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

Michel Ballevre (1) , Paola Manzotti (2) , Pavel Pitra (1) , Othmar Müntener (3) , Benita Putlitz (3) , Martin Robyr (3)

1 GR - Géosciences Rennes
2 Stockholm University
3 UNIL - Université de Lausanne = University of Lausanne

The sodic amphibole glaucophane, commonly coloured in blue, is generally considered as the mineral indicative of blueschist-facies metamorphism (i.e. occurring in former subduction zones). However, sodic amphiboles display a large range of chemical compositions, due principally to the Fe2+Mg-1 and Fe3+Al-1 substitutions. Therefore, the whole-rock composition, and especially the oxidation state of a rock, strongly controls 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 a misinterpretation of the tectonic framework of the rocks. This work explores the mechanisms that can explain the scarcity of sodic amphibole and sodic pyroxene within the basement of the Dent Blanche Tectonic System (Western Alps), as the result of the Alpine metamorphic history. Field, petrographic and geochemical data indicate that sodic amphiboles crystallize in three different rock types. Firstly, in undeformed pods of ultramafic cumulates (hornblendite), sodic amphibole (magnesioriebeckite) forms coronas around magmatic calcic amphibole. Secondly, in mylonitized granitoids metasomatized along the contact with ultramafic cumulates (amphibole-gneiss and albitite), sodic amphibole (magnesioriebeckite-winchite) mainly forms rosettes or sheaves. Only locally the amphibole needles are aligned parallel to the mylonitic foliation and the stretching lineation. Pale green, patchy zoned aegirineaugite is 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-grained volcanoclastic schists. Bulk rock chemistry of the different lithologies indicates that sodic amphibole and sodic pyroxene developed in Fetotal rich system or in system with a high Fe3+ / (Fe2++Fe3+). Thermodynamic modelling has been performed for different rock types, taking into account the measured Fe2O3 contents. This allows exploring the effect of varying the oxidation state ratio and the water content. Results of these numerical models highlight the role of Fe2O3 on stabilizing sodic amphibole and sodic pyroxene and suggests that sodic amphibole is stable at ~ 8-10 kbar and 400-450°C, i.e. at the transition between the greenschist- and blueschist-facies. Our models suggest lower pressures compared to other estimates based on Si content in muscovite and provide better constraints on the Alpine metamorphic evolution of the Dent Blanche Tectonic System.

Domains: Geochemistry; Petrography

Michel Ballevre, Paola Manzotti, Pavel Pitra, Othmar Müntener, Benita Putlitz, et al.. Greenschist or blueschist facies: the key role of redox conditions for the growth of sodic amphibole in the Dent Blanche Tectonic System (Western Alps). Emile Argand Conference on Alpine Geological Studies 2019, Sep 2019, Sion, Switzerland. pp.52. ⟨insu-02392451⟩