ABSTRACT

Major and trace element and Nd-Sr isotopic data are presented on the granitic rocks known as a basement in the Cheju volcanic island, together with CHIME age data on porphyritic hornblende-biotite granite xenolith from the Byeoldobong area. The CHIME ages from zircon and allanite grains of porphyritic hornblende-biotite granite were determined to be 169 ± 29 Ma and 183 ± 40 Ma, respectively, which are corresponding to a Jurassic age. It is concordant with a K-Ar age of 172.4 ± 5.2 Ma of a biotite granite in the same area, which is comparable to the Daebo granite in the Korean peninsula. Granitic xenoliths from the Byeoldobong area have Nd-Sr isotopic compositions (initial ratio (¹⁴³Nd/¹⁴⁴Nd)i = 0.51156∼0.51158, ε_Nd = −19∼−14 and (⁸⁷Sr/⁸⁶Sr)i = 0.7156∼0.7164, ε_Sr = 101∼179) which are close to those of Jurassic Daebo granite (ε_Nd = −21∼−14, ε_Sr = 104∼171). On the other hand, the micrographic granite from the drilling core in the Hadeokcheon area gives initial ratio (¹⁴³Nd/¹⁴⁴Nd)i = 0.51218 and (⁸⁷Sr/⁸⁶Sr)i = 0.7077), which are comparable to the Cretaceous Bulguksa granite ranging from 0.51184 to 0.51268 in ¹⁴³Nd/¹⁴⁴Nd and 0.7055 to 0.7167 in ⁸⁷Sr/⁸⁶Sr. The T^Nd_DM model ages of 1.0 to 1.7 Ga from the Mesozoic granitic plutons in the Cheju island suggest that the source materials of the plutons in the island have evolved since mid-Proterozoic time as those of the peninsula. In combination with the basement geology and petrochemistry, Nd-Sr isotopes, and radiometric and model ages of granitic rocks, our results imply that the Okchon zone is continuously extended to the Cheju volcanic Island of the southeastern end of the continental margin of the Eurasian plate.