ABSTRACT

Lead isotope ratios have been measured for early Mesozoic granitic rocks from the Hida metamorphic belt, central Japan. A comparison of the data with those of the Hida metamorphic rocks given in a previous work indicates that the lead in some less abundant granitic rocks may have been contaminated with radiogenic leads from the metamorphic rocks, whereas the lead in the predominant intrusives may not have been subjected to such contamination. The lead isotope ratios of the latter group are further compared with the available data for late Mesozoic granitic rocks and Cenozoic volcanic rocks in Japan. The lead in the granitic rocks from the Hida area is similar to that in the basaltic rocks from the Oki Islands in the Japan Sea and is distinctly less radiogenic than the lead in the granitic rocks occurring in central Japan closer to the Pacific coast, suggesting the presence of a lateral isotopic variation analogous to the general trend found in the Cenozoic volcanic rocks. The applicability of the existing models for lead isotopes in island arcs is discussed.