Rb−Sr and Sm−Nd isotopic study of the Seoul granitic batholith in middle Korea

Abstract

We report Rb−Sr and Sm−Nd isotope data to better constrain the origin of the Seoul granitic batholith in Uijongbu-Dongduchon-Pocheon area. The dominant rocks of the Seoul granitic batholith intruding the Precambrian Gyonggi massif are biotite granite and garnet biotite granite. Rb−Sr whole rock data define three subparallel lines in an isochron diagram, which is related to geography rather than to lithology. Biotite granite and garnet biotite granite from the northeastern part of the study area give an isochron age of 170.0±5.2 Ma with an initial Sr isotopic ratio of 0.71669±0.00043, while those from the central part give an isochron age of 167±19 Ma with an initial ratio of 0.71280±0.00075. On the other hand, three biotite granites from the northern part, having low Rb/Sr ratio, plot below the isochron of the central samples with a similar slope (∼168 Ma) and have calculated initial Sr isotopic ratios of 0.7102–0.7109. Whole rock Sm−Nd data do not show isochron relationship, nor do they show distinct initial isotopic ratios for different areas as defined by initial Sr isotope ratio. Calculated εNd (T=170 Ma) values range from −15.6 to −19.9 and are generally higher than those of basement gneisses (an average of −23.4) around the batholith. These isotope data indicate that three distinct magmas with different Sr but similar Nd isotope ratios intruded during middle Jurassic time to form the Seoul batholith in the study area. The Sr and Nd isotope data from the basement rocks of the Gyeonggi massif suggest that the initial Sr and Nd isotope data of the Seoul batholith can be explained by mixing of three components: the mantle component and two crust components with high and low⁸⁷Sr/⁸⁶Sr, which rules out the possibility of its origin from direct partial melting of basement rocks, but requires significant mantle input. The isotope data also suggest that geochemical differences (MnO, Rb, Sc, Nb, Y, and Yb contents) between biotite granite and garnet biotite granite in our previous major and trace element study might be related to some sort of fractionation in each magma.