The high-pressure stability of hydrous phases in orogenic belts: an experimental approach on eclogite-forming processes

• Published in: Tectonophysics 1997-05, Vol.273 (1), p.169-184
• Main Authors: Poli, S., Schmidt, M.W.
• Format: Article
• Language: English
• Subjects: amphibole; eclogite; experimental study; lawsonite; water
• ISSN: 0040-1951; 1879-3266
• DOI: 10.1016/S0040-1951(96)00293-4

Metamorphic evolution of orogenic belts is characterized by reactions involving hydrous phases. Stability relations of hydrous phases depend on H 2O-activity/availability during metamorphism; however, hydrous phases do occur in synthetic and natural systems also at water-undersaturated conditions. H 2O-undersaturated experiments in the CaOAl 2O 3SiO 2H 2O model system were performed in order to predict the role of hydrous phases in H 2O-undersaturated compositions. At constant pressure and temperature, these experiments yielded different assemblages of hydrous + anhydrous phases depending on the experimental bulk H 2O-content. In natural eclogites of mafic to intermediate bulk composition common hydrous phases include amphibole, lawsonite, zoisite, Mg-chloritoid, paragonite, and talc. Complex continuous reactions delimit the stability fields of the different assemblages involving these hydrous phases. Such continuous reactions can be used as powerful petrogenetic tools to derive P-T conditions of natural eclogites. Derivation of H 2O-undersaturated phase relationships for natural and synthetic systems show that hydrous phases also occur in H 2O-undersaturated bulk compositions at eclogite facies conditions. Comparison of experimentally determined phase relationships in mafic rocks with pressure-temperature evolution of orogenic terrains demonstrates that at subsolidus conditions a trend toward H 2O-saturation is necessarily maintained during prograde P-T paths in orogenic belts. As far as dehydration reactions occur, any prograde P-T path necessarily leads to a maximized occurrence of hydrous phases and thus a maximized H 2O-preservation is achieved during metamorphism.