The role of H2O on metamorphism and deformation at high pressure: A combined petrological and thermo-mechanical study based on the Gran Paradiso Unit, Western Alps
Résumé
Quantitative phase petrology analysis of two samples showing variable strain record and different peak pressure (P) conditions show that H2O is a key parameter in the development of contrasting high-pressure (HP) peak conditions in adjacent polymetamorphic rocks. A shear zone in a paragneiss from the Gran Paradiso nappe, Western Alps, shows an Alpine foliation and records a peak P of 1.9 GPa, for a temperature of 500 to 520 °C. A few tens of meters away from the shear zone, a paragneiss showing no apparent Alpine age deformation records a peak P of maximum 1.4 GPa for the temperature range of 500 to 520 °C. The H2O content of the latter has potentially been reduced to low contents following the pre-Alpine, Variscan amphibolite facies, and the absence of re-hydration prior to Alpine orogeny could have inhibited the formation of HP mineral assemblages. The validity of this interpretation is questioned here by considering the mechanical effect of H2O undersaturated rocks on deformation and P during deformation. Based on a thermo-mechanical numerical modelling study, we show that heterogeneities in fluid saturation conditions between rocks lead to strength contrasts that are sufficient to trigger a dynamic P in the range of several hundreds of MPa. In particular, the models successfully reproduce the measured peak P between the two paragneiss studied. This model could be applied to other H2O deficient rocks from HP tectonic units to further explore the role of H2O on the rheology and hence assess its potential impact in the preservation of low P bodies in otherwise HP units from continental collision settings.
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