Initial STSr/S6Sr rations were determined for more than 80 plutonic rocks in Japan, The STSr/ S6Sr ratios of gabbroic and granitic rocks show no significant difference in plutonic terranes where both rocks occur closely associated, implying a genetic relationship between them (e.g., Green Tuff belt) or reequilibration at deep level (e.g., Ryoke belt). Wherever granitic rocks occur independently from gabbroic rocks, the granites have higher ratios than the gabbros.
Initial STSr/S6Sr ratios of the granitic rocks are low (< 0.706) in Northeast Japan but high (< 0.706) in Southwest Japan, the boundary being the Tanakura Tectonic Line. Within Southwest Japan, the ratios are low along the Japan Sea side of the southernmost area. This regional variation is generally correlated with thickness of the continental crust as deduced from the Bouguer anomaly.
Initial STSr/S6Sr ratios of the granitic rocks vary from 0.7037 to 0.7124. The low group (<0.706) is considered to consist of essentially mantle-derived magmas contaminated by crustal material in lesser but varying degree, because of its geological setting and initial STSr/S6Sr values. The high group may have been formed by contamination of a deep-seated magmas by crustal material or by generation of the main part of the magmas within the continental crust. The ratios of individual belts reflect their own history depending upon age and Rb/Sr ratio of the crustal material.
Initial STSr/S6Sr ratios of granitic rocks are generally low for the magnetite-series but high for the ilmenite-series. Thus, a negative correlation is observed between initial ratios and 634S for most Cretaceous-Paleogene granites. However, Neogene ilmenite-series granites are low in both initial STSr/S~Sr and c53~S indicating interaction of the granitic magma with young sedimentary rocks enriched in 32S.