能登半島北部漸新統穴水累層に産するソレアイト質玄武岩,高マグネシア安山岩,古銅輝石安山岩およびアダカイト様安山岩の成因  [in Japanese] Genesis of tholeiitic basalt, high magnesian andesite, bronzite andesite and adakite-like andesite of the Late Oligocene Anamizu Formation, northern Noto Peninsula, Japan  [in Japanese]

能登半島北部の漸新世 (28-27 Ma) 穴水火山岩類中には, 未分化なソレアイト質玄武岩と通常のカルクアルカリ質安山岩の他に, 高マグネシア安山岩, 古銅輝石安山岩およびアダカイト様安山岩などの多様な火山岩がみられる。野外調査, 記載岩石学, 主要成分・微量成分, SrとNdの同位体組成などのデータに基づき, これらの火山岩相互の成因関係を検討した。その結果, ソレアイト質玄武岩と高マグネシア安山岩は, それぞれ異なる含水条件下でのマントルカンラン岩の部分溶融によって生じたマグマを代表することを明らかにした。さらに, アダカイト様安山岩は, 沈み込んだ海洋地殻を構成する玄武岩層 (MORB) の部分溶融によって生じた珪長質マグマが地表へ上昇する過程でマントルと反応し, その組成をやや苦鉄質に組成変化させたものに由来し, 古銅輝石安山岩は, これがさらに苦鉄質に組成変化したマグマに由来する可能性があることを示した。

Lata Oligocene (28-27 Ma) volcanic rocks are widely distributed in the eastern part of the Wajima district of the northern Noto Peninsula in central Japan. These volcanic rocks (the Anamizu volcanic rocks) are composed mainly of calc-alkaline andesite and Primitive tholeiitic basalt with subordinate amounts of andesites with extremely low FeO^*/MgO. On the basis of petrographic and geochemical data, petrogenetic relationships between basalt and the associated low FeO^*/MgO andesites have been evaluated. The Anamizu basalts have high MgO (maximum 11.43%), Ni (302 ppm) and Cr (812 ppm) contents. Olivine basalt W-20 contains a few olivine phenocrysts which show a narrow compositional range (Fo=90.1-83.0) and have high NiO contents (0.44-0.15%). Fe-Mg partitioning suggests that phenocrysts with Fo=89.7 are in equilibrium with the host basalt. These features indicate that olivine basalt W-20 has primary magma characteristics. The Anamizu low FeO^*/MgO andesites comprise high magnesian andesite, bronzite andesite and adakite-like andesite. High magnesian andesite is characterized by high MgO, SiO_2, Ni, and Cr contents (7.67%, 57.12%, 170 ppm and 444 ppm, respectively). Bronzite andesite has small amounts of phenocrysts of orth- and clino- pyroxene with maximum Mg values of 88.2 and 86.9, respectively, and contains higher MgO, Cr and Ni contents compared with calc-alkaline andesite. Adakite-like andesite is characterized by higher Sr contents (around 1000 ppm), lower Y (15-14 ppm) and higher Sr/Y ratios (67-72) than common andesites in island arcs. Petrographic and chemical data show that these volcanic rocks were erupted in the Late Oligocene over a limited area; the primary magmas for tholeiitic basalt and high magnesian andesite were derived by partial melting of upper mantle peridotite under almost anhydrous, andhydrous conditions, respectively. Bronzite andesite and adakite-like andesite may not have formed by direct partial melting of mantle peridotite. Adakite-like andesite has Sr and Nd isotopic compositions similar to N-MORB, and is slightly more mafic than magmas generated by partial melting of the basaltic layer of the subducted oceanic slab. Therefore, adakite-like andesite may result from modification of felsic magma due to interaction with the overlying mantle peridotite during its rise to the surface. Bronzite andesite may also result from interaction between felsic magma and mantle peridotite.