Petrogenetic Evolution of Basaltic Volcanism within the Tertiary Basins of Southeastern Korea and the Opening of the East Sea (Sea of Japan)

We determined the geochemical compositions of volcanic rocks within the Tertiary basins of SE Korea to better understand the petrogenetic evolution of local volcanism. The volcanic rocks are divided into two major groups: Eoil from the Yangnam Basin, and Daljeon from the Pohang Basin. The Eoil rocks (16-22 Ma K-Ar ages) are primarily subalkaline basalt to basaltic andesite, whereas the Daljeon samples (13-15 Ma K-Ar ages) are sodic alkaline phonotephrites. The Eoil rocks exhibit light rare earth element (LREE)-enrichment patterns with chondrite-normalized (La/Yb)N ratios of 3.6-5.0. Daljeon samples are typified by significant LREE-enrichment [(La/Yb)N = 32.0-33.2] and slight negative Sm and Eu anomalies on the REE patterns. The Eoil samples show an arc signature on the primitive mantle-normalized multi-element variation diagrams, with high large-ion lithophile element (LILE) enrichment and the depletion of high field strength elements (HFSE). In contrast, the Daljeon samples are dominated by high ratios of Nb and Ta to LILE and HREE, resembling typical oceanic island basalt but distinguished by a significant trough in K, Rb, Sm, and Ti, and a positive anomaly in P. In Sr-Nd isotope space, all samples fall in the depleted quadrant, but the Eoil rocks are characterized by lower 143Nd/144Nd ratios at a given 87Sr/86Sr ratio than those of the Daljeon phonotephrites. The Eoil rocks have elevated 176Hf/177Hf ratios compared with the Daljeon samples, exhibiting a decoupling of Nd and Hf isotopes in Hf-Nd space. These observations, together with the Eoil rocks showing more radiogenic Pb isotopic compositions than the Daljeon samples, indicate two distinct mantle sources for the volcanism. The source of Eoil magmatism might be mantle wedge metasomatizatized by the addition of subducted pelagic sediment melt, whereas hydrous mineral-bearing sub-continental lithospheric mantle could be the dominant source for the Daljeon volcanism. The difference may be related to changes in the geodynamic setting induced by a back-arc opening in the East Sea, which culminated at around 15 Ma. Lithospheric thinning caused by rifting produced the East Sea and the resultant thermal effect of asthenospheric upwelling may have initiated partial melting of the metasomatized lithospheric mantle, producing the Daljeon volcanism.