Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 1, pp. 43-58Article

Geochemical and Sr-Nd-Pb Isotopic Constraints on the Origin andMagmatic Evolution of Quaternary Lavas ofSakurajima Volcano, Southern Kyushu Island, Japan
Tomoyuki SHIBATA＊, Jun SUZUKI＊, Masako YOSHIKAWA＊, Tetsuo KOBAYASHI＊＊,Daisuke MIKI＊＊＊ and Keiji TAKEMURA＊(Received October 5, 2011 ; Accepted January 7, 2013)
We present the results of a detailed petrogenetic study employing newly determined whole-rock major and traceelement geochemical analyses and Sr-Nd-Pb isotopic compositions of andesitic and dacitic Quaternary lavas of
Sakurajima volcano, a post-caldera volcano situated within the Aira caldera of Japan. Similar geochemical and isotopic
investigations are also carried out on basaltic rocks from pre-caldera stage and monogenetic volcanoes from near
Sakurajima volcano. Quaternary lavas of Sakurajima volcano analyzed in this study are classified as porphyritic
andesites or dacites that contain a mineral assemblage of orthopyroxene, clinopyroxene, and plagioclase, with orwithout olivine, in a groundmass exhibiting either hyalo-ophitic or hyalopilitic textures. The trace element
characteristics of these samples are similar to those of typical island arc magmas, showing clear evidence of Nb
depletion along with enrichments in Rb, K, and Pb, which suggests the addition of aqueous fluids to the mantle wedge
during melt generation. The Sr, Nd, and Pb isotopic compositions plot close to a mixing curve between MORB-type
mantle and sediments of the Philippine Sea Plate, but displaced a bit towards more radiogenic compositions. Plots of Zrversus Nb concentration in these lavas yield a linear trend that falls on a compositional mixing line between the values
for mid-ocean ridge basalt (MORB) and average continental crust. Collectively, these observations indicate that theprimary source magmas for the Quaternary lavas of Sakurajima volcano were initially generated by partial melting of
MORB-type mantle wedge that had already been hydrated by fluids derived from the subducting Philippine Sea Plate.
The additional contribution of significant amounts of crustal material during magma evolution is also evident from theZr/Nb ratios and Sr-Nd-Pb isotopic compositions of the analyzed andesites and dacites of Sakurajima lava samples.
From the mixing relation of Sr-Nd-Pb isotopic compositions, it is suggested that the sedimentary rocks of Shimanto
Group can be a source of the crustal materials. Although most of the major element oxide compositions of these lavas
show a single linear trend on each of the Harker diagrams, two different trends are clearly discernible on each of theP2O5, and TiO2 versus silica variation diagrams, and are subdivided into low-P and high-P geochemical groups. These87
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two groups can also be distinguished when comparing their P2O5 and TiO2 contents and Sr/ Sr ratios, relative to theirphenocrystic plagioclase modal abundances. The magma mixing trends of Sakurajima lavas, which seem to beextended from mono andesitic end-member to two different deictic end-members, are observed from the relationships
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86of major element contents and Sr/ Sr ratios. In addition, the low-P versus high-P groups of lavas show distinctive
distribution patterns, whereby the high-P lavas are surrounded by low-P lavas in the central to southern parts of theSakurajima volcano study area. These observations indicate that mixing of andesitic and dacitic magmas played animportant role in the genesis of Quaternary lavas of Sakurajima volcano, and that multiple dacitic magma chamberswith different geochemical characteristics once existed beneath the Sakurajima area at relatively shallow levels in thecrust. From the relations between SiO2 and Sr isotope ratios, an assimilation and fractional crystallization process isrequired to originate the andesite and dacite end-members.
Key words : Sakurajima volcano; Aira; Kyushu; caldera; arc magmas; trace elements; Sr-Nd-Pb isotopes＊Beppu Geothermal Research Laboratory, Institute forGeothermal Sciences, Kyoto University, Beppu, Oita
874-0903, Japan.
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Graduate School of Science and Engineering, Kagoshima
University, Kagoshima, Kagoshima 890-0065, Japan.

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Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University, Sakurajima,
Kagoshima 891-1419, Japan.
Corresponding author: Tomoyuki Shibata
e-mail: tomo@bep.vgs.kyoto-u.ac.jp