Generally the eclogite facies imprint reveals conditions up to about 3.0 GPa. In detail recent petrological investigations on ZSZ metabasic rocks (from Saas-Fee) indicate pressure around 2.5-3.0 GPa, for temperatures of 550-600°C (Bucher et al., 2005), whereas on the Allalin gabbro indicate 2.5-2.8 GPa and 600- 610°C (Bucher & Grapes, 2009). Highest pressure conditions from 2.7 to higher than 3.2 GPa for temperatures between 590 and 630°C are estimated at Lago di Cignana, which is considered a separate unit (e.g. Groppo et al., 2009; Reinecke, 1991; Reinecke, 1998; Van der Klauw et al., 1997), and in which recently microdiamonds inclusions have been detected in oceanic metasediments (Frezzotti et al., 2011). In the portion of ZSZ south of the Aosta-Ranzola fault (St. Marcel valley) different PT-peak conditions of 2.1±0.3 GPa and 550±60°C have been inferred in metabasics (Martin et al., 2008). HP conditions are also estimated in serpentinites at 2.0-2.5 GPa for temperature of 600-650°C and at 2.2-2.8 GPa for 580-620°C in the northern part of the ZSZ (Li et al., 2004b) and at Valtournanche (Rebay et al., 2012) respectively. In the Zermatt area PT-peak conditions proposed for metarodingites (Li et al., 2008), on the base of coherence between petrologic modelling of rodingites and PT evolution inferred from surrounding metabasic rocks, suggest PT values of 2.5-2.8 GPa and 600-625°C (Bucher et al., 2005).
The peak-conditions and the related PT trajectories reconstructed all over the ZSZ indicate that the metamorphic evolution of this complex has been heterogeneous, even though some authors, after petrologic investigations on metabasic rocks and calcschists, suggested a homogeneous metamorphic evolution for the entire ZSZ, characterised by PT peak conditions of 2.3 ± 0.1 GPa and 540 ± 40°C (Angiboust & Agard, 2010; Angiboust et al., 2009).
Oceanic relict textures are well documented in the ZSZ (Barnicoat & Fry, 1986; Fontana et al., 2008; Martin & Tartarotti, 1989) and the age of these relics ranges between 164 and 153 Ma (Rubatto, 1998). Pre-Alpine structural and mineral relics show that this lithosphere was variously affected by oceanic metamorphism before the Alpine subduction (Bearth, 1976; Dal Piaz et al., 1980; Li et al., 2004a).
As already described in the upper Valtournanche (Dal Piaz, 1965; 1967; 1969), rodingites of the Western Alps are interpreted as former gabbro dykes likely recrystallised during the four Alpine ductile deformation stages affecting the hosting serpentinites (Dal Piaz et al., 1980). Clinopyroxene porphyroclasts with igneous composition are the only mineral relics of the gabbro protolith (Dal Piaz et al., 1980; Li et al., 2004a).
In the Lanzo Massif rodingites derive from gabbro dykes (Bortolami & Dal Piaz, 1968; Dal Piaz, 1969) metasomatised by Cretaceous seawater and subsequently subducted with the hosting serpentinites during the Alpine convergence (Rösli et al., 1991). However late Alpine multi-stage rodingitisation has been described in the Balangero mine of the Lanzo Massif (Castelli et al., 1995), as well as at Bellecombe in the ZSZ, where late greenschist Alpine metasomatism took place, but high-pressure Alpine minerals also occur in rodingites (Ferrando et al., 2010). Finally, a recent petrological work on rodingites from the Zermatt area shows that their main mineral assemblages formed during the Alpine subduction (Li et al., 2008).
In the ZSZ different type of rodingite, mainly distinguished on the basis of their mineral content (Panseri et al., 2008), are ascribed to different protoliths or degrees of metasomatic transformation (Li et al., 2004a). Li et al. (2008) interpret the potential Alpine assemblages exclusively on the basis of consistency between thermodynamic modelling for these rocks and PT evolution constrained in ZSZ metabasic rocks. However their modelling allow identifying near-isothermal reactions in rodingites and they do not describe any structural correlation with superposed fabrics marked by eclogite and greenschist facies minerals in the hosting eclogitised ophiolites.