1999 Fiscal Year Final Research Report Summary
Studies of the Cooling History of a Plutonic Body by K-Ar and Ar40-Ar39 Dating Methods
| Project/Area Number |
09640578
|
|---|
| Research Category |
Grant-in-Aid for Scientific Research (C)
|
| Allocation Type | Single-year Grants |
|---|
| Section | 一般 |
|---|
| Research Field |
地球化学
|
|---|
| Research Institution | YAMAGATA UNIVERSITY |
|---|
Principal Investigator |
SAITO Kazuo Faclty of Science, Yamagata University, Professor, 理学部, 教授 (80125765)
|
|---|
| Project Period (FY) |
1997 – 1999
|
| Keywords | Asahi plutonic body / K-Ar dates / Ar40-Ar39 dates / Biotite / Hornblende / Closure temperature hypothesis / Cretaceous / Yamagata Prefecture |
|---|
| 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
|
