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Timing and duration of Variscan high-pressure metamorphism in the French Massif Central: petrochronological insights from the Najac Massif

Abstract : Precise and accurate investigation of geochronometers in the light of a detailed petrologi- cal context is the best way to fully understand the evolution of metamorphic rocks. Eclogite-facies rocks highlight the early stages of orogenesis; constraining their P-T-t evolution is then of paramount importance to any geodynamic interpretation. An eclogite sample from the Najac Massif (south French Massif Central) displays a zircon-bearing garnet-omphacite-amphibole-rutile-quartz peak assemblage. Pseudosection modelling suggests peak pressure conditions of 15–20 kbar, 560–630°C. Eclogite-facies garnet displays Lu-rich cores and Sm-enriched rims and yields a Lu-Hf age of 382.8 ± 1.0 Ma and a Sm-Nd age of 376.7 ± 3.3 Ma. These ages are interpreted as 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 display chondrite-normalized REE spectra with variably negative, positive or no Eu anomalies and are characterized by either enriched or flat HREE patterns. However, they yield a well constrained in situ LA-ICP-MS U-Pb age of 385.5 ± 2.3 Ma, despite this REE pattern variability. Zr zonation in garnet, Y content in zircon and the diversity of zircon HREE spectra may suggest (i) that zircon crystallized prior to and during incipient garnet growth on the prograde P-T path, in numerous pulses, or (ii) that local REE equilibria were recorded and preserved in zircon. The zircon age of 385.5 ± 2.3 Ma, comparable within error with the Lu-Hf age obtained on garnet, is interpreted as dating the beginning of the eclogite-facies metamorphism. Accordingly, the duration of the prograde part of the eclogite-facies event is estimated at 6.1 ± 4.3 Myr. Coarse crystals of apatite, that formed late in the metamorphic history at significantly lower pressure, yield a U-Pb age of 369 ± 13 Ma interpreted as the timing of the subsequent exhumeation. Combined with the HP dating results obtained, via a similar petrochronological approach, in the adjacent Lévézou Massif (Lotout et al., same volume), constrained at ca. 358 Ma, these results allow to discuss not only the age of the HP metamorphism but also the geodynamics of the French Massif Central.
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Submitted on : Thursday, November 8, 2018 - 10:23:12 AM
Last modification on : Tuesday, December 3, 2019 - 1:54:04 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: petrochronological insights from the Najac Massif. 26ème Réunion des Sciences de la terre - RST, Oct 2018, Lille, France. pp.174. ⟨insu-01915949⟩

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