Complex geochronological record of an emblematic Variscan eclogite (Haut‐Allier, French Massif Central)

Luc de Hoÿm de Marien (1, 2) , Pavel Pitra (2) , Marc Poujol (2) , Nathan Cogné (2) , Florence Cagnard (3) , Benjamin Le Bayon (3)

1 Czech Geological Survey [Praha]
2 GR - Géosciences Rennes
3 BRGM - Bureau de Recherches Géologiques et Minières (BRGM)

Two eclogite samples from the Haut-Allier record a prograde evolution from ~20 kbar, 650 °C to 750 °C, 22-23 kbar followed by heating up to 850-875 °C and partial melting. Incipient decompression in high-pressure granulite facies conditions (19.5 kbar, 875 °C) was followed by exhumation to high-temperature amphibolite facies conditions (<9 kbar, 750–850 °C). Following a detailed geochemical, petrological and geochronological investigation using trace-element data and LA-ICP-MS U-Pb dating of zircon, apatite and rutile, the eclogites reveal an Ordovician (c. 490 Ma) rifting event followed by Devonian (c. 370– 360 Ma) subduction and Carboniferous (c. 350 Ma) exhumation in this part of the French Massif Central (FMC). The previously proposed Silurian age for the subduction, that strongly influenced many tectonic models, is definitively rejected. In the light of other geological data from the FMC, including the lithological and geochemical zoning of calc-alkaline Devonian volcanism, we propose a southward polarity of the subduction and question the very existence of the so-called Massif Central Ocean. Furthermore, we infer that following subduction, the eclogites were relaminated to the upper plate and exhumed at the rear of the magmatic arc pointing to similarities with the geodynamics of the Bohemian Massif. The petrochronological record of zircon is particularly complex. Metamorphic zircon with clear eclogitic REE patterns (no Eu anomaly, flat HREE) and inclusions (garnet, rutile, omphacite) shows concordant apparent ages that spread from c. 380 down to c. 310 Ma. This apparent age pattern strongly contrasts with the well-defined age of apatite and rutile of c. 350 Ma. Apparent zircon ages younger than 350 Ma unequivocally testify that zircon can recrystallize outside the conditions of the eclogite facies, which resets the U–Pb while preserving an apparent eclogitic signature. Local fractures filled by analcite, thomsonite, plagioclase and biotite testify to late interaction of the eclogites with alkaline fluids at relatively low temperatures. This interaction, possibly at c. 310 Ma or later, could lead to the recrystallisation of zircon while leaving apatite unaffected.

Mots clés: Variscan belt; eclogite; petrochronology; zircon; apatite

Domaines: Géochimie

Luc de Hoÿm de Marien, Pavel Pitra, Marc Poujol, Nathan Cogné, Florence Cagnard, et al.. Complex geochronological record of an emblematic Variscan eclogite (Haut‐Allier, French Massif Central). Journal of Metamorphic Geology, 2023, 41 (7), pp.967-995. ⟨10.1111/jmg.12733⟩. ⟨insu-04101059v2⟩