The Rb-Sr Whole Rock Isochron Age and Genesis of Oki Trachyte-rhyolite Group in Oki Dogo Island

Abstract

The precise age for the Oki trachyte-rhyolite group was determined by Rb-Sr whole rock isochron method. This trachyte-rhyolite group was formed by a large scale ring fissured eruption on the Oki Dogo Island in the Japan Sea. A regression of the Rb-Sr isotopic data yields an age of 6.8±0.2 Ma with an initial Sr isotopic ratio (Sri) of 0.707176±0.000080 (2a). This age is consistent with the K-Ar age of 6.3 Ma determined by KANEOKA (1970), and is similar to the activity ages of alkali basalts occurring in Oki Dozen Islands near the Oki Dogo Island. The fission track age of 3.6 Ma by SUZUKI (1970) may be caused by some late annealing events. The Sri determined for the Oki trachyte-rhyolite group is very close to that of basic granulite xenoliths included in some alkali basalts. The lower crust beneath the Oki Dogo Island is considered to be composed of these basic granulites which are very poor in alkali metals. It is proposed that the magma of the Oki trachyte-rhyolite group was formed by partial melting of the lower crust with leaving the basic granulite as a residue.