2003 Volume 54 Issue 11-12 Pages 365-393
Petrographical characteristics such as grain-size composition, mineral assemblages, heavy mineral assemblage, and shape of volcanic glass shards were systematically described for the 101 tephra beds (133 samples) intercalated in the Pliocene Dainenji Formation of the Sendai Group distributed in the Pacific side of Fukushima Prefecture, northeastern Japan. EDS chemical analysis of 1029 points of volcanic glass from 98 tephras (103 samples) was also performed, and the secular changes in petrological feature and chemical composition of glass shards (especially the amount of K2O) were clarified. Tephra beds of the Dainenji formation are mainly fine-grained glassy tephra (Ctype glass subject), and usually contain hornblende phenoclasts. Chemical composition of glass shards is in the range of 68.8-80.0 wt.% SiO2. Tephras in the Dainenji Formation are divided into Low-K,Medium-K and High-K groups on the basis of K2O contents. At least two upward K2O decreases from Medium-K to Low-K are recognized, possibly reflecting the compositional zoning at the top of magma reservoir. Six tephra beds are clearly distinguished from other tephra by higher K2O contents and the presence of biotite phenoclasts, and might be widespread tephra beds derived from central Japan.
