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

K-Ar和Ar40-Ar39测年方法研究深成体的冷却历史

基本信息

批准号：
09640578
负责人：
SAITO Kazuo
金额：
$ 2.24万
依托单位：
YAMAGATA UNIVERSITY
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
1997
资助国家：
日本
起止时间：
1997 至 1999
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-09640578/
关键词：
Asahi 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

项目摘要

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

本研究的主要目的是对位于日本北方山形县和新泻县交界处的朝日深成岩体中的生物化石和人类化石进行年代测定。用Ar ~（38）加标K-Ar法（常规K-Ar法）和快中子辐照K-Ar法（Ar ~（49）-Ar ~（39）法）对样品进行了定年。角闪石的年龄在主体的中心部分约为80 Ma或90 Ma，在周围部分约为60 Ma。相反，黑云母的年龄空间分布非常混乱，即在主体的中部既有年轻（约50 Ma）的年龄，也有老（约100 Ma）的年龄，在主体的周围部分也有老（约90 Ma）的年龄，有些手标本显示黑云母的年龄比共存的黑云母的年龄更老。这与被广泛接受的“闭合温度假说”不一致，该假说坚持认为黑云母的年龄（闭合温度） ...更多信息 re温度约300℃）通常比共存的角质层（闭合温度约500℃）年轻。我们已经报道过，在日本中部的南大窝的几个深成岩体（Tanzawa和秩父岩体）中，黑云母的年代比共存的角闪石的年代更早。朝日新闻的样本又增加了一个这种不一致的例子。此外，很明显，黑云母可以分为两组。第一组表明，随着黑云母中钾含量的降低（从7%到5%），年龄增加（从60 Ma到100 Ma）。这种现象在日本中部的深成岩体中尚未观察到。第二组给出了相似的60 Ma左右的年龄，虽然黑云母的钾含量降低（从7%到4%）。的K，Mg，和Fe的浓度，调查的电子探针微量分析仪，表明蚀变的黑云母发生在边缘的第2组黑云母，但第1组黑云母可能已经从蚀变。黑云母的年龄比共生的角闪石和第一组黑云母的年龄大，这一现象表明，随着钾含量的降低，黑云母年龄增大的现象不能归因于样品的蚀变作用。本工作的结果和野外观察报告的解释是一致的：（1）深成岩体形成于白垩纪;岩体首先侵入岩体的中心部分，随后侵入岩体的周围部分。少