Host: Abstracts of Annual Meeting of the Geochemical Society of Japan
Pages 211-
Late Cenozoic intraplate basaltic rocks in northeastern China have been interpreted as being derived from a mantle source composed of DMM (depleted MORB mantle) and EM1 (enriched mantle type 1) components. To constrain the origin of the enriched mantle component, we have now determined the geochemical compositions (including Sr, Nd, Pb, Hf, and Mg isotopes) of basaltic rocks from the active Mt. Baekdu volcano on the border of China and North Korea. Also obtained are major and trace element concentrations of olivine-hosted rehomogenized melt inclusions in the basalts. The samples show light rare earth element (LREE)-enriched patterns with (La/Yb)N ratios of 3.2–20.0. However, most of the samples have positive Eu and negative Ce anomalies. On a primitive-mantle-normalized trace element distribution diagram, they show typical oceanic island basalt (OIB)-like large-ion lithophile element (LILE) enrichments without significant Nb or Ta depletions. However, compared with OIB, they show enrichments in Ba, Rb, K, Pb, Sr, and P. In addition, the CaO and P2O5 contents of the melt inclusions are positively correlated, indicating the presence of tuite and K-hollandite in the source. The Nb/U ratios of the basalts are generally within the range of OIB, but the Ce/Pb ratios are lower than those of OIB. The Mt. Baekdu basalts do not show meaningful correlations between SiO2 (wt%) and Sr–Nd–Hf isotopic compositions, suggesting a lack of crustal contamination, and this means, therefore, that the low Ce/Pb ratios could be a characteristic of the mantle source. They have elevated Gd/Yb ratios within the range of melts from garnet peridotite. Olivine phenocrysts are characterized by low Ca and high Ni contents when compared with olivines from peridotite-derived melts for a given Fo value. The radiogenic isotopic characteristics (87Sr/86Sr = 0.70449 to 0.70554; εNd = –2.0 to +1.8; εHf = –1.7 to +6.1; 206Pb/204Pb = 17.26 to 18.12) suggest derivation from an EM1-like source together with an Indian MORB-like depleted mantle. The Mg isotopic compositions (δ26Mg = –0.39 ± 0.17‰) are generally lower than the average upper mantle (δ26Mg = –0.25 ± 0.04‰), indicating carbonates in the source. The 87Sr/86Sr ratios decrease with decreasing δ26Mg values whereas the 143Nd/144Nd and (Nb/La)N ratios increase. These observations suggest the mantle source of the Mt. Baekdu basalts contained at least two components that resided possibly in the mantle transition zone (MTZ): (1) recycled subducted ancient (~2.2 to 1.6 Ga) terrigenous silicate sediments, possessing EM1-like Sr–Nd–Pb–Hf isotopic signatures and relatively high values of δ26Mg; and (2) carbonated tuite-bearing eclogites with relatively MORB-like radiogenic isotopic compositions and low values of δ26Mg. These components might have acted as metasomatizing agents in refertilizing the asthenosphere, eventually influencing the composition of the MTZ-derived plume that produced the Mt. Baekdu volcanism.
