Chemical compositions and strontium isotopic ratios of the Tertiary volcanic rocks from southern Kanto, central Japan

Abstract

The major elements and strontium isotopic ratios for nine Tertiary volcanic rocks from southern Kanto, central Japan have been determined. Four of them are bronzite andesites from Choshi, eastern end of Boso Peninsula, and the others are high-alkali tholeiite basalts from Kamogawa (three), South Boso Peninsula, and from Yokosuka (one), Miura Peninsula, and from Oiso (one), Kanagawa. All are submarine volcanic products and intruded into Miocene to Oligocene strata.
The major elements of the bronzite andesites seem quite similar to those of the olivine andesites, one group of the contemporaneous volcanic rocks in Setouchi volcanic provinece. The initial ratios of ⁸⁷Sr/⁸⁶Sr in the Choshi bronizite andesites are for 0.70399-0.70417. In contrast, those in the olivine andesites in the Setouchi province are as high as 0.70494-0.70710. These data are indicative of upper mantle heterogenity for Sr isotopes beneath the Japan arc in Tertiary.
The high-alkali tholeiite basalts have Sr isotopic ratios ranging from 0.7036 to 0.7039. Although some basalts from Kamogawa have the high initial ratios (0.70633-0.70576), this is probably due to the alteration by rock-sea water interaction.