ABSTRACT This study investigates garnet–clinopyroxene double‐layered coronae in a metagabbronorite from the high‐grade metamorphic Gföhl Unit (Bohemian Massif). The coronae formed at the interfaces between relic magmatic orthopyroxene, still preserved in the cores of the coronae, and the plagioclase‐rich rock matrix. They comprise an inner clinopyroxene layer and an outer garnet layer, both of which are polycrystalline. The coronae record a polyphase metamorphic evolution. The first metamorphic stage occurred under high‐pressure–high‐temperature (H–H) conditions and produced the garnet–clinopyroxene coronae. The garnet layer exhibits a pronounced overall compositional zoning, with inward‐decreasing grossular and outward‐decreasing pyrope contents, reflecting chemical potential gradients that drove the necessary chemical mass transfer during corona growth. Subsequent decompression led to a granulite‐facies overprint, which is manifest from discontinuous layers of secondary orthopyroxene and plagioclase along the interface between the stage‐I garnet and clinopyroxene layers. In addition, a secondary compositional zoning developed on the scale of individual garnet grains, which is characterised by a rimward decrease in the grossular content and a concomitant increase in the pyrope and almandine contents. The resulting complex compositional zoning of garnet allows distinguishing between the features generated during the early H–H evolution and those associated with decompression. Thermodynamic modelling yields pressure–temperature conditions indicating formation of the primary garnet–clinopyroxene double‐layered coronae at conditions corresponding to a geothermal gradient of approximately 15°C/km, followed by isothermal decompression and, finally, by rapid cooling. The inferred conditions for the H–H stage likely reflect a transitional geodynamic setting involving deceleration of a subducting slab and associated thermal relaxation.
Asenbaum et al. (2026) studied this question.