Differentiation of peraluminous leucogranites “en route” to the surface

Abstract

Leucogranites display petrological and geochemical heterogeneities that can be related to either primary processes in the source of melts or to secondary processes such as fractional crystallization. In the Armorican Massif, a Hercynian domain in western France, syntectonic leucogranites were emplaced along major shear zones, the so-called South Armorican Shear Zone. The Lizio and Questembert massifs are derived from a similar metasedimentary source ( ε Nd(T) between − 4 and − 6) and have similar magmatic volumes. They display, however, distinct depths of emplacement, with Questembert being emplaced ca. 5–10 km shallower than Lizio. The shallower emplacement of Questembert is recorded by contact metamorphic assemblages and the levels of intrusion in correlated sedimentary sequences. The Questembert magmas are more differentiated than the Lizio ones, as demonstrated by a lower amount of modal biotite, a higher SiO 2 content (72.8–74.2 wt.% vs. 71.7–73.3 wt.%) and a more pronounced peraluminous character. The high-SiO 2 Questembert magmas can be derived from the low-SiO 2 Lizio ones by 15–20 wt.% fractionation of the assemblage alkali feldspar + plagioclase + biotite. Fractionation of zircon and monazite, hosted in biotite, is indicated by a systematic decrease in Zr and Th + LREE contents. The magmatic evolution is well recorded by the oxygen isotope evolution of whole rocks and mineral separates. The Questembert massif records evidence of a two stage fluid–rock interaction. A high-T stage corresponds to the magmatic fluid exsolution, and is recorded in the most evolved samples: appearance of tourmaline in the matrix, changes in the composition of muscovite, increase in whole rock Be, Sn, Rb and W contents, fractionation of K/Rb, Zr/Hf and Nb/Ta ratios, and fractionation of REE patterns. A second post-solidus alteration is indicated by oxygen isotope disequilibrium between quartz and feldspar in the Questembert massif and requires the circulation of low- δ 18O fluids, likely derived from the surface. We propose that the distinct petrological, mineralogical and geochemical evolutions of the Questembert and Lizio magmas are related to processes acting during the ascent of magmas through the continental crust. Questembert is more differentiated than Lizio because it covered a greater distance during its vertical migration, enhancing crystal fractionation from melts.