39Eruption history and magma system of caldera clusters in East HokkaidoJournal of Mineralogical and Petrological Sciences, Volume 107, page 39─43, 2012

doi:10.2465/jmps.111020hT. Hasegawa, hasegawt@mx.ibaraki.ac.jp Corresponding author

LETTER

The eruption history and silicic magma systems of caldera-forming eruptions in eastern Hokkaido, Japan

Takeshi Hasegawa*, Mitsuhiro Nakagawa** and Hiroshi kisHimoto***

*Department of Earth Sciences, College of Science, Ibaraki University 2-1-1, Bunkyo, Mito 310-8512, Japan **Department of Natural History Sciences, Graduate School of Science, Hokkaido University,

N10 W8, Kita-ku, Sapporo 060-0810, Japan ***Department of Disaster Prevention, Asia Air Survey Company with Limited Liability,

1-2-2 Manpukuji, Asao-ku, Kawasaki-shi, Kanagawa Prefecture 215-0004, Japan

The eruptive history and magma systems of large-scale explosive eruptions (VEI >5) in eastern Hokkaido, Ja-pan, are reviewed on the basis of recently reported high-resolution tephrostratigraphy. More than 70 large-scale explosive eruptions have been recorded from the Akan, Kutcharo, Atosanupuri, and Mashu caldera volcanoes in the past 1.7 Ma. The total tephra volume of these eruptions is estimated to be approximately 1000 km3. The dis-charge rate increases remarkably from 0.2 km3/kyr to 2.0 km3/kyr at approximately 0.2 Ma. The discharge rate is still high owing to the recent frequent activity of the Mashu caldera. The silicic magma systems of the Akan, Kutcharo, and Mashu calderas formed independently. On the other hand, the magma of Atosanupuri is associat-ed with that of Kutcharo caldera.

Keywords: Silicic magma, Caldera-forming eruptions, Akan, Kutcharo, Mashu

INTRODUCTION

Four Quaternary caldera volcanoes—Akan, Kutcharo, Atosanupuri, and Mashu—are clustered within the 50 km2 area of eastern Hokkaido at the southern end of the Kurile arc in Japan (Fig. 1). The diameters of Akan and Kutcharo calderas are each more than 20 km, whereas Atosanupuri and Mashu are small calderas located within and on the rim of the Kutcharo caldera, respectively. In eastern Hokkaido, the concentration of large-scale explosive eruptions (VEI >5; this high index could be related to caldera formation) has migrated from Akan to Mashu; that is, from west to east (Hasegawa, et al., 2006). The latest large-scale eruption was that of Mashu approximately 1 ka (Kishi moto, et al., 2009), which resulted in the formation of the 1.5 × 1.2 km Kamuinupuri crater. To forecast the next large-scale, caldera-forming eruption, it is necessary to evaluate the long-term activity and investigate the spatiotemporal changes of the magma systems in this volcanic area. Thus, in this paper, we review the high-resolution tephrostratigraphy data of the four calderas in the ascending order on the basis of recent study results, and we discuss the eruptive history and

silicic magma systems comprehensively.

TEPHROSTRATIGRAPHY OF EACH CALDERA

Akan caldera (Hasegawa and Nakagawa, 2007; Hasegawa et al., 2009a)

This caldera is a rectangular-shaped structure (24 km × 13 km) with a complex history of caldera-forming eruptions. The pyroclastic deposits from the Akan caldera (Akan PD) are divided into at least 40 eruptive units, some of which are separated by paleosols that represent significant time intervals between eruptions. These units are summarized into 17 eruptive groups of Ak1-Ak17 in the descending stratigraphic order, each of which is composed of a series of eruptive units (Fig. 2). Although the juvenile materials of the Akan PD are commonly composed of two-pyroxene andesite to dacite pumice and scoria, these 17 groups are each chemically distinct. Some eruptive units are intercalated with pyroclastic deposits related to the formation of the Kutcharo caldera (Kutcharo PD) and six distal air-fall ash layers from central Hokkaido (HR-1−HR-6). The radiometric ages of the exotic tephras range from 1.46 Ma to 0.21 Ma, suggesting that the caldera-forming eruptions of the Akan volcano

40 T. Hasegawa, M. Nakagawa and H. Kishimoto

occurred over a period of more than 1 million years.On the basis of lithic component analysis of its

pumice fall deposits, the petrology of its juvenile materials, and the Bouguer anomaly, the Akan caldera is considered to be a composite caldera with a rectangular shape reflecting the distribution of multiple vent areas.

Kutcharo caldera (Katsui and Sato, 1963; Okumura, 1991; Hasegawa et al., 2011)

The 26 km × 20 km Kutcharo caldera is the largest caldera in Japan. The caldera-forming eruption began with a large-scale pyroclastic flow that produced Furume Welded Tuff (FWT) at 400 ka (Fig. 2). Subsequent cal-dera-forming eruption units, the Kutcharo pumice flows VIII–I in ascending order (KpVIII to KpI), proceeded during 210 ka–35 ka, with dormancies of 20 kyr to 40 kyr. KpIII and KpII are the products of the same eruption unit, known as KpII/III. KpIV (115 ka-120 ka) is the most voluminous unit at 175 km3 [It should be noted that volumes in this paper are not corrected to dense rock equivalent (DRE)]. Five eruption units composed of only pumice fall (Kpfall-V-I) are also recognized above KpIV. The juvenile materials of the Kutcharo PD are mainly two-pyroxene dacitic to rhyolitic white pumice. The FWT characteristically contains quartz phenocrysts.

Atosanupuri caldera (Hasegawa et al., 2009b)

The Atosanupuri caldera, relatively small at approxi-mately 6 km in diameter, formed within the Kutcharo caldera rim. Large-scale pyroclastic deposits associated with the Atosanupuri caldera formation (Atosanupuri PD) are Chanai-c (Ch-c) and Nakashumbetsu Upper-a –c and -e (Nu-a, Nu-c and Nu-e). The largest eruption unit is Ch-c at 6.9 km3, ~ 20 ka. The juvenile material of the Atosanupuri PD is commonly two-pyroxene dacitic white pumice.

Mashu caldera (Kishimoto, et al. 2009; Hasegawa et al., 2009b)

The 7.5 km × 5.5 km Mashu caldera was formed on the eastern rim of the Kutcharo caldera and includes the Kamuinupuri crater in its eastern region. The present Mashu caldera and Kamuinupuri crater were formed by large-scale explosive eruptions after 12 ka. These deposits are described as Ma-b (~ 1 ka) to Ma-l (~ 12 ka) in descending order. The largest eruption unit at 18.6 km3, known as the Mashu main caldera-forming eruption (Ma-mcf), is composed of deposits Ma-j–Ma-f, the age of which was estimated to be 6.7 ka by accelerator mass

100km

45°

145°

43°

143°141°

Hokkaido

Akan

Atosanupuri

MashuKamui-nupuri

Kutcharo

10 km

Figure 1

Figure 1. Shaded relief map including caldera rims, demonstrating the digital topography of the eastern Hokkaido caldera cluster il-luminated by light from the northwest direction.

Figure 2. Diagram showing the Quaternary tephrostratigraphy and chronology in eastern Hokkaido. Note that the eruption center has shifted eastward with time. The ages of the Mashu PD were determined by Hasegawa et. al., (2009b) and Yamamoto et al. (2010). * Machida and Arai (2003); ** Hasegawa et al., (2010); *** Hasegawa et al. (2008); † Ishii et al. (2008); ‡ Matsui and Mat-suzawa (1985); § Sagawa et al. (1984).

Spfa-1

Aso-4

Toya

HR-1

HR-2

HR-4

HR-5

Ds-Oh

HR-6

HR-3

Ak1

Ak2

Ak3

Ak4

Ak5

Ak6

Ak7

Ak8

Ak9

Ak10

Ak11

Ak12

Ak13

Ak14

Ak15

Ak16

AK17

Ak6-aAk6-bAk6-cAk6-dAk7-aAk7-bAk7-cAk7-dAk7-e

Ak9-aAk9-bAk9-c

Ak12-aAk12-bAk12-c

Ak14-aAk14-bAk14-cAk14-dAk14-eAk14-fAk14-gAk15-aAk15-bAk15-cAk15-dAk15-eAk15-f

Ak13-aAk13-b

Ch-c

Nu-a

Nu-c

Nu-e

Ma-bMa-c

Ma-eMa-d

Ma-mcfMa-kMa-lCh-aCh-b

Ch-d

Nu-b

Nu-d

Nu-f

Nu-hNu-iNu-l

Nu-mNu-nNu-oNu-pNu-r

1.3 Ma‡

1.46 Ma§

0.51 Ma**

1.0 Ma†

400 ka**

115~120 ka*

0.76 Ma***

40~45 ka*

85~90 ka*

112~115 ka*

210 ka**

Kp IV

Kp V

Kp VI

Kp VIIKp VIII

FWT

Kp I

Kp II/III

Kpfall I

Kpfall IIKpfall III

Kpfall IVKpfall V

Distalash Akan Kutcharo

Atosa-nupuri Mashu

eruption unitincludingpyroclastic flow

eruption unit ofpyroclastic fall

west eastFigure 2

HkP

3,660 yBP

6,510~6,920 yBP10,130 yBP11,930 yBP

23,430 yBP

27,380 yBP

27,970 yBP

32,640 yBP35 ka

4,720 yBP

30,320 yBP

980 yBP1700~1850 yBP

41Eruption history and magma system of caldera clusters in East Hokkaido

spectrometry (AMS) 14C dating. The 4.6 km3 Ma-b eruption resulted in the formation of the Kamuinupuri crater. Ma-l is also a large eruption unit at 6.6 km3. Between Ma-l and KpI, at least 15 large-scale eruption units are described and include Nu-r, Nu-p, Nu-o, Nu-n–Nu-l, Nu-i, Nu-h, Higashikayano Pumice (HkP), Nu-f, Nu-d, Nu-b, Ch-d, Ch-b, and Ch-a. Geological and petrological data indicate that these units were produced by the Mashu volcano. Widespread 32.6 ka Daisetsu-

Ohachidaira (Ds-Oh) tephra from central Hokkaido is sandwiched between Nu-r and Nu-p. The juvenile materials of the Mashu PD are commonly two-pyroxene andesite to dacite.

DISCUSSION

Discharge rate and eruption history

The tephra volumes of the more than 70 eruptive units were calculated by the common method as follows. The volumes of pyroclastic flow deposits excluding co-

ignimbrite ash were estimated by their distributions and average thicknesses. The volumes of pyroclastic fall deposits were determined from detailed isopach maps using the method of Hayakawa (1985). The volumes of poorly exposed layers were roughly estimated by the ratio of their thicknesses to clearly defined layers at the same location. The total volume of these deposits was estimated to be approximately 1000 km3 (Fig. 3). In addition, we determined the age of each eruption unit through the loess chronometry method (Hayakawa, 1995) on the basis of radiometric age data and thickness of loess/soil layers at the type localities. The average discharge rate during the past 1.7 Myr in this area is estimated to be approximately 0.6 km3/kyr. Global compilation of 170 volcanoes and volcanic fields revealed that the long-term rates for silicic

eruptions worldwide range from <10−5 km3/kyr to 101 km3/kyr (White et al., 2006). According to these data, the East Hokkaido Caldera Cluster (EHCC) represents a high rate of silicic output. The discharge rate of EHCC is comparable to that of the Altiplano-Puna Volcanic Complex (APVC) of the Central Volcanic Zone of the Andes (de Silva et al., 2007) where 21 large-scale eruptions have occurred during the past 10 Ma. The highest rates in the world are recorded at Yellowstone in North America and the Taupo Volcanic Zone (TVZ) in New Zealand, both of which are well known as regions with the most intense Quaternary silicic volcanism. Altho-ugh the volcanisms of Yellowstone and the TVZ are similar in size (~ 100 km square area), longevity (~ 2 Myr), and magma output rate (5-7 km3/kyr), the TVZ contrasts with Yellowstone in its high frequency (>34 eruption units), but relatively small size (>75 km3/unit), of caldera-forming eruptions (Houghton, 1995). In

0

200

400

600

800

1000

1200

0200400600800100012001400160018002000

Age (ka)

Vol

ume

(km

3 )

Ak17Ak13-a

Ak10Ak7-c

Ak4

FWTKpVIII

Ak2

KpIV

KpI

30 20 10

200

150

100

50

Ma-mcfMa-lCh-c

Nu-eNu-r

KpI

Figure 3

Figure 3. A diagram showing the age versus cumulative volume of large-scale eruptions derived from the East Hokkaido caldera cluster (Akan, Kutcharo, Atosanupuri, and Mashu).

FWT

SiO2 (wt.%)

Ba/

ZrK

2O (w

t.%)

3.0

2.5

2.0

1.5

1.0

0.5

0.0

7

6

5

4

3

2

阿寒屈斜路

アトサヌプリ摩周

Akan PDKpII/III-VIII, FWTKpIAtosanupuri PDMashu PD

50 55 60 65 70 75 80

Low-KMedium-K

Figure 4Figure 4. Diagrams of the variations in SiO2 versus K2O (upper) and Ba/Zr (lower) in the whole-rock chemistry of juvenile mate-rials from Akan, Kutcharo, Atosanupuri, and Mashu PDs. The boundary between low- and medium-K was determined by Gill (1981).

42 T. Hasegawa, M. Nakagawa and H. Kishimoto

comparison, the EHCC could be evaluated as a region of exceptionally frequent caldera-forming eruptions (>70 eruption units during the past 1.7 Myr), even though the scales of the eruptions are relatively small. These contrasts in eruption types by frequency and size reflect the differences in geological setting among stable continental (Yellowstone and APVC) and arc-trench (TVZ and EHCC) systems.

The time vs. cumulative volume diagram in this area indicates that the discharge rate increases significantly from 0.2 km3/kyr to 2.0 km3/kyr at the dormancy between FWT (0.40 Ma) and KpVIII (0.21 Ma). Although this high output rate is attributed to the existence of Kutcharo PD, Ak2 and Ak1 from Akan caldera also contribute. Moreover, the high output rate does not appear to have decreased during the last 35 kyr, which represents the active stages of Atosanupuri and Mashu. The Mashu caldera’s eruptions were frequently more explosive and voluminous than those of Atosanupuri.

Silicic magma systems beneath each caldera

As mentioned in the previous subsection, the juvenile materials of the more than 70 eruption units show similar petrography of two pyroxenes without hydrous minerals. However, the chemical compositions of the calderas are each distinctive. The whole-rock compositions (nor-malized to 100% anhydrous) of the juveniles of the Mashu PD are classified into a low-K group (Gill, 1981) and are clearly distinguishable from the rocks of the other calderas (Fig. 4). The SiO2 contents of the Mashu PD are also clustered in a dactic (65-72 wt%) field, whereas those of the Kutcharo PD are in a rhyolitic (72-76 wt%) field. The Akan PD shows a wide range of SiO2 and various K2O contents. The Kutcharo PD can also be identified by a higher Ba/Zr ratio (>3.5) that in the Akan and Mashu PDs (No data are available on the compositions of Atosanupuri PD trace elements). These results suggest that the magma systems of the Akan, Kutcharo, and Mashu calderas differ and have been independently constructed. In this case, the previously mentioned increase of discharge rate may be affected by the change in local stress field of this volcanic region rather than the evolution of magma system.

The petrological features of the Atosanupuri PD are similar to those of the Kutcharo PD, particularly in the youngest KpI, which implies that the Atosanupuri magma originated from the residual KpI magma. This result is consistent with the location of the Atosanupuri caldera. The petrology of these silicic magma systems will be dealt with in greater details in future studies.

CONCLUSION

Our analysis of the four caldera volcanoes in eastern Hokkaido is summarized in the following points: (1) The studied area has experienced more than 70

large-scale eruptions during the past 1.7 Myr, with a total volume of approximately 1000 km3.

(2) The discharge rate increased from 0.2 to 2.0 km3/kyr at 0.2 Ma and has not since decreased.

(3) The silicic magma systems of the Akan, Kutcharo, and Mashu calderas formed independently, and the Atosanupuri magma appears to have been formed from the Kutcharo magma.

ACKNOWLEDGMENTS

We thank the editor Y. Tamura and an anonymous re-viewer for their constructive comments.

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Manuscript received October 20, 2011Manuscript accepted January 7, 2012

Manuscript handled by Yoshihiko Tamura