# Step-by-step exhumation of a high-pressure granulite revealed by sequential replacement of Ti-bearing mineral (Variscan French Massif Central)

Luc de Hoÿm de Marien, Pavla Štípská, Pavel Pitra, Andrew Kylander-Clark, Vít Peřestý, Martin Racek, Jonas Vanardois

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Luc de Hoÿm de Marien, Pavla Štípská, Pavel Pitra, Andrew Kylander-Clark, Vít Peřestý, et al. Step-by-step exhumation of a high-pressure granulite revealed by sequential replacement of Ti-bearing mineral (Variscan French Massif Central). 29 ème édition de la réunion des sciences de la Terre (RST 2025), Société Géologique de France (SGF), Oct 2025, Montpellier, France. pp.659022. insu-05376535

HAL Id: insu-05376535

https://insu.hal.science/insu-05376535v1

Submitted on 21 Nov 2025

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Step-by-step exhumation of a high-pressure granulite
revealed by sequential replacement of Ti-bearing
mineral (Variscan French Massif Central)

Luc De Hoÿm De Marien*¹, Pavla Štípská¹, Pavel Pitra², Andrew Kylander-Clark³, Vít Peřestý¹, Martin Racek⁴, and Jonas Vanardois⁵

¹Czech Geological Survey – République tchèque

²Observatoire des sciences de l'environnement de Rennes – Université de Rennes, Institut National des Sciences de l'Univers, Université de Rennes 2, Centre National de la Recherche Scientifique, Institut National de Recherche pour l'Agriculture, l'Alimentation et l'Environnement – France

³Department of Earth Science [Santa Barbara] – États-Unis

⁴Institute of petrology and strucRAL geology, Charles University, Prague – République tchèque

⁵Institut des sciences de la terre [Lausanne] – Suisse

Résumé

Post-peak-pressure P–T paths of high-pressure units provide important constraints about the processes of exhumation and orogenic building. The P–T path of a high-pressure unit of the Variscan French Massif Central is poorly constrained from symplectites in former eclogites. While the peak eclogite facies conditions (20–25 kbar, 850–900 °C) were previously determined, the present study of a mafic granulite from the same unit provides further details about the subsequent P–T evolution.

The studied sample consists of pristine high-pressure granulite facies domains of garnet–diopside–plagioclase grading to domains where amphibole is common in replacement textures. Ti-bearing accessory minerals are rutile, titanite or ilmenite. Rutile is included in garnet, plagioclase and titanite, whereas titanite and ilmenite occur in the matrix. Titanite is commonly texturally related to amphibole, suggesting the introduction of a fluid. Titanite is partially or totally replaced by vermicular ilmenite. The observations constrain the sequential replacement of rutile by titanite, followed by the replacement of titanite by ilmenite.

Phase equilibrium modelling indicates the conditions 10–15 kbar and 800–1000 °C for the peak high-pressure granulite facies event. Amphibole and titanite were associated with incomplete hydration during the introduction of a fluid. Titanite stability is modelled at T < 800 °C in a range of pressure of 10–15 kbar, suggesting the replacement of rutile by titanite during cooling and limited decompression. On the other hand, ilmenite is modelled at lower pressure, below 7–8 kbar, suggesting a subsequent decompression along a steeper P–T path.

Petrological data and P–T modelling suggest three metamorphic stages during the exhumation: 1) decompression from the eclogite (20–25 kbar, 850–900 °C) to the granulite facies (10–15 kbar, 800–1000 °C); 2) cooling under 800 °C with limited decompression; and 3)

<footnote>*Intervenant</footnote>
steeper decompression below 8 kbar. This sequence points to at least two main decompres-
sion stages separated by cooling. This sequence is compatible with exhumation from mantle
to crustal depth, followed by partial cooling in the lower orogenic crust and subsequent
crustal thinning or redistribution within the crust.