Abstract
Tuffs occur interbedded with coal-bearing clastic sedimentary rocks of the Upper Member of the Paleogene Ube Group, SW Japan. We examined the eruptive source of these tuffs using geochronological, petrochemical and Sr isotopic data. The tuffs consist of crystal and lithic fragments, and glassy matrix altered to clay minerals. The crystal fragments comprise plagioclase (An : 34 45%), quartz, magnetite and ilmenite, whereas the lithic fragments are andesite lava and felsite. The tuffs yield fission-track zircon ages of 36.0±2.1 and 34.6±1.5 Ma, which are consistent with the age indicated by the vertebrate fossil, Amynodon watanabei and plant remains. Eight plagioclase samples separated from the tuffs have low initial Sr isotope ratios, ranging from 0.7043 to 0.7049. These ratios are similar to those of Cretaceous-Paleogene granitic rocks from the North Zone of SW Japan, as delineated by other work. The data indicate that the tuffs were probably derived from a volcano in the Paleogene volcanic front, which extended along the Japan Sea from the eastern San-in district to Tsushima Strait.
