口ノ津層群における凝灰岩層および竜石層中の安山岩のジルコンのフィッション・トラック年代

Abstract

Fission track ages of zircon crystals separated from the tuff layers in the Kuchinotsu Group and those from the andesite of the Tatsuishi Formation in the Shimabara Peninsula, northwest Kyushu have been determined in order to bring some light on the history of the Hohi and the Sanin volcanic activities (MATSUMOTO, 1963) in the Early to Middle Pleistocene age in Kyushu and to clarify the absolute ages of the mammalian and floral zones discriminated in the Kuchinotsu Group.As the diameter of the fission track has been considerably shortened by annealing, correction was made on each given age in the present study, based on the results of the annealing examination.The fission track ages of the Kuchinotsu Group and the Tatsuishi Formation are as follows:Fission track ages (m.y.)Tatsuishi Formation 0.24±0.06Kuchinotsu Group {Kitaarima Formation (Saishoji tuff breccia) 1.43±0.27 Oya Formation {Oya tuff (Tuff VIII) 1.76±0.22 Mukaigoya tuff breccia 1.89±0.16The fission track age of the Mukaigoya tuff breccia (1.89±0.16 F.T.m.y) indicates the beginning of the Early Pleistocene Hohi volcanic activities widely recognized as lavas or pyroclastics in the Early Pleistocene formations in north Kyushu. Furthermore, all the fission track ages of the Kuchinotsu Group seem to be consistent with the results of magnetostratigraphy and biostratigraphy based on the mammalian and plant fossils of this group.The Tatsuishi Formation is mainly composed of mud flow of hornblende andesite which belongs to the early volcanic products of the Unzen volcano representing one of the Middle Pleistocene Sanin volcanic activities of west Japan (MATSUMOTO, 1963). The fission track age of the Tatsuishi Formation (0.24±0.06 F.T.m.y) almost corresponds to those of the hornblende andesite of the Tara-dake (MATSUMOTO et al., 1977) of the Nagasaki Prefecture, northwest Kyushu.