Emplacement, burial, exhumation – deciphering the fate of the Lévézou massif via petrochronological studies of felsic and mafic eclogites

Abstract : Determining timescales, rates and duration of metamorphic processes together with the pressure – temperature (P–T) evolution of metamorphic rocks is a prerequisite for deciphering the geodynamic evolution of mountain belts. We present here a complete study from the Lévézou massif (French Massif Central), based on the investigation of mafic and granitic rocks that preserved various metamorphic stages during the variscan orogeny. Both rocks types emplaced during Ordovician times at ca. 470 Ma (zircon U-Pb dating). Phase diagram modelling performed on the whole rock composition of undeformed granites allowed to constrain the emplacement conditions at ca. 2 kbar and 670°C. The underformed granites locally developed metamorphic textures, as tiny garnet coronae surrounding magmatic biotite, rutile aggregates pseudomorphing magmatic ilmenite or rare rectangular clusters dominated by phengite, kyanite, garnet and quartz that pseudomorphcordierite. Pseudosections calculated for the local bulk composition of the cordierite pseudomorphs suggest that they developed at 15-17 kbar and 650-670°C, i.e. under eclogite-facies conditions. The neighbouring mafic eclogites recorded slightly higher pressure conditions at 18-23 kbar and 680-800°C, and were later equilibrated at 8-9.5 kbar and ca. 600°C. A large panel of geochronometers (U-Pb on zircon, rutile and apatite, Lu-Hf and Sm-Nd on garnet) were investigated and combined to tightly constrain the P–T conditions of the mafic metamorphic rocks. Lu-Hf garnet dating from a fresh and a retrogressed eclogite yield identical dates of 357 ± 13 Ma and 357.5 ± 4 Ma, that approximate the age of the HP metamorphic peak. Fresh and retrogressed samples yield respectively 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, while garnet Sm-Nd dating yield an age of 350.4 ± 7.7 Ma. The similarity between all recorded ages from distinct chronometers and radiometric methods testify to very fast exhumation, highlighting a major nearly adiabatic decompression, ranging from 8.5 to 15 kbar (av. 12 ± 3 kbar) in less than 6 My. Finally, a campaign-style Ar-Ar study on granitic rocks highlights localized migmatisation enhanced by fluid circulation, contemporaneous with two deformational events at ca. 350 and 342 Ma, and constrains the final geodynamic evolution of the massif. Further details can be found in [1] and [2]. References: [1] Lotout et al. (2017) International Journal of Earth Sciences [2] Lotout (2017) PhD dissertation, 374p
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Submitted on : Tuesday, March 12, 2019 - 8:51:45 AM
Last modification on : Tuesday, August 13, 2019 - 11:14:21 AM

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  • HAL Id : insu-02064556, version 1

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Caroline Lotout, Pavel Pitra, Marc Poujol, Gilles Ruffet, Robert Anczkiewicz, et al.. Emplacement, burial, exhumation – deciphering the fate of the Lévézou massif via petrochronological studies of felsic and mafic eclogites. Research in Progress - Metamorphic Studies Group, Mar 2019, Portsmouth, United Kingdom. 2019. ⟨insu-02064556⟩

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