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Deceiving zircon REE patterns and U-Pb dates from migmatite-hosted eclogites (example from the Montagne Noire, France)

Abstract : Zircon REE patterns are widely used to attribute specific U-Pb dates to eclogite-facies metamorphism. Eclogites hosted in sillimanite-bearing migmatites in the Montagne Noire dome (French Massif Central) reached c. 750°C, 21 kbar before significant decompression at high temperature. Zircon crystals from an eclogite sample yield three distinct clusters of dates at 360 Ma, 340 Ma and 315 Ma. Whatever their apparent ages, however, all these grains display identical REE patterns (no Eu anomaly, flat HREE), usually described as typical for eclogite-facies equilibration. This could be related to eclogite-facies zircon crystals affected by a partial to total solid-state recrystallization during the subsequent exhumation, preserving their REE signature, but responsible for a partial to complete resetting of their U-Pb system. Alternatively, zircon may have grown in distinct pulses at different stages of the P-T evolution, characterized by the presence of significant proportions of stable or metastable garnet. In the absence of other criteria, none of these dates can be unequivocally attributed to the HP event. The 315 Ma zircon dates obtained from the embedding sillimanite-bearing migmatitic rocks are interpreted as the age of the HT-LP metamorphism. This strongly suggests that this 315 Ma date obtained from the eclogite sample is related to the HT-LP overprint and cannot be attributed to the eclogite-facies metamorphism. We caution against the unequivocal use of REE patterns in zircon to determine the age of HP metamorphism in eclogites occurring in migmatitic domes. Misinterpreted young ages may lead to inferring erroneous fast exhumation rates and consequently questionable geodynamic models.
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Contributor : Isabelle Dubigeon <>
Submitted on : Thursday, November 8, 2018 - 1:57:08 PM
Last modification on : Thursday, June 18, 2020 - 3:25:16 AM

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  • HAL Id : insu-01916389, version 1

Citation

Pavel Pitra, Eloise Bretagne, Jean van den Driessche, Caroline Lotout, Marc Poujol, et al.. Deceiving zircon REE patterns and U-Pb dates from migmatite-hosted eclogites (example from the Montagne Noire, France). 26ème Réunion des Sciences de la terre - RST, Oct 2018, Lille, France. pp.466. ⟨insu-01916389⟩

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