Dok island comprises Pliocene volcanic products such as a series of volcanoclastic rocks and lavas ranging in composition from alkali basalts, and trachyandesites to trachytes. Compositional variation of the basaltic rocks can be attributed to fractional crystallization of olivine, clinopyroxene, plagioclase, and magnetite. Chemical variations among the trachyandesites are caused by fractionation of clinopyroxene, plagioclase, and magnetite with minor amphibole, while trachytes are controlled mainly by feldspar fractionation. Incompatible element abundance ratios and chondrite normalized LREE/HREE ratios (e.g., (La/Yb)c: 24.8 to 32.8 for basalts, 15.6 to 31.2 for trachyandesites) suggest that the origins of the basalts and trachyandesites involve both different degrees of partial melting and subsequent fractional crystallization processes. Trace element ratios of the basalts from Dok island are characterized by high Ba/Nb, La/Nb, Ba/Th and Th/U and isotopic ratios (Tasumoto and Nakamura, 1991) that are similar to the EM 1 type of oceanic island basalts such as Gough and Tristan da Cunha basalts.
Cenozoic alkaline volcanic rocks are distributed along the northeast margin of Eurasian plate, possibly the result of regional rifting which formed in response to the subduction of the western Pacific plate beneath the north eastern part of the Eurasian plate. Major, trace, rare earth elements concentrations and Sr, Nd, Pb isotopic systematics are reported for Cenozoic volcanic rocks from Korea (group I), Korea and China border (group II) and NE China (group III). These volcanic rocks are significantly enriched in highly incompatible elements, and are enriched relative to bulk silicate earth (BSE) based on 1 4 3 Nd/ 1 4 4 Nd vs. 8 7 Sr/ 8 6 Sr diagram. The clear negative and positive correlations are observed between 1 4 3 Nd/ 1 4 4 Nd and 8 7 Sr/ 8 6 Sr vs. 2 0 6 Pb/ 2 0 4 Pb ratios of the volcanic rocks. The 2 0 7 Pb/ 2 0 4 Pb and 2 0 8 Pb/ 2 0 4 Pb vs. 2 0 6 Pb/ 2 0 4 Pb plots show linear arrays parallel to the Northern Hemisphere Reference Line (NHRL), and 2 0 8 Pb/ 2 0 4 Pb data points are displaced considerably above NHRL in pattern similar to that of the oceanic island basalts (OIB) that show the Dupal signatures. Trace element ratios and isotopic signatures indicate that the group III rocks were derived from a different magma source which is more enriched compared to that of group I and II. Group I and II rocks have trace element signatures such as Rb/Sr, Sm/ Nd, Ba/Nb and La/Nb ratios are similar to OIB, while group III rocks are characterized by high La/Nb, Th/Nb and Zr/Nb ratios similar to EM I OIB. Geochemical data including Sr, Nd and Pb isotopic compositions of the group I and II rocks suggest two mantle reservoirs in the origin of the rocks: asthenospheric MORB-like sources and EM lithospheric component. Group III rocks are typical of magma derived from a reservoir predominantly consisting EM I component. The subcontinental lithospheric mantle source similar to the postulated EM 1 component must have played a significant role in magma generation of group III rocks.
