He-Ar and Nd-Sr isotopic compositions of ultramafic xenoliths and host alkali basalts from the Korean peninsula

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Author(s)

    • Kim Kyu Han
    • Department of Science Education, Ewha Womans University
    • Nagao Keisuke
    • Laboratory for Earthquake Chemistry, Graduate School of Science, University of Tokyo
    • Sumino Hirochika
    • Laboratory for Earthquake Chemistry, Graduate School of Science, University of Tokyo
    • Okuno Mitsuru
    • Department of Earth System Science, Faculty of Science, Fukuoka University
    • Song Jeehye
    • Department of Science Education, Ewha Womans University

Abstract

Noble gas, Nd and Sr isotopic ratios and major and trace element compositions were determined for ultramafic xenoliths and their host Cenozoic alkali basalts from Baegdusan, Baegryongdo, Jogokri, Jejudo in the Korean peninsula, and Long Quan, close to the Baegdusan in northeastern China, to characterize the lithospheric mantle and the source of alkali basaltic magmatism beneath the active continental margin of the southeastern part of the Eurasian plate. The xenolith samples yield significantly variable <SUP>3</SUP>He/<SUP>4</SUP>He ratios ranging from <0.2 to 16.8 R<SUB>A</SUB>, with most samples (3.5-7.9 R<SUB>A</SUB>) lower than the MORB value (~8 R<SUB>A</SUB>). Among them, high <SUP>3</SUP>He/<SUP>4</SUP>He ratios obtained by melting the samples are considered to reflect cosmogenic contribution. The <SUP>40</SUP>Ar/<SUP>36</SUP>Ar ratios are much lower than the MORB values. Enriched Nd-Sr isotopic compositions in xenoliths and host basalts from the Baegdusan and Baegryongdo areas suggest assimilation of EMII lithosphere and/or continental crust. Widely ranging trace element concentrations in the xenoliths and highly saturated incompatible elements in the host alkali basalts are observed. K-Ar age data show that Cenozoic alkali volcanism in the Korean peninsula occurred intermittently, ranging in age from 21 Ma through 11.5-5.0 Ma to 0.1 Ma, and becoming gradually younger toward the south of the peninsula. Our geochemical and isotopic data suggest the presence of heterogeneously metasomatized enriched lithospheric mantle generated at an ancient subduction zone within the continental margin of the southeastern end of the Eurasian plate. Degree of enrichments by the metasomatism is discussed based on the observed <SUP>3</SUP>He/<SUP>4</SUP>He ratios in the xenolithic olivines.

Journal

  • GEOCHEMICAL JOURNAL

    GEOCHEMICAL JOURNAL 39(4), 341-356, 2005

    GEOCHEMICAL SOCIETY OF JAPAN

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