Metamorphic Evolution of the Baekdong Metabasite in the Hongseong Area, South Korea and its Relationship with the Sulu Collision Belt of China

Abstract

Ultramafic rocks in the Hongseong area, southwest Gyeonggi massif, South Korea, occur as lenses within Precambrian granitic gneiss. The ultramafic bodies contain metabasite boudins that have undergone at least three stages of metamorphism. The first stage mineral assemblage is garnet + augite (X Jd+Aeg = 0.06–0.11) + hornblende + plagioclase + rutile + titanite, and represents a transitional condition between the eclogite (EG), high-pressure granulite (HG) and amphibolite facies (AM). The second stage is characterized by an assemblage of garnet + augite (X Jd+Aeg = 0.01–0.06) + hornblende + plagioclase + titanite, representing the transitional condition between low-pressure granulite facies (LG) and AM. A third stage is identified by an AM assemblage of hornblende + plagioclase ± garnet in symplectites formed during garnet breakdown. The P-T conditions of the first, second, and third stages are 11.8–16.3 kb 690–780°C, 8.2–8.7 kb 660–820°C, and 4.0–6.3 kb 490–610°C, respectively. Cores and rims of two garnets in the country rock granitic gneiss give P-T estimates of 8.9–13.5 kb 640–815°C and 5.5–6.3 kb 575–680°C, indicating a retrograde P-T path similar to that of the metabasite boudins. Ti-Zr-Y data indicate that the Baekdong metabasites probably had an island arc paleotectonic setting. While it may be concluded that the Hongseong area in the Gyeonggi massif is a possible extension of the Sulu collision belt in China, Sm-Nd whole-rock - garnet isochron ages of the Baekdong metabasites (268–297 Ma) are significantly older than the age of UHP metamorphism (208–245 Ma) in the Dabie-Sulu collision belt. Hence, collision between the North and South China blocks may have occurred earlier in the Korean peninsula than in China.