No 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 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 in zircon, rutile and apatite, Lu–Hf and Sm–Nd in garnet) in a petrological context tightly constrained by petrographic observations, trace element analyses and phase equilibrium modelling. Both samples recorded similar HP conditions at 18–23 kbar and 680–800°C, whereas the retrogressed eclogite later equilibrated at 8–9·5 kbar and c.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 element 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, a 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.