ABSTRACT

K-Ar age determinations were carried out on two lamprophyre dikes intruded into the Sambagawa metamorphic rocks at Shingu in the central part of Shikoku Island, Southwest Japan. These dikes are very distinctive among Neogene igneous rocks in Shikoku because of their abundant mantle xenoliths and carbonate minerals in the fresh groundmass, and of their unique chemical compositions. In order to eliminate the effect of extraneous argon from xenoliths and carbonates, removal of large crystals and acid leaching experiments were done for three grain size fractions. Apparent ages ranging from 18.6±0.7Ma to 21.6 ±2.0Ma were obtained for both rocks. However, positive correlation between apparent ages and atm. ⁴⁰Ar% suggests that samples contain initial argon which is not atmospheric. The well defined isochron age of 17.7±0.5Ma is obtained from all samples with the initial ⁴⁰Ar/³⁶Ar ratio of 310±5 in the ⁴⁰Ar/³⁶Ar-⁴⁰K/³⁶Ar diagram. As the initial ⁴⁰Ar/³⁶Ar ratio is higher than that of atmosphere, apparent ages do not show any geologic meaning, and the isochron age of 17.7±0.5Ma gives the intrusive age of dikes. The Shingu lamprophyres are much older than other Miocene igneous rocks in Shikoku ranging in age from 14 to 12Ma, and both magmatisms could be genetically independent. As the Shikoku Basin kept opening in a northeast-southwest direction at 18Ma, it could not subduct significantly beneath Southwest Japan from the south when the Shingu lamprophyres intruded into the Sambagawa metamorphic belt. Petrogenesis of the Shingu magma could be related to the rifting of the back arc basin, the Japan Sea.