Microstructural and Geochemical Evolution of Spherulitic Rhyolitic Dykes in Cheongsong, South Korea

Near Galpyeong Reservoir in northern Cheongsong County, mid-eastern Korean Peninsula, a rhyolitic stock with an elliptical shape is exposed, measuring about 2 km by 1 km. Surrounding this stock, rhyolitic dykes radially intrude the Cretaceous sedimentary rocks within a 4 to 5 km range. These dykes are mainly grayish-blue or brown, with occasional white dykes. They contain well-developed spherulitic textures, with spherulites up to 1 meter or more in diameter. Due to their diverse floral-like shapes, they have been traditionally called "flower stones" and are recognized as a key geosite of the Cheongsong Global Geopark.

The rhyolitic dykes, with widths of 0.5 to 3 m, display non-linear, curved paths. Brown dykes crosscut grayish-blue ones, and larger spherulites occur in dyke cores, while smaller ones are located at the margins. Flow banding, induced by shearing, is more prominent at the margins and in the brown dykes. Elongated cavities are frequently observed along flow bands in brown dykes, but these bands do not penetrate the spherulites. The spherulites in brown dykes are generally larger and often encased in mafic outer crusts, facilitating easy separation from the host rock.

The chemical composition of the rhyolitic dykes corresponds to the subalkaline series of rhyolites. Spherulites show relatively lower SiO₂ content than the matrix, but in the spider diagram, there are minimal compositional differences between the spherulites and the matrix.

Spherulites primarily formed through a combination of radial and spherical quartz growth. Their nucleation centers include quartz, orthoclase, or flow structures, though in some cases, no nucleus is identifiable. Some spherulites exist as individual units, while others are aggregates of smaller spherulites forming larger ones. In the brown dykes, most large spherical spherulites are clusters of numerous smaller ones.

Chemical composition analysis from the center of the spherulites to the matrix indicates SiO₂ and Na₂O contents are highest in the spherulite interiors, decreasing sharply in the nearby matrix and stabilizing at low levels at greater distances. In contrast, Al₂O₃, K, and FeO contents are lowest in the spherulite interiors, peaking at the surfaces, and decreasing gradually to relatively high levels further away.

The zircon saturation geothermometer suggests that in grayish-blue dykes, spherulites and matrix formed at nearly the same temperature, while in brown dykes, spherulites formed at higher temperatures than the matrix.

These findings indicate that the margins of a rhyolitic magma chamber cooled rapidly, enabling spherulites to form under relatively high internal temperatures. The initial dykes likely formed without incorporating chamber spherulites. In a subsequent phase, dykes transported pre-existing spherulites, during which flow structures developed around the already solid spherulites.