Tartèse, R. and Boulvais, P.
Differentiation of peraluminous granites 'en route' to the surface.
In: 19th V.M. Goldschmidt conference, 21-26 June 2009, Davos, Switzerland.
Leucogranites display petrological and geochemical heterogeneities that can be related to either primary processes acting at the source, or to secondary processes of 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. Questembert rocks are more differentiated than Lizio rocks, 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 low-SiO2 Lizio samples by a 15 wt.% fractionation of the assemblage kfs + pl + bt. The magmatic evolution is well recorded by the oxygen isotope evolution of whole rocks and mineral 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 rising. Questembert is more differentiated than Lizio because it covered a longer distance during its vertical migration, allowing more crystal-liquid segregation. Regional deformation likely enhanced this process through a filter-press type mechanism. The shallower emplacement of Questembert liquids led to the exsolution of a fluid phase, which further interacted with surrounding rocks, developing a pervasive deuteric alteration. A further influx of fluids from the surface was possible because of the very shallow depth of emplacement.
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