Marked change of Sr-Nd isotopic compositions of granitoids in Sanin Belt of SW Japan and Gyeongsang Basin of Korea during the latest Cretaceous, and geologic significance

Temporal variations in the whole-rock chemistry and Sr-Nd isotope ratios of Cretaceous-Paleogene granitoids forming composite batholiths in the central Sanin district of SW Japan are examined and compared with correlated granitoids in the Gyeongsang Basin of Korea. The granitoids show marked temporal variations in wholerock chemistry, particularly of some large-ion lithophile and high field-strength element concentrations and Sr-Nd isotope ratios. The Older Inbi intrusive rocks (78-68 Ma) are characterized by having higher K2O, Rb, Y, Zr, and F contents, higher K2O/Na2O ratios, lower MgO, P2O5, and Sr contents, and lower K/Rb and Ti/Zr ratios than the Younger Inbi intrusive rocks (68-53 Ma) and Tamagawa intrusive rocks (44-30 Ma). Initial 87Sr/86Sr ratios (SrI) and εNd(T) values of the Older Inbi intrusive rocks are 0.7052-0.7055 and –1.5 to –0.1, respectively, and those of the Younger Inbi and Tamagawa intrusive rocks are 0.7037-0.7053 and +0.4 to +6.1, respectively. Comparable changes in Sr-Nd isotope ratios at ca. 68 Ma are also observed in the granitoid suite of the Gyeongsang Basin, Korea. It is hypothesized that the granites were derived from partial melting of the lower crust, and that the temporal changes in the isotopic ratios of the lower crust were imparted during the latest Cretaceous. These changes can be explained by the injection of depleted, mantle-derived mafic magmas from the asthenosphere into the lower crust beneath the Sanin Belt of SW Japan and Gyeongsang Basin of Korea at that time.