K-Ar ages and petrology of the Takinoue stage volcanic rocks in central Hokkaido, Japan: Geochemistry of volcanic rocks that form a shallow reservoir in the Yufutsu Oil and Gas Field

Abstract

The K-Ar ages and chemical compositions of Early Miocene volcanic rocks in southern central Hokkaido were determined to clarify the tectono-magmatic evolution of the back-arc basins of the Japan and Okhotsk seas. In the Yufutsu Oil and Gas Field, andesitic tuffs of the Takinoue Formation range in age from 14.2 ± 0.4 to 19.9 ± 2.4 Ma. Andesitic lavas of the Takinoue Formation, where they occur in the Umaoi Hills, are 19.1 ± 0.5 Ma in age, and andesitic breccias of the Sakae Formation in the Sakae region yield an age of 17.3 ± 0.5 Ma, which is within error of the age of the Takinoue Formation. These ages, combined with microfossil data, indicate that volcanism of the Takinoue stage occurred from 14 to 21 Ma. The Takinoue volcanic rocks in central Hokkaido show a wide range of trace-element abundances and isotopic ratios. Volcanic rocks of the Yufutsu field are characterized by depleted isotopic ratios (low ⁸⁷Sr/⁸⁶Sr and high ¹⁴³Nd/¹⁴⁴Nd values) and high Zr/Nb ratios, similar to those of mid-Miocene lavas from the NE Honshu arc associated with opening of the Japan Sea, and consistent with an origin via melting of a depleted mantle source. In contrast, other Takinoue stage volcanic rocks from central Hokkaido do not show depleted isotopic ratios and have low-medium Zr/Nb ratios. The wide range of geochemical abundances in the Takinoue stage volcanic rocks suggests that central Hokkaido may record a different tectono-magmatic history to that of the NE Honshu arc.