Fission-track ages of the Miocene pumice tuff in the region around Matsushima Bay, Miyagi Prefecture

Fission-track age determinations were carried out on five Miocene pumice tuff samples which were collected from the region around Matsushima Bay, Miyagi Prefecture. The obtained ages by grain-by-grain method (proposed by Nishimura, 1977), using zircon crystals separated from the samples, are as follows; (1) Sample no. Sh 1 from the middle to upper horizon of the Shiogama Formation (23.1 +- 2.4 Ma). (2) Aj 1 from the lowermost horizon of the Ajiri Formation (24.4 +- 1.7 Ma). (3) Mt 1 from the lower pumice tuff member of the Matsushima Formation (15.8 +- 1.0 Ma). (4) Nk 1 from the middle horizon of the Neko Formation (13.3 +- 1.2 Ma). (5) Hys 1 from the Shirasakayama Tuff Member of the Hataya Formation (11.7 +- 1.1 Ma). The ages of the samples except for Aj 1 are concordant with the lithostratigraphy and the previously reported K-Ar age (Konda and Ueda, 1980) or diatom biostratigraphy (Akiba et al., 1982). The age of Aj 1, older than the expected age, may be due to accidental zircon crystals. It is generally thought that tuff samples contain reworked or xenocryst zircon crystals as well as essential ones. (author)