High initial Sr-isotopic ratios of gabbro and metadiabase in the Ryoke Belt, Southwest Japan

Abstract

Rb and Sr contents and Sr-isotopic ratios as well as SiO₂ and K₂O contents were determined for basic rocks (gabbro, cortlandite, anorthosite and metadiabase) from the Ryoke Belt in the Kinki and Setouchi Inland Sea districts, Southwest Japan. The basic rocks are characterized by lower K₂O and Rb contents and by higher initial Sr-isotopic ratios than the mantle-derived igneous rocks in the oceanic region. The high initial Sr-isotopic ratios of the basic rocks are explained by the high isotopic ratios of their source materials.