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

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1 Journal of Mineralogical Eclogite and Petrological metamorphism Sciences, in the Kii Volume Peninsula 18, page 189 2, 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 , Japan ** Institute of Geological Sciences, Polish Academy of Sciences INGPAN, Podwale 75, Wroclaw, Poland *** Department of Earth and Planetary Sciences, Nagoya University, Chikusa - ku, Nagoya , Japan Quantitative analyses of garnet - bearing epidote amphibolite (hereafter garnet amphibolite) from the Funaokayama 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 garnet 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.8 GPa, 57 C to 1.3 GPa, 59 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 + H 2 O) give consistent peak - P estimates of GPa. The garnet contains quartz inclusions retaining residual pressures of up to ~.7 GPa, which is as high as the values reported in well - characterized eclogite samples in central Shikoku. The inferred 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 ~.8 GPa, 58 C suggest decompression to the epidote - amphibolite facies (M2). The M2 minerals 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 doi: /jmps S. Endo, s - endo@aist.go.jp Corresponding author The Sanbagawa belt of SW Japan is a region of Cretaceous oceanic subduction - type metamorphism (e.g., Isozaki and Itaya, 199; Wallis et al., 29). The most deeply subducted portions of this orogen include eclogite or the eclogite assemblage garnet + omphacite + quartz. To understand 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 eclogite 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 occupies highest structural levels of the Sanbagawa belt (Wallis and Aoya, 2). Moreover, close examination of inclusions in garnet has further extended the Eclogite unit

2 19 S. Endo, I. Nowak and S.R. Wallis to domains of pelitic schist, which is the predominant lithology in the Sanbagawa belt (Zaw et al., 25; Kouketsu et al., 21). In these recent studies, laser Raman spectroscopy has played an important role in the identification of microscopic inclusions diagnostic of the eclogite facies (e.g., aragonite, omphacite) or detection of high residual pressures of quartz inclusions (Enami et al., 27). In addition, calculated equilibrium phase - assemblage diagrams (pseudosections) have been used to recognize eclogite facies assemblages in various rock compositions (Aoki et al., 29; Kouketsu and Enami, 211; Endo et al., 212). Despite the improved understanding of how to recognize rocks that have undergone eclogite facies metamorphism, 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 Sanbagawa belt show retrogression in the epidote - amphibolite facies (e.g., Takasu, 1989; Ota et al., 24). In addition, 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., 212), 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, 199; Aoya et al., 23; Okudaira and Yoshitake, 24; Wallis et al., 29). Therefore, 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 Sanbagawa 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 (Hirota, 1991). This area lies about 1 km east of the nearest known eclogite locality in eastern Shikoku (Matsumoto et al., 23), 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 Funaokayama unit in western Kii Peninsula reached the eclogite facies at its maximum burial. OUTLINE OF GEOLOGY The Sanbagawa belt of southwest Japan extends around 8 km from the Kanto Mountains through Chubu Region, Kii Peninsula to Shikoku and eastern Kyushu (Fig. 1a). The regional metamorphism in this belt has been discussed in terms of four zones based on index minerals in pelitic schists: the chlorite (<36 C, GPa), garnet (44 ± 15 C, GPa), albite - biotite (52 ± 25 C, GPa) and oligoclase - biotite (61 ± 2 C, GPa) zones, in ascending order of metamorphic grade (Enami, 1983; Higashino, 199; Enami et al., 1994). The full set of the four metamorphic zones is exposed in the Besshi and Asemigawa areas of central Shikoku. Parts of the high - grade zones in central Shikoku (a) Kyushu (b) Sanbagawa belt M.T.L. 1 km Ryumon peridotite Amurian Plate Shikoku Kii Chubu Kanto Philippine Sea Plate 2 km Quaternary sediment Pelitic schist Siliceous schist Mafic schist N Ultramafic rocks Sample locality Grt amphibolite Pelitic schist Funaokayama unit R. Kinokawa Iimori synform Iimori unit Tomobuchi unit 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. (c) Pole to S2 L2 (stretching) Crenulation? N?

3 Eclogite metamorphism in the Kii Peninsula 191 have mineral parageneses demonstrating eclogite facies metamorphism before a regional epidote - amphibolite facies overprint (e.g., Takasu, 1989; Ota et al., 24; Aoya et al., 213). In western Kii Peninsula, the Sanbagawa belt consists mainly of mafic, pelitic - psammitic and siliceous schists and ultramafic bodies. The area has been divided into the Tomobuchi (structurally lower) and Iimori (structurally upper) units (Hirota, 1991) (Fig. 1b). Metamorphism in this area is equivalent to the chlorite zone (Tomobuchi 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 distinctly high - grade rocks and the extent of these rocks have been treated as an independent tectonic unit (Funaokayama unit) (Hirota, 1991; Takasu et al., 1996) (Fig. 1b). The Funaokayama unit consists of mafic, pelitic and minor siliceous schists. The main schistosity in the Funaokayama 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 metamorphism of the Funaokayama unit is comparable to the albite - biotite and oligoclase - biotite zones of central Shikoku. However, the phengite 4 Ar/ 39 Ar cooling age of the Funaokayama unit (74.8 ±.4 Ma, Takasu et al., 1996) is significantly younger than that of the oligoclase - biotite zone of the Asemigawa area, central Shikoku (84.3 ±.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 samples used in this study were collected from southern slope of the Mt. Funaoka (6 m) (Fig. 1b). PETROGRAPHY OF GARNET AMPHIBOLITE Garnet amphibolite from the Funaokayama unit is composed mainly of garnet, amphibole, epidote, quartz, phengite, chlorite, plagioclase, and subordinate amounts of rutile, 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 lineation is parallel to the stretching direction. Monomineralic 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 - 89/88R (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.

4 192 S. Endo, I. Nowak and S.R. Wallis versity and AIST. The accelerating voltage, specimen current and beam diameter were 15 kv, 12 na and 2-1 µm, respectively. Representative microprobe analyses are reported in Table 1. Hereafter, abbreviations for minerals and end-member components follow Whitney and Evans (21). quartz (Figs. 2b and 2c). Garnet crystals generally suffered varying degrees of fracturing and chloritization. A representative compositional profile for garnet is shown in Figure 3a. It ranges within Alm59 64Sps1 1Grs23 3Prp6 1 and displays well-preserved growth zoning characterized by a monotonous decrease in Mn from core (Sps1) to rim (Sps1). Pyrope component increases gradually from the Mn-richest center (Prp6) to a middle zone (Prp9 1) and then decreases slightly toward the crystal margin (Prp8 9). Garnet Garnet occurs as euhedral porphyroblasts that vary in size from.5 mm to 5 mm. It contains inclusions of amphibole, epidote, quartz, phengite, zircon, apatite, titanite and rutile. No inclusions of albite or omphacite were observed. There is no systematic change in inclusion paragenesis from core to rim, but some garnet grains show an abrupt increase in the size and population of quartz inclusions 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 garnet 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 stages in the tectonic evolution of the area. Pressure shadows around the garnet porphyroblasts are filled mainly by (b) (a) Amphibole Prismatic amphibole crystals occur as the main constituent 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 magnesiohornblende and tschermakite fields according to Leake et al. (1997) (Fig. 3b). In retrogressed samples, these amphibole crystals are partially rimmed by actinolite. Amphibole crystals within garnet are generally larger (>2 μm in length) than other inclusion minerals (e.g., epidote, titanite, rutile), and commonly form composite inclusions with quartz. The compositional range of am- (d) S2 Ep S2 Qz Hbl Grt S1 1 mm (c) Hbl 1 mm 1 mm D1 (e) S2 S1 Titanite Epidote Amphibole Hbl+Ep+Chl S2 Garnet Quartz Chlorite Phengite Amphibole 2 mm M2 M3 Hbl Hbl Act Epidote Grt S1 Qz Albite Garnet Quartz Amphibole D2 (=Ds) M1 Porphyroblast Albite Chlorite Phengite Stilpnomelane Calcite Ti-phase Rt+Ttn Ttn Ttn Quartz 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 minerals. (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 center.org/dn/jst.jstage/jmps/

5 Eclogite metamorphism in the Kii Peninsula 193 (a).7 Rim Core Rim phibole included in garnet largely overlaps with that of the matrix amphibole (Fig. 3b). Mole fraction (b) Mg/(Mg+Fe 2+ ) [A] Na+K (apfu) [B] Na (apfu) (c) Mg/(Mg+Fe) Wtc Act 7.5 Almandine Grossular Spessartine Distance (µm) Ktp/Ed Brs/Hbl Brs/Ktp Hbl/Ed 7. Si (apfu) Phengite Inc. Grt Inc. Ab Pyrope Amphibole Inc. Grt Matrix (Core) Matrix (Rim) Si (apfu) 6.5 Trm/Prg Ts Trm Prg/Ts Matrix (Core) Matrix (Rim) Figure 3. Mineral chemistry of garnet amphibolite from the Funaokayama unit. (a) Rim - core - rim compositional profile of a garnet crystal. (b) Amphibole compositions plotted in terms of Na content in the B site, A - site occupancy and Mg/(Mg + Fe 2+ ) against Si content. (c) Phengite compositions plotted in terms of Mg/(Mg + Fe) against Si content. 6. Other minerals Phengite in the matrix has a compositional range of Si = apfu, Mg# = , X Na [= Na/(Na + K)] = Some phengite flakes in the matrix have thin Fe - rich rims with a compositional range of Si = apfu, Mg# = , X Na = Rare phengite inclusions in garnet have compositions of Si = apfu, Mg# = , X Na = (Fig. 3c). Chlorite in the matrix has a compositional range of Si = apfu, Mg# = 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 crystals composed of a core with X Ps [= Fe 3+ /(Fe 3+ + Al)] = , a mantle with X Ps = and a thin outermost rim with X Ps = Epidote included in garnet shows the same compositional range as the core and mantle of the matrix epidote (Fig. S1a; Fig. S1 is available online from jmps/121125). Prograde titanite is rich in Fe 3+ (Al 2 O 3 = wt%, Fe 2 O 3 = wt%) (Fig. S1b) and occurs as minute (<2 μm) inclusions in garnet. Retrograde titanite (Al 2 O 3 = wt%, Fe 2 O 3 = wt%) occurs as subhedral crystals (<5 μm) in the matrix and included in albite porphyroblasts. Relic rutile is rarely preserved within the matrix titanite. Pristine rutile crystals are exclusively present as inclusions in garnet. Metamorphic and deformational stages Three metamorphic stages (M1, M2 and M3) can be defined based on the petrographic observation presented above (Fig. 2e). The first metamorphic stage (M1) is defined 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 metamorphic stage (M2) is characterized by pervasive recrystallization 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 expense 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-

6 194 S. Endo, I. Nowak and S.R. Wallis formation phase D2, which corresponds to the main deformation phase of the Sanbagawa belt, D S (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., 27; Kouketsu et al., 21). The Raman spectroscopic parameter Δω 1 defined in Enami et al. (27) is the measure of residual pressure retained by a quartz inclusion. We analyzed quartz microinclusions in three samples of garnet amphibolite, following the procedure of Enami et al. (27). The histogram of Δω 1 is shown in Figure 4a, Number of analyses Intensity (a) (b) Number of analyses (c) Upper limit of Ep-amp facies data (Enami et al., 27) Max ω1 range of eclogite facies samples in the Besshi area (Enami et al., 27; Kouketsu et al., 21) Mean R2=.262 SD= µm Arg Arg Cal ω 1 (cm -1 ) R2 = [D1/(G + D1 + D2)] area Raman shift (cm -1 ) Figure 4. (a) Histogram of Δω 1 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 microphotograph (Plane - polarized light) of the garnet with aragonite inclusions (Arg) which shows a bimodal distribution. There is no clear correlation between the Δω 1 value and position of inclusions within garnet crystals. The higher Δω 1 group ranges from cm 1, corresponding to residual pressures of GPa. This result is comparable to the well - characterized eclogite samples in the Besshi area ( cm 1, Enami et al., 27; Kouketsu et al., 21). P-T constraints from metapelite Thermometry using Raman spectra of carbonaceous matter (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. Funaoka (Fig. 1b) and adjacent to the garnet amphibolite in the Funaokayama unit. Grain - shape fabric of quartz in this sample indicates deformation and dynamic recrystallization of this mineral and thus suggests deformation of pelitic schists continued during relatively low - T conditions 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, following the procedure of Aoya et al. (21). 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. (21) gives T = 537 ± 3 C Inclusions in garnet in this sample were also examined 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 inclusions appear to have transformed to calcite. Pseudosection modeling Phase equilibria at M1 stage of garnet amphibolite were modeled in the system K 2 O - Na 2 O - CaO - MnO - FeO - MgO - Al 2 O 3 - SiO2 - H2 O - O 2 (KNCMnFMASHO). Gibbs free energy minimization has been carried out using the software Perple_X_ (Connolly, 29), the internally consistent thermodynamic dataset of Holland and Powell (1998, updated 22) and the following solid - solution models: garnet (White et al., 27), clinopyroxene and amphibole (Diener and Powell, 212), epidote (Holland and Powell, 1998), chlorite (Holland et al., 1998), phengite (Coggon and Holland, 22), biotite (Tajcmanová et al., 29) and feldspar (Fuhrman and Lindsley, 1988). Fluid was considered to be pure H 2 O and in excess. The effective bulk composition was constructed by combining modes and mean compositions of minerals: garnet (3.34 vol%; Alm 63 Sps 3 Prp 8 Grs 26 ), amphibole (65.55

7 Eclogite metamorphism in the Kii Peninsula 195 Pressure (GPa) Pressure (GPa) (a) (c) Mol.%: SiO , Al 2O , FeO 12.69, MnO.5, MgO 1.61, CaO 14.29, Na 2O 1.91, K 2O.13, O 2.86 Ph Hbl Gln Omp Chl Ph Omp Qz Chl Ph Hbl Qz Chl Bt Hbl Qz Prp6 Prp7 Ph Hbl Qz Ph Hbl Omp Qz Bt Hbl Qz Bt Pl Hbl Qz Mgt Bt Pl Hbl Qz Bt Pl Hbl Di Mgt (-Ep) Prp8 Prp8 3 vol% Grt 1 vol% Grt Amp [B] Na =.4 apfu [B] Na = 1. Temperature ( o C) Grs27 [B] Na =.8 Grs26 +Cpx +Ep, Grt, H 2O 3 vol% Grt [B] Na =.6 Grs25 (b) Pressure (GPa) (d) Pressure (GPa) Mol.%: SiO , Al 2O , FeO 12.4, MnO.2, MgO 1.76, CaO 14.36, Na 2O 1.95, K 2O.13, O Lws Chl Ph Omp Qz Ph Si=3.3 apfu Chl Ph Hbl Qz Chl Bt Hbl Qz Sps2 2Pl Ph Hbl Qz Bt Hbl Qz [B] Na = 1. Bt Pl Hbl Qz Sps3 Prp9 Si=3.25 Si=3.15 +Gln Si=3.2 Grs27 Temperature ( o C) Grs28 Sps1 Rim Ph Hbl Omp Qz [B] Na =.6 +Cpx Bt Pl Hbl Qz Mgt [B] Na =.8 Bt Hbl Di Qz Amp [B] Na =.4 apfu Bt Pl Hbl Di Mgt (-Ep) Grs25 Grs26 Grs27 Prp1 +Ep, Grt, H 2O Grs28 Grs29 Grs3 1.1 Sps1 Sps6 Grs27 Sps3 Prp8 Core (Outer) 1.1 Prp8 Core (Outer).8 Core (Inner) Grs26 Grs27.8 Core (Inner) Temperature ( o C) Temperature ( o C) 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 (Sps 3 1 Prp 6 8 Grs ) are shown on (a). Gray solid stars indicate P - T conditions at which inner core and outer core compositions are reproduced. (d) Compositional isopleths for the garnet rim (Sps 1 3 Prp 8 1 Grs 25 3 ) 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. vol%), epidote (22.7 vol%; Cz 5 Ep 5 ), phengite (2.1 vol%; Ms 58 Cel 2 Fcel 1 Pa 12 ), chlorite (.33 vol%; Clc 37.5 Ame 14 Dph 48 MnClc.5 ), albite (.67 vol%) and quartz (6.2 vol%). The modal proportion (vol%) was estimated using elemental map data ( μm 2 area) from a sample 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 converted 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 zoning profile assuming a spherical geometry. Ti, Mn and K

8 196 S. Endo, I. Nowak and S.R. Wallis 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 (Sps 3 1 Prp 6 8 Grs ) and rim (Sps 1 3 Prp 8 1 Grs 25 3 ) 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.8 GPa, 57 C to 1. GPa, 58 C (Fig. 5c). The garnet growth with increasing pressure 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, 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 pseudosection 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. GPa, 58 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 pseudosection modeling, a crosscheck on the quantitative P - T conditions using simpler methodologies is desirable. Here we use inclusion assemblage Ttn + Rt + Ep + Qz and the garnet rim composition to estimate the peak - P conditions based on the following reaction: 5Ttn + 2Cz = 5Rt + 3Grs + 2Qz + H 2 O. The equilibrium constant (K Eq ) of the reaction is written as (a 5 Rt a 3 Grs a 2 Qz a H 2O)/(a 5 Ttn a 2 Cz). Isolated euhedral inclusions of epidote and titanite seem to preserve their prima- (a) Pressure (GPa) Arg Cal Grt-Ep- Ttn-Rt-Qz G96+I VH93+M RSCM (Metapelite) Omp+Qz Di ss +Ab D2=Ds M1 D1 M2 Hbl-Ab-Qz (b) Pressure (GPa) Thermal structure along the slab surface before onset of ridge subduction Myr Steady state Grt and Bt zones Eclogite Subduction-stage P-T paths of the Sanbagawa schists in Shikoku Funaokayama.4 M3 Act-Ab-Qz.5 Chl zone Chl-Ph-Qz Temperature ( o C) Temperature ( o C) 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 garnet (Vance and Holland, 1993) and ideal molecular - mixing for titanite (VH93 + M). The univariant reaction Omp + Qz = Di ss + Ab is shown as the low - P limit of eclogite facies. Thermodynamics of mixing in clinopyroxene follows Diener and Powell (212). P - T constraints from metapelite by Raman spectra of carbonaceous matter (RSCM) thermometry and the aragonite - calcite reaction are also shown. (b) Comparison of subduction - stage P - T paths between the Funaokayama garnet amphibolite (this study) and the Sanbagawa schists in Shikoku (compiled by Aoya et al., 23). Gray curves show model P - T conditions at the slab surface before ridge subduction (Endo et al., 212, calculated with the thermal model of Uehara and Aoya, 25).

9 Eclogite metamorphism in the Kii Peninsula 197 ry compositions. This is because that the epidote inclusions 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, central Shikoku (Endo, 21) (Fig. S1b). Rutile and quartz are almost pure in composition. In addition, fluid was assumed to be pure H 2 O. It follows that the equilibrium constant can be simplified to a 3 Grs/(a 5 Ttn a 2 Cz). Taking the subregular solution model for garnet (Ganguly et al., 1996) and ideal ionic - mixing models for titanite (a Ttn = X Ca X Ti X Si X O[O1] ) and clinozoisite (a Cz = X Ca[A2] X Al[M3] ), compositions of the coexisting three minerals give - lnk Eq = , which is comparable to the value of titanite - and rutile - bearing eclogite from the Western Iratsu body (Endo, 21). Using the thermodynamic data of Holland and Powell (1998), the equilibrium curve lies at GPa at a temperature range of 5-6 C (Fig. 6a). The use of the regular solution model for garnet (all interaction energies set to zero except for W CaMg = 33 kj/mol: Vance and Holland, 1993) and ideal molecular - mixing model for titanite (a Ttn = X Ti ) lowers the pressure estimates at most.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 58 ± 15 C at.8 GPa for M2 (Hbl + Ab + Qz) and 45 ± 5 C at.4 GPa for M3 (Act + Ab + Qz) (Fig. 6a). The Chl - Ph - Qz thermobarometry (Vidal et al., 21; Parra et al., 22) gives conditions of M3 as 4 ± 1 C,.4 ±.1 GPa (Fig. 6a). DISCUSSION AND CONCLUSION The areal extent of the eclogite facies metamorphism The eclogite assemblage Grt + Omp + Qz in the Sanbagawa belt has only been recognized in the Besshi (central Shikoku) and Kotsu (eastern Shikoku) areas (e.g., Takasu, 1989; Matsumoto et al., 23). Indirect evidence (i.e., geothermobarometric results) for the eclogite facies metamorphism in the Sanbagawa belt has been proposed for garnet - rich pods and garnet amphibolite from the oligoclase - biotite zone of the Asemigawa area, central Shikoku (Aoki et al., 29; Uno and Toriumi, 21) and garnet - glaucophane schist from the Bizan area, eastern Shikoku (Takasu et al., 212). All previous recognized and proposed locations of eclogite facies metamorphism in the Sanbagawa belt have been restricted to Shikoku Island. Petrographic observations combined with geothermobarometric calculations indicate that the Funaokayama garnet amphibolite preserves a record of a hairpin - like P - T path of metamorphism with the prograde evolution from.8 GPa, 57 C to 1.3 GPa, 59 C (M1) followed by near isothermal decompression to ~.8 GPa (M2) and subsequent cooling to ~ 4 C,.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 facies, even if the most conservative pressure estimate is adopted (Fig. 6a). The discovery of aragonite in associated metapelite strongly supports the idea that the Funaokayama 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 Funaokayama garnet amphibolite are somewhat lower than those of the eclogite - facies rocks in the Besshi and Kotsu areas (Matsumoto et al., 23; Aoya et al., 213 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 direction, and thus tectonic models on the exhumation of the high - grade Sanbagawa belt should take this into account. High-pressure garnet amphibolite The assemblage Grt + Omp + Qz in metabasite is commonly used to define the eclogite facies. However, omphacite - free metabasite is likely to be common in the low - P and moderate - T range of the eclogite facies. Metamorphic gradients of the Sanbagawa belt match the formation conditions of such rocks. For example, metabasaltic rocks in the Western Iratsu body of the Besshi area recrystallized to plagioclase - free garnet amphibolitic assemblages during the eclogite facies metamorphism, and the occurrence of the Grt + Omp assemblage is restricted to bulk composition with relatively high (CaO + Na 2 O)/ (FeO + MgO) ratio (Endo et al., 212). The (CaO + Na 2 O)/(FeO + MgO) ratio of the Funaokayama garnet amphibolite (.7) falls within the range of the eclogite - facies garnet amphibolite from the Western Iratsu body (<.72). The result of pseudosection modeling suggests that omphacitic clinopyroxene stabilizes in this bulk composition at pressure in excess of 1.7 GPa at C (Fig. 5). It remains unclear whether albite porphyroblast - rich layers of the Funaokayama garnet amphibolite represent

10 198 S. Endo, I. Nowak and S.R. Wallis 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 Funaokayama unit suffered pervasive deformation (D2 = Ds) during exhumation, which enhanced the omphacite - breakdown reactions to form syn - D2 albite porphyroblasts (Fig. 2d). The petrographic features (mineral assemblages, mineral chemistry and banded structures of alternating albite porphyroblast - rich and - free layers) of the Funaokayama garnet amphibolite are similar to those of garnet amphibolite in the oligoclase - biotite zone of the Asemigawa area (Aoki et al., 29; Uno and Toriumi, 212). 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 metamorphism in the Sanbagawa belt and also in other similar geological regions. Tectonic implications The newly derived subduction - stage P - T path for the Funaokayama unit is characterized by thermal gradients (14-22 C/km) that are high for the typical range of subduction zones and a very high dp/dt ratio (.5 GPa/2 C) at P >.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., 23; Uehara and Aoya, 25). 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 arrived 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., 212 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 requires 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 comments 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 awarded to SW. DEPOSITORY MATERIALS Figure S1 and color version of Figure 2 are available online from jmps/ REFERENCES Aoki, K., Kitajima, K., Masago, H., Nishizawa, M., Terabayashi, M., Omori, S., Yokoyama, T., Takahata, N., Sano, Y. and Maruyama, S. (29) Metamorphic P - T - time history of the Sanbagawa belt in central Shikoku, Japan and implications for retrograde metamorphism during exhumation. Lithos, 113, Aoya, M., Uehara, S., Matsumoto, M., Wallis, S.R. and Enami, M. (23) Subduction - stage pressure - temperature path of eclogite from the Sambagawa belt: prophetic record for oceanic - ridge subduction. Geology, 31, Aoya, M., Kouketsu, Y., Endo, S., Shimizu, H., Mizukami, T., Nakamura, D. and Wallis, S.R. (21) Extending the applicability of the Raman carbonaceous - material geothermometer using data from contact metamorphic rocks. Journal of Metamorphic Geology, 28, Aoya, M., Noda, A., Mizuno, K., Mizukami, T., Miyachi, Y., Matsuura, H., Endo, S., Toshimitsu, S. and Aoki, M. (213) Geology of the Niihama district. Quadrangle Series, 1:5,. Geological Survey of Japan, AIST, Tsukuba (in Japanese with English abstract). Coggon, R. and Holland, T.J.B. (22) Mixing properties of phengitic micas and revised garnet - phengite thermobarometers. Journal of Metamorphic Geology, 2, Connolly, J.A.D. (29) The geodynamic equation of state: what and how. Geochemistry, Geophysics, Geosystems, 1, Q114. Diener, J.F.A. and Powell, R. (212) Revised activity - composition models for clinopyroxene and amphibole. Journal of Metamorphic Geology, 3, Enami, M. (1983) Petrology of pelitic schists in the oligoclase - biotite zone of the Sanbagawa metamorphic terrain, Japan:

11 Eclogite metamorphism in the Kii Peninsula 199 phase equilibria in the highest grade zone of a high - pressure intermediate type of metamorphic belt. Journal of Metamorphic Geology, 1, Enami, M., Wallis, S.R. and Banno, Y. (1994) Paragenesis of sodic pyroxene - bearing quartz schist: implications for the P - T history of the Sanbagawa belt. Contributions to Mineralogy and Petrology, 116, Enami, M., Nishiyama, T. and Mouri, T. (27) Laser Raman microspectrometry of metamorphic quartz: A simple method for comparison of metamorphic pressures. American Mineralogist, 92, Endo, S. (21) Pressure - temperature history of titanite - bearing eclogite from the Western Iratsu body, Sanbagawa metamorphic belt, Japan. Island Arc, 19, Endo, S., Wallis, S.R., Tsuboi, M., Aoya, M. and Uehara, S. (212) Slow subduction and buoyant exhumation of the Sanbagawa eclogite. Lithos, , Fuhrman, M.L. and Lindsley, D.H. (1988) Ternary - feldspar modeling and thermometry. American Mineralogist, 73, Ganguly, J., Cheng, W.J. and Tirone, M. (1996) Thermodynamics of aluminosilicate garnet solid solution: new experimental data, an optimized model, and thermodynamic applications. Contributions to Mineralogy and Petrology, 126, Higashino, T. (199) The higher - grade metamorphic zonation of the Sanbagawa metamorphic belt in central Shikoku, Japan. Journal of Metamorphic Geology, 8, Hirota, Y. (1991) Geology of the Sambagawa metamorphic belt in western Kii Peninsula, Japan. Memoires of the Faculty of Sciences, Shimane University, 25, (in Japanese). Holland, T.J.B. and Blundy, J. (1994) Non - ideal interactions in calcic amphiboles and their bearing on amphibole - plagioclase thermometry. Contributions to Mineralogy and Petrology, 116, Holland, T.J.B. and Powell, R. (1998) An internally consistent thermodynamic data set for phases of petrological interest. Journal of Metamorphic Geology, 16, Holland, T.J.B., Baker, J. and Powell, R. (1998) Mixing properties and activity - composition relationships of chlorites in the system MgO - FeO - Al 2 O 3 - SiO2 - H2 O. European Journal of Mineralogy, 1, Isozaki, Y. and Itaya, T. (199) Chronology of the Sanbagawa metamorphism. Journal of Metamorphic Geology, 8, Kouketsu, Y., Enami, M. and Mizukami, T. (21) Omphacite - bearing metapelite from the Besshi region, Sambagawa metamorphic belt, Japan: Prograde eclogite facies metamorphism recorded in metasediment. Journal of Mineralogical and Petrological Sciences, 15, Kouketsu, Y. and Enami, M. (211) Calculated stabilities of sodic phases in the Sambagawa metapelites and their implications. Journal of Metamorphic Geology, 29, Kunugiza, K., Takasu, A. and Banno, S. (1986) The origin and metamorphic history of the ultramafic and metagabbro bodies in the Sanbagawa Metamorphic Belt. Geological Society of America Memoir, 164, Leake, B.E., Woolley, A.R., Arps, C.E.S., Birch, W.D., Gilbert, M.C., Grice, J.D., Hawthorne, F.C., Kato, A., Kisch, H.J., Krivovichev, V.G., Linthorne, K., Laird, J., Mandarino, J., Maresch, W.V., Nickel, E.H., Rock, N.M.S., Schumacher, J.C., Smith, D.C., Stephenson, N.C.N., Ungaretti, L., Whittaker, E.J.W. and Youzhi, G. (1997) Nomenclature of amphiboles: Report of the subcommittee on amphiboles of the International Mineralogical Association, Commission on New Minerals and Mineral Names. European Journal of Mineralogy, 9, Matsumoto, M., Wallis, S., Aoya, M., Enami, M., Kawano, J. and Seto, Y. (23) Petrological constrains on the formation conditions and retrograde P - T path of the Kotsu eclogite unit, central Shikoku. Journal of Metamorphic Geology, 21, Nakajima, T., Shirahase, T. and Shibata, K. (199) Along - arc lateral variation of Rb - Sr and K - Ar ages of Cretaceous granitic rocks in Southwest Japan. Contributions to Mineralogy and Petrology, 14, Okudaira, T. and Yoshitake, Y. (24) Thermal consequences of the formation of a slab window beneath the Mid - Cretaceous southwest Japan arc: A 2 - D numerical analysis. Island Arc, 13, Ota, T., Terabayashi, M., Katayama, I. (24) Thermobaric structure and metamorphic evolution of the Iratsu eclogite body in the Sanbagawa belt, central Shikoku, Japan. Lithos, 73, Parra, T., Vidal, O. and Agard, P. (22) A thermodynamic model for Fe - Mg dioctahedral K white micas using data from phase - equilibrium experiments and natural pelitic assemblages. Contribution to Mineralogy and Petrology, 143, Tajcmanová, L., Connolly, J.A.D. and Cesare, B. (29) A thermodynamic model for titanium and ferric iron solution in biotite. Journal of Metamorphic Geology, 27, Takasu, A. (1989) P - T histories of peridotite and amphibolite tectonic blocks in the Sambagawa metamorphic belt, Japan. In The Evolution of Metamorphic Belts (Daly, J.S., Cliff, R.A. and Yardley, B.W.D. Eds.). Geological Society of London Special Publication, 43, Takasu, A. and Dallmeyer, R.D. (1994) 4 Ar/ 39 Ar mineral age constraints for the tectonothermal evolution of the Sambagawa metamorphic belt, central Shikoku, Japan: a Cretaceous accretionary prism. Tectonophysics, 185, Takasu, A., Dallmeyer, R.D. and Hirota, Y. (1996) 4 Ar/ 39 Ar muscovite ages of the Sambagawa schists in the Iimori district, Kii Peninsula, Japan: Implications for orogen - parallel diachronism. Journal of Geological Society of Japan, 12, Takasu, A., Kabir, M.F., Nakamura, K., Kondo, Y. and Kainuma, M. (212) Metamorphic P - T path of the garnet glaucophane schists in the Bizan district, Sambagawa metamorphic belt, eastern Shikoku, southwest Japan. Abstracts with Programs 212 Annual Meeting of Japan Association of Mineralogical Sciences, R8-1 (in Japanese). Uehara, S. and Aoya, M. (25) Thermal model for approach of a spreading ridge to subduction zones and its implications for high - P/high - T metamorphism: Importance of subduction versus ridge approach ratio. Tectonics, 24, TC47. Uno, M. and Toriumi, M. (21) Quantitative analysis of material transfer during the ascent of garnet - amphibolite mass in the Sambagawa metamorphic belt, Japan. Abstracts with Programs of American Geophysical Union, Fall Meeting, V33A Vance, D. and Holland, T. (1993) A detailed isotopic and petrological study of a single garnet from the Gassetts Schist, Vermont. Contributions to Mineralogy and Petrology, 114, Vidal, O, Parra, T. and Trotet, F. (21) A thermodynamic model

12 2 S. Endo, I. Nowak and S.R. Wallis for Fe - Mg aluminous chlorite using data from phase equilibrium experiments and natural pelitic assemblages in the 1º to 6ºC, 1 to 25 kb range. American Journal of Science, 31, Wallis, S.R. (1998) Exhuming the Sanbagawa metamorphic belt: the importance of tectonic discontinuities. Journal of Metamorphic Geology, 16, Wallis, S.R. and Aoya, M. (2) A re - evaluation of eclogite facies metamorphism in SW Japan: proposal for an eclogite nappe. Journal of Metamorphic Geology, 18, Wallis, S.R., Anczkiewicz, R., Endo, S., Aoya, M., Platt, J.P., Thirlwall, M. and Hirata, T. (29) Plate movements, ductile deformation and geochronology of the Sanbagawa belt, SW Japan: tectonic significance of Ma Lu - Hf eclogite ages. Journal of Metamorphic Geology, 27, White, R., Powell, R. and Holland, T.J.B. (27) Progress relating to calculation of partial melting equilibria for metapelites. Journal of Metamorphic Geology, 25, Whitney, D.L. and Evans, B.W. (21) Abbreviations for names of rock - forming minerals. American Mineralogist, 95, Manuscript received November 25, 212 Manuscript accepted February 26, 213 Published online May 16, 213 Manuscript handled by M. Satish - Kumar