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Timing and duration of Variscan high-pressure metamorphism in the French Massif Central: A multimethod geochronological study from the Najac Massif

Abstract : 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.
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Submitted on : Monday, April 9, 2018 - 4:36:41 PM
Last modification on : Tuesday, December 3, 2019 - 1:54:03 PM

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Caroline Lotout, Pavel Pitra, Marc Poujol, Robert Anczkiewicz, Jean van den Driessche. Timing and duration of Variscan high-pressure metamorphism in the French Massif Central: A multimethod geochronological study from the Najac Massif. Lithos, Elsevier, 2018, 308-309, pp.381 -394. ⟨10.1016/j.lithos.2018.03.022⟩. ⟨insu-01762154⟩

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