Evolution of magma mixing in an alkaline suite: the Grande Cascade sequence (Monts-Dore, French Massif Central). Geochemical modelling

Abstract

Abstract Trachyandesitic lavas from an alkaline suite (Grande Cascade sequence, Monts-Dore, French Massif Central) have been investigated for evidence of mechanical magma mixing. A wide variety of textures are present in the sequence, as pillow-like enclaves, bandings and emulsified facies. The Grande Cascade flows and pyroclasts range from basalt to trachyte (44–67 wt.% SiO 2 ). Pillow-like enclaves, intricate commingling and local diffusive mixing at the mafic-felsic interface in emulsified types, show that mafic inclusions were incorporated in their trachytic host as blobs of magma. Such inclusions were probably dispersed throughout the trachytic magma by mechanical mixing. Different stages of the evolution of a mechanical mixing process were recognized in relation with the stratigraphy (and chronology) of the products: the most heterogeneous features (enclaves, bandings) are restricted to the bottom of the sequence, where basalt and trachyte coexist; at the top, texturally homogeneous trachyandesites occur. Relict phenocrysts are observed in both basaltic enclaves (oligoclase, Fe-rich clinopyroxene, sanidine of the trachyte) and trachytic hosts (Mg-rich olivine, Mg-rich clinopyroxene, Ca-rich plagioclase of basalt). The intermediate rocks (basic trachyandesites) contain xenocrysts of Mg-rich olivine, clinopyroxenes, oligoclase, sanidine, together with reversely zoned plagioclase and pyroxene phenocrysts. This assemblage is presumed to result from the incorporation of xenocrysts related both to mafic and felsic endmembers. All mineral assemblages are out of equilibrium. Reversed compositional zoning profiles in plagioclase and clinopyroxene crystals were produced by the mixing event. We are able to identify the compositional characteristics of the components that were mixed. A primary alkaline basalt (Mg-olivine, labrador, Mg-clinopyroxene, magnetite) and a silicic trachyte (oligoclase, Fe-clinopyroxene, sanidine) are the endmembers of mixing. Observed chemical variations in major and trace elements can be modelled as pure binary mixtures between basaltic and trachytic endmembers. Using the “best mixing line” method (Provost and Allegre, 1979), the calculations emphasize that trachyandesites are the result of complete hybridization between basalt and trachyte. The data on mesostasis show that melt mixing (and not only crystal transfers) has produced the intermediate lavas of the Grande Cascade sequence. Textural criteria, petrography, mineral chemistry and geochemical data (major and trace elements) support the magma mixing model.