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

Post-peak-pressure P–T paths of high-pressure units provide important constraints about the processes of exhumation and orogenic building. An isothermal decompression followed by cooling was proposed for high-pressure unit of the Variscan French Massif Central using secondary assemblage in partially retrogressed eclogite. Yet, this path is poorly constrained due to the localised and partial character of retrograde equilibria. While the peak eclogite facies conditions (20–25 kbar, 850–900 °C) were determined by previous work, 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 that the peak high-pressure granulite facies assemblage, mineral chemistry and proportions are best reproduced around 10–15 kbar and 800–1000 °C. Since zircons was not identified in the rock the result of Zr-in-rutile thermometry only indicates a minimum temperature of ~ 680 °C. Modelling the influence of H₂O on the equilibrium assemblage shows that amphibole and titanite were associated to incomplete hydration during an external fluid influx. Titanite stability is modelled at T < 800 °C in a range of pressure of 8–15 kbar, suggesting the replacement of rutile by titanite during cooling and limited decompression. On the other hand, ilmenite is modelled at lower pressure, under 7–8 kbar, suggesting a subsequent decompression along 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) steeper decompression below 8 kbar. Contrary to what was suggested previously, this sequence point to at least two main decompression 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.