Pre-Variscan granulite under Alpine high-P:the Ruitor Massif, W-Alps

The Ruitor Massif in the Western Alps is part of the Grand-Saint-Bernard Nappe, across Savoy (F), Valle d'Aosta (I) and Valais (CH) (Sartori et al 2006). This pre-Carboniferous basement crops out between the internal border of the Zone Houillère (Upper Carboniferous), to the W, and the Combin-type Piemontese calcschists (~ Mesozoic), to the E. Along a W to E section from Tarentaise to Valgrisenche, the Alpine main foliation rotates from E-dipping, through vertical, to W-dipping at the Ruitor internal border, and is classically interpreted as backthrust-related (Platt et al 1989). Near the SE end of the Ruitor Massif in Valgrisenche, peak Alpine metamorphic conditions have recently been estimated at 493-560°C and 2 GPa, based on Raman spectra of carbonaceous matter and on phengite Si-content (samples RU of Mendes et al., 2023).The Ruitor Massif consists of orthogneisses derived from granitic intrusions that yield U-Pb zircon ages close to 460 Ma (Guillot et al 2002, Bergomi et al 2017). The host rocks are a several kilometer thick stack of Al-rich, garnet-chloritoid-paragonite-phengite micaschist alternating with amphibolite and quartz-albitite layers. Preserved migmatite textures are visible in the field, with mineral relics such as andalusite pseudomorphs or muscovite attesting to mid- to lower crustal, pre-Alpine, HT conditions. Some amphibolite bands enclose garnet-blue amphibole boudins, suggesting an eclogitic episode of currently unknown age.In this presentation, we will focus on the peculiar mineralogy inherited from the protracted, polyphased history, detailing the assemblage from dark ocelli of a variegated micaschist, sampled at the SE-border of the Massif near the Rifugio degli Angeli in Valgrisenche: chloritoid Fe83 Mn4 Mg13, garnet Alm58 Sps23 Grs15 Prp4, paragonite with Na/(Na+k)>0.95, minor phengite with Si=3.33 apfu, plus ilmenite, allanite and rare zircon. Albite and quartz are not observed in the ocelli, but instead make up large portion of the leucocratic inter-ocelli walls. This anomalously Al-rich assemblage is compatible with a restite chemistry, and the past occurrence of Al-silicates.Overall, the detailed geology of the Ruitor Massif, although of interest, remains poorly understood, with little geochemical and lithostratigraphic data.References:Bergomi MA et al (2017) Tectonics 36, 2950-2972Guillot F et al (2002) Int J Earth Sci 91: 964-978Mendes K et al (2023) Geology 51, 1153–1157Platt JP et al (1989) Geol Soc Spec Pub 45, 135-152Sartori M et al (2006) Eclog Geol Helv 99, 363-407