40Ar-39Ar Studies on Some Tanzawa Tonalite Samples.

40Ar/39Ar analyses on hornblende and biotite samples separated from the Tanzawa tonalite demonstrate that most samples have highly disturbed K-Ar systematics. Expect for Mt. Ishiwari samples, unrealistically old ages are commonly observed in lower temperature fractions. Especially, Omatazawa samples showed severely disturbed age spectra, which strongly suggests that one should not use the previously reported 10 Ma hornblende K-Ar ages in the southwestern margin of the Tanzawa pluton in a discussion of the cooling history of the pluton. In the eastern part, although showing a clear evidence of excess argon in the lower temperature fractions, the biotite and hornblende in the Yushin sample yield poorly defined high temperature plateaus about 5 Ma. On the other hand, the biotite and hornblende separated from the Mt. Ishiwari sample reveal apparently good plateaus in age spectra. However, the biotite has a plateau age older than that for the cogenetic hornblende, which is consistent with our previous K-Ar results on these samples, but is inconsistent with the widely accepted closure temperature hypothesis. Comparing with our previous K-Ar results, the 6.26 +/- 0.17 plateau age for the hornblende could be interpreted as the time when the sample cooled to the closure temperature of hornblende (about 500°C). The older age of the biotite should be due to excess argon.