For the first time, an albite orthogneiss has been recognised and dated
within the HP-LT blueschist facies metabasites and metapelites of the
Ile de Groix. It is characterised by a peraluminous composition, high
LILE, Th and U contents, MORB-like HREE abundances and moderate Nb and Y
values. A U-Pb age of 480.8 +/- A 4.8 Ma was obtained by LA-ICP-MS
dating of zircon and titanite. It is interpreted as the age of the
magmatic emplacement during the Early Ordovician. Morphologically
different zircon grains yield late Neoproterozoic ages of 546.6-647.4
Ma. Zircon and titanite U-Pb ages indicate that the felsic magmatism
from the Ile de Groix is contemporaneous with the acid, pre-orogenic
magmatism widely recognised in the internal zones of the Variscan belt,
related to the Cambro-Ordovician continental rifting. The magmatic
protolith probably inherited a specific chemical composition from a
combination of orogenic, back-arc and anorogenic signatures because of
partial melting of the Cadomian basement during magma emplacement.
Besides, the late Devonian U-Pb age of 366 +/- A 33 Ma obtained for
titanite from a blueschist facies metapelite corresponds to the age of
the HP-LT peak metamorphism.