Fission track ages of tuffaceous sandstone from the Toki Lignite-bearing Formation of the Mizunami Group in the Tono district, Gifu Prefecture, central Japan

Abstract

The depositional age of the Toki Lignite-bearing Formation of the lowermost part of the Miocene Mizunami Group, which is distributed in the southeastern part of Gifu Prefecture, central Japan, has been investigated by fission track FT dating and petrographic description of four tuffaceous sandstones.An investigation of the refractive indices of plagioclase suggests that the tuffaceous sandstones are a mixture of andesitic tuff and granitic detritus. Two age populations, -Ma early Miocene and -Ma late Cretaceous to early Tertiary were obtained by a statistical treatment of the distribution of zircon grain ages. The younger age cluster is regarded as the age of andesitic tuff and the older one is probably the age of detrital zircon derived from the Toki Granite -Ma . The younger age implies the depositional age of the middle part of Toki Lignite-bearing Formation. Based on the obtained and previously reported FT ages, the depositional age of the Toki Lignite-bearing Formation except for the basal conglomerate is inferred to range from to Ma. AbstractCThe Geological Society of Japan 2006 459

Spatial distribution of the apatite fission‐track ages in the Toki granite, central Japan: Exhumation rate of a Cretaceous pluton emplaced in the East Asian continental margin

“…After passing the AFT PAZ, the t–T path in each sample (Figure ) became low‐gradient and then attained the same temperature level as that in the present day at 30–20 Ma, indicating that the Toki granitic body was exposed to the surface prior to this time. The depositional age of the Mizunami Group overlying the Toki granite corresponds to the period of 20–18 Ma (Sasao et al, ). These temporal relations can explain the time sequence from the exposure of the Toki granite to the formation of the Mizunami Group.…”

“…The Nohi rhyolite, a pyroclastic flow deposit, formed prior to the intrusion and emplacement of the Toki granite (zircon CHIME age 85 AE5 Ma and allanite CHIME age 86 AE7 Ma; Suzuki, Nakazaki, & Adachi, 1998). The Miocene Mizunami Group, which has an age of 20-18 Ma (Sasao, Iwano, & Danhara, 2006), is composed of fluvial, lacustrine, and neritic sediments (Itoigawa, 1974(Itoigawa, , 1980. The Toki granite is unconformably overlain by the Mizunami Group (Itoigawa, 1974(Itoigawa, , 1980.…”

Timing of clockwise rotation of Southwest Japan: constraints from new middle Miocene paleomagnetic results

“…Zircon fission track results suggest that the group was deposited in the early to middle Miocene (Kobayashi 1989;Sasao et al 2006Sasao et al , 2011, although no data have been reported for the Oidawara Formation. Paleomagnetic polarity and marine microfossil data, including biostratigraphic data for planktonic foraminifera, radiolaria, and diatoms, constrain the age of the shallow marine sediments within the Mizunami Group (Kawamura et al 2011, and references therein).…”

Petrographic study of the Miocene Mizunami Group, Central Japan: Detection of unrecognized volcanic activity in the Setouchi Province

“…) is younger than the depositional age of the Mizunami Group (20–15 Ma; Itoigawa ; Sasao et al . ). Therefore, at present, there are no candidate volcanic sources for the supply of volcanic materials to the Mizunami Basin.…”

“…More recently, Sasao et al . () reported that tuffaceous sandstones in the lowermost part of the Mizunami Group are composed of andesitic tuff and clastic sediments, which were likely derived from granitic rocks, indicated by their heavy mineral and plagioclase compositions.…”