Garnet–Clinopyroxene Double-Layered Coronae in a Metagabbronorite From the High-Grade Metamorphic Gföhl Unit, Moldanubian Zone

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 (Formula presented.) –H (Formula presented.)) 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 (Formula presented.) –H (Formula presented.) 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 (Formula presented.) –H (Formula presented.) stage likely reflect a transitional geodynamic setting involving deceleration of a subducting slab and associated thermal relaxation.