Cretaceous granitoids and their zircon U-Pb ages across the south-central part of the Abukuma Highland, Japan

摘要

Zircons separated from Cretaceous granitoids are dated from a south-central transect of the Abukuma metamorphic and granitic terrane. The zircon ages do not follow older' and younger' granitoid ages that are used conventionally. In the western part of the study area (Zones I, II and III) where the Takanuki and Gosaisho metamorphic rocks are exposed, the Iritono quartz dioritic stock intruding the greenschist facies rocks in Zone III exhibits the oldest age of 121Ma in the studied region. Quartz diorite located northward shows 112Ma, but the other four granitoids intruding into the Takanuki and Gosaisho metamorphic rocks are younger and 103-99Ma. Two-mica and biotite granites belong to the youngest age group of 99Ma. The granitic activities of both the Abukuma and Ryoke belts were initiated by intrusion of quartz dioritic magmas and were ended by two-mica granite activity. The ages of the eastern two batholiths vary from 110 to 106Ma (four samples), and show no age common to the Kitakami granitoids farther to the north. Throughout the Japanese Islands arc, Cretaceous granitic activities became younger toward the marginal sea side from the Kitakami Mountains, to the Abukuma Highland, and the Ryoke Belt, then to the Sanin belt of the Inner Zone of Southwest Japan.