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From burial to exhumation: evolution of eclogitic terranes through multimethod petrochronology, Lévézou Massif, French Massif Central

Abstract : To unravel the evolution of metamorphic rocks, the understanding of the mineral growth through time is required. However, dating early metamorphic stages is a challenge, due to subsequent retrograde overprinting. This is particularly true for HP metamorphism that highlights the early stages of orogenesis. 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. 620°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. This is interpreted as the emplacement age of the mafic protoliths and, in agreement with other data available for the Variscan belt, is considered to represent the extreme thinning of continental margins during a rifting event or a back-arc extension. 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 357.5 ± 4 Ma, that approximate the age of the high pressure metamorphic peak. Fresh and retrogressed samples respectively yield 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. All these ages are identical within error and, when combined with P-T estimations, highlight a major decompression ranging from 8.5 to 15 kbar in less than 6 My, suggesting exhumation rates in excess of 5 mm/yr up to more than 1 cm/yr. Decompression is nearly adiabatic with gradient ranging from 7.8 to 1.3°C/km, followed by cooling rates after major decompression exceeding 50°C/Ma.
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Submitted on : Thursday, November 8, 2018 - 1:48:10 PM
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Caroline Lotout, Marc Poujol, Pavel Pitra, Robert Anczkiewicz, Jean van den Driessche. From burial to exhumation: evolution of eclogitic terranes through multimethod petrochronology, Lévézou Massif, French Massif Central. 26ème Réunion des Sciences de la terre - RST, Oct 2018, Lille, France. pp.459. ⟨insu-01916376⟩

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