Initial ⁸⁷Sr/⁸⁶Sr ratio heterogeneity in Kamihara Tonalite, Ryoke belt, southwest Japan: Evidence from strontium isotopic analysis of apatite

抄録

Strontium isotopic analysis was carried out on magmatic apatite in the Kamihara Tonalite obtained from the Cretaceous Ryoke belt, southwestern Japan. ⁸⁷Sr/⁸⁶Sr isotopic compositions of euhedral apatite indicated that the initial ⁸⁷Sr/⁸⁶Sr ratio (SrI) in the tonalite varies from 0.70695 to 0.70790. This provides direct evidence of SrI heterogeneity in the Kamihara Tonalite. The SrI heterogeneity is inherited to Rb-Sr whole-rock age determination, and makes the age discrepancy between the Rb-Sr age and CHIME, K-Ar ages. By combining the results of the isotopic analysis, knowledge on the whole-rock chemistry, and field observations, it can be inferred that the SrI heterogeneity in the Kamihara Tonalite was caused by the assimilation of high SrI country metamorphic rocks of the Ryoke belt.