On the Korean Peninsula, located in the eastern region of East Asia, the Gyeongsang Basin, a representative sedimentary basin formed during the Cretaceous Period, occupies about a fourth of South Korea. In the Gyeongsang Basin, the Kusandong Tuff (ca. 103 Ma; Kim et al., 2013), a representative marker bed, is distributed across approximately 200 km from north to south. About 2.5 m-thick tuff layer also develops in Sinsu and Changseon islands, located in the southern part of the basin. There is controversy as to whether it is the southernmost extension of the Kusandong Tuff or a separate tuff body. To resolve this, a total of 4 samples were collected from two islands, and LA-MC-ICP-MS zircon U-Pb dating was performed. As a result, all samples were slightly contaminated with common lead, and lower intercept ages of ca. 99 Ma were calculated, indicating a systematically younger than the reported ages of Kusandong Tuff (ca. 103 Ma). And zircons used in age calculations are largely divided into two domains accoring to the significant differences in brightness in cathodoluminescence (CL) images. Based on the combination of these domains, zircons show: 1) dark overall, 2) dark in the core with bright rims (known as reverse zoning), and 3) only bright oscillatory zoning with/without some inherited cores. In general, dark domains have a uranium content of ca. 3000-7000 ppm, while bright ones range from 34-541 ppm. And in the CL image, some boundary of the dark domains in the cores had melted and infiltrated by bright ones. To understand the evolutionary history of the magma that formed these zircons, trace and rare earth elements were analyzed using LA-ICP-MS. The results indicate that the dark domains of zircons were formed in relatively highly evolved and cooler magma, while the bright domains of zircons were formed in less evolved and hotter magma. To summarize the magma evolution model, crystallization of dark domains occurred first in relatively highly evolved magma, and then domains showing bright and distinct oscillatory zoning were formed with the injection of less evolved magma from deeper sources. At the same time, bright domains grew on the rims of existing dark zircons due to the injected magma. It is thought that the new magma injected into the existing magma chamber caused an increase in internal pressure, which ultimately led to the volcanic eruption. Considering the dating results, it is believed that all of these processes occurred over a short period of time, roughly limited to about 99 Ma on the geological time scale.
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