Differentiation of peraluminous granite en route to the surface
Résumé
Leucogranites display petrological and geochemical heterogeneities that can be related to either primary processes at the source, or to secondary processes such as differentiation. In the Armorican Massif, a Hercynian domain in western France, syn-tectonic leucogranites were emplaced along major shear zones. The Lizio and Questembert massifs are derived from a similar metasedimentary source and emplaced at distinct depths, ca. 3-4 kbar and 1-2 kbar, respectively. The Questembert granite is more differentiated than the Lizio one, as demonstrated by a lower amount of modal biotite, a higher SiO2 content and a more pronounced peraluminous character. The high-SiO2 Questembert rocks can derive from the low-SiO2 Lizio ones by a 15 wt.% fractionation of a kfs + pl + bt + zrn + mnz assemblage. This magmatic evolution is well recorded by the oxygen isotope evolution of whole rocks, as well as quartz and muscovite separates, demonstrating that these granites crystallized from almost crystal-free liquids. We propose that the differences between the Questembert and the Lizio granites are related to processes acting during magmas ascent through the crust. Questembert is more differentiated than Lizio because the crystal-melt mixture covered a longer distance during its vertical migration, allowing for more crystal-liquid segregation. Regional deformation likely enhanced this process through a filter-press type process. Associated shear heating can explain the shallower depth of emplacement for the Questembert pluton as it moved the brittle-ductile transition zone toward the surface (Leloup et al., 1999). The shallower emplacement of the Questembert melt led to the exsolution of a fluid phase, which further interacted with surrounding rocks, developing a pervasive deuteric alteration. Leloup, P.H., et al. (1999). Geophys. J. Int. 136, 19-40.