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

本研究的主要目的是从位于日本北部山形县和新泻县交界处的朝日深成岩体中分离出生物体和混闪石。对样品进行了常规K-Ar定年和快中子辐照K-Ar定年(Ar49-Ar39定年)。角闪岩的年龄在中部为80 Ma或90 Ma左右，周围为60 Ma左右。与之相反，黑云母在空间分布上表现出较大的扰动，既有年轻的(约50 Ma)和老的(约100 Ma)的数据出现在身体中部，也有老的(约90 Ma)出现在周围，一些手部标本显示黑云母的年龄比共生的黑云母更老。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日期(闭合…更多的Re温度(约300℃)通常比共存的角闪石的Re温度(约500℃的关闭温度)年轻。我们已经报道过，在日本中部南大福萨山的几个深成岩体(坦泽体和七步体)中，黑云母的年龄比共存的角闪石的年龄更早。朝日的例子增加了这种不一致的另一个例子。此外，很明显，黑云母可以分为两类。第一组年龄增加(从60 Ma到100 Ma)，随着黑云母中钾含量的减少(从7%到5%)。这种现象在日本中部的深成体中没有观察到。第二组获得了大约60 Ma的相似年龄，尽管黑云母的钾含量下降了(从7%降到4%)。电子探针分析表明，2组黑云母边缘发生了黑云母蚀变，而1组黑云母可能未发生蚀变。黑云母比角闪石和1族黑云母的年龄偏早的现象表明，随着钾含量的降低，年龄增加，这不能归因于样品的变化。这项工作的结果和野外观测的报告与如下解释是一致的：1)深成岩体形成于白垩纪；2)深成岩体首先侵入身体中部，然后侵入身体周围。较少

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