Late Variscan (315 Ma) subduction or deceptive zircon REE patterns and U-Pb dates from migmatite-hosted eclogites? (Montagne Noire, France)

Eclogites hosted in sillimanite-bearing migmatites in the Montagne Noire dome (French Massif Central) have an oceanic protolith and recorded a prograde P-T path from similar to 19.5 kbar, 700 degrees C to the pressure peak at similar to 21 kbar, 750 degrees C (pseudosection modelling), suggesting metamorphism in a subduction setting. Subsequent exhumation to low-P high-T (LP-HT) conditions (similar to 6 kbar, 730 degrees C) is constrained by the equilibration conditions of the embedding migmatite. In samples of a fresh and a retrogressed eclogite, all zircon crystals but one display similar REE patterns (no Eu anomaly, flat HREE), usually ascribed to crystallization under eclogite-facies conditions. Yet, the U-Pb apparent ages of zircon crystals from both eclogites spread from c. 360 Ma to a dominant data cluster at c. 315 Ma. The c. 315-310 Ma zircon U-Pb dates obtained from the embedding migmatite are interpreted as the age of crystallization of the partial melt during the LP-HT metamorphic stage. First-order geological evidence, in particular the sedimentary record, excludes the existence of a subduction zone in the region at this period. Unless calling upon a major reappraisal of the tectonics of the European Variscan orogen, this suggests an ambiguous relation between REE patterns and U-Pb dates in the zircon population. Various scenarios that could account for the observations are discussed. Combining our data with the results of a previously published Sm-Nd dating of garnet, and regional considerations, we consider that 360 Ma is the best approximation of the minimum age of the eclogite-facies event. We hypothesize that the eclogites formed farther north and were transferred to their present location by lower-crustal flow. It is inferred that during the subsequent exhumation, eclogite-facies zircon grains recrystallized and underwent partial to total resetting of their U-Pb system, whereas the REF system remained mostly unmodified. These results caution against the use of REE patterns as the only criterion to associate a specific zircon age with HP metamorphism in eclogites occurring in migmatitic domes.

Automated summary

Eclogites in the Montagne Noire dome in the French Massif Central are believed to have formed in a subduction setting, with a prograde P-T path recorded. However, the discrepancy between REE patterns and U-Pb ages in zircon crystals suggests potential geological complexities in the region.