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