Using multiscale structural analysis to infer high-/ultrahigh-pressure assemblages in subducted rodingites of the Zermatt-Saas Zone at Valtournanche, Italy

Davide Zanoni, Gisella Rebay, Maria Iole Spalla, and Jacopo Bernardoni

Rodingites hosted in serpentinites of the upper Valtournanche (Zermatt-Saas Zone) record four stages of ductile deformation of which the first three are associated with new mineral growth; D1 and D2 developed under high-pressure to ultrahigh-pressure conditions and D3 under lower pressure conditions.

On the base of the modal amount of the main mineral phases three types of rodingites are individuated: epidote-, garnet-chlorite-clinopyroxene-, and vesuvianite-bearing rodingites. They are interpreted as deriving from ocean floor metasomatism of former gabbro dykes intruded in mantle rocks, successively affected by subduction-related metamorphism during the Alpine convergence.

Field mapping allowing continuous structural correlation shows that the deformation history recorded in the serpentinites affects also rodingites. On the basis of syn-D2 peak conditions already inferred in serpentinites (2.5 ± 0.3 GPa and 600 ± 20 °C) the following syn-D2 assemblages in rodingites are interpreted as developed under the same physical conditions, during the Alpine subduction: epidoteII, clinopyroxeneII, Mg-chloriteII, titaniteI, ± garnetII, ± tremoliteI in epidote-bearing rodingites; Mg-chloriteII, garnetII, clinopyroxeneII, ± vesuvianiteII, ± opaque minerals, in garnet-chlorite-clinopyroxene-bearing rodingites; vesuvianiteII, Mg-chloriteII, clinopyroxeneII, garnetII, ± titaniteI, ± epidoteI in vesuvianite-bearing rodingites. Pre-D1 mineral relicts such as Cr-rich spinel and cm-sized clinopyroxene porphyroclasts are interpreted as magmatic vestiges of the rodingitised and eclogitised gabbro dykes, whereas Cr-rich garnet and Cr-Ti-Ca-rich vesuvianite as developed during oceanic metasomatism. Distinct chemical compositions characterise the same minerals recrystallised during the Alpine subduction. Finally, multiscale structural analysis shows that the dominant structural and metamorphic imprint recorded by the Valtournanche eclogitised rodingites developed during the Alpine subduction and that, in the favourable bulk composition (Ca-rich), vesuvianite is stable also at metamorphic conditions close to ultrahigh-pressure.