Microstructure of garnet in calc-silicate rocks from Cheongsong, South Korea: Effects of fluid and deformation

Abstract

Garnet is a common mineral found in metamorphic rocks of various lithologies in the continental basement, subducting slab, and upper mantle. In such environments, fluid–rock interactions, metamorphism, and deformation occur dynamically, disrupting garnet chemical distribution and its microstructure. Investigating the garnet microstructure in fluid-rich environments provides key insights into the role of fluids in the deep crust. In this study, the garnet microstructure in calc-silicate rocks from Cheongsong, South Korea, was analyzed to elucidate its petrofabric evolution during contact metamorphism and concurrent plastic deformation. Three garnet types were categorized based on their microstructures using scanning electron microscopy, electron backscattered diffraction, and electron microprobe analysis. Type I garnet is characterized by garnet clusters formed by multiple nucleation and coalescence at the preferred nucleation site. Type II garnet shows intense deformation due to dislocation creep. Type III garnet exhibits chemically zoned structures with light-colored garnet surrounding dark fragmented garnet, interpreted as fractured relict garnet partly dissolved and replaced by Fe-richer garnet. Transmission electron microscopy images of types II and III garnet showed evidence of dislocation creep. Deformation temperatures were estimated using the quartz c-axis fabric opening angle thermometer, ranging from 708 to 741 ± 50 °C. Considering the water-rich environment and metamorphic mineral assemblage, the actual deformation temperature of calc-silicate rock is suggested to be approximately 650–700 °C, consistent with garnet plastic deformation conditions. The nucleation and deformation processes of garnet suggested in this study provide insights into the petrogenesis and tectonometamorphic processes in contact-metamorphosed rocks within fluid-rich environments.