Summary and conclusions
The Antrona ultramafic body derives from mantle harzburgites - lherzolites associated with dunite pods (± pyroxenite layers; Fig. 9a). The LPO of olivine and the chemical composition of olivine, clinopyroxene porphyroblasts and spinel suggests a sub-oceanic nature for these mantle-derived rocks. Though the Alpine tectonic transposition has obliterated the primary structure of the Tethyan lithosphere, thus preventing a clear interpretation of the original stratigraphy, however we envisage that the mantle portion of the Antrona ophiolite may have constituted a section of the Jurassic Tethyan oceanic lithosphere, together with gabbros, basalts and sediments (Fig. 9b), now constituting the highly strained amphibolite, metagabbro and metasedimentary horizons (Fig. 9c).
Figure 9. Simplified model of the evolution of the Antrona Ophiolite, from mantle to Alpine convergence stage.
Except for the scarce occurrence of rodingites in the Antrona area, this scenario is comparable to that envisaged for the Mount Avic serpentinite massif (Tartarotti and Martin, 1991; Fontana et al., 2008; Panseri et al., 2008) and although their extension is smaller, the Antrona ophiolite seems to be comparable with a coherent ophiolitic slice, such as the Zermatt Saas ophiolite (e.g., Angiboust et al., 2011 and refs. therein), more than with a serpentinite mélange. Turco and Tartarotti (2006) suggest for amphibolites a P-T evolution characterized by early Alpine prograde blueschist path (T = 372° C for P = 1 GPa), followed by an eclogitic metamorphic stage (T = 386° C for P = 1.5 GPa), which was further followed by a decompression accompanying the exhumation history in the Tertiary (Pfeifer et al. 1989; Turco and Tartarotti, 2006), testified by a re-equilibration under epidote amphibolite to amphibolite facies conditions. A similar P-T evolution may be indicatively addressed for clinopyroxene-rich layers and amp-bearing chloriteschists, probably originated by original gabbros transformed during the Alpine tectonics.