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From burial to exhumation: emplacement and metamorphism of mafic eclogitic terranes constrained through multimethod petrochronology, case study from the Lévézou massif (French Massif Central, Variscan belt)

Abstract : Linking mineral growth and time is required to unravel the evolution of metamorphic rocks. However, dating early metamorphic stages is a challenge, due to subsequent retrograde overprinting. A perfectly fresh eclogite and a former eclogite retrogressed under amphibolite facies from the southern French Massif Central (Lévézou massif, Variscan belt) were investigated with a large panel of geochronometers (U-Pb on zircon, rutile and apatite, Lu-Hf and Sm-Nd on garnet) in a petrological context tightly constrained by petrographic observation, trace element analyses and phase equilibrium modelling. Both samples recorded similar HP conditions at 18-23 kbar and 680-800°C, while the retrogressed eclogite later equilibrated at 8-9.5 kbar and ca. 600°C. In the retrogressed sample, most of the zircon grains are characterized by negative Eu anomalies and HREE enrichment, and yield an Ordovician U-Pb date of 472.3 ± 1.7 Ma, interpreted as the emplacement age of the mafic protolith. In agreement with other data available for the Variscan belt, and based on zircon trace elements record and whole rock geochemistry, this age is considered to represent the magmatism associated with the extreme thinning of the continental margins during the Ordovician. In the same sample, few zircon rims show a weaker HREE enrichment and yield a date of 378 ± 5.7 Ma, interpreted as a prograde pre-eclogitic age. Lu-Hf garnet dating from both samples yields identical dates of 357 ± 13 Ma and 358.0 ± 1.5 Ma inferred to approximate the age of the high-pressure metamorphic peak. Fresh and retrogressed samples yield respectively 350.4 ± 7.7 Ma and 352 ± 20 Ma dates for Sm-Nd garnet dating, and 367.8 ± 9.1 Ma and 354.9 ± 9.5 Ma for U-Pb rutile dating. Apatite grains from the retrogressed sample give a mean age of 351.8 ± 2.8 Ma. The similarity between all recorded ages from distinct chronometers and radiometric methods (U-Pb – rutile, apatite; Lu-Hf – garnet; Sm-Nd – garnet) combined with P–T estimations from high-pressure metamorphic rocks equilibrated under different conditions testifies to very fast processes that occurred during the Variscan orogeny, highlighting a major decompression of 15-8.5 kbar in less than 7 Myr, and suggesting mean exhumation rates in excess of 6.3 mm/yr.
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Caroline Lotout, Marc Poujol, Pavel Pitra, Robert Anczkiewicz, Jean van den Driessche. From burial to exhumation: emplacement and metamorphism of mafic eclogitic terranes constrained through multimethod petrochronology, case study from the Lévézou massif (French Massif Central, Variscan belt). Journal of Petrology, Oxford University Press (OUP), inPress, pp.egaa046. ⟨10.1093/petrology/egaa046⟩. ⟨insu-02539986⟩

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