Several episodes of magmatic activity recorded within the Tertiary sedimentary basins in the southeastern part of Korea are interpreted to have formed in a framework of separation of Japan arc from Eurasia. The volcanic rocks are Early Eocene or Early-Middle Miocene in age, showing a distinct chronological gap. The Miocene volcanics are regionally divided into two groups along the Hyeongsan fault; younger Daljeon basalts (13.6-15.2 Ma, K-Ar age) within the Pohang basin, north of the fault, and older volcanics (16-22 Ma) within the Janggi, Eoil and Haseo basins, south of the fault. The Daljeon basalts contain olivine as a major phenocryst, and are alkalic, ne-normative, trachy basaltic, “Within Plate Basalt”-like and enriched in incompatible elements. The older volcanics have plagioclase as a major phenocryst, and are subalkalic, q-normative, andesitic, “Arc Basalt”-like, and depleted in incompatible element. ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios of the younger basalts are 0.7044-0.7045 and 0.512831-0.512835, respectively, whereas those of the older andesitic rocks are 0.7041-0.7048 and 0.512643-0.512809, respectively. Sm/Nd and Rb/Sr ratios of the basalts 0.154-0.158 and 0.026-0.034 whereas those of the andesitic rocks are 0.013-0.298 and 0.052-0.123, respectively. Of the andesitic rocks, samples of the Eoil basin are more refractory. Campared with available data, the Daljeon basalts are geochemically and isotopically similar to Enriched mantle type ocean island basalt (OIBs) or worldwide intraplate alkaline basalts, whereas the andesitic rocks are similar to those from the andesites of the Andes, South America. Overall characteristics suggest that source magma of the Daljeon basalts may be derived from upwelling asthenospheric magma with additional fractional crystallization. On the other hand, genesis of the andesitic rocks can be explained through a complex history at an active continental margin. Some geochemical differences among the andesitic rocks may be explained with different degrees of fractional crystallization and heterogeneity of initial source compositions. Based on the separation history of the Japan arc from Eurasia, magmatism of the Tertiary sedimentary basins can be explained by andesitic succeeded by alkali basaltic activity, after intermittent cessation of subduction due to changes of plate motion.

抄録全体を表示