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Title :Stratigraphy and K-Ar ages of Tertiary volcanic rocks in the Hamamasu area northwestern Hokkaido, Japan
Title alternative :北海道西北部、浜益地域に分布する第三紀火山岩の層序とK-Ar年代
Authors :Aoki, Atsushi
Shuto, Kenji
Itaya, Tetsumaru
Publisher :日本地質学会
Issue Date :May-1999
Journal Title :地質学雑誌
Volume :105
Issue :5
Start Page :341
End Page :351
ISSN :0016-7630
Abstract :Tertiary volcanic rocks in the Hamamasu area along the Japan Sea coast in northwestern Hokkaido are lithologically divided 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.
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。
Keywords :Hamamasu area
northern Hokkaido
debris flow deposits
fluvial deposits
sector collapse
K-Ar age
NE Japan arc
Kurile arc
Zr/Y ratio
Type Local :学術雑誌論文
Language :eng
Format :application/pdf
URI :http://hdl.handle.net/10191/17800
fullTextURL :http://dspace.lib.niigata-u.ac.jp/dspace/bitstream/10191/17800/1/ZC_105(5)_341-351.pdf
Appears in Collections:10 査読済論文 = Postprint

Please use this identifier to cite or link to this item: http://hdl.handle.net/10191/17800