2020 Volume 71 Issue 1 Pages 33-48
Laser ablation–inductively coupled plasma–mass spectrometry zircon U–Pb ages were acquired for three felsic tuffaceous beds, one from the upper Takikubo Formation (sample IT01) and two from the lower Horita Formation (IT02 and IT03), to determine depositional ages of the Izumi Group in the Ikeda district, eastern Shikoku, southwestern Japan. The weighted mean 206Pb/238U ages and 2σ errors are 78.3 ± 1.3 Ma (IT01), 80.8 ± 1.2 Ma (IT02), and 79.3 ± 1.1 Ma (IT03). Two of the three ages (78.3 ± 1.3 Ma and 79.3 ± 1.1 Ma) passed the χ2 red (reduced) statistical test, but the other (80.8 ± 1.2 Ma) failed.
These U–Pb ages indicate that the maximum depositional age of the Izumi Group in this district is middle Campanian (magnetostratigraphic chron C33n). These ages are similar to those reported from the lower Takikubo Formation in the Kan-onji district (80.8–78.3 Ma). Although an apparent stratigraphic thickness from the lower Takikubo Formation to the lower Horita Formation reaches 12 km, there is no younging trend of the zircon U–Pb ages through these formations. This suggests that either the sedimentation rate of the Izumi Group was high or there was a lack of volcanic activity that could produce new zircon crystals in the hinterland during deposition of the succession.
