• Search Research Projects
  • Search Researchers
  1. Back to previous page

Studies of the Cooling History of a Plutonic Body by K-Ar and Ar40-Ar39 Dating Methods

Research Project

Project/Area Number09640578
Research Category

Grant-in-Aid for Scientific Research (C)

Allocation TypeSingle-year Grants
Section一般
Research Field 地球化学
Research InstitutionYAMAGATA UNIVERSITY

Principal Investigator

SAITO Kazuo  Faclty of Science, Yamagata University, Professor, 理学部, 教授 (80125765)

Project Period (FY) 1997 – 1999
Project Status Completed(Fiscal Year 1999)
Budget Amount *help
¥3,500,000 (Direct Cost : ¥3,500,000)
Fiscal Year 1999 : ¥600,000 (Direct Cost : ¥600,000)
Fiscal Year 1998 : ¥500,000 (Direct Cost : ¥500,000)
Fiscal Year 1997 : ¥2,400,000 (Direct Cost : ¥2,400,000)
KeywordsAsahi plutonic body / K-Ar dates / Ar40-Ar39 dates / Biotite / Hornblende / Closure temperature hypothesis / Cretaceous / Yamagata Prefecture / 冷却史 / K-Ar年代測定 / ^<40>Ar-^<39>Ar年代測定 / 深成岩
Research Abstract

The major aim of this study is to dates biotie and homblende separates from collected in the Asahi plutonic body which is located on the border of Yamagata and Niigata Prefectures, Northern Japan. The samples were dated by the K-Ar dating method with spiking Ar38 (the conventional K-Ar dating) and the K-Ar dating with irradiation by fast neutrons (Ar49-Ar39 dating method). The hornblendes yielded the dates about 80 Ma or 90 Ma in the central part of the body, showing younger dates around 60 Ma in the surrounding parts. On the contrary, the biotite showed much disturbed spatial distribution of dates, that is, both young (about 50 Ma) and old (around 100 Ma) dates appeared in the central part of the body and, also, old dates (about 90 Ma) appeared in the surrounding part.
Some hand specimens showed that the date of the biotite is older than the coexisting biotite. This is inconsistent to the widely accepted 'Closure Temperature Hypothesis' which insists that the date of biotite (the closu … More re temperature of about 300℃) is usually younger than that of the coexisting hornblende (the closure temperature of about 500℃). We have already reported that, in several plutonic bodies in the South Fossa Magna, central Japan (Tanzawa and Chichibu bodies), biotite yielded a date older than that of coexisting hornblende. The Asahi sample adds another example of this inconsistency. In addition, it became clear that the biotite can be classified into two groups. The first group shows increasing dates (from 60 Ma to 100 Ma) with decreasing potassium content (from 7% to 5%) in the biotites. This kind of phenomenon has not be observed in the plutonic bodies in the central Japan. The second group yields similar dates of around 60 Ma though the potassium content of the biotites decrease (from 7% to 4%). The concentration of the K, Mg, and Fe, investigated by an electron probe micro analyser, suggests that alternation of biotite took place in the margins of the group-2 biotite, but the group-1 biotite may have been free from alteration. The phenomena that the biotite has the dates older than that for the coexisting hornblende and the group-1 biotites show the increasing dates with decreasing potassium content could not be attributed to the alteration of the sample.
The results of this work and the reports of the field observations are consistent with the explanation that 1) the plutonic body formed in Cretaceous and 2) the pluton first intruded the central part of the body and subsequently intruded into the surrounding parts of the body. Less

Report

(4results)
  • 1999 Annual Research Report   Final Research Report Summary
  • 1998 Annual Research Report
  • 1997 Annual Research Report

Research Products

(3results)

All Other

All Publications

  • [Publications] 斎藤和男 大村卓史: "朝日花崗岩体の冷却年代と黒雲母中の過剰アルゴン"地球惑星科学関連学会1998年合同大会予稿集. 245 (1998)

    • Description
      「研究成果報告書概要(和文)」より
    • Related Report
      1999 Final Research Report Summary
  • [Publications] Kazou SAITO and Takashi Omura: "The cooling chronology of the Asahi granitoid and extraneous Ar in biotite"Abstracts 1998 Japan Earth and Planetary Science Joint Meeting. 245 (1998)

    • Description
      「研究成果報告書概要(欧文)」より
    • Related Report
      1999 Final Research Report Summary
  • [Publications] 齋藤和男、大村卓史: "朝日花崗岩体の冷却年代と黒雲母中の過剰アルゴン"地球惑星科学関連学会1998年合同大会予稿集. 245 (1998)

    • Related Report
      1999 Annual Research Report

URL :

Published : 1997-04-01   Modified : 2016-04-21  

Information FAQ News Terms of Use

Powered by NII kakenhi