K-Ar Age Determination of the Granitic Plutons in Ulsan-Kyeongju Area, the Mid-Eastern Kyeongsang Basin, Korea

K-Ar mineral ages have been determined for late Cretaceous to early Tertiary granitic plutons in Ulsan-Kyeongju area, the mid-eastern Kyeongsang Basin. The granitic plutons in the area can be grouped into two plutonic units; the Ulsan and Kyeongju plutons. The Ulsan plutons consist of hornblende biotite granodiorite, Ulsan granitic complex and porphyritic biotite granite, and the Kyeongju plutons are composed of hornblende biotite granodiorite, biotite granite and alkali granite. K-Ar biotite ages (67∼51 Ma) of the Ulsan plutons are significantly older than those (50∼47 Ma) of the Kyeongju plutons. Combined with the previous results about the cooling history of adjecent plutons and petrological information of the studied plutons, Ulsan hornblende biotite granodiorite and Ulsan granitic complex, which have K-Ar biotite ages older than 62 Ma, are thought to have been emplaced during late Cretaceous. On the other hand, Ulsan porphyritic biotite granite, Kyeongju biotite granite and Kyeongju alkali granite, which show K-Ar biotite ages younger than 54 Ma, seem to have been formed during early Tertiary. However, it is impossible to infer the emplacement age of Kyeongju hornblende biotite granodiorite, because its K-Ar biotite ages are considered to have been reset by later thermal effect. Considering the existence of early Tertiary plutons in the study area, particularly Kyeongju area, is likely to be the youngest granitic province or segment in the Kyeongsang Basin.