Highly oblique subduction and multiple events of the eclogite
metamorphism: The Sambagawa belt
基礎設計室 Kabir Md Fazle

Introduction
Subduction zones are the location ofmany important processes on Earth, including
recycling of crustal material and volatiles into
the mantle, the creation of new crust that ultimately forms continents, and generation ofsome of Earth’s deadliest earthquakes andchains of volcanoes. At the interface between
the subducting slab and the overlying mantle
wedge, significant heat transfer and physicochemical exchange occur. To better understand these processes, it is critically important
to ascertain how crustal and mantle materials
move and interact at these boundaries. Exhumed rocks from the subduction interface
provide a primary means of testing models of
subduction and are recognized by distinctivehigh–P low–T blueschist-eclogite facies mineral assemblages.
Eclogite is a metamorphic rock consistingof omphacitic clinopyroxene and pyrope-richgarnet. It is a rare but geologically significant
rock type. It is common in the upper mantle,
especially in regions occupied by subductedoceanic plates. Eclogite is unusually dense fora silicate rock (3.4–3.5 g/cm3) which suggeststhat very high pressure was involved in theformation of this rock type. Eclogite is a common rock type at deeper parts of subducting
slabs and helps to keep the plate tectonic conveyor belt in operation by dragging down the
subducting slab but it is rare at the surface. It
is not very common for the rocks once buried
that deep to somehow raise again to the upperparts of the lithosphere.

The Sambagawa belt
The Sambagawa metamorphic belt is oneof the best studied high–P type metamorphic
belts in the world. The belt extends for about

800 km east-west along the south-west Japanarc, stretching from Saganoseki Peninsula in
Kyushu to the Kanto Mountains in the northeast (Fig. 1). It is widely accepted that this belt
represents the deepest part of an accretionary
wedge complex that developed in an ancient
subduction along the eastern margin of theEurasian continent (e.g. Isozaki & Itaya, 1990;
Takasu et al., 1994; Wallis, 1998).

Figure 1. Distribution of eclogites and eclogite
facies metamorphic rocks in the Sambagawa metamorphic belt.

Several coarse-grained eclogite-bearing
bodies occur within the albite-biotite and oligoclase-biotite zones in the central Shikoku,
i.e. the Higashi-akaishi and Nikubuchi peridotite bodies; the Western Iratsu, Quartz Eclogite(Gongen) masses, Seba eclogitic basic schists(including Onodani eclogite); the Gazo eclogite; and the Sebadani, Eastern Iratsu and Tonaru metagabbro masses (Banno et al., 1976;
Takasu, 1984; Kunugiza et al., 1986; Takasu,
1989; Aoya, 2001; Enami et al., 2004; Ota etal., 2004; Miyagi & Takasu, 2005; Miyamoto
et al., 2007; Sakurai & Takasu, 2009; Kabir &Takasu, 2010a, b) (Fig. 2).

Metamorphic zonation

133°21´E

MTL
Kokuryo RiverN

Eclogite bodies

Oligoclase-biotite zone

Peridotites & serpentinites (Upper-mantle origin)

Albite-biotite zone

Epidote amphibolites (Metagabbro origin)

Garnet zone

Epidote-amphibolites (Accretionary complex
e.g. sedimentary volcanoclastic rock)

Chlorite zone
33°54´N

EI
Mt. Futatsudake

WI
HA

Mt. MonosuminoatamaTN

GZ

Mt. Higashiakaishi

QE

HE

NB

Seba River

Onodani area

SB
Seba basic schists

Sambagawa belt

nR

za
Do

r

ive

Japan Sea

35°N

Pacific Ocean
Study area

135°E

200 km2 km

Figure 2. Geological map and metamorphic zonation of the Besshi district in the Sambagawa metamorphic
belt, central Shikoku, Japan (compiled by Kabir & Takasu, 2010a). SB-Sebadani metagabbro mass; TNTonaru metagabbro mass; WI-Western Iratsu mass: EI-Eastern Iratsu mass; HA-Higashi-akaishi peridotite
mass; QE-Quartz eclogite mass; HE-Hornblende eclogite mass; NB-Nikubuchi peridotite mass and GZGazo eclogite mass.

These eclogites mainly occur as block-likebodies with varying size and lithology. Metamorphic P–T conditions of the eclogites are
distinct from those of the surrounding Sambagawa schists, and they show variable P–T histories (Kunugiza et al., 1986; Takasu, 1989;
Takasu et al., 1994). Other eclogite localities
in the Sambagawa belt have been reported approximately 80 km to the east in the KotsuBizan areas in eastern Shikoku (Matsumoto etal., 2003; Tsuchiya & Hirajima, 2013; Kabir& Takasu, 2016) (Fig. 1).
Multiple metamorphic events
In the previous research, I have clarified
the high–P metamorphism (including eclogite
facies metamorphism) in the Sambagawa metamorphic belt, Besshi district, central Shikoku,
Japan. I have distinguished four metamorphic
events from the Sambagawa eclogites (Fig. 3).

These are the precursor metamorphic event(amphibolite facies), the first high-pressure
metamorphic event (eclogite facies) and thesecond high-pressure metamorphic event(eclogite facies) and the third high-pressure
metamorphic event (epidote-amphibolite facies) (Kabir & Takasu, 2010a, 2016).
Eclogite metamorphism in the Besshi andKotsu-Bizan areas in the Sambagawa belt are
divided into two types based on the metamorphic geothermal gradient, one is high–T typeof metamorphic temperature over 600 °C at 20
kbar; and the other is low–T type of the metamorphic temperature below 600 °C at 20 kbar
(Kabir & Takasu, 2016) (Fig. 3).
The high-T type eclogites include the Onodani eclogite, the Tonaru eclogitic metagabbro, the Quartz Eclogite (Gongen) masses andthe Western Iratsu mass (Fig. 3).

Figure 3. Summary of metamorphic P–
T conditions for the eclogites in the
Sambagawa metamorphic belt. 1, jadeite–garnet glaucophane schists, Bizan
area (Kabir & Takasu, 2016); 2, lawsonite-bearing eclogite, Kotsu area
(Tsuchiya & Hirajima, 2013); 3, Onodani eclogite (Kabir & Takasu, 2010a);4, Tonaru eclogite (Miyagi & Takasu,
2005); 5, jadeite-bearing kyanite eclogite, Eastern Iratsu (Endo & Tsuboi,2013); 6, Quartz eclogite (Miyamoto etal., 2007) and 7, Western Iratsu (Endoet al., 2012).

The Onodani eclogites in the Seba eclogitic basic schists preserve two-fold eclogite
facies metamorphism (Kabir & Takasu,
2010a), and the second eclogite facies metamorphic conditions are 630–680 °C and 20–22
kbar (Aoya, 2001; Kabir & Takasu, 2010a).
The Tonaru mass represents the peak metamorphic conditions of 700–730 °C and ≥ 15
kbar (Miyagi & Takasu, 2005). The quartzrich kyanite eclogites in the Quartz Eclogite
mass record the peak P–T conditions of 23–24
kbar and 675–740 °C (Miyamoto et al., 2007).
The Western Iratsu mass represents eclogite
facies and preserves pressure peak at low–T
condition followed by a thermal peak at 620 °C and 16 kbar (Endo et al., 2012) (Fig. 3).

In contrast, the other eclogites in the Sambagawa metamorphic belt experienced relatively low–T eclogite facies metamorphic conditions. The first eclogite facies metamorphism of the Onodani eclogites represents thepeak metamorphic conditions of 530–590 °C
and 19–21 kbar (Kabir & Takasu, 2010a) (Fig.
3). The Western Iratsu mass represents eclogite facies pressure peak at a lower temperature (510–560 °C and 18 kbar) followed by athermal peak at higher–T conditions (Endo etal., 2012). The jadeite-bearing kyanite eclogiteof the Eastern Iratsu mass are estimated to be
24–25 kbar and 570–610 °C (Endo & Tsuboi,2013). Glaucophane eclogite and lawsonitebearing glaucophane eclogite in the Kotsu area
represents their peak metamorphic conditions
are estimated to be c. 600 °C and 20 kbar
(Matsumoto et al., 2003; Tsuchiya &Hirajima, 2013). Kabir & Takasu (2016) haverecently reported the occurrence of Jadeitebearing eclogites from the Bizan area, Tokushima Prefecture and the eclogite experienced low–T eclogite facies metamorphism,
the peak metamorphic conditions of 580–600 °C and 18–20 kbar.

Thus, there are two types of the eclogite
facies metamorphism in the Sambagawa metamorphic belt, i.e. low–T type eclogite facies
metamorphism and subsequent high–T typeeclogite facies metamorphism. Beside two
types eclogite facies metamorphism; the Sambagawa metamorphic rocks suffered precursor
metamorphism and the Sambagawa metamorphism together with the surrounding schists.
Discussion: highly oblique subduction of
Izanagi plate
The precursor metamorphic events of high–T metamorphic conditions have been produced by the heat from the wedge mantle/hot
hanging wall at the inception of subduction.
Subduction of cold oceanic plate disturbed theinitial thermal conditions of the continental
plate and resulted in a cooling of the hanging
wall (Peacock, 1996).
The low–T type eclogite facies metamorphism was caused by subduction of the oceanic plate probably soon after the onset of subduction, due to highly oblique subduction of
Izanagi plate (Engebretson et al., 1985; Wallis
et al., 2009; Kabir & Takasu, 2010a). The laterwarm subduction event is associated with achange in plate motion (normal subduction)and the arrival of spreading ridge to the subduction zone (Aoya et al., 2009; Endo et al.,2012).

The peak metamorphic age of the noneclogitic Sambagawa schists has been estimated at 100–90 Ma (40Ar/39Ar amphibole,phengite, and whole rock ages) (Takasu &Dallmeyer, 1990; Dallmeyer et al., 1995). If
the 89 Ma age for the peak eclogitic metamorphism is reliable, it is similar to or slightly
younger than the peak metamorphic age of the
Sambagawa non-eclogitic schists. This implies
a very short time interval between the two eclogite facies metamorphic events. Exhumation
following an initial period of nearly isothermal
decompression may support high exhumation
rates in the order of 2.5 cm yr–1 (Wallis et al.,2009).
The protoliths of the Sambagawa schists
have been subducted as the Izanagi plate
passed beneath the eastern margin of Eurasia.
Between 135 and 85 Ma, the Izanagi plate was
subducted towards the north-northwest andnorth with a high migration rate of more than
20 cm yr–1 (Engebretson et al., 1985; Maruyama & Seno, 1986) (Fig. 4).

These lithologies of above-mentioned eclogites originated from oceanic protoliths suchas basaltic volcaniclastic rocks, pelitic sediments (shales) and siliceous sediments(cherts). All of these lithotypes are constituents of accretionary complexes. The eclogite
facies metamorphism was caused by subduction of the oceanic plate, with the metamorphism occurring beneath an accretionary prism
at a depth of more than 70 km.
Wallis et al. (2009) reported a Lu-Hf garnet-omphacite isochron age of 89 ± 0.6 Mafrom the Sebadani eclogitic basic schists. The
eclogite body was juxtaposed with the Sambagawa schists at glaucophane stability conditions during the third HP metamorphic event.

Figure 4. Velocity vectors of the relative motionsof the Farallon, Izanagi, Kula and Pacific plateswith respect to the Eurasian plate after Maruyama& Seno (1986).

This rapid migration of the oceanic plateprobably caused intense tectono-metamorphic
activity. According to reconstructions of platemotions, highly oblique subduction (>60º fromthe normal subduction) of the Izanagi plate

occurred 115–100 Ma (Wallis et al., 2009)(Fig. 5). This resulted in oscillatory movements of eclogite bodies between the oceanic
plate and its hanging wall, leading to the plural/dual eclogite metamorphic event.

Figure 5. Motion history of plates in western Pacific realm with respect to a point with present location
33.8 °N 133.3 °E within the Sanbagawa belt in central Shikoku constructed using the poles of Engebretson
et al. (1985) (after Wallis et al., 2009).

References
Aoya, M., 2001. P–T–D path of eclogite from the
Sambagawa belt deduced from combinationof petrological and microstructural analyses.
Journal of Petrology, 42, 1225–1248.
Aoya, M., Mizukami, T., Uehara, S. & Wallis,
S.R., 2009. High-P metamorphism, patternof induced flow in the mantle wedge, andthe link with plutonism in paired metamorphic belts. Terra Nova, 21, 67–73.
Banno, S., Yokohama, K., Enami, M., Iwata, O.,
Nakamura, K. & Kasashima, S., 1976. Petrography of the peridotite-metagabbro complex in the vicinity of Mt. Higashi-akaishi,
Central Shikoku. Part I. Megascopic texturesof the Iratsu and Tonaru epidote amphibolite
masses. The Science Reports of Kanazawa
University, 21, 139–159.
Dallmeyer, R.D., Takasu, A. & Yamaguchi, K.,
1995. Mesozoic tectonothermal development of the Sambagawa, Mikabu and Chichibu belts, south-west Japan: evidence from40
Ar/39Ar whole-rock phyllite ages. Journalof Metamorphic Geology, 13, 271–286.
Enami, M., Mizukami, T. & Yokoyama, K., 2004.
Metamorphic evolution of garnet-bearing

ultramafic rocks from the Gongen area,
Sanbagawa belt, Japan. Journal of Metamorphic Geology, 22, 1–15.
Endo, S. & Tsuboi, M., 2013. Petrogenesis andimplications of jadeite-bearing kyanite eclogite from the Sanbagawa belt (SW Japan).
Journal of Metamorphic Geology, 31, 647–
661.
Endo, S., Wallis, S.R., Tsuboi, M., Aoya, M. &Uehara, S., 2012. Slow subduction andbuoyant exhumation of the Sanbagawa eclogite. Lithos, 146–147, 183–201.
Engebretson, D.C., Cox, A. & Gordon, R.G., 1985.
Relative motions between oceanic and continental plates in the Pacific basins. Geological Society of America Special Paper, 206, 1–59.
Isozaki, Y. & Itaya, T., 1990. Chronology ofSanbagawa metamorphism. Journal of Metamorphic Geology, 8, 401–411.
Kabir, M.F. & Takasu, A., 2010a. Evidence formultiple burial–partial exhumation cycles
from the Onodani eclogites in the Sambagawa metamorphic belt, central Shikoku, Japan. Journal of Metamorphic Geology, 28,
873–893.

Kabir, M.F. & Takasu, A., 2010b. Glaucophanic
amphibole in the Seba eclogitic basic
schists, Sambagawa metamorphic belt, central Shikoku, Japan: implications for timingof juxtaposition of the eclogite body with
the non-eclogite Sambagawa schists. Earth
Sciences, 64, 183–192.
Kabir, M.F. & Takasu, A., 2016. Jadeite–garnet
glaucophane schists in the Bizan area, Sambagawa metamorphic belt, eastern Shikoku,
Japan: significance and extent of eclogite
facies metamorphism. Journal of Metamorphic Geology, 34, 893–916.
Kunugiza, K., Takasu, A. & Banno, S., 1986. The
origin and metamorphic history of the ultramafic and metagabbro bodies in the Sanbagawa Metamorphic Belt. In: Blueschists andEclogites (eds Evans, B. W. & Brown, E.
H.), 164, 375–386, Geological Society ofAmerica Memoir.
Maruyama, S. & Seno, T., 1986. Orogeny and relative plate motions: example of the Japaneseislands. Tectonophysics, 127, 305–329.
Matsumoto, M., Wallis, S.R., Aoya, M., Enami,M., Kawano, J., Seto, Y. & Shimobayashi,N., 2003. Petrological constraints on the formation conditions and retrograde P–T pathof the Kotsu eclogite unit, central Shikoku.
Journal of Metamorphic Geology, 21, 363–
376.
Miyagi, Y. & Takasu, A., 2005. Prograde eclogites
from the Tonaru epidote amphibolite mass in
the Sambagawa Metamorphic Belt, central
Shikoku, southwest Japan. Island Arc, 14,
215–235.
Miyamoto, A., Enami, M., Tsuboi, M. & Yokoyama, K., 2007. Peak conditions of kyanitebearing quartz eclogites in the Sanbagawa
metamorphic belt, central Shikoku, Japan.
Journal of Mineralogical and Petrological
Sciences, 102, 352–367.
Ota, T., Terabayashi, M. & Katayama, I., 2004.
Thermobaric structure and metamorphic
evolution of the Iratsu eclogite body in the
Sanbagawa belt, central Shikoku, Japan.
Lithos, 73, 95–126.
Peacock, S.M., 1996. Thermal and petrologic
structure of subduction zones. Geophysical

Monograph, 96, 119–133.
Sakurai, T. & Takasu, A., 2009. Geology and metamorphism of the Gazo mass (eclogitebearing tectonic block) in the Sambagawa
metamorphic belt, Besshi district, central
Shikoku, Japan. Journal of Geological Society of Japan, 115, 101–121.
Takasu, A., 1984. Prograde and retrograde eclogites in the Sambagawa metamorphic belt,
Besshi district, Japan. Journal of Petrology,
25, 619–643.
Takasu, A., 1989. P–T histories of peridotite andamphibolite tectonic blocks in the Sanbagawa metamorphic belt, Japan. In: Evolution of
Metamorphic Belts (eds Daly, J. S., Cliff, R.
A. & Yardley, B. W. D.), 43, 533–538,
Blackwell Scientific Publications, Oxford,
Geological Society, London, Special Publications.
Takasu, A. & Dallmeyer, R.D., 1990. 40Ar/39Ar
mineral age constraints for the tectonothermal evolution of the Sambagawa metamorphic belt, central Shikoku, Japan: a Cretaceous accretionary prism. Tectonophysics,
185, 111–139.
Takasu, A., Wallis, S.R., Banno, S. & Dallmeyer,
R.D., 1994. Evolution of the Sambagawa
metamorphic belt, Japan. Lithos, 33, 119–
133.
Tsuchiya, S. & Hirajima, T., 2013. Evidence of thelawsonite eclogite facies metamorphismfrom an epidote-glaucophane eclogite in the
Kotsu area of the Sanbagawa belt, Japan.
Journal of Mineralogical and Petrological
Sciences, 108, 166–171.
Wallis, S.R., 1998. Exhuming the Sanbagawa metamorphic belt: the importance of tectonic
discontinuities. Journal of Metamorphic Geology, 16, 83–95.
Wallis, S.R., Anczkiewicz, R., Endo, S., Aoya, M.,
Platt, J.P., Thirlwall, M. & Hirata, T., 2009.
Plate movements, ductile deformation andgeochronology of the Sanbagawa belt, SW
Japan: tectonic significance of 89–88 Ma Lu–Hf eclogite ages. Journal of Metamorphic
Geology, 27, 93–105.