Secondary compositional zoning of garnet from the high-grade metamorphic Gföhl Unit, Moldanubian Zone: constraints on relative cation diffusivities and geodynamic processes

The garnets in garnet pyroxenites from centimetre- to several-hundred-metre-sized mafic lenses embedded in felsic high-pressure granulites of the Gföhl Unit (Moldanubian Zone, Bohemian Massif) are relics of an early high-pressure–high-temperature metamorphic stage related to Variscan subduction and continental collision. Subsequent isothermal decompression to granulite-facies conditions led to the partial replacement of garnet by plagioclase-bearing assemblages. Associated with the partial replacement, a pronounced secondary compositional zoning developed in the relic garnets, which indicates relatively fast diffusion of Fe and Mg and comparatively slow diffusion of Ca. Based on inverse diffusion modelling, cooling rates in the range of 7–C/Myr were estimated for the garnet pyroxenites, indicating rapid cooling and short-lived granulite-facies overprint after decompression. The petrological evidence is compatible with the extrusion of partially molten, buoyant felsic lithologies, which incorporated slivers of mafic lithologies en route. Through the heat they transported advectively, these lithologies produced perturbations of the thermal structure at mid-crustal levels, the decay times of which varied depending on the volumes of the hot material exhumed in different regions.