Multiple crust reworking in the French Armorican Variscan belt: implication for the genesis of uranium-fertile leucogranites
Résumé
In the French Armorican Variscan belt, most of theeconomically significant hydrothermal U deposits are spatiallyassociated with peraluminous leucogranites emplacedalong the south Armorican shear zone (SASZ), a dextral lithosphericscale wrench fault that recorded ductile deformationfrom ca. 315 to 300 Ma. In the Pontivy-Rostrenen complex, acomposite intrusion, the U mineralization is spatially associatedwith brittle structures related to deformation along theSASZ. In contrast to monzogranite and quartz monzodiorite(3 < U < 9 ppm; Th/U > 3), the leucogranite samples arecharacterized by highly variable U contents (~ 3 to 27 ppm)and Th/U ratios (~ 0.1 to 5) suggesting that the crystallizationof magmatic uranium oxide in the more evolved facies wasfollowed by uranium oxide leaching during hydrothermal alterationand/or surface weathering. U-Pb dating of uraniumoxides from the deposits reveals that they mostly formed betweenca. 300 and 270 Ma. In monzogranite and quartzmonzodiorite, apatite grains display magmatic textures andprovide U-Pb ages of ca. 315 Ma reflecting the time of emplacementof the intrusions. In contrast, apatite grains from theleucogranite display textural, geochemical, and geochronologicalevidences for interaction with U-rich oxidized hydrothermalfluids contemporaneously with U mineralizingevents. From 300 to 270 Ma, infiltration of surface-derivedoxidized fluids leached magmatic uranium oxide from fertileleucogranite and formed U deposits. This phenomenon wassustained by brittle deformation and by the persistence ofthermal anomalies associated with U-rich granitic bodies.
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