Geological setting
The Antrona Ophiolite is located in the Pennine Alps (western part of the Central Alps; Fig. 1). On the Italian side, the Antrona ophiolite crops out in the Antrona, Loranco, Bognanco, Anzasca, and Vigezzo valleys, near Domodossola; on the Swiss side, it is exposed in the Laggintal-Simplon area, to the south of the Simplon Line. In the internal Pennine domain, the Antrona ophiolite lies on the footwall of the huge Monte Rosa recumbent anticline (upper Penninic) where it forms two large-scale structures, i.e., the Antrona synform (or Gabbio synform) and the Vanzone antiform (Laduron and Merlin, 1974; Milnes et al., 1981). The tectonic contact between the Antrona ophiolite and the Monte Rosa unit is underlined by the Furgg Zone (Argand, 1911; Bearth, 1954; Jaboyedoff et al., 1996; Escher et al., 1997; Froitzheim, 1997, 2001; Keller and Schmid, 2001; Steck et al., 2001; Kramer, 2002) consisting of micaschists, albitic schists, and leucocratic gneisses (Permian-Carboniferous) with eclogitic to greenschist-facies mafic boudins, thin micaceous quartzites and dolomitic marbles of probable pre-Mesozic age (Bearth, 1954). The Monte Rosa anticline is wrapped at its top by the Zermatt-Saas ophiolite nappe (e.g., Ernst and Dal Piaz, 1978; Oberhansli, 1980; Barnicoat and Fry, 1986; Reinecke, 1991, 1998; Bucher et al., 2005; Angiboust et al., 2009) which disappears at depth below the middle Penninic Mischabel backfold (Milnes et al., 1981). The Antrona Ophiolite overlays the middle Penninic Camughera-Moncucco Nappe (included in the “Lepontine basement nappes” of Fig. 1; Bearth, 1956; Laduron, 1976; Bigioggero et al., 1981; Milnes et al., 1981; Escher et al., 1997; Steck et al., 1997; Keller et al., 2005a). The Antrona ophiolite consists of N-MORB to T-MORB-type metabasalts retaining relict pillow structures, minor serpentinite, and gabbro (Laduron, 1976; Laduron and Merlin, 1974; Colombi and Pfeifer, 1986; Colombi, 1989; Pfeifer et al., 1989; Martin et al., 1994; Carrupt and Schlup,1998; Turco and Tartarotti, 2006). These rocks are covered by garnet-micaschists, metaradiolarite, calcschists, sedimentary breccia, marbles, and graphitic marbles. These ophiolitic rocks have experienced regional subduction-related high-pressure metamorphism during the Alpine orogeny (Colombi and Pfeifer, 1986; Keller et al., 2005a; Keller et al., 2005b; Turco and Tartarotti, 2006). Eclogite rocks in the Antrona ophiolite have been documented by Colombi and Pfeifer (1986) and Turco and Tartarotti (2006). The occurrence of lawsonite pseudomorphs within amphibolites have led Turco and Tartarotti (2006) to suggest a qualitative P-T evolution characterized by early Alpine prograde blueschist path followed by a high pressure (eclogitic) metamorphic peak. P-T estimates for the metamorphic peak give T = 372° C for P = 1 GPa and T = 386° C for P = 1.5 GPa (Turco and Tartarotti, 2006). During the decompression accompanying the exhumation history in the Tertiary, the Antrona ophiolite was re-equilibrated under epidote-amphibolite to amphibolite facies conditions (Pfeifer et al. 1989; Turco and Tartarotti, 2006).