日本最古の堆積岩年代472Ma（オルドビス紀前期末）とその意義

Abstract

The Hitoegane Formation, with the Middle Ordovician conodont, in the Hida marginal belt, Southwest Japan, represents the oldest sedimentary unit in Japan. In order to date the lowest part of the formation, U-Pb age of igneous zircon from tuff beds was measured by LA-ICP-MS. A felsic tuff bed from the lower part of the Hitoegane Fm was newly dated 472 Ma (latest Early Ordovician) that marks the oldest age for sedimentary unit hitherto reported from Japan. This result suggests that the basement of proto-Japan was probably older than the Ordovician. The Iwatsubodani Formation, an apparently underlying unit below the Hitoegane Fm, however, yielded U-Pb zircon age of 280 Ma (Early Permian). Thus this unit is too young to be the sedimentary basement of the Hitoegane Fm. Newly added also were some pieces of information on the U-Pb ages and stratigraphy of the Paleozoic-Mesozoic sedimentary units of the Hida marginal belt.