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

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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.