Introduction
The origin of rodingites contained within serpentinite massifs is generally considered as a side effect of serpentinisation of mantle rocks (e.g. O’Hanley et al., 1992) taking place mostly in the ocean floor environment. During this process, metasomatic fluids, enriched in Ca2+ (e.g. Honnorez & Kirst, 1975; Nicolas, 1966) by the destabilisation of clinopyroxene (e.g. Austrheim & Prestvik, 2008; Coleman, 1967), drive the transformation of mafic dykes, emplaced in mantle rocks, into Si-undersaturated and Ca-rich rodingites.
Even if rodingitisation may also take place in subduction environments, as in the case of the Othrys massif (Tsikouras et al., 2009), most of the models consider rodingitisation to be a metasomatic ocean floor process (e.g. Bach & Klein, 2009; Palandri & Reed, 2004; Panseri et al., 2008). Geochemical analyses from the present ocean floor support this last model (e.g. Frost et al., 2008). Therefore, metamorphic transformations involving rodingites during the evolution of a convergent system are generally neglected, except modelling at pressure ≤ 1 GPa (Castelli et al., 1995; Rice, 1983) or studies on the effects of contact metamorphism on rodingites (Frost, 1975).
The earlier authors, who investigated rodingites in the orogenic environment of the Alpine belt (Amstutz, 1962; Cornelius, 1912; Rondolino, 1937; Staub, 1915; Weinschenk, 1894), described mineral assemblages of these rocks as metasomatic relicts unaffected by regional metamorphism. Later Dal Piaz (1967) pointed out that rodingites were widely re-equilibrated during their Alpine high-pressure (HP) evolution together with their country rocks. In addition, rodingites affected by HP metamorphism have been described in the Voltri massif (Messiga et al., 1983; Piccardo et al., 1980) in the Mulhacén complex of Southern Spain (Puga et al., 1999), and in the Pfulwe region and south of the Aosta-Ranzola fault in the Zermatt-Saas Zone (Ferrando et al., 2010; Li et al., 2008).
In order to define the HP mineral assemblages in the boudinaged rodingites of the Valtournanche area, embodied in HP/ultrahigh pressure (UHP) partly de-hydrated serpentinites of Zermatt-Saas Zone (Rebay et al., 2012), we undertook a multiscale analysis to recognize the combined structural and metamorphic evolution of these rocks.
Relationships between deformation and metamorphism have been used as a tool to carry out structural correlation in metamorphic terrains (e.g. Hobbs et al., 2010; Pitcher & Flinn, 1965; Turner & Weiss, 1963; Williams, 1985) and in recent times also to define shape and size of tectono-metamorphic units in orogenic belts (e.g. Spalla et al., 2000; 2005). We used multiscale structural analysis to individuate stable mineral assemblages marking superposed fabrics developed during multistage deformation in rodingites. Correlation among superposed structures affecting rodingites, hosting serpentinites, and associated metagabbros is based on a structural mapping, performed at 1:5,000 scale. The deformation history provides the reference frame to ensure a meaningful correlation of mineral assemblages in rocks with different bulk compositions; a detailed microstructural analysis on fabrics scaled in time validates the relative chronology at the regional scale. Furthermore, we took advantage of the already acquired knowledge on the thermodynamic constraints on mineral phase stability for chemical systems characterising ultramafic and mafic rocks and of numerous studies on the HP metamorphic conditions of the Zermatt-Saas Zone to infer the HP/UHP mineral assemblages developed in rodingite.
Mineral abbreviations in the text and figures are according to Whitney and Evans (2010).