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Pétrologie et géochimie des formations quartzofeldspathiques de la série métamorphique mésozonale du Chassezac (Lozère, Massif Central français)

Abstract : Petrographic studies, supported by a detailed cartography, make it possible to propose a schema of the lithostratigraphic and metamorphic history of the studied sector. The crystallophyllian series includes, from base to summit: - a caxoniferous quartzofeldspathic ensemble with sillimanite (± cordierite) surmounted by a discontinuous horizon of biotite - sillimanite gneiss; a mesozonal staurotide-garnet, andalusite-cordierite sequence, consisting of three units: .gneiss eyed at the base, a median horizon of rubano-eyed leptynites associated with levels of muscovite leptynites (old leucogranites), a micaschistous upper set; - a powerful series of epimetamorphic schists (schists from the Cevennes to albite, chlorite, muscovite). The metamorphic evolution of these formations is complex: . At a first episode of low-pressure intermediate staurotide-garnet type, synchronous with the first phase of deformations (isoclinal folds P1 to foliation S1 axial plane), follows a second episode of low-pressure type at andalusite-cordierite, contemporary of the second phase deformations, resulting in local remobilization (late phase 2) of the quartzofeldspathic catazonal formations. A third phase is accompanied by a dynamic retromorphosis around the Villefort accident. . The series is also influenced by the granite massifs, which is marked in the micaschists by a halo with chiastolitic andalusite; this phenomenon post phase 1 and ante phase 2 in contact with the Margeride is later (syn phase 2) in contact with the Borne. A more specific study, mineralogical and chemical, concerns the quartzofeldspathic formations eyed. It highlights: - the identity of the eyed gneiss of Cchassezac and the arc of Fix, - their probably granitic origin with a granodioritic tendency whose establishment would not be linked to a distensive phase like in Montagne Noire.
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Jacques Negron. Pétrologie et géochimie des formations quartzofeldspathiques de la série métamorphique mésozonale du Chassezac (Lozère, Massif Central français). Pétrographie. Université Claude Bernard - Lyon 1, 1979. Français. ⟨tel-02071789⟩

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