Eclogite facies metaultramafite from the Veporic Unit (Western Carpathians, Slovakia)

Abstract

Metaultramafic rocks closely associated with eclogites in the Veporic unit of the Western Carpathians record a complex P–T evolution, including the effects of high-pressure (HP) metamorphism. The investigated metaultramafite is chemically similar to pyroxenite, has a fineto medium-grained texture, is composed predominantly of olivine and amphibole, and contains minor amounts of garnet, orthopyroxene, spinel, chlorite, ilmenite and carbonates. The highpressure mineral assemblage is garnet (XMg = 0.46–0.47) + olivine (XMg = 0.71–0.73) + low-Al orthopyroxene (XMg = 0.77–0.78; Al = 0.02–0.03 apfu) + ilmenite + chlorite (XMg = 0.87–0.89) + Cr-spinel. Chromium-rich spinel is most likely a relict from the pre-HP metamorphic stage, possibly of magmatic origin. Calculations using a garnet– orthopyroxene Fe–Mg exchange thermometer, Al-in-orthopyroxene barometer, and thermodynamic modelling in the system SiO2–TiO2– Al2O3–FeO–MgO–CaO–H2O indicate that the peak conditions of metamorphism reached 2.4±0.4 GPa and 702±20 °C. Subsequent decompression and retrogression is recorded by the formation of aluminous orthopyroxene, replacement of garnet by symplectites of Al-spinel and amphibole (hornblende), transformation of Cr-spinel to Al-spinel and formation of abundant amphibole in the matrix. Metaultramafic rocks in the Veporic unit thus provide evidence, in addition to that from associated eclogites, for high-pressure metamorphism in the pre-Alpine basement of the Western Carpathians, which is most likely of Variscan age.