山陰帯の宝物山花崗岩におけるストロンチウム同位体比初生値と西南日本内帯の白亜紀-古第三紀火成岩類のストロンチウム同位体の地理的分布

The Hobutsu-san granite stock, the southwestem part of Tottori prefecture, SW Honshu, has a Rb-Sr whole rock isochron age of 72.3±2.8Ma with an initial ratio of 0.70681. The initial ratio is remarkably high for granitoids of the San'in Belt. High Sr isotope ratios of some lower crustal granulite xenoliths included in an alkali basalt of the Oki islands, suggest that the lower crust could have been a magma source for the Hobutsu-san granite. Initial Sr isotopic ratios of Cretaceous-Paleogene igneous rocks in the Inner Zone of Southwest Japan decrease from south to north through a transitional zone. This supports a previous suggestion by KAGAMI et al. (1987) that the magmas for the granites of SW Honshu and North Shikoku have been derived from three source regions with different geochemistry.