CHIME monazite ages of the Otagiri and Ichida Granites in the Komagane area, Nagano Prefecture

Abstract

The Otagiri Granite comprises peraluminous fine-grained two-mica granites to granodiorites, and intrudes into the Ichida Granite within the eastern part of the Ryoke metamorphic belt. Monazites from three samples of the Otagiri Granite and a sample of the Ichida Granite were dated by the Chemical Th-U-total Pb Isochron Method (CHIME). The ages are 77.4±3.1 Ma, 76.3±2.8 Ma and 77.2±3.5 Ma for the Otagiri Granite and 79.4±3.1 Ma for the Ichida Granite. The CHIME monazite ages, interpreted as the time of the emplacement, agree with the intrusive relation. The present dating shows that the 100-90 Ma Rb-Sr whole-rock isochron ages (Yuhara, 1994) for the Otagiri Granite is fortuitous. The CHIME monazite ages support the previous correlation of the Otagiri Granite with the Kadoshima and Busetsu Granites of ca. 77 Ma age (Kagami, 1972 ; Shibata and Ishihara, 1979 ; Nakai et al., 1985 ; Suzuki et al., 1994b).