40Ar-39Ar and K-Ar mineral ages of the Tabito composite mass in the southern Abukuma Mountains, northeast Japan.

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).