Abstract
The underground volcanic activity before middle Miocene in the Niigata oil and gas fields are divided into three periods; late Oligocene (27-23Ma), early Miocene (19-16Ma) and middle Miocene (16-13Ma).
These volcanism might have taken place in close association with multiple formations of NS-trending grabens. The rifting of the Japan Sea might have caused the formation of the first stage graben of late Oligocene to early Miocene. During the spreading of the Japan Sea, the second stage graben has been formed in submarine environments. Within this submarine deep graben, middle Miocene basaltic and rhyolitic rocks were extensively accumulated and they have been buried in the level deeper than 5, 000-000m since middle Miocene.
The deep seated early to middle Miocene basatts can be divided into three different groups based on initial Sr isotopic ratios (Sri) and εNd values. Sri and εNd values expected for unaltered samples of each group are as follows; Sri=0.7032-0.7033 and εNd=+6-+8 for group I, Sri=0.7038-0.7040 and εNd=+4-+5 for group II, Sri=0.7045-0.7050 and εNd=0-+2 for group III. Middle Miocene rhyolites from the Tsugawa and Tadami areas show significantly higher Sri ratios (0.7055-0.7080) and lower εNd values (-8-0) than the basaltic rocks.
Basaltic magmas of group III are considered to have formed from the sub-island arc lithospheric mantle with an undepleted isotope signature, while those of groups I and II are from the hot mantle with a depleted isotope signature, upwelling from the athenosphere to the sub-island arc lithospheric mantle during the spreading of the Japan Sea. Genesis of rhyolitic rocks from the underground and surface outcrops in the Tsugawa and Tadami areas may be ascribed to the partial melting of the lower crust.
