Timing and duration of Variscan high-pressure metamorphism in the French Massif Central: A multimethod geochronological study from the Najac Massif
Résumé
Accurate dating of eclogite-faciesmetamorphismis of paramount importance in order to understand the tectonicevolution of an orogen. An eclogite sample fromtheNajac Massif (FrenchMassif Central, Variscan belt) displays azircon-bearing garnet-omphacite-amphibole-rutile-quartz peak assemblage. Pseudosection modeling suggestspeak pressure conditions of 15–20 kbar, 560–630 °C. Eclogite-facies garnet displays Lu-enriched cores and Smrichrims and yields a Lu-Hf age of 382.8±1.0Ma and a Sm-Nd age of 376.7±3.3Ma. The ages are interpretedas marking the beginning of the prograde garnet growth during the initial stages of the eclogite-facies metamorphism,and the high-pressure (and temperature) peak reached by the rock, respectively. Zircon grains displaychondrite-normalized REE spectra with variably negative, positive or no Eu anomalies and are characterized byeither enriched or flat HREE patterns. However, they yield a well constrained in situ LA-ICP-MS U-Pb age of385.5±2.3 Ma, despite this REE pattern variability. Zr zonation in garnet, Y content in zircon and the diversityof zircon HREE spectra may suggest that zircon crystallized prior to and during incipient garnet growth on theprograde P-T path, recording the initial stages of the eclogite-facies conditions. Consequently, the zircon age of385.5±2.3Ma, comparable within error with the Lu-Hf age obtained on garnet, is interpreted as dating the beginningof the eclogite-faciesmetamorphism. Accordingly, the duration of the prograde part of the eclogite-faciesevent is estimated at 6.1±4.3 Myr. Subsequent exhumation is constrained by an apatite U-Pb age at 369±13Ma.