| 1.
論文(リポジトリ) |
論文(リポジトリ)
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Kondo, Hiromi ; Shuto, Kenji ; Fukase, Masayuki
概要:
Andesitic rocks, of the Pliocene Myojin-iwa volcanic field, in the northern part of Niigata Prefecture, Northeast Japan,
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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.<br />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.
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| 2.
論文(リポジトリ) |
論文(リポジトリ)
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深瀬, 雅幸 ; 周藤, 賢治
概要:
Oligocene volcanic rocks from the middle part of the Monzen Formation in the Oga Peninsula, northeastern Japan consist o
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f lava flows and pyroclastic rocks. These volcanics are composed of basalts and andesites which can be divided into four groups based on differences in their TiO_2 contents and petrographic characteristics；high TiO_2 basalt group (HTG), low TiO_2 basalt and andesite group (LTG), quartz bearing medium TiO_2 andesite group (MTG) and porphyritic andesite group (PTG). Olivine basalts of the HTG have geochemical characteristics similar to those of basaltic rocks from continental rift zone, whereas other basalts of the HTG and LTG have characteristics of both continental rift zone basalts and of island arc basalts. These compositional differences of basaltic rocks can be attributed mainly to differences in the geochemical characteristics of their source mantle. The variations in HFS (high field strength) element contents among olivine basalts in the HTG may have resulted from either differences in degree of partial melting of source mantle or differences in degree of fractional crystallization mainly of clinopyroxene from the primary basaltic magma. Andesitic rocks from the study area are characterized by the wide variations in Zr/Nb ratio and initial ^143Nd/^144Nd ratio (Nd I value), suggesting that the production of these andesitic rocks were affected by assimilation of crustal materials with low Zr/Nb ratio and low ^143Nd/^144Nd ratio. The accidental materials such as granitic fragments, quartz crystals having corroded form and phenocrystic plagioclases having core part with very low An% are contained in the MTG and LTG andesites. These features indicate that granitic rocks probably constituting wall rocks around the magma chamber are the candidate for the assimilants.<br />男鹿半島西部地域に分布する漸新統門前層中部の火山岩類について，岩石学的検討を行った．門前層中部の火山岩類は，9タイプの斑晶鉱物組み合わせをもつ玄武岩～安山岩からなる．これらの火山岩類をTiO_2含有量と記載岩石学的特徴の違いに基づいて4つの岩石グループに区分した．玄武岩にはリフト帯に産する玄武岩と類似する微量元素比をもつものと，リフト帯と島弧の玄武岩の中間的な性質をもつものがある．これらの性質の違いは，Nd同位体比の検討から起源マントルの違いを反映したものと考えられる．安山岩には花崗岩質の岩片，これに由来するとみられる融食した石英とAn組成の著しく低い斜長石などが含まれることや，安山岩は幅広いNdI値とZr/Nb比を有することから，安山岩の形成には，これらの値と比の低い花崗岩質岩石の同化作用の関与があったことを論じた．
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| 3.
論文(リポジトリ) |
論文(リポジトリ)
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Aoki, Atsushi ; Shuto, Kenji ; Itaya, Tetsumaru
概要:
Tertiary volcanic rocks in the Hamamasu area along the Japan Sea coast in northwestern Hokkaido are lithologically divid
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ed into three formations. These are, in ascending order, the Iwao, Horo and Hamamasu Formations. The Iwao Formation is composed mainly of lavas and pyroclastic rocks of basalt and andesite with subordinate amounts of debris flow deposits. The Formation is divided into six units, which are in ascending order,A,B,C,D,E and F units. K-Ar ages include 3.8±0.1 Ma for andesite from the A unit and basalts from the B and C units as well as an andesite dike intruding the B unit. The andesites of the D and F units are dated to be 2.6±0.1 Ma. These stratigraphical results and K-Ar age determinations show that volcanic rocks from the Iwao Formation, and the dike were derived from a volcano which has been erupted during a relatively short period of 3.7 to 3,8 Ma in the eastern part of this area, and the volcano seems to have collapsed, resulting in debris flow deposits and fluvial deposits. After completion of the volcano, the fluvial deposits accumulated on the southern part of this area. Thereafter, at 2,6 Ma the basaltic lavas overlay the volcanic products formed during the 3.7-3.8 Ma volcanic activity, mainly at the southern and northern parts of this area. The geochemical characteristics of basalts from this area, different from those of the Pliocene to Quaternary basalts from the NE Japan arc and the Quaternary basalts from the Kurile arc, may suggest a different origin of the magma from those in the NE Japan and Kurile arcs.<br />北海道西北部、日本海沿岸の浜益地域には、鮮新世の署寒別火山噴出物に覆われて第三世紀火山岩が広く分布しているが、それらは層序学的に下位から岩老層、幌層、浜益層に分けられ、さらに岩老層は、AからFの6ユニットに細分される。K-Ar年代は下位の5ユニットの玄武岩と安山岩が3.7±0.1 Maと3.8±0.1 Ma, Bユニット中の安山岩岩脈が3.8±0.1 Ma,最上位の浜益層の玄武岩が2.6±0.1 Maを示す。岩老層の溶岩および火砕岩は、3.7～3.8 Maに本地域東部に形成された火山から噴出し、土石流堆積物や河川性積物は、この火山の山体崩壊によって形成されたと推定される。2.6Maには浜益層の溶岩が噴出し、3.7～3.8 Maの火山生成物を覆った。本地域の玄武岩は、東北日本・千島弧の鮮新世～第四紀の玄武岩と異なる地球化学的性質をもっている。これは、本地域の鮮新世のマグマが、これらの鮮新世～第四紀のマグマとは異なる成因により形成されたことを示唆する。
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