Garnet peridotites from Pohorje: Petrography, geothermobarometry and metamorphic evolution

Abstract

Ultrahigh-pressure (UHP) metamorphism has been recorded in Eo-Alpine garnet peridotites from the Pohorje Mts., Slovenia, belonging to the Eastern Alps. The garnet peridotite bodies are found within serpentinized metaultrabasites in the SE edge of Pohorje and are closely associated with UHP kyanite eclogites. These rocks belong to the Lower Central Austroalpine basement unit of the Eastern Alps, exposed in the proximity of the Periadriatic fault system. Garnet peridotites show signs of a complex four-stage metamorphic history. The protolith stage is represented by a low-P high-T assemblage of olivine + Al-rich orthopyroxene + Al-rich clinopyroxene + Cr-spinel. Due to metamorphism, primary clinopyroxene shows exsolutions of garnet, orthopyroxene, amphibole, Cr-spinel and ilmenite. The UHP metamorphic stage is defined by the assemblage garnet + olivine + Al-poor orthopyroxene + clinopyroxene + Cr-spinel. Subsequent decompression and final retrogression stage resulted in formation of kelyphitic rims around garnet and crystallization of tremolite, chlorite, serpentine and talc. Pressure and temperature estimates indicate that garnet peridotites reached the peak of metamorphism at 4 GPa and 900 °C, that is well within the UHP stability field. Garnet peridotites in the Pohorje Mountains experienced UHP metamorphism during the Cretaceous orogeny and thus record the highest-pressure conditions of all Eo-Alpine metamorphism in the Alps.