Timing and duration of variscan high-pressure metamorphism in the French Massif Central: a petrochronological study from the Najac Massif

International audienceUnderstanding and constraining the time dimension of the history of a rock - absolute ages, timescales, rates or durations - is one of the major challenges in Earth Sciences. In eclogite-facies rocks, the time dimension helps to better understand the early stages of the building of orogens and is therefore of paramount importance. An eclogite sample from the Najac Massif (French Massif Central, Variscan belt) 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 and titanite in garnet fractures are inferred to have formed contemporaneously, late in the metamorphic history, at significantly lower pressure. The Zr-in-Titanite thermometer yields 615-640 °C for the crystallization of titanite. The U-Pb apatite age at 369 ± 13 Ma then records the timing of the subsequent exhumation. Further details in Lotout et al., 2018. Lotout, C., Pitra, P., Poujol, M., Anczkiewicz, R. & Van Den Driessche, J. (2018). Timing and duration of Variscan high-pressure metamorphism in the French Massif Central: A multimethod geochronological study from the Najac Massif. Lithos 308, 381-394