火山砕屑性堆積物から推定される海洋性島弧の発達過程 : 伊豆・小笠原弧を例として  [in Japanese] Oceanic arc evolution estimated from volcaniclastic sand and sandstones : an example from the Izu-Bonin Arc  [in Japanese]

火山弧の特徴を砕屑性堆積物から復元するためには, 火山岩岩片を多量に含む未成熟な砂岩に対して鉱物学的及び火山岩岩石学的検討を加えることが有効であり, 層位学的な検討によって, 未知の島弧の発達過程を詳しく知ることができる。ODP Leg 126において伊豆・小笠原弧から得られた砂及び砂岩は火山源物質と少量の生物源物質からなり, 陸源砕屑物を欠く。このことはこれらの物質の供給源は漸新世から更新世を通じ伊豆・小笠原火山弧であったことを意味する。砕屑性斑晶鉱物の組み合わせ, 砕屑性輝石及び斜長石の化学組成, 砂岩の全岩の主成分及び微量成分元素分析結果は, これらの火山砕屑性物質の供給源が漸新世のカルクアルカリ玄武岩-安山岩の火山活動から鮮新世-更新世のソレアイトのバイモーダル火山活動へと変化したことを示唆しており, アルカリ岩や無人岩的な要素は認められない。

The study of volcanic-lithic fragment rich immature sand and sandstones, using mineralogical and petrological criteria is considered important in order to reconstruct volcanic arc characteristics. It enables us to estimate the volcanic rock series and tectonic setting, and together with stratigraphic studies it is possible to recognize changes in arc evolution which were previously unknown. The sand and sandstones recovered around the Izu-Bonin Arc during ODP Leg 126 consist dominantly of volcanic-lithic fragments, plagioclase feldspar grains, volcanic glass both rhyolitic and basaltic, pumice, scoria, pyroxene, fewer olivine and hornblende, and bioclastic materials, and a lack of terr igenous material. Modal composition of sand and sandstones imply that these materials were derived from the Izu-Bonin magmatic arc during Oligocene to Pleistocene time. The sand and sandstones reflect the petrologic characteristics of their source. The association of detrital phenocryst minerals, chemical composition of detrital pyroxene and plagioclase, and major and trace element chemistry of sand and sandstones suggest that volcaniclastic materials were derived from low-K tholeiite or calc-alkali igneous rocks, instead of an alkalic or boninitic source. The abundance of detrital mafic minerals in the sand and sandstones, in order of decreasing frequency, changed from CPX>OPX>HB in Oligocene samples to OPX>CPX>PIG>OL in Plio-Pleistocene samples. OPX and CPX belong to the Mg-rich rock series in Oligocene samples, and the Fe-rich rock series in Plio-Pleistocene samples. Basic and acidic andesites are predominant in Oligocene rocks; in contrast, Plio-Pleistocene sediments are richer in SiO_2. These differences may correlate with a switch after the Oligocene from calc-alkali basaltic-andesitic volcanism to tholeiitic bimodal volcanism.