第６回 国際ワークショップ “WATER DYNAMICS”

１． 開催趣旨および開催概要

変動地球惑星科学グローバル COE の支援を受け，平成 21 年 3 月 4 日～6 日まで，仙台

国際センター 萩 において表記国際ワークショップを開催した．招待講演者 15 名，キー

ノート講演者 3 名，ポスター発表者 49 名，参加者はのべ 150 であった．

この国際ワークショップ“WATER DYNAMICS”は，「みず」を軸に，環境材料，エネルギ

ー，地球プロセス，生態など多元的な分野を横断的に網羅し，変動地球惑星科学グローバ

ル COE に先立つ先端地球科学技術 21 世紀 COE の時から開催されており，今回で６回目

を数える．21 世紀 COE のときは，流動ダイナミクス国際研究教育拠点と共催であったが，

今回から本 GCOE 単独での開催となっている．

今回は，- Life & Environment –をメインテーマに据え，カテゴリーＡは「材料」カテゴリ

ーＢは「地球システム」，カテゴリーＣ「生命と生態」，カテゴリーＤは「アジア諸国の環

境問題」に関わるセッションとした．学際的領域をカバーし，活発でかつ有意義な講演と

議論が行われた．

ワークショップ web サイト http://geo.kankyo.tohoku.ac.jp/wd/wd6/index.html

２．組織委員会

組織委員会

大谷栄治 教授 （理学研究科 組織委員長 GCOE 研究代表者）

土屋範芳 教授 （環境科学研究科 プログラム委員長 カテゴリーＢ担当）

掛川 武 准教授 （理学研究科 カテゴリーＣ担当）

井奥洪二 教授 （環境科学研究科 カテゴリーＡ担当）

井上千弘 教授 （環境科学研究科 カテゴリーＤ担当）

平野伸夫 助教 （環境科学研究科 会場設営担当）

小川泰正 助教 （環境科学研究科 会場設営担当）

渡邊典昭 助教 （環境科学研究科 プログラム担当）

岡本 敦 助教 （環境科学研究科 野外巡検担当）

３．招待講演およびキーノート講演

カテゴリーＡ招待講演者

K. Yanagisawa Kochi University JAPAN

H. Fujimori Yamaguchi University JAPAN

カテゴリーB 招待講演者 キーノート講演者

E. Ohtani Tohoku University (key note)

N. Tsuchiya Tohoku University (key note)

Roland Hellmann Environmental Geochemistry group, Observatory for Earth and Planetary

Sciences Grenoble France LGIT, Maison des Geosciences France

Nick Oliver James Cook University Australia

Brian Rusk James Cook University Australia

Konstantin Litasov COE fellow in Tohoku University

Anton Shatsky COE fellow in Tohoku University

John Hernlund British Columbia University Canada

カテゴリーC 招待講演者 キーノート講演者

T. Kakegawa Tohoku University (key note)

Y. Watanabe The NASA Astrobiology Institute and Department of Geosciences,

The Pennsylvania State University USA

Pinti L Daniele GEOTOP Research Center and Department des Sciences de la Terre et de

I'Atmosphere University Quebec Montreal Canada

Lilik Eko Widodo Institute of Technology Bandung, Indonesia

Agus Jatnika Effendi Institute of Technology Bandung, Indonesia

Batkhishig Bayara Mongolian University of Science and Technology

Phan Van Minh Fishery Faculty, Nong Lam University

K. Kaya Tohoku University Japan

T.Komai AIST Japan

４．過去の開催

第１回 ２００４年３月１７日～１９日 青葉記念会館

第２回 ２００４年１１月１１，１２日 仙台国際センター

第３回 ２００５年１１月１６，１７日，仙台国際センター

第４回 ２００６年１１月７，８日 青葉記念会館

第５回 ２００７年９月２５日～２７日 仙台国際センター

５．野外巡検

ワークショップ終了後，3/7-3*9 の日程で，四国四万十帯および三波川変成岩体を中心に

野外巡検を行った．

参加者：土屋範芳，岡本敦，平野伸夫，Roland Hellmann， Nick Oliver，Brian Rusk

巡検案内書を次ページ以下に示す．

WWaatteerr DDyynnaammiiccss

FFiieelldd ttrriipp,, SShhiikkookkuu 7-9, March, 2009

Shimanto accretionary complex

Sanbagawa metamorphic belt

Schedule 7 th (Saturday)

08:10 Sendai Airport Departure

11:50 Kochi Airport Arrival

13:00 Lunch

13:30-15:30 Yokonami mélange (Shimanto accretionary complex)

17:00-18:00 Oboke metaconglomerate (Shimanto Metamorphic rocks)

19:00 Hotel Arrival (Awa-no-Sho)

8th (Sunday)

8:30 Hotel departure

9:30 chlorite zone 1 (Sanbagawa belt, Asemigawa river)

10:30 chlorite zone 2 (highest vein fraction zone, Asemigawa river)

11:30 garnet zone (metamorphosed pillow lava, Asemigawa river)

13:00 Albite-biotite zone (Dozangawa river)

Lunch

15:00 Nikubuchi (eclogite floating rocks, Dozangawa river)

16:30 Minetopia Besshi (small museum of Besshi Mine)

18:30 Hotel Arrival (Hosho-Hotel) at Matsuyama

9th Monday

11:45 Matsuyama Airport departure

13:00 Haneda Airport Arrival

A-1: Shimanto accretionary complex The accretionary complex of southwest Japan has been divided into the Jurassic Chichibu, the

Cretaceous Shimanto and the Tertiary Shimanto complexes, and is bounded by the BTL (Butsuzo

Tectonic Line) and the ATL (Aki Tectonic Line). The complex is composed of four facies; slope

basin, forearc basin, trench-fill and melange facies. Trench-fill facies are dominant, and three units

of melange facies, the Yokonami, Kure and Okitsu melanges, are interleaved within the trench-fill

facies. The slope and forearc basin sediments are scattered; the Cretaceous Monobegawa Group and

Torinosu Group overlie the Chichibu complex, and the Doganaro and Uwagumi formations and the

Uwajima Group unconformably overlie the Shinjogawa Group in the Cretaceous Shimanto complex

(Fig. A-2). The sedimentary succession of the trench fill, forearc basin and slope facies are relatively

coherent sedimentary sequences, with only locally tight fold structures. In contrast, the melange

facies have suffered much greater deformation, and include ‘exotic’ oceanic blocks.

Fig. A-1 Schematic diagram illustrating the seismogenic and aseismic portions of subduction thrust

fault for young oceanic crust beneath continents (Hyndman et al. 1997).

Fig. A-2 Geological setting of Yokonami mélange (Sakaguchi 1996).

The uppermost Cretaceous melange in the Shimanto complex involves Mid-Oceanic Ridge Basalt

(MORB-type) blocks without any pelagic sediment cover. This also supports the idea that a

mid-ocean ridge approached the trench and subducted beneath an accretionary prism of the

Cretaceous Shimanto complex.

The thermal conditions of Shimanto belt (170-270˚C) is consistent with that of

seismogenic zones (Fig. A-1, Hynman et al. 1997), and also recently several pseudotachylyte have

been discovered along the subduction thrust in the Okitsu and Mugi mélanges in Shikoku. The

thermal structure of the Cretaceous age is similar to that of active subduction zone in the southwest

Japan (Nankai Trough). Now the Ocean Drilling Program is drilling boreholes toward the

seimogenic depth in Nankai Trough.

References

A. Taira, J. Katto, M. Tashiro, M. Okamura, K. Kodama, Mod. Geol. 12 (1988) 5–46.

Sakaguchi A, (1999) Earth Planet Sci Lett, 173, 61-74.

Hyndman RD, Yamano, M., Oleskevich DA (1997) The Island Arc, 1997, 244-260.

A-3 Characteristic of veins

The Yokonami mélange contains two types of veins; veins that develop within sandstone blocks and

along minor faults (Fig. A-3). Here, we call the former and latter types of veins to be Type I and II,

respectively. Type I veins occur only within boudinaged sandstone blocks, and do not cut through

surrounding mudstone. These veins develop in direction normal to the elongation of sandstone block,

and show thickness of < 2 mm and length of < 50 cm. Type II veins occur in association with minor

faults along foliation of mudstone, or cutting mélange fabric. Type II veins commonly develop over

10 m, and which thickness is 10- 50 mm. Both Type I and II veins are commonly composed of

quartz and calcite, but show contrasting microstructures.

Fig. A-3 An outcrop photo of Type I and II veins.

Type I veins contain euhedral to subhedral quartz grains that elongate in direction normal to the

vein wall, and widen toward the center of the vein (Fig. A-4). Individual elongate quartz grains

include small CL-bright regions on the vein wall, and these regions are surrounded by CL-dark

region (Fig. A-4), indicating that the elongate quartz grains nucleated on the both side of the vein

wall and grew toward the center of the veins. Such elongate-blocky texture indicate that these grains

grew within an open cavity (Bons 2001). Quartz grains growing from the both side of the vein wall

impinge on each other in thin Type I veins (< 0.1 mm) (Fig. A-4), but do not in thicker ones. In thick

Type I veins, the central parts are commonly filled with anhedral calcite or quartz grains. In Type I

veins, there is no evidence for multiple

crack-seal events.

Fig. A-4 (a)

Optical

photomicrograph

of Type I vein. (b)

SEM-CL image of

the area shown by

open rectangle in

(a).

Type II veins

show more

complicated

structure than

Type I veins. They contain cumulative structure of lenticular segments of veins that align parallel to

the vein length and the host rock fragments (Fig. A-5). Individual vein segments commonly are filled

with quartz grains with elongate-blocky textures as similar to Type I veins (Fig. A-5), indicating that

these vein segments did not form as a result from breakdown of large vein, but preserve repeated

fracturing along the minor fault plane. In the central part of thick vein segments, there are large

quartz grains with euhedral shape (> 5 mm). These euhedral quartz grains contain growth zoning that

were identified by CL-intensity contrast. The CL-intensity of quartz varies between the different

portions of the vein interior. The elongate quartz grains growing from vein wall show high

CL-intensity (CL-bright), large euhedral grain grains in central part of the veins show low

CL-intensity (CL-dark), and other blocky quartz grains show intermediate CL-intensity (CL-grey),

respectively (Fig. A-5). The quartz within the veins contain 0.1 – 0.9 wt% of Al and 0.05 – 0.35 wt%

of Fe. The Al/Si value is 0 – 2 x 10–3 in the CL-bright quartz, and increases with decreasing

CL-intensity, and is 2 – 4.5 10–3 in the CL-dark quartz.

Fig. A-5 micorstructurs of Type II veins.

Fig. A-6 P-T conditions estimated from H2O-rich and CH4-rich fluid inclusions.

B-1 Sanbagawa metamorphic belt The Sanbagawa metamorphic belt (Fig. 1) is a high-pressure intermediate

metamorphic belt that extends for seven hundred kilometers in the southwestern area of the Japanese islands. The northern boundary is the major strike slip fault of the Median Tectonic Line (MTL). The Sanbagawa belt is composed mainly of pelitic schists, psammitic schists, and mafic schists with small amount of metachert, a lithological association formed from an oceanic sedimentary sequence with underlying basaltic crust.

The Sanbagawa belt is divided into four mineral zones, defined by the index minerals in metapelites: the chlorite, the garnet, the albite-biotite and the oligoclase-biotite zones in order of increasing metamorphic grade (Higashino 1990, Fig.B-2). The Sanbagawa rocks experienced from greenschist to epidote-amphibolite facies metamorphism (Enami et al. 1994; Okamoto and Toriumi 2004; 2005), and some highest-grade rocks record eclogite-facies metamorphism (e.g. Takasu 1989; Aoya 2001). In central Shikoku, the peak P-T conditions are estimated to be 300-360˚C, 0.55-0.65 GPa for the chlorite zone, 425-495 ˚C, 0.7-1.0 GPa for the garnet zone, 495-590˚C, 0.8-1.1 GPa for the albite-biotite zone (Fig. B-1, Enami et al. 1994). Comparison between petrological studies and numerical simulations on the thermal structure of the subduction zone revealed that the Sanbagawa metamorphism resulted from the subduction of warm young slab (Aoya 2003). K-Ar and 40Ar-39Ar dating of phengitic mica yields cooling ages ranging from 90 Ma to 63 Ma (Itaya and Takasugi 1988; Takasu and Dallmeyer 1990), indicating that the Sanbagawa belt was exhumed during the late Cretaceous. The dominant structural features of the Sanbagawa metamorphic rocks are a gently northward-dipping foliation and an east-west subhorizontal mineral lineation. The mineral lineation is generally considered to have formed during or after the peak metamorphism (Wallis 1990; 1998). In central Shikoku, the highest-grade oligoclase-biotite zone is located in the middle of the structural sequence, and the metamorphic grade decreases both northwards and southwards (Fig. 6). This pattern is thought to be a consequence of a large south-vergent recumbent fold that formed during exhumation of the Sanbagawa belt (Banno and Sakai 1989).

Fig. B-1 P-T path of the Sanbagwa metamorphic belt (modified after Aoya, 2003).

References: Aoya, M., 2003, Subduction-stage pressure path of eclogite from the Sambagawa belt: Prophetic

record for oceanic-ridge subduction: Geology, v. 31, p.1045–1048.

Enami, M., Wallis, S.R., and Banno, Y., 1994, Paragenesis of sodic pyroxene-bearing quartz schists:

implications for the P-T history of the Sanbagawa belt: Contributions to Mineralogy and

Petrology, v. 116, p. 182–198.

Higashino T (1990) The higher-grade metamorphic zonation of the Sanbagawa metamorphic belt, in

central Shikoku Japan. J Metamorphic Geol 8: 413-423.

B-2 Oboke Unit The Oboke conglomerate contains clasts of granitic material and the associated psammitic schists

contain abundant zircon grains. This is in contrast to the common Sanbagawa schists, that are

composed of pelitic and mafic schists. This unit has been considered to the member of the

Sanbagawa belt; howver, Aoki et al. (2007) undertook U–Pb dating on detrital igneous zircon, and

showed that the depositional ages of 90–80 Ma of the Oboke low-grade metamorphic rocks post-date

the peak metamorphic ages of the Sanbagawa belt (120–110 Ma: Okamoto et al., 2004; 90-60 Ma:

Itaya and Talasugi, 1988), and revealed that the Oboke metamorphic rocks belong not to the

Sanbagawa belt, but to the Northern Shimanto belt. The P-T condition of this area is estimated to be

240-270 ˚C, 0.4-0.45GPa (Aoki et al. 2008).

B-3 Characteristic of veins hosted by Sanbagawa schists

In the Nagatoro area of the Kanto Mountains, veins in rocks of the chlorite zone are

commonly subvertical, intersecting the foliation and the stretching lineation within the host rocks at

a high angle (60–90°). Petrological and fluid-inclusion studies reveal that these veins formed at

200–400°C and 0.1–0.4 GPa, during the late stages of exhumation of the Sanbagawa belt (Okamoto

et al., 2008). Because the fold axes of exhumation-related upright folds are oriented normal to the

high-angle veins, it is likely that the veins formed vertically.

The high-angle veins are classified into two types: (1) quartz-albite-chlorite-K-feldspar veins

(Qtz-Ab-Chl-Kfs) that show stretched crystal to elongate-blocky textures, and constitute more than

70 % of veins in the area (Fig. B-3), and (2) Qtz-Ab-calcite (Cal) veins with blocky textures (Fig.

B-4). Okamoto et al. (2008) proposed that the chemical components of the Qtz-Ab-Chl-Kfs veins

were derived from adjacent host rocks, whereas those of the Qtz-Ab-Cal veins were derived from

distant sites.

The spatial distribution of the high-angle veins are not homogeneous in the regional

scale (Fig. 4). Since P-T conditions of the vein formation is similar (200-400 ˚C, 0.1-0.4GPa) during

the exhumation, the vein frequency is not controlled by metamorphic grade (peak temperature), but

by the vertical distance from the fluid source.

Reference

Okamoto, A., Kikuchi, T., and Tsuchiya, N., 2008, Mineral distribution within polymineralic veins

in the Sanbagawa belt, Japan: Implications for mass transfer during vein formation:

Contributions to Mineralogy and Petrology, v. 156, p. 323–336.

Morohashi, K., Okamoto, A., Satish-Kumar, M., Tscuchiya, N., 2008, Variation in stable isotope

compositions (δ13C, δ18O) of calcite within exhumation-related veins from the Sanbagawa

metamorphic belt: Journal of Mineralogical Petrological Sciences, v.103, p. 361-364.

Fig. B-2 (a) Metamorphic zonation in the central Shikoku (modified after Hogashino, 1990) with

volume fraction of veins. (b) Cross section along Sarutagawa-Asemigawa rivers. (c) Ar-Ar age along

the section (Itaya and Takasugi 1988).

(a)

(b)

(c)

Fig. B-3 Microstructures of Type I (stretched-crystal veins).

Fig. B-4 Microstructures of Type III veins (Blocky texture).

Figure B-4 Carbon and oxygen isotope compositions (δ13C, δ18O) of calcite in veins and host rocks.

The variation of δ18O is very small (14-16 ‰ in Kanto, 15-20‰ in Shikoku), whereas the δ13C value ranges from -20-2, indicating that the fluid of vein formation is H2O rich and oxygen isotope

composition was buffered by the host rocks.

B-4 Bedded Cu-Fe sulfide ores In the Sanbagawa Belt, there are numerous (>100) deposits and mineral occurrences of bedded

Cu-Fe sulfide ores (Besshi-type deposits). The representative Besshi type deposits in Shikoku are the

Besshi, Sazare and Shirataki deposits and the Besshi deposit is the largest. The Besshi deposit was

mainly mined at four orebodies (Besshi, Yokei, Ikadazu and Sekizen) during the period from 1690 to

1973 and produced approximately 30 million metric tons of ore at 2.5 % Cu on average. According

to Kase and Yamamoto (1988), the main features of Besshi-type deposits are (1) a close association

with the basic schist, (2) stratiform or lenticular ore beds that were deformed and metamorphosed

together with host rocks, (3) the absence of stockwork veins, and (4) the frequent presence of

ferruginous or manganiferous chert beds close to the ore horizons.

Based on the whole-rock chemistries, Nozaki et al. (2006) suggest that (1) the HFSE

contents of the Besshi basic schist are comparable to those of N-MORB, (2) The REE signatures

may be attributed to seafloor weathering prior to accretion and subsequent metamorphism, and (3)

the Besshi sulfide deposit was very likely formed by hydrothermal activity related to MOR

volcanism.

References:

Kase K, Yamamoto M (1988) Mining Geol, 8, 401-411.

Nozaki T, Nakamura K, Awaji S, Kato Y (2006) Resource Geol, 56,423-432.