@article{oai:niigata-u.repo.nii.ac.jp:00003217,
 author = {Kondo, Hiromi and Shuto, Kenji and Fukase, Masayuki},
 issue = {6},
 journal = {地質学雑誌},
 month = {Jun},
 note = {Andesitic rocks, of the Pliocene Myojin-iwa volcanic field, in the northern part of Niigata Prefecture, Northeast Japan, show transitional chemical characteristics between typical calc-alkaline and tholeiitic series. The most primitive basaltic andesite among these andesitic rocks (the Myojin-iwa Formation) has geochemical characteristics similar to those of other Pliocene-Quaternary basaltic rocks from the back-arc side of the NE Japan, and has initial Sr isotope ratio slightly higher than those of the latter basaltic rocks. Andesites from the Myojin-iwa area show SiO_2 contents ranging from 53.3% to 62.8, and initial Sr and Nd isotope ratios vary from 0.70324 to 0.70378 and 0.512791 to 0.512926, respectively. The initial Sr isotope ratios display positive correlation with SiO_2, and their initial Nd isotope ratios gradually decrease with increasing SiO_2. This suggests that the genesis of andesites from the area can not be attributed to simple fractional crystallization of the primary basaltic magma, but to an assimilation and fractional crystallization (AFC) process. Based on petrographical evidence, major and trace element compositions and Sr-Nd isotope systematics, the Paleogene granitoid rocks constituting the upper crust beneath the Myojin-iwa and surrounding areas are the possible candidates for assimilants. An AFC model using the granitoid rocks as the assimilant can successfully reproduce the chemical variations of the andesites from the Myojin-iwa Formation by assuming the γ values (ratio of the rate of assimilation to the rate of fractional crystallization) of less than 0.2., 東北日本の背弧側（新潟県北部）には,鮮新世（約3Ma）の主に安山岩質の溶岩および火砕岩からなる明神岩層が分布する.これら安山岩類の斑晶鉱物組み合わせ,斑晶鉱物の化学組成,全岩の主要・微量元素組成などの特徴は,安山岩の組成変化の主な要因が,斑晶鉱物の分別にあることを示唆している.しかし,安山岩類のSrI値はSiO_2量の増加とともに高くなり,NdI値は逆にSiO_2量の増加とともに低くなる傾向を示す.このことは,これらの安山岩類は玄武岩質マグマからの単純な分別結晶作用によって形成されたものではないことを示している.これらのことから,明神岩層の周辺に分布する漸新世の花崗岩質岩石と先第三系堆積岩（これらは新潟地域の上部地殻の主な構成岩である）の同化作用の影響について検討した.その結果,安山岩類の化学組成および同位体組成の変化は,玄武岩質マグマが花崗岩質岩石を同化しながら分別結晶作用したことによって説明されることが明らかにされた.},
 pages = {426--441},
 title = {An AFC (assimilation and fractional crystallization) process as the petrogenesis of andesites from the Pliocene Myojin-iwa Formation, the back-arc side of the Northeast Japan : combined major-and trace-element and Sr-Nd isotope constraints},
 volume = {106},
 year = {2000}
}