<scp>U</scp>–<scp>Pb</scp> zircon age from the radiolarian‐bearing <scp>H</scp>itoegane <scp>F</scp>ormation in the <scp>H</scp>ida <scp>G</scp>aien <scp>B</scp>elt, <scp>J</scp>apan

Abstract

Abstract The dating of radiolarian biostratigraphic zones from the S ilurian to D evonian is only partially understood. Dating the zircons in radiolarian‐bearing tuffaceous rocks has enabled us to ascribe practical ages to the radiolarian zones. To extend knowledge in this area, radiometric dating of magmatic zircons within the radiolarian‐bearing H itoegane F ormation, J apan, was undertaken. The H itoegane F ormation is mainly composed of alternating beds of tuffaceous sandstones, tuffaceous mudstones and felsic tuff. The felsic tuff and tuffaceous mudstone yield well‐preserved radiolarian fossils. Zircon grains showing a U–Pb laser ablation–inductively coupled plasma–mass spectrometry age of 426.6 ± 3.7 Ma were collected from four horizons of the H itoegane F ormation, which is the boundary between the P seudospongoprunum tauversi to F utobari solidus– Z adrappolus tenuis radiolarian assemblage zones. This fact strongly suggests that the boundary of these assemblage zones is around the L udlowian to P ridolian. The last occurrence of F . solidus is considered to be P ragian based on the reinterpretation of a U–Pb sensitive high mass‐resolution ion microprobe ( SHRIMP ) zircon age of 408.9 ± 7.6 Ma for a felsic tuff of the K urosegawa belt, S outhwest J apan. Thus the F . solidus– Z . tenuis assemblage can be assigned to the L udlowian or P ridolian to P ragian. The present data also contribute to establishing overall stratigraphy of the P aleozoic rocks of the Fukuji–Hitoegane area. According to the O rdovician to C arboniferous stratigraphy in this area, O rdovician to S ilurian volcanism was gradually reduced to change the sedimentary environment into a tropical lagoon in the early D evonian. And the quiet C arboniferous environment was subsequently interrupted, throwing it once more into the volcanic conditions in the M iddle P ermian.