福島県東北部の霊山火山岩類のSr同位体比

抄録

Rb and Sr concentrations and Sr isotopic compositions (Sr⁸⁷/Sr⁸⁶) have been determined for five basalts, four andesitic basalts, one andesite and one obsidian of the Ryozen formation.
These eleven volcanic rocks have initial Sr⁸⁷/Sr⁸⁶ ratios ranging from 0.7033 to 0.7046. These ratios may be nearly identical among the different rock types.
The Sr⁸⁷/Sr⁸⁶ ratios of the basaltic rocks are slightly higher than those of occeanic ridge tholeiites but very similar to those of tholeiitic basalts from Hawaiian Islands and Type II island arc such as the Izu-Bonin Islands, Mariana Islands, Fiji Islands and New Britain Islands.
On the other hand, the Sr⁸⁷/Sr⁸⁶ ratios of acid members are very resemble to those of acid volcanic rocks from Type II island arc and they also lie in the lower part of the range measured in acid volcanic rocks from Type I island arc such as the Japanese Islands and New Zealand, and from Cenozoic orogenic belt in the United States and Central America.
The Rb concentrations and K/Rb ratios of basaltic rocks in the Ryozen formation are
nearly comparable to those of the tholeiitic basalts from Hawaiian Islands and Type II island arc, while the Sr concentrations are close to those of oceanic ridge tholeiites.
Considering from the results of the present work and previous petrological studies, it may be concluded that the siliceous basaltic and acid volcanic rocks of the lower and upper horizons are derived by fractional crystallization of olivine tholeiitic magma which produces olivine basalts in the middle horizon. An alternatively possibility is that olivine tholeiitic, siliceous basaltic and acid magmas may be derived directly from certain upper mantle material presumably of peridotitic composition, which has Sr⁸⁷/Sr⁸⁶ ratios around 0.704.