Abstract
The Tabito composite mass in the Abukuma Mountains was dated for its biotite and hornblende separates by 40Ar-39Ar and K-Ar methods. K-Ar biotite ages ranging from 111 to 95 Ma are significantly older than the coexisting hornblendes in two of four rock samples examined. 40Ar-39Ar age spectra of biotite and hornblende show argon-loss and convex-upward patterns. However, higher temperature fractions of hornblende yielded well defined plateau ages of 103.1±1.4 Ma for the Iritabyuto granodiorite and 101.7±1.2 Ma for Myojin-ishi tonalite, which are consistent with a conventional K-Ar age of hornblende from the Komuro tonalite by Shibata and Uchiumi (1983). 40Ar-39Ar plateau ages of hornblende support the cooling rate (∼15°C/m.y.) of the Tabito mass by Tanaka et al. (1999).
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