Rb-Sr and K-Ar geochronology and petrogenesis of the Aji Granite in the eastern Sanuki district, Ryoke Belt, southwest Japan

Abstract

The Rb-Sr and K-Ar geochronological study was made for the Aji Granite in the eastern Sanuki district, one of the fine-grained granites in the Ryoke Belt. This granite is divided into hornblende-free and hornblende-bearing fades. The former gives a Rb-Sr whole-rock isochron age of 82.9±8.0 Ma with Sri of 0.70773 ±0.00007 which is interpreted as the intrusive age of the Aji Granite and therefore the final plutonism in the eastern Sanuki district. Biotites separated from six samples give K-Ar biotite ages between 80.4 Ma and 79.4 Ma. K-Ar hornblende age is of 83.5 Ma. These age data suggest rapid cooling after the emplacement of the granite. The initial Sr and Nd ratios of the Aji Granite fall within the range of the Ryoke granitoids. They are close to those of the Fine-grained Granite in the Awaji Island, but differ from those of the Busetsu, Kadoshima and Otagiri Granites in the Chubu district. The Aji Granite and Finegrained Granite in the Awaji Island have I-type granite chemical characteristics, whereas other fine-grained granites in the Ryoke Belt have S-type characteristics. This suggests varying source characteristics for the fine-grained granitic magmas in the Ryoke Belt.