THE LITHOFACIES AND THE RADIOMETRIC AGES OF THE NYODOZAKI IGNEOUS ROCK (TERTIARY), OGA PENINSULA, NORTHEAST JAPAN

Abstract

The occurrence, lithofacies and radiometric ages of the NyfldBzaki Igenous Rock, Akashima Formation, the basal part of the Monzen Group (so-called Neogene) were examined. The present study reveals that the Member was largely composed of the welded dacitic pyroclastic flow deposits, and it was unconformably overlying the pre-Tertiary granitic rocks. The K-Ar ages of 30 and 38 m. y. and the zircon fission track ages of 67.2±9.9 and 61.2±9.0m.y. were obtained from the wleded tuff of the Member.
It is possible to correlate the Nyûdôzaki Igneous Rock with the Futomiyama Group, Hokuriku Province, and the Sumikawa Formation, Asahi mountain land, based upon the radiometric ages, the lithofacies, and the stratigraphic relation to the underlying Upper Cretaceous pyroclastic flow deposits and granitic rocks.
In northeast Japan, the equivalent pyroclastic deposits of the Nyûdôzaki Igenous Rock are extending only in the region along the Japan Sea coast, and not developing in the Ôu backbone mountain range. This difference seems to suggest that the different volcanic activities in time or space existed in the former region.
K-Ar age of the welded tuff, obtained from the Nyûkawa Formation, Sado Island, which were correlated with the Nyûdôzaki Igneous Rock, and that of biotite, separated from the Shinzan Rhyolites, the upper-most part of the Monzen Group Oga, Peninsula, were also determined to compare with the age of the NyfldBzaki Igneous Rock. However, the results of the measurement; 21 and 39m.y., respectively, were not coincide with the lithological and biostratigraphical correlation of their formation.