Sr isotopic compositions of some Miocene volcanic rocks from the basin of the Kitakami river, northeast Japan

Abstract

Rb and Sr concentrations and Sr isotopic compositions (8YSr/86Sr) have been determined for Lower and Middle Miocene basalt, andesitic basalt, andesite and dacite of the Inase, Ishikoshi, Nonodake and Matsushima volcanic rocks in the Kitakami river basin and for Upper Miocene basalt in the Jyoge district to the west of Sendai city. The volcanic rocks of the former region are composed of those belonging to the hypersthenic and pigeonitic rock series proposed by KUNO (1950). The analysed ten volcanic rocks have 87Sr/86Sr ratios ranging from 0.7036 to 0.7040. This range is nearly identical for both series and is similar to that of Lower Miocene Ryozen volcanic rocks from the northeastern part of Fukushima Prefecture. The strontium isotopic data suggest that the primary magmas which produced the volcanic rocks in the Kitakami river basin and the Jyoge district might have been originated from the upper mantle peridotitic material chemically similar to the source material of the Ryozen volcanic rocks. Five new chemical analyses and the published data for these volcanic rocks are not divided into the distribution fields of the hypersthenic and pigeonitic rock series in the AFM, (FeO+Fe2O3×0.9)/MgO-SiO2 and (FeO+Fe2O3×0.9)/MgO-(FeO+Fe2O3×0.9) diagrams. Therefore it is not valid to regard that the hypersthenic and pigeonitic rock series are equivalent respectively to the calc-alkalic and tholeiitic rock series. It is reasonable to consider, in the case of these volcanic rocks, that the essential factor for producing the hypersthenic or pigeonitic rock series is ascribed not to the apperance of magnetite or amphibole with high Fe/Mg ratio at the early stage or the middle stage of fractionation of magma but to a slight difference in the temperature condition of the magma at the stage of crystallization of minerals in the groundmass. There is also no systematic difference in K2O/Na2O ratios between the above two rock series. This evidence and Sr isotopic feature show that the genesis of the volcanic rocks of the hypersthenic rock series can not be ascribed to contamination of granitic material in the magma.