Rb-Sr whole rock isochron age of the Habu granodiorite in the eastern Yamaguchi Prefecture

Abstract

The Habu granodiorite mass (Habu mass) occurs in the western part of the Sanyo belt, Chugoku district, Southwest Japan. It is composed of the following rock types; porphyritic hornblende tonalite to granodiorite (marginal facies) and fine-to medium-grained biotite granite (main facies). This mass forms a normally zoned pluton with a compositional variation from the mafic margin to the felsic center.
   Six granitic rocks from the Habu mass give a Rb-Sr whole rock isochron age of 124.2±10.8 Ma with an initial ratio (SrI) of 0.70597±0.00024, which is the oldest age among granitic rocks from Sanyo belt. The SrI of the Habu mass (0.7060) is similar to the granitic rocks from the western part of Sanyo belt, suggesting that the activity of granitic magma in Sanyo belt had started at least 124 Ma.