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Fig. 6


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Whole-rock geochemistry of the Cézarenque–Joyeuse gneisses. (a) Al/3-K vs. Al/3-Na cationic diagram of de La Roche (1968) aiming at discriminating meta-igneous from metasedimentary rocks. (b) Total alkali vs. silica classification diagram of Le Bas et al. (1986). The dashed line is the subalkaline–mid-alkaline boundary following Rittman (1957). (c) B–A cationic classification diagram of Debon and Le Fort (1988) with the subdivisions of Villaseca et al. (1998). (d) SiO2 vs. Na2O + K2O – CaO and (e) SiO2 vs. FeOt/(FeOt + MgO) (in wt.%) diagrams of Frost et al. (2001). (f) K/(K + Na) vs. B. cationic classification diagram of Debon and Le Fort (1988). The dotted lines represent the contours encompassing 85% of the data for the following distributions: Variscan plutonic and volcanic rocks from the French Massif Central with SiO2 between 63–72% (n = 1350, Moyen et al., 2017), the Ollo de Sapo gneisses (n = 141, based on the compilation of García-Arias et al., 2018), the Cévennes mica schists (n = 48, data from Weisbrod, 1970; Harlaux, 2016; Couzinié, 2017). The contour levels were drawn using the kde2d function of R (Venables and Ripley, 2002). The B–A values for sediment-derived experimental melts at 2–15 kbar and 700–900 °C were taken from the compilation in Couzinié et al. (2017). The black arrow in (c) depicts the increase in Fe solubility observed by Gaillard et al. (2001) in a subaluminous 930 °C granitic melt when fO2 decreases from NNO + 1.5 down to NNO-0.7 (∼QFM, NNO sanding for Ni–NiO buffer).

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