Rb-Sr and K-Ar age determinations of the granitic rocks in the southern part of the Kyeongsang basin, Korea: Implication for cooling history and evolution of granitic magmatism during late Cretaceous.

Abstract

Abstract-Rb-Sr whole rock isochron ages and K-Ar mineral ages have been determined for late Cretaceous granitic rocks distributed in the southern part of the Kyeongsang basin, Korea. The granitic rocks are grouped into seven in four areas (Masan, Jinhae, Kimhae and Busan) based on their petrological features. In the five plutons, Rb-Sr whole rock isochron ages and isotopic initial ratios were determined as follows: Rb-Sr whole rock age Initial Sr ratioMasan Hbbtgr 100.1 ± 7.1 Ma 0.70489 ± 0.00012Jinhae Btgr 70.5 ± 1.9 Ma 0.70497 ± 0.00006Kimhae Hbbtgr 83.9 ± 9.0 Ma 0.70538 ± 0.00021Busan Btgr (YS) 70.6 ± 4.2 Ma 0.70531 ± 0.00029Busan Btgr (KP) 68.8 ± 3.4 Ma 0.70579 ± 0.00028 Rb-Sr whole rock ages become younger toward more felsic and smaller plutons, which is thought to reflect progressive lowering in melting degree at the source with time. Initial Sr ratios of each pluton progressively increase from the western to eastern areas, regardless of their ages. This means that the granitic magmas have been more evolved toward the eastern area due to much contribution from lower crustal materials. K-Ar ages of biotite are about 3 to 15 m.y. younger than Rb-Sr whole rock isochron ages in each plutonic unit. Average cooling rate of each body is directly proportional to the volume of granitic body. The conduction-loss of heat due to large thermal contrast and the circulating meteoric water between a granitic body and the country rock at shallow levels are thought to have directly affected the average cooling rate of each body. Initial Sr isotopic ratios of the granitic rocks in the study area are well correlated with those of the central to northern Kyushu and the North Zone of Southwest Japan. Rb-Sr isochron ages of the Cretaceous granitic rocks became younger from central Kyushu (120-100 Ma) through northern Kyushu (115-90 Ma) to southern part of the Kyeongsang basin (100-70 Ma). The granitic magmatism having 70 Ma ages on the eastern side of the Yangsan Fault is thought to be better correlated with those of the North Zone of Southwest Japan.