Stratigraphic comparisons of the chemical characteristics and K-Ar ages of Miocene volcanic rocks from the Daigo and the Motegi districts

Abstract

Chemical composition and K-Ar age of Miocene volcanic rocks were determined in the Daigo district, Ibaraki Prefecture. K-Ar ages of biotite for the volcanism range from 18.0 to 15.5 Ma. Compiling stratigraphy and isotopic ages revealed three magma types between N6 and N8 of planktonic foraminineferal biostratigraphy in the Daigo district. Early and middle volcanisms in the district were active as calc-alkali magma and island arc tholeiitic magma. Late volcanism is of oceanic tholeiitic magma.
We compared the chemical characteristics of volcanism and the stratigraphic succession in the Daigo district with that found in the Motegi district of Tochigi Prefecture based on a review of the literature. CHRON stratigraphy placed the Early Miocene formations of the Motegi district between C5En and C5Br which corresponds to the zone between N6 and N8 of planktonic foramineferal biostratigraphy. In addition, isotopic age analysis of the volcanism revealed that these formations range in age from 18.6 to 15.9 Ma, indicating that the Motegi Formation is thus comparable with the early Miocene formations of the Daigo district. Moreover, icelandites from the Motegi Formation are comparable with icelandites of the Nantaisan Volcanic Breccia in the Daigo district, and stratigraphy revealed that twice oceanic-type volcanisms occurred from N6 to N8 in these districts. Twice oceanic-type volcanisms took place in the opening-stage of the Japan Sea. The former is related with the rifting, and the latter with the strike-slip fault during the opening of the Japan Sea.