Strontium isotopic ratios of Tertiary volcanic rocks of northeastern Honshu, Japan: implication for the spreading of the Japan Sea

Strontium isotopic ratios of sixty-seven Tertiary volcanic rocks from the northeastern Honshu, Japan, were determined for the purpose of examining the genesis among the volcanic rocks. Two distince suites of volcanic rocks occur in the northeastern Honshu; the rocks older than 16 Ma (Monzen-Daijima Stege) of predominantly intermediate composition and the rocks younger than 16 Ma (Nishikurosawa-Funakawa Stege) with bimodal suite of mafic and felsic composition. Initial values of 87Sr/86Sr in the Teriary volcanic rocks from the northeastern Honshu, lie in the range from 0.7033 to 0.7068. High (87Sr/86Sr)I ratios are observed for the rocks older than 16 Ma from the Japan Sea side (H zone). It is noteworthy that the rocks younger than 16 Ma show significantly lower (87Sr/86Sr)I ratios in the Dewa Hill, Japan Sea coast and North Akita areas in the northeastern Honshu (L zone). The rocks younger than 16 Ma from the L zone can also be interpreted as having been originated as a mantle-diapir associated with the spreading of the Japan Sea basin. If the basaltic magma was formed from the diapir, the 87Sr/86Sr ratio would be close to the range from 0.7033 to 0.7037 as the low-Sr isotopic ratio zone (L zone) in the northeastern Honshu, Japan. (author)