An ^<40>Ar-^<39>Ar Chronological Study on a Formation and Coling History of the Tanzawa Tonalitic Body

我对位于坦泽调性体最西北端的石wari山地区与调性体接触的调性岩和角砾石进行了取样。对上述样品中分离的黑云母和角闪石进行了K-Ar年龄研究。石wari山地区北部的小卫星体Shishidome体的tonalite样品也为K-Ar年龄测定提供了角闪石。这些角闪石样品的K-ar年龄在6.7 ~ 7.0 Ma之间。石笠山黑云母年龄为8.4 Ma。比角闪石的年代要久远得多。由于深成角闪石的年龄往往比共存的黑云母更老，这种老的黑云母年龄可能是由于样品中含有过量的氩。我们之前的研究清楚地表明，位于该岩体西南部的Omatazawa的角闪石样品的年龄可达10 Ma。我在其中一些样品上应用了40Ar-39Ar法。遗憾的是，由于这些样品的K含量较低，Ar40-Ar39的研究并不是很成功。OM-02是我唯一能从中获得一些地质信息的样本，在最低温度部分显示出极其古老的年代。样品中一定含有过量的氩。上述结果结合前人的研究结果，表明该岩体形成于约7 Ma以前。坦泽地块和本州弧之间碰撞的开始可能是这个岩体形成的原因。上述成果发表于1990年地磁、地电与行星科学春季学术会议、金泽AGU西太平洋会议和1991年1月在广州举行的第二届中日地球化学、宇宙化学和同位素地质联合学术研讨会。我还向J. Geomag提交了一篇关于这个主题的论文。Geoelectr。对于石wari山的黑云母样本，我计划在不久的将来确定40Ar-39Ar的年龄。

I sampled tonalite and hornfels, which contacts with the tonalite, from Mt. Ishiwari area which consists of the north-westernmost part of the Tanzawa tonalitic body. K-Ar ages were studied on biotite and hornblende separated from the above samples. A tonalite sample from the Shishidome body, a small satellite body locating to the north of Mt. Ishiwari area, also provided hornblende for K-Ar age determination. These hornblende samples yield K-ar ages between 6.7 to 7.0 Ma. In contrast, biotite fron Mt. Ishiwari showed an age of 8.4 Ma. which is significantly older than that for the hornblende. Since plutonic hornblende frequently shows an age older than coexisting biotite, this old biotite age may be due to excess argon in the sample.Our previous studies clearly showed that hornblende samples from Omatazawa, which is located in the south western part of the pluton, yield ages as old as 10 Ma. I applied the 40Ar-39Ar method on some of these samples. Unfortunately, because of low K content of these samples, the Ar40-Ar39 study was not very successful. OM-02, the only sample from which I can obtain some geological information, demonstrates extremely old age in the lowest temperature fraction. The sample must have contained excess argon.Above results, with considering our previous results, suggest that the pluton was formed about 7 Ma ago. The start of a collision between the Tanzawa block and the Honshu arc may have been responsible of the formation of the pluton.Above results were presented at the 1990 spring conference of Geomagnetism, Geoelectricity, and Planetary Science, and at AGU Western Pacific Conference held in Kanazawa city, and at 2nd China-Japan Joint Symposium on Geochemistry, Cosmochemistry, and Isotope Geology held in Guanaghzou on January 1991. I also submitted a paper on this topic to J. Geomag. Geoelectr.For the biotite sample from Mt. Ishiwari, I am planning to determine an 40Ar-39Ar age in near future.