Prograde and retrograde P–T evolution of a Variscan high-temperature eclogite, French Massif Central, Haut-Allier

Abstract

<jats:p>The P–T evolution of a mafic eclogite sample from the Haut-Allier was studied in order to constrain the dynamic of the Variscan subduction in the eastern French Massif Central. Three successive metamorphic stages M1, M2 and M3, are characterized by assemblages comprising garnet1-omphacite-kyanite, garnet2-plagioclase, and amphibole-plagioclase, respectively, and define a clockwise P–T path. These events occurred at the conditions of eclogite (M1; ∼ 20 kbar, 650 °C to ∼ 22.5 kbar, 850 °C), high-pressure granulite (M2; 19.5 kbar and 875 °C) and high-temperature amphibolite facies (M3; &amp;lt; 9 kbar, 750–850 °C), respectively. Pseudosection modelling of garnet growth zoning and mineralogy of the inclusions reveal a prograde M1 stage, first dominated by burial and then by near isobaric heating. Subsequent garnet1 resorption, prior to a renewed growth of garnet2 is interpreted in terms of a decompression during M2. High-pressure partial melting is predicted for both the M1 temperature peak and M2. M3 testifies to further strong decompression associated with limited cooling. The preservation of garnet growth zoning indicates the short-lived character of the temperature increase, decompression and cooling cycle. We argue that such P–T evolution is compatible with the juxtaposition of the asthenosphere against the subducted crust prior to exhumation driven by slab rollback.</jats:p>

Metamorphism and linked deformation in understanding tectonic processes at varied scales

“…The FMC eclogites are generally severely retrogressed under lower-pressure and HT conditions, resulting in the disappearance of HP peak equilibrium assemblages. However, since the discovery of small-sized relicts of coesite within garnets from the Mont du Lyonnais eclogites [Lardeaux et al, 2001], the application of modern methods of thermodynamic modelling to these rocks demonstrated their formation under UHP to HP conditions, thus under paleo-gradients typical of subduction zones [Benmammar et al, 2020, Berger et al, 2010, De Hoÿm de Marien et al, 2020, Lotout et al, 2018, 2020.…”

Complex geochronological record of an emblematic Variscan eclogite (Haut‐Allier, French Massif Central)

“…The eclogites occur within a variably retrogressed hectometre lens embedded in a sillimanite‐bearing diatexite in close association with a felsic orthogneiss. The eclogites underwent prograde metamorphism in the eclogite facies from 650°C at 20 kbar to 750°C at 22–23 kbar and crossed the solidus during an isobaric temperature increase up to 850–875°C (de Hoÿm de Marien et al, 2020). Following incipient decompression to ~19 kbar in the HP granulite facies, further decompression to 5–10 kbar at 750–850°C led to partial recrystallization in the HT amphibolite facies.…”

“…Both eclogite samples display comparable petrography. A detailed description of the sample BR18 is presented in de Hoÿm de Marien et al (2020). In summary, the textural relationships in both samples point to a peak mineral assemblage of garnet (g1), omphacite (cpx1), kyanite (ky), phengitic muscovite (mu), and accessory quartz (q), rutile (ru), sulphide, apatite, and zircon.…”

“…The thermometry results were plotted against P and T using the minimum and maximum values of Ti in zircon and Zr in rutile (Figure 12). Ti in zircon from BR19 yields temperatures between 600°C and 850°C (at P > 19 kbar), a wide interval that encompasses the whole temperature range modelled for the eclogite and granulite facies metamorphism (de Hoÿm de Marien et al, 2020). The results obtained on zircon from BR18 indicate a narrower range of temperatures ( T = 770–860°C at P = 20–22.5 kbar, inferred from the phase‐diagram modelling) above the modelled solidus of the sample.…”

“…Ti‐in‐zircon and Zr‐in‐rutile thermometry results. The P–T path is drawn after pseudosection modelling in de Hoÿm de Marien et al (2020). The hatched areas correspond to the stability fields of the critical mineral assemblages.…”