Lead isotopic studies of metamorphic and sedimentary rocks from the Hida metamorphic terrain, Japan

Abstract

The isotopic compositions of lead and the approximate concentrations of lead and uranium were determined for some gneisses and Paleozoic sandstones from the Hida metamorphic terrain, Japan. All the Hida gneisses studied have similar ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb ratios. The lead in these rocks is significantly more radiogenic than that from the Precambrian metamorphic complex in the eastern part of Asian continent. This difference suggests that these two rock systems developed in different geochemical environments. The lead of the sedimentary rocks from the Hida metamorphic terrain shows a large variation in ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb ratios, and generally appears more radiogenic than that of the gneisses. These isotopic data indicate some genetic correlation between the metamorphic and sedimentary rocks but are not sufficient to demonstrate that these sedimentary rocks were the original materials of the Hida metamorphic rocks. The lead isotopic data for the sedimentary rocks can be interpreted by means of a two-stage evolutional model. The time of formation of the second evolution system is estimated to be (2, 300 ± 350)m.y. This value may be related to some Precambrian igneous activity in the present site of the Japanese Islands.