Discrepancy between equilibrium thermodynamics-based P-T calculations and quartz-in-garnet elastic barometry in coesite-bearing eclogite (Śnieżnik Massif, NE Bohemian Massif)

The Śnieżnik Massif forms the eastern part of the Orlica-Śnieżnik Dome (OSD), located in the north-eastern part of the European Variscan Belt. The OSD, which exposes the root zone of the Variscan Orogen, comprises mostly orthogneisses containing small bodies of ultra-high pressure (UHP) eclogites. Previous studies on the metamorphic conditions recorded by these eclogites yielded inconsistent results. Some authors suggest that they were metamorphosed in conditions of ~1.9-2.2 GPa and ~700-750 °C ¹. Others, however, argue that the eclogites experienced nearly-UHP peak metamorphic conditions of ~2.6-3.0 GPa and 800-930 °C.²

This study provides the first evidence of UHP metamorphic episode recorded in eclogites from the OSD, as coesite inclusions were discovered in garnet and omphacite grains. This finding is consistent with our results obtained using Grt-Cpx-Ky-Ph-Coe/Qtz geothermobarometry and phase equilibria modelling, which both indicated conditions of peak metamorphism of ~2.8 – 3.2 GPa and ~830-870 °C, partially overlapping the coesite stability field.

We also applied quartz-in-garnet elastic barometry to provide additional constraints on the pressure conditions of metamorphism. About 60 inclusions of quartz were identified using Raman spectroscopy. The residual pressure calculated from the spectral shifts of 464 cm^-1 characteristic quartz Raman band reaches a maximum of ~0.73 GPa. This corresponds to the entrapment pressure of ~2.1 GPa, calculated based on the elastic solution for an isotropic spherical inclusion. This estimation contradicts the results coming from methods based on equilibrium thermodynamics. Moreover, such low peak pressure would not explain the presence of the observed coesite inclusions. We hypothesize that the discrepancy might be related to viscous relaxation of garnet host grains under such high peak metamorphic temperatures.

 

References

[1]     Štípská, P. et al. The juxtaposition of eclogite and mid-crustal rocks in the Orlica-Śnieżnik Dome, Bohemian Massif. J. Metamorph. Geol. 30, 213–234 (2012).

[2]     Majka, J. et al. Integrating X-ray mapping and microtomography of garnet with thermobarometry to define the P-T evolution of the (near) UHP Międzygórze eclogite, Sudetes, SW Poland. J. Metamorph. Geol. 37, 97–112 (2019).