Isotopic ages of alkali rocks from the Nemuro Group in Hokkaido, Japan: Late Cretaceous time-scale points

Abstract

KAr and RbSr age determinations were carried out on igneous rocks from five horizons in the Late Cretaceous Nemuro Group in easternmost Hokkaido, Japan. The results are as follows: dolerite and basalt from the Middle-Upper Campanian Nokkamappu Formation, 75.0–76.2 and 72.0–73.9 Ma, respectively; monzonite from the uppermost Campanian-lowermost Maastrichtian Otamura Formation, biotite KAr and RbSr ages of 70.3 and 69.5 Ma, respectively; dolerite from the Middle Maastrichtian Oborogawa Formation, 69.8 Ma (monzonite from this formation: biotite KAr and RbSr ages of 70.4 and 69.8 Ma, respectively); dolerite from the Late Maastrichtian Hamanaka Formation, 66.0 and 67.6 Ma. These age results are generally consistent with the biostratigraphy, and provide valuable calibration points for the Late Cretaceous time scale.