# Strike-slip tectonic-driven peraluminous rare-metal magmatism: insights from the late-Variscan Blond granitic complex (French Massif Central)

Johan Ramirez Briones, Christophe Ballouard, Gaétan Milesi, Patrick Carr, Marie Daoulas-Gerardin, Matthieu Harlaux, Oscar Laurent, Alexis Plunder, Jérémie Melleton, Jean Vallance, et al.

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Johan Ramirez Briones, Christophe Ballouard, Gaétan Milesi, Patrick Carr, Marie Daoulas-Gerardin, et al.. Strike-slip tectonic-driven peraluminous rare-metal magmatism: insights from the late-Variscan Blond granitic complex (French Massif Central). GeoTolosa 2025, Jun 2025, Toulouse, France. hal-05288892

HAL Id: hal-05288892

https://cnrs.hal.science/hal-05288892v1

Submitted on 30 Sep 2025

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# Strike-slip tectonic-driven peraluminous rare-metal magmatism: insights from the late-Variscan Blond granitic complex (French Massif Central)

Johan Ramirez-Briones *<fnref n="1" />, Christophe Ballouard <fnref n="1" />, Gaétan Milesi <fnref n="1" />, Patrick Carr <fnref n="1" />, Marie Gérardin <fnref n="1" />, Matthieu Harlaux <fnref n="2" />, Oscar Laurent <fnref n="3" />, Alexis Plunder <fnref n="2" />, Jérémie Melleton <fnref n="2" />, Jean Vallance <fnref n="4" />, Julien Mercadier <fnref n="1" />

During the late Carboniferous to early Permian, major crustal magmatism in the internal domains of the European Variscan belt was locally associated with the emplacement of small volume of highly-fractionated peraluminous magmas strongly enriched in rare metals such as Li, Be, Nb, Ta, Sn, and W known as rare-metal granites and pegmatites. The northern part of the French Massif Central (FMC) is particularly well endowed with such deposits, and significant scientific research has focused on their formation conditions and magmatic-hydrothermal evolution. However, the tectonic context of rare-metal magmas emplacement, including the role of deformation in melt extraction, ascent and metal enrichment, remains poorly understood. The Blond peraluminous granitic complex, located in the NW FMC, represents an ideal natural laboratory for studying the relationship between late-Variscan tectonics and rare-metal magmatism. The main granitic intrusion consisting of two-mica and muscovite facies was syntectonically emplaced at 319±7 Ma along the Oradour fault, a major late-Variscan N135-striking dextral strike-slip shear zone. Both facies are crosscut by several F-Sn-(Li)-rich topaz-cassiterite granitic dykes and stocks ("ongonite") likely emplaced in relation to N60-striking sinistral shear zones. New structural measurements, petrogeochemical analyses, and U-Pb dating are currently in progress to investigate the magmatic and tectonic history of the Blond complex and its relationship with the emplacement of strongly evolved melts.

In the northwest part of the complex (La Puelle quarry), the muscovite facies of the Blond granite shows a ~N100 oriented subvertical magmatic fabric and is affected by two sets of high- dipped strike-slip shear bands (mean: N033-77°E and N072-82°S) and one set of steep ongonite dykes (mean: N006-62°E). The shear bands are sinuous and show sinistral plastic to brittle- plastic deformation as protomylonitic zones with S-C fabrics and cores of (ultra)mylonite or ultracataclasite. They bear sub-horizontal lineations or striations, and are locally overprinted by extensional fractures developed along the shear band and filled by comb quartz. The ongonite dykes (1 to 20-cm thick) are commonly sinuous, locally arranged in en échelon pattern, and become thinner in the propagation tip. The contacts between the dykes and the host rock are either sinuous or sharp. One ongonite dyke is plastically deformed within a sinistral shear zone, which supports a syntectonic emplacement. The reduced stress tensor calculated through inversion of structures (n=34) is consistent with reverse strike-slip deformation (sub-vertical) with a sub-horizontal N010-oriented. The structures in the quarry can be explained with the classical Riedel shear model, where the ongonite dykes represent tension gashes, and the N033 and N072-oriented families the R' and P' antithetic shear zones,

<footnote n="1">¹ GeoResources, University of Lorraine, France</footnote>

<footnote n="2">² Bureau de Recherches Géologiques et Minières (BRGM), France</footnote>

<footnote n="3">³ Géosciences Environnement Toulouse, University of Toulouse, France</footnote>

<footnote n="4">⁴ Department of Engineering, Pontifical Catholic University of Peru, Peru</footnote>
respectively, linked at depth to the Oradour fault.

In situ U-Pb dating of magmatic cassiterite constrains the emplacement of ongonites at 310±6 Ma. The episode of tectonically-assisted rare-metal magmatism seems synchronous with the N-S-striking emplacement of the Richemont rare-metal rhyolitic dyke (313±3 Ma) and wolframite-cassiterite-bearing quartz veins (310±4 Ma). These observations underscore the significant role of late-Variscan strike-slip tectonics in creating structures of localized extension, which facilitated the vertical migration of rare-metal-rich melts and the focalization of mineralizing fluids at upper crustal levels.

This work is funded by ANR-22-EXSS-0014.