Sr-Nd-Pb isotopic and major and trace element compositions of the Yufu-Tsurumi volcanic rocks : implications for the magma genesis of the Yufu-Tsurumi volcanoes, northeast Kyushu, Japan

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We have characterized the Sr-Nd-Pb isotopes and major and trace element compositions of 10 Quaternary volcanic rocks from the Yufu and Tsurumi volcanoes in northeast Kyushu, Japan. The enriched incompatible elements, negative Nb, and positive Pb and Sr anomalies are generally interpreted to be from island arc affinities of the lavas. However, the LREE/HREE ratio of the lavas (La/Yb = 8.5 ± 1.3) is greater than that from the island arc intermediate volcanics from northeast Japan (La/Yb = 3.8 ± 0.6), suggesting a different origin. One dacite sample had a Sr/Y ratio > 40, and the SiO2, Al2O3, MgO, Y, Yb, Sr, and 87Sr/86Sr compositions fell within the range of typical adakites. Other lavas were classified as normal island arc-type magmas. These results suggest that a partial melting of the subducting Philippine Sea Plate played a role in the genesis of the Yufu-Tsurumi volcanic rocks. The Sr, Nd, and Pb isotopic compositions show evidence of mixing between melts, derived from oceanic basaltic crust and subducted terrigenous sediments for the adakite magma, whereas another source is required to explain the enrichment in Sr-Nd isotopes and depletion in Pb isotopes of the island arc-type magma. Although the island arc-type magma appears to have a different source, a mixing of the adakite magma with this magma is apparent from our observations of the trace elements and isotopes.
日本鉱物科学会の論文
2006-10-01