Journal of the Virtual Explorer

At Valtournanche four stages of ductile deformations have been detected in serpentinites and in the hosted rodingite dykes, in agreement with earlier structural reconstructions (Dal Piaz et al., 1980). In serpentinites the first two stages are responsible for the development of S₁ and S₂ or the composite S₁/S₂ foliations, and the third one for a fold system and a differentiated S₃ crenulation cleavage or, locally, a disjunctive cleavage. S₁ and S₂ developed in eclogite facies conditions, whereas S₃ developed in epidote amphibolite facies conditions (Rebay et al., 2012). The fourth stage is not associated with new mineral growth, it produces open folds, and is only rarely associated with a disjunctive cleavage. The full structural history is coherently recorded both in the rodingite dykes and in the surrounding serpentinite.

The mineralogical support of the S₂ differentiated foliation in serpentinites indicates PT conditions of 2.5 ± 0.3 GPa, for T of 600 ± 20 °C (Rebay et al., 2012); its structurally coeval counterpart in rodingitised gabbro dykes is marked by the assemblages:

- Ep_II, Cpx_II, MgChl_II, Ttn_I, ± Grt_II, ± Tr_I in epidote-bearing rodingites;

- MgChl_II, Grt_II, Cpx_II, ± Ves_II, ± opaque minerals in garnet-chlorite-clinopyroxene-bearing rodingites;

- Ves_II, Mg-Chl_II, Cpx_II, Grt_II, ± Ttn_I, ± Ep_I in vesuvianite-bearing rodingites.

On the ground of structural correlation, these assemblages are interpreted to have formed under the same PT conditions estimated for S₂ in serpentinites at the transition between HP/UHP conditions.

The syn-D₃ mineral assemblages are instead interpreted as formed at P of ~0.7 GPa for T ~550°C, coherently with the inferred syn-D₃ PT conditions in serpentinites (Rebay et al., 2012); the entire metamorphic evolution of the three rodingite types, associated with their syn-D₂ metamorphic assemblages, is shown on Fig. 15.

Mineral and structural relicts predating the S₁ foliation are related to the oceanic magmatic and metasomatic history. Cm-sized clinopyroxene porphyroclasts and fine grained Cr-rich spinel (e.g. Allan et al., 1988) crystallised during the emplacement of the gabbro dykes, which are the rodingite protoliths, in the mantle rocks as already pointed out by previous authors (e.g. Dal Piaz, 1967). Moreover uvarovite-rich garnet and Ca- Cr-rich vesuvianite developed during the oceanic metasomatism responsible for the serpentinisation of the mantle rocks and the rodingitisation of the hosted gabbro dykes. Uvarovite-rich garnet is believed to form from the reaction between Cr-spinel and Ca-rich garnet assisted by the mobilisation of Cr by fluids circulation (e.g. Akizawa et al., 2011; Mogessie & Rammlamair, 1994), or by reactions involving Cr-spinel, augitic pyroxene, and anorthitic plagioclase, both during ocean floor metasomatism. Being rich in Cr and Ti, pre-D₁ vesuvianite is consistent with being formed by the destabilisation of Cr-rich spinel during ocean floor metasomatism as well (Kobayashi & Kaneda, 2010); this is also supported by the full occupancy of the X+X’ site (high Ca content), which is characteristic of low grade metamorphism (Gnos & Armbruster, 2006).

In general, since rodingitisation processes involve enrichment in CaO and MgO and depletion in SiO₂ of the mafic protoliths, the inferred bulk rock compositions from vesuvianite- to epidote-bearing rodingites suggest a decrease in oceanic metasomatism (see also Dubińska et al., 2004; Schandl et al., 1989). This is consistent with: a) pre-D₁ clinopyroxene porphyroclasts containing the lowest and highest amount of Ca in epidote- and vesuvianite-bearing rodingites respectively, which indicates a stronger chemical reworking of these porphyroclasts in vesuvianite-bearing rodingites; b) uvarovite-rich garnet and Ca- Cr-rich vesuvianite occurring only in garnet-chlorite-clinopyroxene- and vesuvianite-bearing rodingites.

Figure 15. Rodingite HP/UHP mineral assemblages and their PTdt path as deduced from hosting serpentinites

[View full size]

PTdt path (dashed line) of Alpine HP/UHP subduction-related (D₂) and LP exhumation-related (D₃) stages for serpentinites (Rebay et al., 2012). The main HP/UHP mineral phases forming the assemblages of the three types of rodingites are shown in ACF projections, in which A = (Al^VI + Fe³⁺)/2; C = Ca; F = Fe²⁺ + Mg + Mn. In Vesuvianite-bearing rodingites garnet + chlorite + vesuvianite assemblage is the most common. The double black arrow indicates the increasing direction of Ca and Mg and the Piemonte-Ligurian oceanic metasomatism throughout the bulk compositional systems of the three rodingite types. Metamorphic facies are after Spear (1993).

On the other hand, different bulk rodingite composition may be also due to different composition of protoliths, but since a single rodingite boudin may consists of different rodingite rock types (Fig. 6), we argue that the three different rodingite types most likely manifest a different progression in the rodingitisation process.

In all rodingite types, both pre-D₁ clinopyroxene and garnet porphyroclast rims show a chemical zoning with compositions that progressively approach those of the matrix clinopyroxene and garnet. This suggests a chemical reworking during the development of the S₂ foliation under the Alpine elcogite facies conditions. In this frame, the increase of andradite at the rim of pre-D₁ garnet may suggest an increase of oxygen fugacity during garnet transformation occurring during S₂ development.

The occurrence of Ca- Cr-rich pre-D₁ vesuvianite exclusively as inclusions in pre-D₁ uvarovite-rich garnet suggests that this phase is less resistant to recrystallisation during the Alpine subduction than uvarovite-rich garnet.

These data show that in rodingitised gabbro dykes at Valtournanche the pervasive structural and metamorphic imprint formed during the Alpine subduction. Subsequently these boudins have been poorly re-equilibrated both texturally and mineralogically during the Alpine exhumation under epidote amphibolite facies conditions.

Mineral relicts such as the core of pre-D₁ clinopyroxene and garnet porphyroclasts, Cr-rich spinel, and pre-D₁ vesuvianite are the only vestiges of the Piemonte-Ligurian oceanic history.

These data show that in Ca-rich systems vesuvianite can be stable also under HP/UHP conditions, as already suggested for the Pfulwe area of the Swiss portion of the ZSZ (Li et al., 2008) and observed in jadeitites from Myanmar (Nyunt et al., 2009). Finally, in the Valtournanche rodingites the detailed multiscale structural analysis made possible to discriminate HP/UHP-vesuvianite grains from oceanic relicts and late Alpine vesuvianite. This structural discrimination allowed individuating the significant variations in Cr-, Ti-, Ca-, and Mg-contents critical to detect vesuvianite grown in different metamorphic environments.