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Abstract: We have determined the Sr, Nd, Pb and Hf isotopic compositions of 13 samples from six late Cenozoic centers of basaltic volcanism in South Korea, including Baengnyeong Island, Jogokni, Ganseong, Jeju Island, Ulleung Island and Dok Island, in order to understand the nature of the mantle source and melt-generating processes. The basalts have OIB-like trace element abundance patterns, and also contain mantle-derived xenoliths of primarily spinel peridotite. Combining our new isotopic data with previously published data from areas with late Cenozoic basalts throughout East Asia shows that the mantle source has a DMM–EM1 array for northeast China and a DMM–EM2 array for Southeast Asia. The Korean Peninsula basalts we have studied are geographically located between these two broad domains, and interestingly, define an array between DMM and an intermediate end member between EM1 and EM2 on various isotopic correlation diagrams. Variations along the array on isotopic correlation diagrams are systematic regionally, with EM2 signatures predominant on Jeju Island and EM1 becoming increasingly more important toward Ulleung and Dok Islands, the Ganseong area, and Baengnyeong Island. This is without any corresponding changes in the basement and the lithospheric mantle beneath the region. These observations suggest that the Korean Peninsula late Cenozoic basalts we have studied and other East Asia late Cenozoic intraplate volcanism are sourced in the asthenospheric mantle. This mantle is characterized by two distinct, large-scale domains — one a mixture of DMM and EM1 components, and the other a mixture of DMM and EM2. Previous studies on East Asian Cenozoic volcanic rocks have advocated origins by either plume activity or decompressional melting of the mantle lithosphere in a rift environment. On the basis of our new trace element and isotopic compositions, which have OIB-like characteristics, we rule out an origin in the mantle lithosphere. Furthermore, because of the absence of typical hotspot tracks with clear age progressions, and so far, lack of unequivocal tomographic images delineating distinct, deep-seated thermal anomalies that could be interpreted as plumes, we suggest that the late Cenozoic alkalic magmatism in East Asia originated in the shallow asthenosphere, largely in response to dramatic changes in stress regimes. The interplay between the India–Eurasia collision and subduction of the Pacific Plate beneath the eastern margin of Eurasia resulted in a late Cenozoic tensile setting in East Asia capable of inducing melting in the asthenosphere by decompression. This is contrary to previous studies which advocated a plume or a mantle lithospheric origin for the East Asia late Cenozoic alkalic volcanism. [Copyright &y& Elsevier]

Abstract: We report the Sr, Nd and Pb isotopic ratios of clinopyroxene separates from spinel peridotite xenoliths hosted by late Cenozoic alkali basaltic rocks from Baengnyeong Island, Jogokni, and Jeju Island, all near the southeastern margin of the Sino-Korean Craton in South Korea. Measured Sr, Nd and Pb isotopic compositions in the clinopyroxene separates show, in general, simple trends between depleted MORB-like mantle (DMM) and relatively enriched components, which can be explained by old event(s) of infiltration by metasomatic melts/fluids that enriched a precursor time-integrated depleted mantle in incompatible trace elements and overprinted its isotopic composition. Comparison of the xenoliths with the host basalts suggests that infiltration of carbonatitic melts can explain the characteristics of the Baengnyeong xenoliths while reaction with residual slab-derived melt/fluid subjected to amphibole and phlogopite fractionation explains the Jogokni xenoliths best, ruling out direct influence of the magmatism that produced the host basalts. On the other hand, the Jeju clinopyroxene separates display a mixing hyperbola between DMM and the host basalts in Sr–Pb and Nd–Pb isotope correlation diagrams, suggesting some influence by liquids similar to the host basalts. The Korean xenoliths tend to be similar in the overall range of their mineralogic modes, elemental compositions, and Sr–Nd–Pb isotopic ratios to those of the fertile peridotite xenoliths derived from the post-Archean lithospheric mantle of eastern China, and yet without any compositional evidence for a refractory Archean mantle root. Overall comparison of the measured isotopic compositions for these Korean xenoliths with available data on lavas from eastern China, indicates that the dominant enriched mantle source components for the Cenozoic intraplate volcanism in the East Asia (EM1 for northeast China, and EM2 for Southeast Asia) are absent in the lithospheric mantle beneath the region. The Cenozoic intraplate volcanism, therefore, must have originated in the asthenosphere. [Copyright &y& Elsevier]