Late Variscan (315 Ma) subduction or deceptive zircon REE patterns and U-Pb dates from migmatite-hosted eclogites? (Montagne Noire, France) - Archive ouverte HAL Access content directly
Journal Articles Journal of Metamorphic Geology Year : 2022

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

(1) , (1) , (1) , (1) , (2, 3) , (1)
1
2
3

Abstract

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 ~19.5 kbar, 700°C to the pressure peak at ~21 kbar, 750°C (pseudosection modelling), suggesting metamorphism in a subductional setting. Subsequent exhumation to low-pressure high-temperature (LP–HT) conditions (~6 kbar, 730°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 crystallisation 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 crystallisation 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. Confronting 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 hypothesise 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 recrystallised and underwent partial to total resetting of their U-Pb system, whereas the REE 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.
Fichier principal
Vignette du fichier
Pitra_et_al_21-MNeclogites-JMG+suppl.pdf (4.49 Mo) Télécharger le fichier
Origin : Files produced by the author(s)

Dates and versions

insu-03188533 , version 1 (14-06-2021)

Identifiers

Cite

Pavel Pitra, Marc Poujol, Jean van den Driessche, Eloise Bretagne, Caroline Lotout, et al.. Late Variscan (315 Ma) subduction or deceptive zircon REE patterns and U-Pb dates from migmatite-hosted eclogites? (Montagne Noire, France). Journal of Metamorphic Geology, 2022, 40 (1), pp.39-65. ⟨10.1111/jmg.12609⟩. ⟨insu-03188533⟩
181 View
72 Download

Altmetric

Share

Gmail Facebook Twitter LinkedIn More