(Table 1) K and Ar content, and age determination of ODP Holes 126-792E and 126-793B

Results of conventional K-Ar dating on five samples from two sites from the Izu-Bonin forearc are presented. Two samples recovered from a volcanic edifice and overlying sediments drilled on the western side of the forearc basin (Site 792) indicate a basement age of 34 Ma. This is consistent with early Oligocene biostratigraphic ages from the overlying sediments. Three samples from the basement of Hole 793B at the center of the basin are not analytically distinguishable, with a best age of 27.1 +/- 0.6 Ma. This is slightly younger than the 30-33 Ma biostratigraphic and magnetostratigraphic estimates from the overlying sediments, suggesting that alteration processes have lowered the apparent K-Ar ages. These ages suggest that syn-rift volcanism occurred in a forearc location during the middle Oligocene.

本文呈现了采自伊豆-小笠原（Izu-Bonin）弧前带两个站位的5件样品的常规钾氩（K-Ar）定年结果。从弧前盆地西侧钻探获取的火山建造及其上覆沉积物中获得的2件样品（站位792）显示其基底年龄为34 Ma。该结果与上覆沉积物的早渐新世生物地层年龄相符。盆地中心793B钻孔基底的3件样品在分析层面无法区分，其优选年龄为27.1 ± 0.6 Ma。该年龄略低于上覆沉积物的生物地层与磁地层估算年龄（30~33 Ma），表明蚀变作用降低了表观钾氩定年值。上述年龄结果表明，中渐新世时期弧前带发生了同裂谷火山作用。