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Paper Number: 1512

U–Pb evidence for rapid formation of the Nohi Rhyolite, one of the largest caldera-forming volcanic complexes on Earth

Hoshi, H.1, Iwano, H.2, Danhara, T.2 and Sako, K.3

1 Aichi University of Education, Kariya, Aichi 448-8542, Japan; hoshi@auecc.aichi-edu.ac.jp 2 Kyoto Fission-Track Co., Ltd, Kita-ku, Kyoto 603-8832, Japan
3 Aichi University of Education, Kariya, Aichi 448-8542, Japan

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The Nohi Rhyolite in central Japan consists of voluminous late Cretaceous felsic rocks (ignimbrites, lavas, volcaniclastics, intrusives, etc.) that form a caldera-related volcanic complex, and is thought to have an original volume attaining 5000–7000 km3 [1]. This complex contains some large ignimbrite sheets, with the largest one that has an original volume of more than 2200 km3, roughly equivalent to the Fish Canyon Tuff in North America [2] and the Toba Tuff in Sumatra [3]. Repeated caldera-forming eruptions resulted in the emplacement of huge volumes of erupted materials [4]. Previous K-Ar, Ar-Ar, Rb-Sr, fission-track and CHIME (chemical Th-U-total Pb isochron method) age determinations have led to an interpretation that the volcanic activity is divided into two major episodes [1]; the early episode corresponds to lower two units (Nohi-1 and Nohi-2; 85–80 Ma) and the late episode corresponds to upper four units (Nohi-3 to Nohi-6; 75–68 Ma). However, our new zircon U-Pb dates presented here indicate a much shorter duration of volcanic activity (Figure 1). On the basis of the new results, caldera-forming eruptions in the early episode are interpreted to have occurred between 72 and 70 Ma. At present we have only one age determination for the late episode units (we will have more data at the congress), and the result for Nohi-4 shows a date of about 70 Ma. These new results, together with previous data for the late episode, suggest that the Nohi Rhyolite formed within a few million years, probably 4 Myr or less, at around 70 Ma. If this is correct, then we obtain the average magma production rate of caldera-forming eruptions per unit area for the Nohi Rhyolite, which is estimated as 3–5 × 10−4 km3/yr·km2. This estimate is one order of magnitude greater than the previous calculation [5] and similar to the rates for the Yellowstone [6] and San Juan [7] volcanic fields in North America.

References:

  1. Yamada N et al. (2005) Nohi Rhyolite: Association for the Geological Collaboration in Japan, Tokyo, p.183

  2. Steven TA and Lipman PW (1976) Calderas of the San Juan Volcanic Field, Southwestern Colorado: USGS Professional Paper 958, p.35

  3. Chesner CA (1998) J Petrol 39: 397-438

  4. Koido Y (1991) Bull Volcanol 53: 132-146

  5. Yamamoto T (2003) Island Arc 12: 294-309

  6. Christiansen RL (2001) The Quaternary and Pliocene Yellowstone Plateau volcanic field of Wyoming, Idaho, and Montana: USGS Professional Paper 729-G, p.145

  7. Lipman PW (1984) J Geophys Res 89: 8801-8841

Figure 1: Zircon U-Pb dates of the Nohi Rhyolite (red symbols). Se, Selandian; Da, Danian; Ma,

Maastrichtian; Ca, Campanian.