K-Ar ages of the Miocene Ryozen basalts from the northern margin of the Abukuma Highland, Japan.

Abstract

K-Ar ages were determined for Miocene Ryozen basalts from east of Fukushima City. Northeast Japan. The Ryozen basalts mainly comprise primitive basalt lava flows of the island are tholeiite series with subordinate amounts of pyroclastic flows. Ten basalt specimens gave almost identical K-Ar ages of 12.4-16.0 Ma, corresponding to Middle Miocene age. If we combine the data of previous studies with the present ones, we can conclude that the Ryozen district have two periods of activities in Miocene. A weak andesitic volcanism took place about 22 Ma, whereas very intence basaltic activity occurred about 13-16 Ma. The volcanic front of 13-16 Ma was estimated to lie 30-50km east of the Quaternary volcanic front.

Contribution of slab melting and slab dehydration to magmatism in the NE Japan arc for the last 25 Myr: Constraints from geochemistry

“…The age of Takadate samples in this study is 12.2–14.7 Ma, corresponding to the more recent phase. Two eruption phases have been also reported for Ryozen at 21.3–22.1 Ma and 12.4–16.0 Ma [ Tsunakawa et al , 1983; Ohki et al , 1993a]. Among the samples from Ryozen, RZ‐4 shows a younger age than the others.…”

“…Previous K‐Ar age data dated magmatic activity at Choshi at 21 Ma [ Seno et al , 1986], the Tomari volcanism at 14 Ma [ Watanabe et al , 1993] and the Mitaki volcanism at 8 Ma [ Uto et al , 1989]. The binary volcanic activities have been identified for Ryozen formations (22 and 15 Ma [ Tsunakawa et al , 1983; Ohki et al , 1993a]) and the Takadate formations (22 and 15 Ma [ Uto et al , 1989]). We chose the samples from the Iwate Volcano as a representative reference of the Quaternary NE Japan arc (Figure 1).…”

Shifting of the volcanic fronts during Early to Late Miocene in the Northeast Japan arc

“…These volcanic rocks can be classified mostly to the island arc tholeiite series and calc-alkaline series, however, from their mineralogy and major and trace element chemistry, they can be partially classified to the alkaline series. Ohki et al 1993). This evidence suggests that the Ryozen sequence has two different groups of ages of -22Ma and 13-16Ma.…”

“…However recent K-Ar age determinations for the Miocene volcanic rocks showed that they were formed by a nearly simultaneous volcanism of Volcanic fronts in the Northeast lapan arc 89 Middle Miocene age (13-16Ma) (Shuto et al 1992a;Ohki et al 1993;Watanabe et al 1993). These rocks were composed mainly of andesites with subordinate amounts of basalts and dacites and occurred in the trench side of the NE Japan arc.…”

“…However recent K-Ar age determination for the Miocene volcanic rocks from the trench side of the NE Japan arc (Shuto et al 1992a;Ohki et al 1993;Watanabe et al 1993), and the available radiometric age data for the Tertiary volcanic rocks provided by many previous workers, have made it possible to distinguish the three different volcanic fronts in the NE Japan arc in temporal and spatial distribution. This paper shows the three different temporal and spatial distributions of the volcanic fronts in the NE Japan arc and discusses the tectonic significance of the shifting of volcanic fronts during the Early to Late Miocene.…”