中央北海道南部の後期漸新世火山岩の岩石学的検討―背弧海盆拡大の初期火山活動―

Abstract

The Oligocene volcanism in central Hokkaido may have taken place as the volcanic arc of the Eurasian continental margin prior to the formation of the Japan Sea back-arc basin. The Oligocene volcanic rocks in the Minami-Naganuma Formation recovered from the Nanporo drill cores consist of alkali olivine basalt, basaltic andesite, and andesite, and is classified into Lower (3742.2-3744.6 m depth), Middle (3488.0-3489.4 m), and Upper (3243.2-3245.1 m) members. The Oligocene volcanic rocks are characterized by greater large ion lithophile element and rare earth element enrichments, and also show a depletion in high-field strength elements. The Oligocene volcanic rocks have enriched isotopic compositions (higher 87Sr/86Sr and lower 143Nd/144Nd), similar to the Early Miocene lavas in the precursory stage of the Japan Sea. The geochemical characteristics of the Oligocene volcanic rocks are consistent with magma generation beneath central Hokkaido area from subduction-modified lithospheric mantle. The felsic andesites from the Middle member have adakite-like characteristics; high Sr/Y ratios, low Y contents, which can be produced in garnet stability field. In contrast to the felsic andesites, the basalts and basaltic andesites in the Lower and Upper members have similar isotopic compositions with systematically different REE abundances. Alkali basalts from the Upper member was inferred to have generated through small degrees of partial melting at pressure >2 GPa (>70 km depth), which is similar to that for the Quaternary NE Honshu back-arc basalts. In comparison with the NE Honshu arc magma, petrological evidence of the Oligocene basalts is consistent with the existence of a thick lithospheric lid beneath central Hokkaido.