Tectonic record, magmatic history and hydrothermal alteration in the Hercynian Guérande leucogranite, Armorican Massif, France
Résumé
The Guérande peraluminous leucogranite was emplaced at the end of the Carboniferous in the southern part of
the Armorican Massif. At the scale of the intrusion, this granite displays structural heterogeneities with a weak
deformation in the southwestern part, whereas the northwestern part is marked by the occurrence of S/C and
mylonitic extensional fabrics. Quartz veins and pegmatite dykes orientations as well as lineations directions in
the granite and its country rocks demonstrate both E–WandN–S stretching. Therefore, during its emplacement
in an extensional tectonic regime, the syntectonic Guérande granite has probably experienced some partitioning
of the deformation. The southwestern part is characterized by a muscovite–biotite assemblage, the presence of
restites and migmatitic enclaves, and a lowabundance of quartz veins compared to pegmatite dykes. In contrast,
the northwestern part is characterized by a muscovite–tourmaline assemblage, evidence of albitization and
gresenization and a larger amount of quartz veins. The southwestern part is thus interpreted as the feeding
zone of the intrusion whereas the northwestern part corresponds to its apical zone. The granite samples display
continuous compositional evolutions in the range of 69.8–75.3 wt.% SiO2. High initial 87Sr/86Sr ratios and low
εNd(T) values suggest that the peraluminous Guérande granite (A/CNK N 1.1) was formed by partial melting of
metasedimentary formations. Magmatic evolution was controlled primarily by fractional crystallization of Kfeldspar,
biotite and plagioclase (An20). The samples from the apical zone show evidence of secondary
muscovitization. They are also characterized by a high content in incompatible elements such as Cs and Sn, as
well as lowNb/Ta and K/Rb ratios. The apical zone of the Guérande granite underwent a pervasive hydrothermal
alteration during or soon after its emplacement. U–Th–Pb dating on zircon and monazite revealed that the
Guérande granite was emplaced 309.7± 1.3 Ma ago and that a late magmatic activity synchronous with hydrothermal
circulation occurred at ca. 303 Ma. These new structural, petrological and geochronological data presented
for the Guérande leucogranite highlight the interplay between the emplacement in an extensional tectonic
regime, magmatic differentiation and hydrothermal alteration, and provide a general background for the understanding
of the processes controlling some mineralization in the western European Hercynian belt.