Evidence of the Lawsonite Eclogite Facies Metamorphism from an Epidote-Glaucophane Eclogite in the Kotsu Area of the Sanbagawa Belt, Japan

166 Journal of Mineralogical and S.Petrological Tsuchiya and Sciences, T. Hirajima Volume 108, page 166─ 171, 2013

LETTER

Evidence of the lawsonite eclogite facies metamorphism from an epidote-glaucophane eclogite in the Kotsu area of the Sanbagawa belt, Japan

Shigeki Tsuchiya and Takao Hirajima

Department of Geology and Mineralogy, Graduate School of Science, Kyoto University, Kitashirakawa Oiwake-cho, Kyoto 606-8502, Japan

The lawsonite (Lws) eclogite facies mineral assemblage [Lws + omphacite (Omp)] was discovered as an inclu- sion in garnet (Grt) within a unique epidote (Ep)-glaucophane (Gln) eclogite in the Kotsu area of the Sanbaga- wa belt in eastern Shikoku, Japan. One of the unique characteristics of this eclogite is the occurrence of Grt, 2+ displaying a distinct Mn-bell shaped zoning pattern {Xsps [= Mn/(Fe + Mn + Mg + Ca)] > 0.4 at the core}. Lws inclusions are columnar shaped crystals of ~ 20 μm in size, and are present from the Outer Core to Inner 2+ Rim {0.03 < Xprp [= Mg/(Fe + Mn + Mg + Ca)] < 0.08}. Omp inclusions are anhedral, 10-100 μm in size, and

are present from the Inner Rim to Outer Rim (0.06 < Xprp < 0.11). The mineral assemblage of the unique eclog- ite in the matrix (i.e., Grt, Omp, Ep, quartz, Gln/barroisite, and phengite) is identical to that of the common

eclogite in the relevant area. However, the Mn bell-shaped zoning of Grt is weak in the common eclogite (Xsps < 0.1 throughout the grain). Combining a model petrogenetic grid within the NCKFMASH system and para- genesis of inclusion minerals in the zoned Grt allowed us to propose a new prograde P-T path for the Kotsu eclogite [i.e., (1) ~ 400 ºC and 0.8 GPa represented by an assemblage of Lws + albite + Gln + Ep + paragonite (Pg) in the Inner Core, (2) a subsequent P and T increase from ~ 450 ºC and 1.3 GPa to 550 ºC and 1.8 GPa by an assemblage of Lws + Omp + Gln + Ep + Pg in the Inner Rim, and (3) 600 oC and 2.0 GPa by an assemblage of Grt + Omp + Gln + Ep + Pg in the Outer Rim/matrix]. The derived prograde P-T path is comparable to the thermal structure along the top of the present-day Philippine Sea slab beneath Shikoku Island, as calculated by Peacock (2009).

Keywords: Lawsonite eclogite, Sanbagawa metamorphic belt, Kotsu eclogite

INTRODUCTION high water content in the rock is required for Lws-bearing equilibria to form at an elevated pressure where there is a Eclogite is a high P metamorphic rock that provides in- geothermal gradient of ~ 6-8 ºC/km (Clarke et al., 2006), dispensable geological and geophysical information relat- or because the preservation of the Lws-bearing eclogite ing to the area beneath plate convergent boundaries. Such requires exhumation with substantial cooling, which pro- information aids in the understanding of the subduction/ vides a strong constraint on the tectonic regime of exhu- exhumation mechanisms of crustal rocks and the scale of mation (Clarke et al., 2006). solid/fluid material cycling between the crust surface and Eclogite rocks have previously been reported in two the upper mantle. areas in the Sanbagawa belt; the Besshi area in central Most eclogite that is cropped out on the earth’s sur- Shikoku and the Kotsu area in eastern Shikoku (Takasu, face contains epidote (Ep)-group minerals as the major 1984; Takasu and Kaji, 1985; Aoya, 2001; Matsumoto et calc-alumino silicate, instead of lawsonite (Lws) (Tsuji- al., 2003). According to these studies, the eclogites were mori et al., 2006). The reason for the rare occurrence of a mainly formed under quartz (Qtz) and Ep stable condi- Lws-bearing eclogite, although Lws equilibria are pre- tions (600-700 ºC at around 2.0 GPa) in close association dicted to occur over a broad P-T range, is either because a with glaucophanic/barroisitic amphiboles, although some - - doi:10.2465/jmps121022b garnet (Grt) bearing ultramafic rocks from the Higashi S. Tsuchiya, [email protected] Corresponding akaishi body in the Besshi area show very high P equilib- author rium conditions, which suggests coesite stability (700-810 Evidence of the lawsonite eclogite facies metamorphism in Japan 167

ºC and 2.9-3.8 GPa; Enami et al., 2004). 1.2 km, in the vicinity of Mt. Kotsu (Fig. 1). Matsumoto The lack of a jadeite (Jd) + Qtz assemblage, and the et al. (2003) referred to this unit as the ʻKotsu Basic limited occurrence of sodic amphibole, which is found Schist (KBS) unitʼ. Ep blueschist is the main lithotype of mainly in highly oxidized metabasites/metacherts in the the KBS unit, and eclogite crops out only within a limited Sanbagawa belt, suggests that the Sanbagawa belt belongs section of this unit, i.e., around the Enokidani valley (Fig. to a high P intermediate type (Miyashiro, 1961; Hirajima, 1). According to our geological studies, the main schistos- 1983a; Otsuki and Banno, 1990). Therefore, the prograde ity of metabasites exposed along the Enokidani valley P-T path of the Kotsu eclogite, as proposed by Aoya et al. show a more or less north-south orientation of strike, (2003), which passes through the Ep stability field, ap- whereas most of that within the Ep blueschist from the pears to fit with the mineralogy of the surrounding schists KBS unit predominantly shows an east-west strike. in the belt. By applying the geothermobarometers of Ellis and However, we discovered a relic of a Lws eclogite Green (1979) and Waters and Martin (1993), Matsumoto facies assemblage for the first time, [i.e., isolated inclu- et al. (2003) estimated the peak P-T conditions for the sions of Lws, omphacite (Omp), Ep, paragonite (Pg), and formation of the Kotsu eclogite as being around 600 ºC a multiphase solid inclusion (MSI) mainly composed of and 2.0 GPa. Ep and Pg], in the chemically zoned Grt in an Ep-glauco- phane (Gln) eclogite within the Kotsu area. This discov- PETROGRAPHY AND CHEMICAL ery caused us to consider re-evaluating the prograde P-T ZONING OF GARNET path of the Kotsu eclogite. In this paper, therefore, we de- scribe the detailed mode of occurrence of Lws and other In this report, we focus on the petrography of a unique inclusion minerals in Grt, and then propose a new pro- eclogite specimen (KT23), composed mainly of intercala- grade P-T path of the Kotsu eclogite. tions of 5 mm-thick, pinkish- and bluish-colored layers. Throughout this paper, we use the term ʻeclogiteʼ for Well-developed isoclinal foldings are observed with any metabasite in which Grt and Omp are present as an wavelengths of a few centimeters, and an axial plane of equilibrium assemblage, regardless of their abundance. these isoclinal folding are nearly parallel to the main The mineral abbreviations are after Kretz (1983), except schistosity of the surrounding rocks (N35°E, 38°N). This for phengite (Phn). eclogite specimen is in contrast with the common eclogite from the Kotsu area which is mostly blue-green and has a REGIONAL GEOLOGY clear schistosity composed of Gln, Ep, Omp, and Phn, and is the same type as the sample studied in previous The Kotsu area in the Sanbagawa metamorphic belt is lo- works (Aoya et al., 2003; Matsumoto et al., 2003). cated in eastern Shikoku (Fig. 1), and within this area, Ta- KT23 is mainly composed of a matrix of Grt, Omp, kasu and Kaji (1985) first reported the occurrence of an Ep, Qtz, Gln, Pg, and Phn. Fine-grained Grt, with a diam- Ep-barroisite eclogite. Wallis and Aoya (2000) later iden- eter of ~ 0.1 mm, predominates within the pinkish-col- tified an - Ep Gln eclogite on the southeastern edge of a ored layer. The bluish-colored layer is composed of flat-lying basic schist unit with a maximum thickness of coarse-grained Grt with a diameter of ~ 1 mm, along with the other matrix-forming minerals mentioned above. Some pieces of coarse-grained Grt are rich in inclusion minerals, most of are identical to the matrix forming phases. It is noted, however, that Lws is identified only as an inclusion phase in coarse-grained Grt (Fig. 2a). Coarse-grained Grt generally shows a distinct chem- ical profile, with Mn bell-shaped and Fe/Mg bowl-shaped

patterns, [i.e., from the core (Alm35Sps43Prp01Grs21) to the

rim (Alm63Sps01Prp11Grs25)], suggesting a temperature in- crease during its growth. Since the major elements (Fe, Mn, Mg, and Ca) of coarse-grained Grt show clear con- centric patterns parallel to the surface of the grain, we as- sume that this particular Grt was not significantly affected by later stage diffusion. For this reason, the contours of Figure 1. Geological map of the Kotsu area after Matsumoto et al. the major element concentrations can be treated as iso- (2003) and the sampling point of KT23. chronal planes. We examined about 30 coarse Grt grains 168 S. Tsuchiya and T. Hirajima

and tentatively defined four growth stages within the Grt with minor amounts of components of K (K2O = 0.03- on the basis of mineral assemblages. Since Xprp [= Mg/ 2.79 wt% and 0.42 wt% on average), whereas the Pg in- (Fe2+ + Mn + Mg + Ca)] monotonically increases from the clusions developed along visible cracks and the Pg in the core to the rim, the value of Xprp is used to quantify each matrix are slightly richer in K component (K2O = 0.50- stage: Inner Core (Xprp = 0.01-0.03), Outer Core (Xprp = 1.08 wt% and 0.76 wt% on average).

0.03-0.06), Inner Rim (Xprp = 0.06-0.08) and Outer Rim Omp inclusions in Grt have some variations in their

(Xprp = 0.08-0.11), in the order of the development of Grt chemistry, but there is no clear relation between their growth. growth stages and Omp compositions. The compositional Based on the above-mentioned Grt growth stage range of the Omp core in the matrix is the same as that of classifications, distinct features of the mode of occurrence the Omp inclusions in Grt (Jd45-55DiHd38-46Acm02-05). The of inclusion phases are identified. In the Inner Core the composition of the Omp rim in the matrix, on the other following occur: Qtz (~ 10 μm); Ep (~ 10-50 μm); Pg hand, shows a negligible amount of an acmite (Acm) - - - (~ 10 50 μm); the MSI of Ep Pg (10 150 μm); and ti- component (Jd54-60DiHd40-56Acm00). Note that the amount tanite (Ttn) (~ 10 μm). Lws (~ 20 μm), albite (Ab) (~ 10 of ferric iron is calculated based on the charge balance μm), Chlorite (Chl) (10-50 μm), Gln (~ 15 μm), and Phn (the total cation being 4.00 assuming oxygen = 6.00 ba- (~ 30 μm) firstly appear in the Outer Core in addition to sis). the minerals present in Inner Core. The size of Qtz grains The Fe-Mg distribution coefficients between the grows within this zone, i.e., 10-100 μm. In the Inner Rim, Omp inclusions (39 grains) and the nearby Grt, KD [= the appearance of Omp (~ 10 μm), rutile (Rt) (~ 20 μm), (Fe2+/Mg)Grt/(Fe2+/Mg)Omp], decrease from the Inner Rim and a symplectitic MSI composed of tremolite and Ab, (KD = 20.26 at Xprp = 0.06) to the Outer Rim (KD = 15.60 which seems to be a reaction product of Omp breakdown, at Xprp = 0.10), indicating a temperature increase during is noted. In the Outer Rim, Lws disappears and dolomite Grt growth. Based on the Grt-clinopyroxene (Cpx) geo- (Dol) (~ 20 μm) appears, and Omp (10-100 μm), Qtz (5- thermometer by Ellis and Green (1979), the temperature 100 μm) and Ttn (10-30 μm) grains become larger in this estimate of the Inner Rim was ~ 580 ºC for 1.5 GPa, and zone. A representative coarse-grained Grt with inclusions that of the Outer Rim was ~ 600 ºC for 2.0 GPa. Since is shown in Figure 2a, and the inclusion mineral paragen- Phn in our sample contains the Pg component [= Na/(Na esis is summarized in Figure 2b. + K + Ca)] of ~ 0.1, the Grt-Cpx-Phn geobarometer of The common Kotsu eclogite also contains coarse- Waters and Martin (1993) did not work well (the pressure grained Grt of ~ 1 mm in diameter, but it shows a weak estimate calculated with too much Al3+ component was a zoning from the core (Alm58Sps06Prp10Grs26) to the rim coesite stability condition of ~ 2.7 GPa). Matsumoto et al.

(Alm60Sps03Prp15Grs23). This Grt is rich in Ep inclusions (2003) estimated the peak P-T conditions for the Kotsu (Aoya et al., 2003; Matsumoto et al., 2003), but until now, eclogite as being around 600 ºC and 2.0 GPa based on the Lws had never been reported either from these eclogites geothermobarometers of Ellis and Green (1979) and Wa- or from the surrounding schists of the Kotsu area. ters and Martin (1993), and the peak temperature estimate is in good agreement with our result. CHEMISTRY OF INCLUSION MINERALS THE PETROGENETIC GRID IN THE NCKFMASH Lws inclusions show an almost ideal composition, with SYSTEM minor amounts of Fe2O3 [SiO2, TiO2, Al2O3, Fe2O3, MnO, CaO and total are 38.52, 0.20, 31.61, 0.86, 0.11, 17.86, and The P-T conditions of each Grt growth stage can be esti- 3+ 89.16 wt%, respectively: Ca1.00Al1.94Fe0.03Si2.01O7(OH)2･ mated by the combination of inclusion mineral assem-

H2O]. However, the Ep, Pg, and Omp inclusions in the blages (Fig. 2b) and a petrogenetic grid in an appropriate coarse-grained Grt show some variations in terms of their system. The studied rock contains various minerals as de- chemistry. This report provides a simple description of scribed above (Fig. 2), and these minerals can be ex- their chemical trends and a more detailed description will pressed in the system of Na2O-CaO-K2O-FeO-MgO- be done elsewhere. MnO-Al2O3-SiO2-H2O-TiO2. Since K2O, MnO, and TiO2 3+ 3+ The value of YFe [= Fe /(Al + Fe )] in the Ep inclu- components are present only in Phn, Grt, and Rt/Ttn, re- sions in the Inner Core is 0.02-0.09 (0.08 on average), spectively, we can ignore them. Additionally, we assume and that in the Ep in the Outer Rim is 0.08-0.16 (0.14 on SiO2 and H2O components can be treated as excess phas- average). Values of YFe in the Ep in the matrix (0.13-0.15) es. Thus, we need a petrogenetic grid within an NCKF- are similar to that in Ep in the Outer Rim. MASH system with excess Qtz, Phn, and H2O. Wei and Most of the Pg inclusions show an ideal composition Powell (2006) reported a petrogenetic grid within this Evidence of the lawsonite eclogite facies metamorphism in Japan 169

Figure 2. (a) Chemical map of the Mn content of a representative coarse-grained garnet, showing concentric contours and its inclu- sion minerals. Note that all the inclusions without labeling are ti- tanite. (b) Paragenetic diagram of the Kotsu eclogite (KT23). All the inclusion minerals seem to coexist with the host garnet. Qtz, Ep, Pg, MSI of Ep-Pg, and titanite are commonly identified as inclusions from the Inner Core to the Outer Rim of coarse- grained garnet.

Figure 3. Modified petrogenetic grid for the system of NCKFMASH, projected from phengitic muscovite,

quartz and H2O after Wei and Pow- ell (2006). Mineral assemblages of three stages,(i.e., Outer Core, Inner Rim, and Outer Rim/Matrix), are shown in italics. system, and we modified it so that only the minerals pres- Lws + Ab/Jd ent in mafic environments are visible (as shown in Fig. 3). = Grt + Omp + clinozoisite (Cz) + Pg (1) Note that this petrogenetic grid is constructed under satu- and/or ration with pure H2O. Thus, the Lws stability field, for ex- Lws = Cz + Kyanite (2). ample, shows its maximum stability. It is usually considered that the breakdown of Lws The MSI of Ep-Pg can be considered as a product of re- would be caused by an overstepping toward the higher T action (1). Since the Lws-terminal reaction (2) is located side of the following two reactions: on the slightly higher T side of the reaction (1), Lws, Ep 170 S. Tsuchiya and T. Hirajima and Pg can coexist in the limited P-T conditions bounded an active ridge to duplicate such a shaped prograde path. by these two reactions (Fig. 3). Another key metamorphic In contrast, hereby, we propose a new prograde P-T mineral, Omp, can coexist with Pg, but only between the path for the Kotsu eclogite, with a clockwise shape, i.e., reaction (3); Omp + Pg = Grt + plagioclase + Chl + Cz, from 400 °C and 0.8 GPa, through 500 °C and 1.5 GPa, up and the reaction (4); Grt + Gln + Jd + Lws = Omp + Pg. to 600 °C and 2.0 GPa as described above in Figure 3.

According to the petrogenetic grid (Fig. 3), the min- The mineral assemblage in the Inner Rim (Xprp = 0.06- eral assemblage enclosed in the Outer Core (Xprp = 0.03- 0.08) (Grt + Lws + Omp + Gln + Ep + Pg) suggests that 0.06) (Grt + Lws + Ab + Gln + Ep + Pg) was firstly this part grew under the Lws eclogite facies, especially in formed at around 400 ºC and 0.8 GPa or below. With an consideration of the P-T conditions of reaction (5). increase of P and T, the mineral assemblage in the Inner P-T conditions of subducting slabs under various

Rim (Xprp = 0.06 - 0.08) (Grt + Lws + Omp + Gln + Ep + tectonic settings have been calculated by many authors Pg) was then formed, along with reaction (5); Lws + Omp (Peacock and Wang, 1999; Iwamori, 2000; van Keken et = Grt + Cz + Pg + Gln, i.e., from ~ 450 ºC and 1.3 GPa to al., 2002; Peacock, 2009; Wada and Wang, 2009; Syra- ~ 550 ºC and 1.8 GPa, which is located in between reac- cuse et al., 2010). Our newly proposed P-T path of the tion (1) and (2). Lastly, the mineral assemblage in the Kotsu eclogite best fits to the P-T conditions found at the Outer Rim/matrix (Grt + Omp + Gln + Ep + Pg) was top of the present-day Philippine Sea plate subducting be- formed during the peak P-T conditions of around 600 ºC neath Shikoku Island (Peacock, 2009). The calculation of and 2.0 GPa. the P-T conditions by Peacock (2009) based on the fossil ridge subduction model, leads us to consider that the DISCUSSION AND CONCLUSION Izanagi plate, which is supposed to have produced the Sanbagawa metamorphic belt, was also accompanied by a The barric type Sanbagawa belt has been considered as a fossil ridge. high P intermediate type, in spite of the occurrence of Grt in the common Kotsu eclogite shows a homoge- eclogites in the highest grade part (Banno et al., 1978). neous chemical profile (Xsps < 0.1 throughout the grain), Lws rarely occurs in metapelites of the Chl zone of the while Grt in the unique eclogite (KT23) shows a Mn bell- Sanbagawa belt (Watanabe et al., 1983; Ueno, 1999) and shaped chemical profile. The Mn bell-shaped profile indi- in metabasites of the Mikabu Greenstone complex (Seki, cates nucleation and growth of Grt in relatively low T 1958); the grade of which is mostly equivalent to the Chl conditions (e.g., Banno et al., 1986; Matsumoto et al., zone. However, the occurrence of Lws was ascribed to the 2005), and this can account for the preservation of an local bulk control, namely the difference in their chemis- amount of Lws. try and/or fluid compositions (Hirajima, 1983b; Goto et Furthermore, the common Kotsu eclogite contains al., 2007; Shinjoe et al., 2009). Therefore, the major calc- abundant Hem and calcite (Cal), indicating that its chemi- 3+ almino silicate in the Sanbagawa schists and eclogites has cal system is Fe -rich, and which gives it a relatively low been considered to belong to the Ep-group minerals. H2O fugacity. In contrast, KT23 does not contain either Aoya et al. (2003) proposed that a prograde P-T path Hem or Cal, but contains sulfide minerals, although a few of the Kotsu eclogites reached 600 ºC and 2.0 GPa with a Dol inclusions are present in the Grt rim. These contrast- dP/dT > 7.1 kbar/100 ºC, based on a unique reaction, in ing features are undoubtedly the key factors controlling order to explain the chemical zoning patterns of amphi- whether Lws crystallization takes place or not. boles and clinopyroxenes: ACKNOWLEDGMENTS

4Acm + 2tschermakite + 2Qtz + H2O → 2Gln + 2Ep + hematite (Hem) (6). We gratefully acknowledge M. Enami, A. Takasu, and S. Endo for their constructive comments and editorial advice Because Enami et al. (1994), Enami (1998) and Inui and in the improvement of the first draft. We also thank A. Toriumi (2002) reported that the prograde P-T paths of Nakamura and K. Yoshida for their help with the EPMA the Sanbagawa schists had a relatively small dP/dT, Aoya analyses. This study is partly supported by the JSPS et al. (2003) proposed that the prograde P-T paths of the (Grant-in Aid for Science Research Nos. 21109004 and Sanbagawa metamorphic rocks, including eclogites, lay 22244067). on a P-T path with a larger dP/dT with increasing pres- sure. Additionally, they performed two-dimensional nu- REFERENCES merical modeling and concluded that the Sanbagawa met- amorphic rocks were formed just before the subduction of Aoya, M. (2001) P-T-D path of Eclogite from the Sanbagawa Evidence of the lawsonite eclogite facies metamorphism in Japan 171

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