Decompression of host-inclusion systems in UHP rocks: insights from observations and models

Polymorphic transformations are key tracers of metamorphic processes, also used to estimate the pressure and temperature conditions reached by a rock. In particular, the quartz-coesite transition is commonly used to define the lower boundary of the ultrahigh-pressure (UHP) metamorphic field. The partial preservation of coesite included in garnets from UHP rocks bring considerable insights into the burial and exhumation mechanisms of the continental crust involved in convergent zone. Coesite was first described in the Western Alps by Chopin^[1], in the Dora-Maria whiteschist, one of the most emblematic UHP rock worldwide. Although the partial preservation of coesite inclusions in garnet has long been attributed to the pressure vessel effect, the interrelationship and relative timing between fracturing and retrogression is still contentious.

Here we study the reaction-deformation relationships of coesite inclusions initially enclosed in garnet and transforming into quartz during the decompression process. We combine 2D numerical thermo-mechanical models constrained by pressure-temperature-time (P-T-t) estimates from the Dora-Maira whiteschist. The model accounts for a compressible visco-elasto-plastic rheology including a pressure-density relationship of silica based on thermodynamic data. This allows us to study the effect of reaction-induced volume increase during decompression. Our results capture the typical fracture patterns of the host garnet radiating from retrogressed coesite inclusions and can be used to study the relative role of volume change associated with a change of P-T conditions on the style of deformation during decompression.

The mechanisms of the coesite-quartz transformation and geodynamic implications are presented and validated against geological data. The effect of fluids on the phase transition and the conditions of access of fluids during the transformation are discussed in the light of the results of the thermo-mechanical models.

This study demonstrates the high potential of thermo-mechanical modelling in enhancing our understanding of the processes involved in the formation and evolution of metamorphic minerals.

 

[1]Chopin (1984) Contributions to Mineralogy and Petrology 86, 2, 107-118