Pohorje eclogites revisited: Evidence for ultrahigh-pressure metamorphic conditions

Kyanite eclogites from the Pohorje Mountains, Slovenia, are providing the first evidence of ultrahigh-pressureEo-Alpine metamorphism in the Eastern Alps. Polycrystalline quartz inclusions in garnet, omphacite and kyaniteare surrounded by radial fractures and exhibit microtextures diagnostic for the recovery after coesite breakdown.The non-stoichiometric supersilicic omphacites found in Pohorje eclogites contain up to 5 mol % of Ca-Eskola molecule.Such clinopyroxenes are known to be stable exclusively at high-pressure conditions exceeding 3 GPa. Theirbreakdown during decompression resulted in exolution of quartz rods and needles that are oriented parallel toomphacite c-axis. The absence of coesite is a consequence of near-isothermal decompression during the first stagesof exhumation.Pressure and temperature conditions for the formation of the peak metamorphic mineral assemblages have beenassessed through a consideration of a) Fe2+-Mg partitioning between garnet and omphacite pairs, based on differentcalibrations; b) the equilibrium between garnet + clinopyroxene + phengite ± kyanite ± quartz/coesite assemblage.Estimated peak pressure and temperature conditions of 3.0-3.1 GPa and 750-783 °C are well within the coesite, i.e.the ultrahigh-pressure stability field.