High-pressure garnet amphibolite from the Funaokayama unit, western Kii Peninsula and the extent of...

189Eclogite metamorphism in the Kii PeninsulaJournal of Mineralogical and Petrological Sciences, Volume 108, page 189─200, 2013

doi: 10.2465/jmps.121125S. Endo, [email protected] Corresponding author

High-pressure garnet amphibolite from the Funaokayama unit, western Kii Peninsula and the extent of eclogite facies

metamorphism in the Sanbagawa belt

Shunsuke Endo*,***, Izabella Nowak**,*** and Simon R. Wallis***

*Institute of Geology and Geoinformation, National Institute of Advanced Industrial Science and Technology (AIST), Central 7, Tsukuba 305-8567, Japan

**Institute of Geological Sciences, Polish Academy of Sciences INGPAN, Podwale 75, 50-449 Wroclaw, Poland***Department of Earth and Planetary Sciences, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan

Quantitative analyses of garnet-bearing epidote amphibolite (hereafter garnet amphibolite) from the Funao-kayama unit, western Kii Peninsula of the Sanbagawa belt reveal a previously unrecognized high-grade part of the Sanbagawa metamorphism. Petrographic observations suggest that the garnet amphibolite underwent three stages of metamorphism (M1, M2 and M3). Records of M1 are preserved as syn-tectonic prograde-zoned gar-net and its inclusions. Pseudosection modeling reproduces the observed M1 assemblage garnet + amphibolite + epidote + phengite + quartz and the growth zoning of garnet records the pressure (P)-temperature (T) evolution from 0.8 GPa, 570 °C to 1.3 GPa, 590 °C. Therefore, this garnet growth and associated deformation (D1) took place during subduction up to eclogite facies conditions. The garnet rim compositions and inclusions of epidote, titanite, rutile and quartz (5titanite + 2clinozoisite = 5rutile + 3grossular + 2quartz + H2O) give consistent peak-

P estimates of 1.3-1.9 GPa. The garnet contains quartz inclusions retaining residual pressures of up to ~ 0.7 GPa, which is as high as the values reported in well-characterized eclogite samples in central Shikoku. The in-ferred peak-P conditions in the eclogite facies is further supported by the occurrence of aragonite in associated pelitic schist. The assemblage hornblende + epidote + titanite + quartz + albite in the matrix and its equilibrium conditions of ~ 0.8 GPa, 580 °C suggest decompression to the epidote-amphibolite facies (M2). The M2 miner-als locally show replacement by the mineral assemblage actinolite + epidote + chlorite + titanite + calcite + quartz + albite, suggesting a partial re-equilibration in the greenschist facies (M3). M2 and M3 are synchronous with the main phase of ductile deformation during exhumation (D2). Despite the absence of the omphacite + quartz assemblage, it is likely that eclogite facies metamorphism in the Sanbagawa belt can be extended to western Kii Peninsula.

Keywords: Eclogite facies, Funaokayama unit, Garnet amphibolite, P-T path, Sanbagawa belt

INTRODUCTION

The Sanbagawa belt of SW Japan is a region of Creta-ceous oceanic subduction-type metamorphism (e.g., Iso-zaki and Itaya, 1990; Wallis et al., 2009). The most deeply subducted portions of this orogen include eclogite or the eclogite assemblage garnet + omphacite + quartz. To un-derstand the nature of large-scale material transport in subduction zones, it is important to know how deep rocks have been buried and to define the spatial extent of eclog-

ite facies metamorphism.Eclogite in the Sanbagawa belt was first recognized

within coarse-grained metagabbro masses in the Besshi area of central Shikoku, and considered to occur as exotic tectonic blocks intruded into the lower pressure schists during the Sanbagawa metamorphism (Kunugiza et al., 1986; Takasu, 1989). However, later discoveries of the eclogite assemblage within fine-grained basic schist around the metagabbro masses led to a reevaluation of the apparently dispersed eclogite occurrences being parts of a large-scale semicoherent unit (the Eclogite unit) that oc-cupies highest structural levels of the Sanbagawa belt (Wallis and Aoya, 2000). Moreover, close examination of inclusions in garnet has further extended the Eclogite unit

190 S. Endo, I. Nowak and S.R. Wallis

to domains of pelitic schist, which is the predominant li-thology in the Sanbagawa belt (Zaw et al., 2005; Kouke-tsu et al., 2010). In these recent studies, laser Raman spectroscopy has played an important role in the identifi-cation of microscopic inclusions diagnostic of the eclogite facies (e.g., aragonite, omphacite) or detection of high re-sidual pressures of quartz inclusions (Enami et al., 2007). In addition, calculated equilibrium phase-assemblage dia-grams (pseudosections) have been used to recognize eclogite facies assemblages in various rock compositions (Aoki et al., 2009; Kouketsu and Enami, 2011; Endo et al., 2012).

Despite the improved understanding of how to rec-ognize rocks that have undergone eclogite facies meta-morphism, there is still uncertainty about the true areal extent of eclogite facies metamorphism. The recognition of eclogite facies rocks is commonly made difficult by strong recrystallization during their exhumation to the earth’s surface and many eclogite localities in the San-bagawa belt show retrogression in the epidote-amphibo-lite facies (e.g., Takasu, 1989; Ota et al., 2004). In addi-tion, pseudosection modeling has shown that eclogite and omphacite-free garnet amphibolite can form at the same P-T conditions for a range of mafic bulk compositions (Endo et al., 2012), implying garnet amphibolite in the Eclogite unit is not always a retrogression product derived from eclogite.

The high-P/T Sanbagawa and low-P/T Ryoke belts are the best studied example of paired metamorphic belts. They were formed in a Cretaceous arc-trench system probably associated with the oblique subduction of a spreading ridge (e.g., Nakajima, 1990; Aoya et al., 2003; Okudaira and Yoshitake, 2004; Wallis et al., 2009). There-fore, there is wide interest in along-arc lateral variations in the nature and timing of metamorphism and plutonism in Cretaceous SW Japan. Although the recognized areal extent of the eclogite facies metamorphism in the San-bagawa belt has increased considerably in the last 15 years, known occurrences are still limited to central and eastern Shikoku. Thus, the next issue to be addressed is the along-arc extent of the eclogite facies metamorphism.

The Funaokayama unit contains some of the highest-grade rocks of the Sanbagawa belt outside Shikoku (Hiro-ta, 1991). This area lies about 100 km east of the nearest known eclogite locality in eastern Shikoku (Matsumoto et al., 2003), and locally contains garnet amphibolite. This study presents detailed petrography and P-T estimates of garnet amphibolite from the Funaokayama unit. Although the typical eclogite assemblages have not been identified in this area, the results of our study suggest that the Funa-okayama unit in western Kii Peninsula reached the eclog-ite facies at its maximum burial.

OUTLINE OF GEOLOGY

The Sanbagawa belt of southwest Japan extends around 800 km from the Kanto Mountains through Chubu Re-gion, Kii Peninsula to Shikoku and eastern Kyushu (Fig. 1a). The regional metamorphism in this belt has been dis-cussed in terms of four zones based on index minerals in pelitic schists: the chlorite (<360 °C, 0.55-0.65 GPa), gar-net (440 ± 15 °C, 0.70-0.85 GPa), albite-biotite (520 ± 25 °C, 0.80-0.95 GPa) and oligoclase-biotite (610 ± 20 °C, 1.0-1.1 GPa) zones, in ascending order of metamorphic grade (Enami, 1983; Higashino, 1990; Enami et al., 1994). The full set of the four metamorphic zones is ex-posed in the Besshi and Asemigawa areas of central Shi-koku. Parts of the high-grade zones in central Shikoku

Figure 1. (a) Index map of the Sanbagawa metamorphic belt. (b) Geological map of the Sanbagawa belt in western Kii Peninsula (after Hirota, 1991). (c) Stereoplots of the main schistosity (S2), stretching lineation (L2) and crenulation lineation (equal area, lower hemisphere projection) within the Funaokayama unit.

Tomobuchi unit

Funaoka-yama unit

Iimori unitIimori unit

Iimori synformIimori synform

Ryumon peridotite

R. Kinokawa?? ??

200 kmPhilippine Sea Plate

Amurian Plate

M.T.L.

(a)

Shikoku Kii

Kanto

Kyushu

Chubu

Sanbagawa belt

1 km

(c) N

Pole to S2L2 (stretching)Crenulation

Quaternary sediment

Pelitic schistSiliceous schistMafic schistUltramafic rocks

Sample localityGrt amphibolitePelitic schist

N

(b)

191Eclogite metamorphism in the Kii Peninsula

have mineral parageneses demonstrating eclogite facies metamorphism before a regional epidote-amphibolite fa-cies overprint (e.g., Takasu, 1989; Ota et al., 2004; Aoya et al., 2013).

In western Kii Peninsula, the Sanbagawa belt con-sists mainly of mafic, pelitic-psammitic and siliceous schists and ultramafic bodies. The area has been divided into the Tomobuchi (structurally lower) and Iimori (struc-turally upper) units (Hirota, 1991) (Fig. 1b). Metamor-phism in this area is equivalent to the chlorite zone (To-mobuchi unit) and the garnet and albite-biotite zones (Iimori unit) (Hirota, 1991). The geological structure in this area is characterized by a westward plunging synform (Iimori synform) (Fig. 1b). Within the northern limb of the Iimori synform, there is a small distribution of dis-tinctly high-grade rocks and the extent of these rocks have been treated as an independent tectonic unit (Funao-kayama unit) (Hirota, 1991; Takasu et al., 1996) (Fig. 1b). The Funaokayama unit consists of mafic, pelitic and mi-nor siliceous schists. The main schistosity in the Funao-kayama unit strikes subparallel to the direction of E-W stretching lineation (Fig. 1c). Mafic and pelitic schists contain albite porphyroblasts with rare development of oligoclase rims (Hirota, 1991), suggesting that metamor-phism of the Funaokayama unit is comparable to the al-bite-biotite and oligoclase-biotite zones of central Shi-koku. However, the phengite 40Ar/39Ar cooling age of the

Funaokayama unit (74.8 ± 0.4 Ma, Takasu et al., 1996) is significantly younger than that of the oligoclase-biotite zone of the Asemigawa area, central Shikoku (84.3 ± 0.6 Ma, Takasu and Dallmeyer, 1994).

The present study focuses on garnet-bearing mafic schist (garnet amphibolite) from the Funaokayama unit. Garnet amphibolite occurs as several-cm thick layers in highly retrogressed mafic schist. Garnet amphibolite sam-ples used in this study were collected from southern slope of the Mt. Funaoka (60 m) (Fig. 1b).

PETROGRAPHY OF GARNET AMPHIBOLITE

Garnet amphibolite from the Funaokayama unit is com-posed mainly of garnet, amphibole, epidote, quartz, phen-gite, chlorite, plagioclase, and subordinate amounts of ru-tile, titanite, zircon, apatite, tourmaline and pyrite. Shape-preferred orientation of amphibole, epidote and phengite in the matrix defines a schistosity and mineral lineation. Boudinage and pressure shadow structures show this lin-eation is parallel to the stretching direction. Monominer-alic calcite veins commonly cut the schistosity. Garnet amphibolite locally shows a banded structure consisting of alternating albite porphyroblast-rich and -free layers.

The compositions of minerals in the Funaokayama garnet amphibolite were analyzed with a JEOL JXA-

8900/8800R (WDS) electron microprobe at Nagoya Uni-

Table 1. Representative analyses (wt%) of minerals in the Funaokayama garnet amphibolite

* Total Fe as FeO. Abbreviations: Grt, garnet; Ep, epidote; Ttn, titanite; Hbl, hornblende; Act, actinolite; Chl, chlorite; Ph, phengite; Pl, plagioclase.


versity and AIST. The accelerating voltage, specimen cur-rent and beam diameter were 15 kV, 12 nA and 2-10 µm, respectively. Representative microprobe analyses are re-ported in Table 1. Hereafter, abbreviations for minerals and end-member components follow Whitney and Evans (2010).

Garnet

Garnet occurs as euhedral porphyroblasts that vary in size from 0.5 mm to 5 mm. It contains inclusions of amphi-bole, epidote, quartz, phengite, zircon, apatite, titanite and rutile. No inclusions of albite or omphacite were ob-served. There is no systematic change in inclusion para-genesis from core to rim, but some garnet grains show an abrupt increase in the size and population of quartz inclu-sions at outer core (Fig. 2a). The inclusion trails within garnet define a sigmoidal internal foliation (S1) (Figs. 2a and 2b) and there is no evidence for any break in the gar-net growth. The curved internal foliation (S1) is at a high angle to and discontinuous with the matrix schistosity (S2), suggesting the two foliations formed at distinct stag-es in the tectonic evolution of the area. Pressure shadows around the garnet porphyroblasts are filled mainly by

quartz (Figs. 2b and 2c). Garnet crystals generally suf-fered varying degrees of fracturing and chloritization. A representative compositional profile for garnet is shown in Figure 3a. It ranges within Alm59−64Sps1−10Grs23−30Prp6−10 and displays well-preserved growth zoning characterized by a monotonous decrease in Mn from core (Sps10) to rim (Sps1). Pyrope component increases gradually from the Mn-richest center (Prp6) to a middle zone (Prp9−10) and then decreases slightly toward the crystal margin (Prp8−9).

Amphibole

Prismatic amphibole crystals occur as the main constitu-ent phase in the matrix and inclusions in garnet (Figs. 2b and 2c). Amphibole crystals in the matrix are chemically homogeneous or slightly zoned with a rimward decrease in the A-site occupancy, and plot mostly within the mag-nesiohornblende and tschermakite fields according to Leake et al. (1997) (Fig. 3b). In retrogressed samples, these amphibole crystals are partially rimmed by actino-lite. Amphibole crystals within garnet are generally larger (>200 μm in length) than other inclusion minerals (e.g., epidote, titanite, rutile), and commonly form composite inclusions with quartz. The compositional range of am-

Figure 2. Petrography of garnet amphibolite from the Funaokayama unit. (a) Thick section of an extracted garnet crystal prepared to expose its geometrical center. Plane-polarized light. (b) Photomicrograph (crossed-polarized light) showing a garnet porphyroblast and the matrix min-erals. (c) Fe X-ray map. Note that hornblende inclusions in garnet align at high angle to the matrix schistosity (S2). (d) Photomicrograph (crossed-polarized light) of a highly-retrogressed sample showing an albite porphyroblast and the matrix minerals. (e) Schematic illustration of microstructures and summary of metamorphic-deformational stages. Color version of Figure 2 is available online from http://japanlink center.org/DN/JST.JSTAGE/jmps/121125.

Albite

Amphibole

Epidote

PhengiteChlorite

Titanite

Quartz

S2

(e)

(b) (d)

1 mm1 mm

GrtGrt

Hbl+Ep+ChlHbl+Ep+Chl

Qz

(a)

(c)

HblHblQz

HblHbl

Ep

GrtGrt

2 mm

1 mm

S2S2

S2S2

S1S1

S2S2 S1S1

Porphyroblast

GarnetAmphiboleEpidoteAlbiteChloritePhengiteStilpnomelaneCalciteTi-phaseQuartz

Hbl Hbl Act

Rt+Ttn Ttn Ttn

M1 M2 M3

D1 D2 (=Ds)

S1

GarnetGarnet

Quartz Amphibole

http://japanlinkcenter.org/DN/JST.JSTAGE/jmps/121125



phibole included in garnet largely overlaps with that of the matrix amphibole (Fig. 3b).

Other minerals

Phengite in the matrix has a compositional range of Si = 3.15-3.32 apfu, Mg# = 0.60-0.67, XNa [= Na/(Na + K)] = 0.09-0.16. Some phengite flakes in the matrix have thin Fe-rich rims with a compositional range of Si = 3.26-3.34 apfu, Mg# = 0.49-0.56, XNa = 0.01-0.02. Rare phengite inclusions in garnet have compositions of Si = 3.09-3.28 apfu, Mg# = 0.38-0.67, XNa = 0.11-0.15 (Fig. 3c). Chlorite in the matrix has a compositional range of Si = 2.70-2.89 apfu, Mg# = 0.47-0.55. Plagioclase forms porphyroblasts of albite composition with less than 4 mol% anorthite content. The albite porphyroblasts enclose inclusions of Hbl, Ph, Chl, Ep, Grt, Ttn and Qz, and their S-shaped trails are continuous with the matrix schistosity (S2) (Fig. 2d). Epidote in the matrix occurs as zoned prismatic crys-tals composed of a core with XPs [= Fe3+/(Fe3+ + Al)] = 0.20-0.26, a mantle with XPs = 0.12-0.18 and a thin outer-most rim with XPs = 0.21-0.26. Epidote included in garnet shows the same compositional range as the core and man-tle of the matrix epidote (Fig. S1a; Fig. S1 is available online from http://japanlinkcenter.org/DN/JST.JSTAGE/jmps/121125). Prograde titanite is rich in Fe3+ (Al2O3 = 1.2-2.1 wt%, Fe2O3 = 0.8-1.6 wt%) (Fig. S1b) and occurs as minute (<20 μm) inclusions in garnet. Retrograde ti-tanite (Al2O3 = 1.2-1.7 wt%, Fe2O3 = 0.3-0.4 wt%) occurs as subhedral crystals (<500 μm) in the matrix and includ-ed in albite porphyroblasts. Relic rutile is rarely preserved within the matrix titanite. Pristine rutile crystals are exclu-sively present as inclusions in garnet.

Metamorphic and deformational stages

Three metamorphic stages (M1, M2 and M3) can be de-fined based on the petrographic observation presented above (Fig. 2e). The first metamorphic stage (M1) is de-fined by prograde-zoned garnet and its inclusions (Hbl + Ep + Qz + Rt + Ttn). Plagioclase was absent in this stage. The syn-tectonic growth of garnet shown by the curved inclusion trails (S1) suggests that M1 was associated with a phase of ductile deformation (D1). The second meta-morphic stage (M2) is characterized by pervasive recrys-tallization to form the matrix assemblage Hbl + Ep + Ab + Ttn + Qz defining S2 foliation. The last metamorphic stage (M3) is defined by minor growth of the greenschist facies assemblage Act + Ep + Chl + Ab + Cal at the ex-pense of the M2 minerals. M3 minerals occur in rims and boudin necks of M2 minerals (Fig. 2e) or as interstitial phases. M2 and M3 stages took place during a single de-

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0 400 800 1200 1600 2000 2400 2800

0

0.5

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6.06.57.07.58.0

[B] N

a (a

pfu)

Si (apfu)

0

0.5

1.0

Mg/

(Mg+

Fe2+

)

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Distance (µm)

Almandine

Grossular

PyropeSpessartine

Rim Core Rim

Mol

e fra

ctio

nM

g/(M

g+Fe

)

Amphibole

Inc. Grt

Matrix (Core)

Matrix (Rim)

ActPrg/Ts

Brs/KtpBrs/KtpWtcWtc TrmTrm

Hbl/EdHbl/Ed

Brs/HblBrs/Hbl

Ktp/EdKtp/Ed

Ts

Trm/Prg

Si (apfu)

Phengite

Inc. Grt

Inc. Ab

Matrix (Core)

Matrix (Rim)

(a)

(b)

(c)

Figure 3. Mineral chemistry of garnet amphibolite from the Funao-kayama unit. (a) Rim-core-rim compositional profile of a garnet crystal. (b) Amphibole compositions plotted in terms of Na con-tent in the B site, A-site occupancy and Mg/(Mg + Fe2+) against Si content. (c) Phengite compositions plotted in terms of Mg/(Mg + Fe) against Si content.




formation phase D2, which corresponds to the main de-formation phase of the Sanbagawa belt, DS (Wallis, 1998).

GEOTHERMOBAROMETRY

Quartz Raman barometry

Residual pressure retained by a quartz inclusion in garnet has recently been used as a useful index of metamorphic pressure in the Sanbagawa belt (Enami et al., 2007; Kouketsu et al., 2010). The Raman spectroscopic parame-ter Δω1 defined in Enami et al. (2007) is the measure of residual pressure retained by a quartz inclusion. We ana-lyzed quartz microinclusions in three samples of garnet amphibolite, following the procedure of Enami et al. (2007). The histogram of Δω1 is shown in Figure 4a,

which shows a bimodal distribution. There is no clear cor-relation between the Δω1 value and position of inclusions within garnet crystals. The higher Δω1 group ranges from 8.3-11.7 cm−1, corresponding to residual pressures of 0.4-

0.7 GPa. This result is comparable to the well-character-ized eclogite samples in the Besshi area (8.5-12.7 cm−1, Enami et al., 2007; Kouketsu et al., 2010).

P-T constraints from metapelite

Thermometry using Raman spectra of carbonaceous mat-ter (RSCM) was applied to a sample of pelitic schist to constrain peak temperature (T) of metamorphism. The outcrop of the sample is on the northern slope of Mt. Fu-naoka (Fig. 1b) and adjacent to the garnet amphibolite in the Funaokayama unit. Grain-shape fabric of quartz in this sample indicates deformation and dynamic recrystal-lization of this mineral and thus suggests deformation of pelitic schists continued during relatively low-T condi-tions during a late stage of exhumation. To minimize the effect of the deformation on CM structure, we use CM grains included in garnet for RSCM thermometry, follow-ing the procedure of Aoya et al. (2010). R2 values [D1/(G + D1 + D2) peak area ratio] of 53 CM Raman spectra show a unimodal distribution (Fig. 4b) and the calibration of Aoya et al. (2010) gives T = 537 ± 30 °C

Inclusions in garnet in this sample were also exam-ined using Raman spectroscopy. These studies revealed the presence of aragonite (Fig. 4c). Other minerals species are titanite, quartz, epidote and CM inclusions. The host garnet is commonly associated with radial fractures around aragonite inclusions, and some aragonite inclu-sions appear to have transformed to calcite.

Pseudosection modeling

Phase equilibria at M1 stage of garnet amphibolite were modeled in the system K2O-Na2O-CaO-MnO-FeO-

MgO-Al2O3-SiO2-H2O-O2 (KNCMnFMASHO). Gibbs free energy minimization has been carried out using the software Perple_X_6.6.6. (Connolly, 2009), the internally consistent thermodynamic dataset of Holland and Powell (1998, updated 2002) and the following solid-solution models: garnet (White et al., 2007), clinopyroxene and amphibole (Diener and Powell, 2012), epidote (Holland and Powell, 1998), chlorite (Holland et al., 1998), pheng-ite (Coggon and Holland, 2002), biotite (Tajcmanová et al., 2009) and feldspar (Fuhrman and Lindsley, 1988). Fluid was considered to be pure H2O and in excess.

The effective bulk composition was constructed by combining modes and mean compositions of minerals: garnet (3.34 vol%; Alm63Sps3Prp8Grs26), amphibole (65.55

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∆ω1 (cm-1)

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Max ∆ω1 range ofeclogite facies samplesin the Besshi area(Enami et al., 2007;Kouketsu et al., 2010)

Upper limit ofEp-amp facies data(Enami et al., 2007)

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Arg Cal

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nsity

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Mean R2=0.262SD=0.059

Num

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f ana

lyse

s

(b)

(c)

(a)

Figure 4. (a) Histogram of Δω1 value of quartz inclusions in garnet from the Funaokayama garnet amphibolite. (b) Histogram of R2 value of carbonaceous matter (CM) in a sample of pelitic schist from the Funaokayama unit. (c) Raman spectrum of an aragonite inclusion in garnet of the pelitic schist sample. Inset shows a mi-crophotograph (Plane-polarized light) of the garnet with arago-nite inclusions (Arg).


vol%), epidote (22.07 vol%; Cz50Ep50), phengite (2.01 vol%; Ms58Cel20Fcel10Pa12), chlorite (0.33 vol%; Clc37.5

Ame14Dph48MnClc0.5), albite (0.67 vol%) and quartz (6.02 vol%). The modal proportion (vol%) was estimated using elemental map data (1024 × 1024 μm2 area) from a sam-ple that is free from albite porphyroblasts that is likely to

be the least affected by late stage retrogression and non-

isochemical changes. The volumetric proportion was con-verted to molar proportion using the molar volume data of Holland and Powell (1998). The mean composition of garnet was calculated with a representative core-rim zon-ing profile assuming a spherical geometry. Ti, Mn and K

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Temperature (oC)P

ress

ure

(GP

a)

Ph Hbl Qz

Bt Hbl Qz

Ph Hbl Omp Qz

Chl Bt H

bl Qz

Bt Pl Hbl Qz

Chl Ph Hbl Qz

Chl Ph Omp Qz

Bt Pl Hbl Di Mgt (-Ep)Bt Pl Hbl Qz Mgt

+Lws

2Pl

Bt HblDi Qz

+Cpx

Amp [B]Na = 0.4 apfu

[B]Na = 0.6

[B]Na = 0.8[B]Na = 1.0

Ph Hbl Qz

Bt Hbl Qz

Ph Hbl Omp Qz

Bt Pl Hbl Qz

Chl Ph Hbl Qz

Chl Ph Omp Qz

Bt Pl Hbl Di Mgt (-Ep)Bt Pl Hbl Qz Mgt

Chl Bt Hbl Qz

Ph Hbl Gln Omp

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1.4

1.7

2.0

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Temperature (oC)

Pre

ssur

e (G

Pa)

+Cpx

Amp [B]Na = 0.4 apfu

[B]Na = 0.6

[B]Na = 0.8[B] Na = 1.0

0.5

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1.7

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Temperature (oC)

Pre

ssur

e (G

Pa)

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Temperature (oC)

Pre

ssur

e (G

Pa)

Core (Inner)

Core (Outer)

Rim

Prp6Prp6

Prp7Prp7

Prp8Prp8

Prp8Prp8

Grs27Grs27Grs26Grs26

Grs25Grs25

Grs27Grs27


Sps10Sps10

Sps3Sps3

Sps6Sps6

Grs25Grs25


Sps3Sps3

Prp8Prp8


Grs30Grs30

Sps1Sps1

Prp10Prp10

Prp9Prp9Sps2

Sps2

1 vol% Grt

3 vol% Grt

Mol.%: SiO2 49.10, Al2O3 10.37, FeO 12.69, MnO 0.05, MgO 10.61, CaO 14.29, Na2O 1.91, K2O 0.13, O2 0.86

Mol.%: SiO2 49.22, Al2O3 10.29, FeO 12.40, MnO 0.02, MgO 10.76, CaO 14.36, Na2O 1.95, K2O 0.13, O2 0.87+Ep, Grt, H2O +Ep, Grt, H2O

(a) (b)

(c) (d)

Grs

28G

rs28

Grs

27G

rs27

+Gln

Ph Si=3.3 apfu

Si=3.25

Si=3.2

Si=3.15

30 vol% Grt

Core (Inner)

Core (Outer)

Figure 5. Pseudosection modeling for garnet amphibolite from the Funaokayama unit. (a) P-T pseudosection for the early period of garnet growth. (b) P-T pseudosection for the later period of garnet growth. Contours of Na content in the B-site of amphibole are shown as a guide for compositions of amphibole. The clinopyroxene-in line is also indicated by a bold dashed line. (c) Compositional isopleths for garnet core (Sps3−10Prp6−8Grs25−27) are shown on (a). Gray solid stars indicate P-T conditions at which inner core and outer core compositions are repro-duced. (d) Compositional isopleths for the garnet rim (Sps1−3Prp8−10Grs25−30) are shown on (b). Gray solid stars indicate P-T conditions at which the compositions of the garnet inner rim and outer rim are reproduced.


in the mean amphibole composition were neglected to fit the solid-solution models used. To model equilibrium volume during early and late periods of garnet growth, the composition and molar proportion of matrix minerals were combined with those of either whole garnet or outer half volume of garnet.

The calculated P-T presudosections for early and late periods of garnet growth are presented in Figures 5a and 5b. The compositional ranges of garnet core (Sps3−10Prp6−8

Grs25−27) and rim (Sps1−3Prp8−10Grs25−30) were contoured on these pseudosections (Figs. 5c and 5d). The measured garnet core compositions (Fig. 3a) are reproduced in the Grt + Ep + Amp + Bt + Qz field (Fig. 5a) and record a segment of P-T path from 0.8 GPa, 570 °C to 1.0 GPa, 580 °C (Fig. 5c). The garnet growth with increasing pres-sure is consistent with the pressure sensitive slopes of garnet isomodes (Fig. 5a). The garnet rim shows a slight increase in Grs and decrease in Prp toward the margin (Fig. 3a), which records a small inflection of the prograde P-T path and the peak-P conditions of 1.3 GPa, 585-590 °C (Fig. 5d) in the Grt + Ep + Amp + Ph + Qz field (Fig. 5b).

The predicted mineral assemblages and mineral compositions during garnet growth are in agreement with the observed inclusion suites. We note that the pseudosec-

tion modeling predicts a change in K-bearing phase from biotite to phengite during garnet growth. Although the predicted minor amount of biotite (1.97 vol% at 1.0 GPa, 580 °C) has not been found in garnet cores, the presence of phengite inclusions with low Si content (Fig. 3c) is consistent with the predicted phengite composition near the biotite/phengite transition (Fig. 5b).

End-member equilibria

Pseudosection modeling has revealed that garnet growth took place during subduction. By considering potential uncertainties in equilibrium volumes and complex solid solution models (e.g., amphibole) used in the pseudosec-tion modeling, a crosscheck on the quantitative P-T con-ditions using simpler methodologies is desirable. Here we use inclusion assemblage Ttn + Rt + Ep + Qz and the gar-net rim composition to estimate the peak-P conditions based on the following reaction:

5Ttn + 2Cz = 5Rt + 3Grs + 2Qz + H2O.

The equilibrium constant (KEq) of the reaction is written as (a5

Rt·a3Grs·a2

Qz·aH2O)/(a5Ttn·a2

Cz). Isolated euhedral inclu-sions of epidote and titanite seem to preserve their prima-

200 400 600 8000

0.5

0

1.0

1.5

2.0

Chl zoneChl zone

Grt and Bt zonesGrt and Bt zones

EclogiteEclogite

Funaokayama

Subduction-stage P-T paths of the Sanbagawa schists in Shikoku

Subduction-stage P-T paths of the Sanbagawa schists in Shikoku

Thermal structure along the slab surfacebefore onset of ridge subduction

Stea

dy s

tate

10 4 2 0 Myr

0

0.4

0.8

1.2

1.6

2.0

300 400 500 600 700

M1

M2

M3

Chl-Ph-Qz

Grt-Ep-Ttn-Rt-Qz

Temperature (oC)

Pre

ssur

e (G

Pa)

Temperature (oC)

Pre

ssur

e (G

Pa)

Omp+Qz

Diss+Ab

G96+I

VH93+MVH93+M

G96+I

D1

Hbl-Ab-Qz

Act-Ab-Qz

(a) (b)

Arg

Cal

RSCM (Metapelite)

D2=Ds

Figure 6. (a) Summary of geothermobarometric calculations and inferred P-T-deformation path for the Funaokayama unit. Subduction-stage P-T path is from Figure 5. The Grt-Ep-Ttn-Rt-Qz equilibrium was calculated with two sets of garnet and titanite activity models: one is the subregular solution for garnet (Ganguly et al., 1996) and ideal ionic-mixing for titanite (G96 + I), and the other is the regular solution for gar-net (Vance and Holland, 1993) and ideal molecular-mixing for titanite (VH93 + M). The univariant reaction Omp + Qz = Diss + Ab is shown as the low-P limit of eclogite facies. Thermodynamics of mixing in clinopyroxene follows Diener and Powell (2012). P-T constraints from metapelite by Raman spectra of carbonaceous matter (RSCM) thermometry and the aragonite-calcite reaction are also shown. (b) Compari-son of subduction-stage P-T paths between the Funaokayama garnet amphibolite (this study) and the Sanbagawa schists in Shikoku (com-piled by Aoya et al., 2003). Gray curves show model P-T conditions at the slab surface before ridge subduction (Endo et al., 2012, calculated with the thermal model of Uehara and Aoya, 2005).


ry compositions. This is because that the epidote inclu-sions can be correlated with inner growth zones of the matrix epidote (Fig. S1a), and the titanite inclusions have distinct compositions that are comparable to those of the eclogite-facies titanite from the Western Iratsu body, cen-tral Shikoku (Endo, 2010) (Fig. S1b). Rutile and quartz are almost pure in composition. In addition, fluid was as-sumed to be pure H2O. It follows that the equilibrium constant can be simplified to a3

Grs/(a5Ttn·a2

Cz). Taking the subregular solution model for garnet (Ganguly et al., 1996) and ideal ionic-mixing models for titanite (aTtn = XCaXTiXSiXO[O1]) and clinozoisite (aCz = XCa[A2]XAl[M3]), com-positions of the coexisting three minerals give -lnKEq = 6.3-7.4, which is comparable to the value of titanite- and rutile-bearing eclogite from the Western Iratsu body (Endo, 2010). Using the thermodynamic data of Holland and Powell (1998), the equilibrium curve lies at 1.5-1.9 GPa at a temperature range of 500-600 °C (Fig. 6a). The use of the regular solution model for garnet (all interac-tion energies set to zero except for WCaMg = 33 kJ/mol: Vance and Holland, 1993) and ideal molecular-mixing model for titanite (aTtn = XTi) lowers the pressure estimates at most 0.2 GPa and gives a comparable result to the pseudosection modeling (Fig. 6a).

Compositions of minerals in the retrogressed matrix were used to estimate P-T conditions during exhumation. The Amp-Pl-Qz geothermometer of Holland and Blundy (1994) gives 580 ± 15 °C at 0.8 GPa for M2 (Hbl + Ab + Qz) and 450 ± 50 °C at 0.4 GPa for M3 (Act + Ab + Qz) (Fig. 6a). The Chl-Ph-Qz thermobarometry (Vidal et al., 2001; Parra et al., 2002) gives conditions of M3 as 400 ± 10 °C, 0.4 ± 0.1 GPa (Fig. 6a).

DISCUSSION AND CONCLUSION

The areal extent of the eclogite facies metamorphism

The eclogite assemblage Grt + Omp + Qz in the Sanbaga-wa belt has only been recognized in the Besshi (central Shikoku) and Kotsu (eastern Shikoku) areas (e.g., Takasu, 1989; Matsumoto et al., 2003). Indirect evidence (i.e., geothermobarometric results) for the eclogite facies meta-morphism in the Sanbagawa belt has been proposed for garnet-rich pods and garnet amphibolite from the oligo-clase-biotite zone of the Asemigawa area, central Shikoku (Aoki et al., 2009; Uno and Toriumi, 2010) and garnet-glaucophane schist from the Bizan area, eastern Shikoku (Takasu et al., 2012). All previous recognized and pro-posed locations of eclogite facies metamorphism in the Sanbagawa belt have been restricted to Shikoku Island.

Petrographic observations combined with geother-mobarometric calculations indicate that the Funaokayama

garnet amphibolite preserves a record of a hairpin-like P-T path of metamorphism with the prograde evolution from 0.8 GPa, 570 °C to 1.3 GPa, 590 °C (M1) followed by near isothermal decompression to ~0.8 GPa (M2) and subsequent cooling to ~ 400 °C, 0.4 GPa (M3) (Fig. 6a). The results of quartz Raman barometry, pseudosection modeling and Grt-Ttn-Rt-Ep-Qz barometry suggest that peak-P conditions of M1 stage reached the eclogite fa-cies, even if the most conservative pressure estimate is adopted (Fig. 6a). The discovery of aragonite in associat-ed metapelite strongly supports the idea that the Funao-kayama unit is a distinct eclogite-facies unit in western Kii Peninsula. However, the slight difference in the peak-

T conditions between the garnet amphibolite and metapelite (Fig. 6a) may suggest a spatial variation of metamorphic P-T conditions within the Funaokayama unit.

The estimated peak-P conditions of the Funaokaya-ma garnet amphibolite are somewhat lower than those of the eclogite-facies rocks in the Besshi and Kotsu areas (Matsumoto et al., 2003; Aoya et al., 2013 and references therein). Accordingly, it is unlikely that the Funaokayama unit forms a continuous unit with the eclogite-facies units in Shikoku. However, the present study greatly expands the extent of eclogite facies metamorphism along-arc di-rection, and thus tectonic models on the exhumation of the high-grade Sanbagawa belt should take this into ac-count.

High-pressure garnet amphibolite

The assemblage Grt + Omp + Qz in metabasite is com-monly used to define the eclogite facies. However, om-phacite-free metabasite is likely to be common in the low-P and moderate-T range of the eclogite facies. Meta-morphic gradients of the Sanbagawa belt match the for-mation conditions of such rocks. For example, metabasal-tic rocks in the Western Iratsu body of the Besshi area recrystallized to plagioclase-free garnet amphibolitic as-semblages during the eclogite facies metamorphism, and the occurrence of the Grt + Omp assemblage is restricted to bulk composition with relatively high (CaO + Na2O)/(FeO + MgO) ratio (Endo et al., 2012). The (CaO + Na2O)/(FeO + MgO) ratio of the Funaokayama garnet amphibolite (0.70) falls within the range of the eclogite-facies garnet amphibolite from the Western Iratsu body (<0.72). The result of pseudosection modeling suggests that omphacitic clinopyroxene stabilizes in this bulk com-position at pressure in excess of 1.7 GPa at 560-590 °C (Fig. 5).

It remains unclear whether albite porphyroblast-rich layers of the Funaokayama garnet amphibolite represent


retrogression products of eclogite. If these layers were isochemical during albite growth, although not recognized in this study omphacite should have been present at the peak of M1. One possible explanation is that the Funao-kayama unit suffered pervasive deformation (D2 = Ds) during exhumation, which enhanced the omphacite- breakdown reactions to form syn-D2 albite porphyrob-lasts (Fig. 2d).

The petrographic features (mineral assemblages, mineral chemistry and banded structures of alternating al-bite porphyroblast-rich and -free layers) of the Funao-kayama garnet amphibolite are similar to those of garnet amphibolite in the oligoclase-biotite zone of the Asemi-gawa area (Aoki et al., 2009; Uno and Toriumi, 2012). Although further work is still needed to identify clear eclogite facies mineral assemblages from these areas, the multidisciplinary approach used in this study is a useful way to explore the extent of the eclogite facies metamor-phism in the Sanbagawa belt and also in other similar geological regions.

Tectonic implications

The newly derived subduction-stage P-T path for the Fu-naokayama unit is characterized by thermal gradients (14-

22 °C/km) that are high for the typical range of subduc-tion zones and a very high dP/dT ratio (0.5 GPa/20 °C) at P > 0.8 GPa, suggesting a pronounced concave-up form of the thermal structure along the slab surface. This type of P-T path is characteristic of ridge subduction settings (Aoya et al., 2003; Uehara and Aoya, 2005). The overall P-T path of the Funaokayama unit is characterized by a curved hairpin-like shape (Fig. 6a), which is in contrast to the clockwise P-T paths proposed for the Sanbagawa schists in Shikoku (e.g., Enami et al., 1994). The prograde P-T path of the Funaokayama unit occurred at higher temperature than the subduction-stage P-T array of the Sanbagawa schists in Shikoku (Fig. 6b). This implies that the subduction and exhumation cycle of the Funaokayama unit occurred immediately before the spreading ridge ar-rived at the subduction zone (Fig. 6b) and that the cycle of subduction and exhumation was complete within a very short time.

The eclogite-facies rocks in the Besshi area probably constituted a thick buoyant subduction channel formed by the accumulation of subducted materials as evidenced by the presence of voluminous metasedimentary rocks and some tectonic slices with older metamorphic histories (Endo et al., 2012 and references therein). In contrast, the spatial extent of the Funaokayama unit is likely to be rather small (Fig. 1b) and it may have formed within a narrowed part of the subduction channel. Compared with

the Eclogite unit in the Besshi area, initial exhumation of the smaller Funaokayama unit by buoyancy forces re-quires more pronounced viscosity reduction, which could be achieved by high temperatures due to closer approach of a spreading ridge as inferred from the prograde P-T path.

ACKNOWLEDGMENTS

We thank Prof. M. Enami and members of the petrology group at Nagoya University for numerous discussions on the Sanbagawa belt. Constructive reviews from Dr. T. Mizukami and Dr. A. Okamoto as well as editorial com-ments from Prof. M. Satish-Kumar greatly improved the manuscript. This work was supported in part by a JSPS research visit to Japan awarded to IN and by JSPS grant-in-aid 24244079 awarded to SW.

DEPOSITORY MATERIALS

Figure S1 and color version of Figure 2 are available on-line from http://japanlinkcenter.org/DN/JST.JSTAGE/jmps/121125.

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Manuscript received November 25, 2012Manuscript accepted February 26, 2013Published online May 16, 2013Manuscript handled by M. Satish-Kumar

High-pressure garnet amphibolite from the Funaokayama unit, western Kii Peninsula and the extent of...

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