Zircon U-Pb ages of the Futomiyama Group in Toyama Prefecture, central Japan

Abstract

The Futomiyama Group is mainly distributed in the southwestern (type locality area) and northeastern (Uozu area) parts of Toyama Prefecture, central Japan, and contains acid volcanic rocks. Despite intensive study, there are no zircon U-Pb ages for the acid volcanic rocks. Here we present the results of laser ablation-inductively coupled plasma-mass spectrometry zircon U-Pb analyses of four acid rocks from the Futomiyama Group in the type locality and Uozu areas. The ²³⁸U-²⁰⁶Pb weighted-mean ages are as follows: 70.2±0.7 Ma for tuff and 66.7±0.9 Ma for intrusive rocks in the type locality area; and 70.0±0.5 Ma for welded tuff and 70.6±1.1 Ma for tuff in the Uozu area. Although the Futomiyama Group was thought to be Paleogene in age, our results clarify that it belongs to the uppermost Cretaceous (Maastrichtian).