Abstract
The Middle Miocene felsic volcanic rocks are widely distributed in the northern part of Tochigi Prefecture, central Japan. The Iiyama Formation and Otaki Tuff Breccia in Imaichi area are composed mainly of rhyolite pyroclastic rocks and lavas and zircon fission-track age determination was carried out for each unit. Age of the pyroclastic flow deposit of the Iiyama Formation is 14.8±0.7 Ma (1σ) which is thought to indicate eruption age of the pyroclastic flow. Age of the Otaki Tuff Breccia corresponds to 13.4±0.6 Ma (1σ), which is obtained from matrix of the debris avalanche deposit. Age of the Otaki Tuff Breccia is disqualified by the chi-square test implying contamination of the zircon from the Iiyama Formation or other factor which disturbs Poisson distribution law. Age of the Otaki Tuff Breccia remains still unclear.
Each dating sample includes detrital zircon possibly from the Late Cretaceous volcanic rocks. Over 20 m thick pyroclastic flow deposit of the Iiyama Formation might be emplaced at relatively low temperature not enough to anneal the tracks in detrital zircon. Although paleo-environment of this area is not fully clarified, low temperature pyroclastic flow is indicative of much water as coolant especially for such a thick deposit. Sporadic occurrence of marine fossils in the Iiyama Formation also implies that sea area might prevail during Middle Miocene. The Middle Miocene felsic volcanic rocks seem to have occurred at different places and ages on the basis of the radiometric age data, however the correlation is still problematic because of the confusion between age data and stratigraphy or dating methods in Utsunomiya area.
