Greenschist or blueschist facies: the key role of redox conditions for the growth of sodic amphibole in the Dent Blanche Tectonic System (Western Alps)

International audienceThe sodic amphibole glaucophane, commonly coloured in blue, is generally considered as the mineralindicative of blueschist-facies metamorphism (i.e. occurring in former subduction zones). However, sodicamphiboles display a large range of chemical compositions, due principally to the Fe2+Mg-1 and Fe3+Al-1substitutions. Therefore, the whole-rock composition, and especially the oxidation state of a rock, stronglycontrols the stability field of the sodic amphibole at the transition from greenschist- to blueschist-facies.Under evaluating this point can lead to the incorrect account of the metamorphic conditions, resulting in amisinterpretation of the tectonic framework of the rocks. This work explores the mechanisms that canexplain the scarcity of sodic amphibole and sodic pyroxene within the basement of the Dent BlancheTectonic System (Western Alps), as the result of the Alpine metamorphic history.Field, petrographic and geochemical data indicate that sodic amphiboles crystallize in three different rocktypes. Firstly, in undeformed pods of ultramafic cumulates (hornblendite), sodic amphibole(magnesioriebeckite) forms coronas around magmatic calcic amphibole. Secondly, in mylonitized granitoidsmetasomatized along the contact with ultramafic cumulates (amphibole-gneiss and albitite), sodic amphibole(magnesioriebeckite-winchite) mainly forms rosettes or sheaves. Only locally the amphibole needles arealigned parallel to the mylonitic foliation and the stretching lineation. Pale green, patchy zoned aegirineaugiteis dispersed in an albite-quartz matrix or forms layers of fine-grained fibrous aggregates. Thirdly,sodic amphibole (magnesioriebeckite-glaucophane) occurs with muscovite-epidote-quartz in fine-grainedvolcanoclastic schists.Bulk rock chemistry of the different lithologies indicates that sodic amphibole and sodic pyroxene developedin Fetotal rich system or in system with a high Fe3+ / (Fe2++Fe3+). Thermodynamic modelling has beenperformed for different rock types, taking into account the measured Fe2O3 contents. This allows exploringthe effect of varying the oxidation state ratio and the water content. Results of these numerical modelshighlight the role of Fe2O3 on stabilizing sodic amphibole and sodic pyroxene and suggests that sodicamphibole is stable at ~ 8-10 kbar and 400-450°C, i.e. at the transition between the greenschist- andblueschist-facies.Our models suggest lower pressures compared to other estimates based on Si content in muscovite andprovide better constraints on the Alpine metamorphic evolution of the Dent Blanche Tectonic System