Hydrogen and oxygen isotopic compositions of the granitic rocks in the southern part of the Kyeongsang Basin, Korea.

Abstract

δD and δ18O values of various minerals and whole-rocks from late Cretaceous granitic rocks in the southern part of the Kyeongsang Basin, Korea have been determined. The granitic rocks are subdivided into seven units in four plutons on the basis of petrological features. The analytical results indicate the isotopic exchange occurred extensively between the ancient meteoric water and the granitic rocks. The isotopic compositions of fresh hornblende and quartz suggest that the initial magmatic δD and δ18O values are -62 to -69‰ and +8.3 to +8.9‰, respectively. Based on the empirical fractionation of δD (about -30 to -40‰) between chlorite-rich mica and water, the δD and δ18O values of meteoric water are calculated to be -50 to -60% and -8 to -9‰, respectively. The largest Masan pluton seems to have experienced the water/rock interaction under almost closed system. The maximum water/rock ratio is calculated to be about 0.2. The isotopic exchange between meteoric water and Kimhae-Busan plutons is thought to have extensively occurred under open system. Whether the isotopic exchanges have occurred under closed or open systems in the study area seems to be closely related to the fracture systems developed around the plutons, the pluton sizes and the amounts of circulating meteoric water.