Hironao Shinjoe *, Atsushi Goto **, Masao Kagitani ***, and Chihiro Sakai ***, INTRODUCTION

Transcription

1 Journal of Mineralogical Ca Al hydrous and silicates Petrological in the Sciences, chlorite grade Volume pelitic 104, schists page , Ca-Al hydrous silicates in the chlorite-grade pelitic schists in Sanbagawa metamorphic belt and a petrogenetic analysis in the model mixed-volatile system Hironao Shinjoe *, Atsushi Goto **, Masao Kagitani ***, and Chihiro Sakai ***, * Tokyo Keizai University, , Minami - cho, Kokubunji, Tokyo , Japan ** Division of Petrology, Department of Life Science, School of Science, University of Hyogo (Branch of Himeji Institute of Technology), 2167 Shosha, Himeji, Hyogo , Japan *** Department of Geology and Mineralogy, Graduate School of Science, Kyoto University, Kyoto , Japan Present address, Sennan Public Health and Welfare Office, Miyagi, , Japan Present address, Itami Department, Nippon Sheet Glass Techno - Research Co. Ltd., , Konoike, Itami, Hyogo , Japan Lawsonite, pumpellyite and epidote coexist in the chlorite zone pelitic schists of the Sanbagawa metamorphic belt. As these pelitic schists contain calcite ubiquitously, the phase relations of hydrous Ca - Al silicate minerals are examined by mixed volatile equilibria in the K 2 O - Na 2 O - CaO - MgO - Al 2 O 3 -SiO 2 -CO 2 -H 2 O system with excess quartz, calcite and a fluid phase. XCO 2 of the stability field of pumpellyite is lower than that of lawsonite. An idealized geometry of an isobaric T - XCO 2 diagram including mixed volatile reactions shows that the stable hydrous Ca - Al mineral changes from lawsonite through pumpellyite to epidote for pelitic schists in the chlorite zone with progressive increase in both temperature and XCO 2 of the fluid phase, though ferric iron expands the stability of epidote throughout the chlorite zone. The fluid phase released by the metamorphic reactions in the chlorite zone is high in H 2 O content even in the presence of CaCO 3. The proposed mixed volatile reactions, instead of the pure dehydration reactions, are applicable directly to the phase relations of CaCO 3 -bearing Sanbagawa basic schists and to the interpretation of the mineral sequence of the hydrous Ca - Al silicates of other high P/T metamorphic belts. Keywords: Hydrous Ca - Al silicates, Pumpellyite, Lawsonite, Pelitic schist, Metamorphic fluid composition, XCO 2, Sanbagawa metamorphic belt INTRODUCTION Hydrous Ca - Al silicate minerals like zeolite group minerals, prehnite, pumpellyite, lawsonite and epidote occur commonly in low - grade metabasites and metagreywackes and play essential roles in the classification of low - temperature metamorphic facies (Brown, 1977; Nakajima et al., 1977; Liou et al., 1987). They also occur in pelitic schists, which have not been paid much attention owing to their sporadic appearance. The occurrence of lawsonite in pelitic schists, however, were exceptionally noticeable (e.g., Landis, 1971; de Roever, 1972; Kawachi, 1975; Glassley et al., 1976; Brown and Ghent, 1983; Watanabe and Kobayashi, 1984; Tsujimori and Itaya, 1999; Ueno, doi: /jmps H. Shinjoe, shinjoe@tku.ac.jp Corresponding author 2001), because its appearance suggests high P/T metamorphic conditions as well as low XCO 2 of the coexisting fluid (e.g., Nitsch, 1968; Ueno, 2001; Goto et al., 2007). Presence of high P/T metamorphic rocks suggested by the appearance of lawsonite contributes to reconstructing the evolution of ancient subduction - related orogenic belts (e.g., Gibbsons and Mann, 1983). Phase equilibria of minerals in the pelitic schist are usually treated as the system K 2 O - FeO - MgO - Al 2 O 3 - SiO 2 -H 2 O (KFMASH) or its sub - system (e.g., Proyer, 2003), because the Ca content of the pelitic rock is usually low in particular in continental shelf sediments. Recently, P - T pseudosections constructed for pelitic rocks in the MnNCKFMASH system are widely used to investigate phase equilibrium in particular for the grossular component in garnet (e.g., Riesco et al., 2004; Storm and

2 264 H. Shinjoe, A. Goto, M. Kagitani and C. Sakai Figure 1. Mineral zone map of central Shikoku region (after Higashino, 1990). Boxes A and B corresponds to locality map of Figures 2a and 2b, respectively. Spear, 2005; Zuluaga et al., 2005). Matsumoto et al. (2005) constructed pseudosection for Sanbagawa pelitic schists and investigated its consistency with previous P - T estimation studies and effect of MnO contents on the appearance of garnet. However, phase relations including hydrous Ca - Al silicates in metapelites have still been obscure, since the thermodynamic database for hydrous Ca - Al silicates are still unclear, in addition to their sporadic appearances. The average CaO content of 198 pelitic schists from the Sanbagawa subduction zone metamorphic belt is as low as about 1 wt% (Goto et al., 1996). Nevertheless, Goto et al. (2002) revealed that most Sanbagawa pelitic schists ubiquitously contain calcite and epidote as Ca - bearing phases, on the basis of the systematic and extensive microscopic observation of 1876 samples from central Shikoku. Occurrence of lawsonite was reported by Watanabe and Kobayashi (1984) and that of pumpellyite by Banno (1998) referring to Sakai (1985) in pelitic schists from the chlorite zone of the Sanbagawa belt in central Shikoku. Ueno (1999, 2001) also described lawsonite - bearing pelitic schists from the chlorite zone of the Sanbagawa metamorphic belt in Kii peninsula, ~ 300 km to the east of the central Shikoku. Accordingly, the sporadic appearance of hydrous Ca - Al silicates in the pelitic schists of the chlorite zone have been widely known, phase relations among them, however, have not been discussed so far. The first objective in this paper is to present the mode of occurrence of pumpellyite in addition to lawsonite and epidote in low - grade carbonaceous pelitic schists of the Sanbagawa metamorphic belt in central Shikoku, southwest Japan. Based on the ubiquitous presence of calcite, Goto et al. (2002) described most of the mineral formation reactions including isograd - defining ones for the Sanbagawa Figure 2. Locality map of specimens with mineral assemblages of hydrous Ca - Al silicates. (a) Asemi - gawa area. (b) Besshi area. Sample codes of analyzed specimens are shown in the map. pelitic schists as mixed volatile equilibria containing both CO 2 and H 2 O instead of pure dehydration equilibria. Their suggestion brought a marked progress in understanding metamorphic reactions forming the Ca - bearing phase in the pelitic schists. For example, they proposed a hornblende formation reaction that had not been suggested before. Goto et al. (2007) analyzed the relation of pressure, temperature, XCO 2 of the fluid phase and chemical composition of the Sanbagawa pelitic, basic and some calcareous schists and showed that calcite - albite - muscovite - clinozoisite (= epidote) is the predominant assemblage in the CaO - NaAlO 2 -Al 2 O 3 tetrahedron. In the chlorite zone, calcite - albite - muscovite - lawsonite might be also a characteristic assemblage. Hence they proposed the lawsonite - out mixed volatile reaction instead of a pure dehydration reaction. The second objective in this paper is to consider the formation and decomposition reactions of pumpellyite using mixed volatile reactions in relation to the lawsonite - out reaction proposed by Goto et al. (2007), and discuss the range in XCO 2 of fluid phase in equilibrium with pumpellyite - bearing assemblages. It will be stressed that mixed volatile reactions are appropriate for the mineral formation reactions in chlorite zone, although ubiquitous appearance of titanite suggests the presence of low XCO 2 fluid. The results will be applied to interpret the mineral assemblages of pelitic schist in chlorite zone which was thought to be lacking index minerals. The arguments are also directly applicable to basic schists in low grade Sanbagawa belt that usually contain pumpellyite, lawsonite and epidote in the chlorite zone. Proposed reactions are

3 Ca - Al hydrous silicates in the chlorite - grade pelitic schists 265 also applied for interpreting the mineral sequence of other subduction zone metamorphic rocks with hydrous Ca - Al silicates. GEOLOGICAL SETTING The Sanbagawa high P/T metamorphic belt distributed along southwest Japan arc was formed within a subduction zone at the Eurasian margin in Late Cretaceous time. Detailed petrologic investigations of the Sanbagawa schists in central Shikoku and inferred metamorphic pressure and temperature evolutions are given in a series of articles (Banno et al, 1986; Banno and Sakai, 1989; Higashino, 1990; Otsuki and Banno, 1990; Enami et al., 1994; Aoya, 2001; Wallis et al., 2001; Inui and Toriumi, 2002). In central Shikoku region, the Sanbagawa belt is divided into the chlorite, garnet, albite - biotite and oligoclase - biotite zones on the basis of the mineral assemblages of the pelitic schists in ascending order of peak metamorphic temperature (Enami, 1983; Higashino, 1990). The chlorite zone occupies the lowest structural horizon and thin layers of the higher - temperature zones overlie it. Our samples were collected from the Besshi and the Asemi - gawa areas, and all come from the high - grade part of the chlorite zone (Figs. 1 and 2). Part of the chlorite zone in the lowest apparent structural horizon exposed in Oboke area, where younger radiometric ages than those of other Sanbagawa schists were reported long ago (Takasu and Dallmeyer, 1990; Shinjoe and Tagami, 1994), was proposed to be the outcrop of the blueschist part of the Northern Shimanto belt as a fenster (Aoki et al., 2008). Since pumpellyite + actinolite + chlorite + epidote is stable in hematite - free basic schists (Nakajima, 1982), our sampling areas belong to the pumpellyite - actinolite facies. According to Enami (1983) and Enami et al. (1994), the maximum pressure and temperature conditions of each mineral zone are as follows, garnet zone, GPa, 440 ± 15 C; albite - biotite zone, GPa, 520 ± 25 C; oligoclase - biotite zone, GPa, 610 ± 25 C. Goto et al. (2007) estimated the maximum pressure and temperature conditions of the chlorite zone as less than 0.8 GPa and less than 360 C with XCO 2 ranging from to PETROGRAPHY AND MODE OF OCCURRENCE OF HYDROUS Ca-Al SILICATES Major constituents of the Sanbagawa pelitic schist are quartz, albite and phengite, the sum of whose modal abundances usually exceeds 90% (Higashino, 1975, 1991). Chlorite, epidote, titanite, tourmaline, apatite, pyrite, pyrrhotite and carbonaceous materials are ubiquitous accessory minerals throughout the Sanbagawa belt. Calcite was also present as a minor but ubiquitous phase during the Sanbagawa metamorphism (Goto et al., 2002). The average CaO content of 35 pelitic schists in the chlorite zone is 0.64 wt% (Goto et al., 1996). Pelitic schists in the low metamorphic grade zone are composed of the alternation of micaceous layers and quartzofeldspathic layers. Constituent minerals in low - Figure 3. Back - scattered electron images of hydrous Ca - Al silicates. (a) Lawsonite and epidote coexistence in a micaceous layer (specimen 81802). Epidote has thin REE - rich rim. (b) Pumpellyite, lawsonite and epidote coexistence in a micaceous layer (specimen ). (c) Columnar pumpellyite crystals in a micaceous layer (specimen 60204). (d) Irregular shape aggregates of small pumpellyite crystals associated with large anhedral epidote crystal (specimen 60207).

4 266 H. Shinjoe, A. Goto, M. Kagitani and C. Sakai Table 1. Whole rock major element compositions of the Sanbagawa pelitic schists Abbreviations in the row of assemblage are: EL, epidote + lawsonite; EP, epidote + pumpellyite; ELP, epidote + lawsonite + pumpellyite. * Total Fe as Fe 2 O 3. Composition was previously reported by Goto et al. (1996). grade zone rocks are usually fine - grained. Hydrous Ca - Al silicates also occur as fine - grained crystals. Further, they are low in modal abundances and also in the frequency of occurrences. Hence, their identification under the optical microscope is often formidable task. In the present work, their identification was, therefore, made using an electron probe micro analyzer. Hydrous Ca - Al silicates occur in both micaceous layers and quartzofeldspathic layers sporadically. Watanabe and Kobayashi (1984) claimed that lawsonite and epidote did not coexist with each other since they had never been observed within the distance less than 0.4 mm. However, we found lawsonite and epidote with the distance of about 10 µm in a micaceous layer (Fig. 3a). Further, a pumpellyite - lawsonite - epidote paragenesis was found from one specimen ( ), occurring within a single micaceous layer less than 0.1 mm thick (Fig. 3b). We, thus, regard assemblages of these hydrous Ca - Al silicates within a micaceous layer in equilibrium. Assemblages of hydrous Ca - Al silicates in pelitic schist specimens in the Asemi - gawa and the Besshi areas are illustrated in Figure 2. The following assemblages for hydrous Ca - Al silicates are identified; pumpellyite + lawsonite + epidote, pumpellyite + epidote, lawsonite + epidote, pumpellyite, lawsonite and epidote (Fig. 2). Pumpellyite occurs as columnar crystals less than 60 µm 20 µm (Fig. 3c). Small (<10 µm) pumpellyite crystals sometimes form aggregates (Fig. 3d). Lawsonite occurs as euhedral prismatic crystals less than 40 µm 10 µm (Figs. 3a and 3b). Epidote appears larger anhedral crystals up to 100 µm (Figs. 3a and 3d). Occasionally tiny epidote crystals form aggregates. Pumpellyite and lawsonite occur only in the chlorite zone but not in the garnet zone, whereas epidote occurs throughout the Sanbagawa belt ubiquitously (Goto et al., 2002). WHOLE ROCK COMPOSITION AND MINERAL CHEMISTRY Whole rock major element compositions of hydrous Ca - Al silicate - bearing pelitic schists were analyzed with an X - ray fluorescence spectrometer (Rigaku Simultix 3550) at Kyoto University. Sample preparations and analytical procedures are after Goto and Tatsumi (1994). Table 1 lists the whole rock compositions, whose ranges are within those in pelitic schist compositions of Goto et al. (1996). Figure 4 shows the relation between CaO and Al 2 O 3 contents of the present samples with those of the chlorite zone pelitic schists by Goto et al., (1996). Six specimens of pelitic schists with two or three hydrous Ca - Al silicates from the Asemi - gawa area were selected for detailed study. Minerals were analyzed with an electron probe micro analyzer of JEOL 5400 with fully quantitative Link System model QX2000 energy - disper- Figure 4. Diagram for the whole rock CaO versus Al 2 O 3 contents of the chlorite zone pelitic schists.

5 Ca - Al hydrous silicates in the chlorite - grade pelitic schists 267 Table 2. Representative analyses of pumpellyite Abbreviations are the same as those in Table1. Table 3. Representative analyses of lawsonite Abbreviations are the same as those in Table1. sive spectrometer of the National Science Museum, Tokyo. The analytical procedure follows Yokoyama et al. (1993). Tables 2 to 6 list representative analyses of pumpellyite, lawsonite, epidote, phengite and chlorite. Compositional variations of pumpellyite are shown in the Al - Mg - Fe * triangular diagram (Fig. 5a). Pumpellyite has high Al/(Al + Mg + Fe * ) ratio (~ 0.8) within the range of those reported from pumpellyite - actinolite facies and glaucophane schist facies metamorphic rocks (Coombs et al., 1976; Schiffman and Liou, 1980). The Al 2 O 3 content of the pumpellyite is close to those of pumpellyite - (Al) reported by Hatert et al. (2007). Chemical compositions of pumpellyite in basic schists in the present area (Nakajima, 1982) are also plotted within the range of those in pelitic schists (Fig. 5b). Pumpellyite in a specimen (60204) contains considerable amount of MnO ranging from 1.0 to 2.1 wt%. The lawsonite composition closely approaches that of the end - member and its Fe 2 O 3 contents are less than 0.7 wt%. (Table 3). Ueno (1999) reported REE - bearing sector - zoned lawsonite from pelitic schists of the Sanbagawa belt in the Kii Peninsula. However, no obvious zonal structure was found in lawsonite of the Asemi - gawa area. The Y Fe [= Fe 3+ /(Fe 3+ + Al)] of epidote ranges from 0.14 to 0.35 with substantial variation within each sample (Fig. 6). Epidote crystals analyzed in this study usually include REE - rich domain as previously reported by Sakai et al. (1984). Some of them have REE - rich thin rim (<10 µm). The Y Fe of epidote seems to decrease towards garnet - in isograd. This trend is consistent with compilation of Y Fe of epidote in pelitic schist of the Sanbagawa belt in central Shikoku (Enami et al., 2004). Phengite has compositional ranges of Si from 6.6 to 7.0 pfu (= per formula unit) and Al from 4.0 to 4.7 pfu (Table 4 and Fig. 7a). The Na 2 O content of phengite is below 0.8 wt%. In rare cases, white micas with low (Fe 2+ + Mg) and high Al contents were found, supposed to be relicts of detrital origin (Fig. 7a). Chlorite has narrow compositional ranges in Si from 5.4 to 5.8 pfu and in Mg/ (Mg + Fe 2+ + Mn) ratio from 0.34 to 0.41, except one specimen (60204) whose chlorite has Mg - richer composi- Table 4. Representative analyses of epidote Abbreviations are the same as those in Table1.

6 268 H. Shinjoe, A. Goto, M. Kagitani and C. Sakai Table 5. Representative analyses of phengite Abbreviations are the same as those in Table1. Table 6. Representative analyses of chlorite Abbreviations are the same as those in Table1. tion of Mg/(Mg + Fe 2+ + Mn) ratio ranging (Table 5 and Fig. 7b). Albite in hydrous Ca - Al mineral - bearing samples has an X An less than DISCUSSION Mixed volatile equilibria among lawsonite, pumpellyite and epidote in the model system Goto et al. (2007) demonstrated the effectiveness of analysis of mineral assemblages of subduction zone meta - sediments as well as basic and some calcareous schists in the CaO - NaAlO 2 -Al 2 O 3 system with excess quartz and the CO 2 -H 2 O binary fluid. To examine phase relations of hydrous Ca - Al silicates including pumpellyite, we analyzed mineral assemblages using mixed volatile equilibria in the simple model system of K 2 O - Na 2 O - CaO - MgO - Al 2 O 3 -SiO 2 -CO 2 -H 2 O as an extension of analysis by Goto et al. (2007). Although oxygen fugacity of carbonaceous material - bearing Sanbagawa pelitic schists is rather low, our model system ignores Fe 2 O 3, which is crucial for the epidote stability. Effect of ferric iron on the appearance of epidote is discussed in a later section. Since the thermodynamic database for low - T mineral is still obscure, solid solution effect of exchanging Mg and Fe 2+ was not considered, which impaired the rigorousness of analysis. However, quite uniform bulk rock Mg/(Mg + Fe) molar ratio of our specimens ( ), except a specimen (60204) with rather higher ratio (0.49), may dilute the effect of Mg/Fe 2+ exchange on min-

7 Ca - Al hydrous silicates in the chlorite - grade pelitic schists 269 Figure 5. Compositional variations of pumpellyite in terms of Al - Mg - Fe * (total Fe). (a) Pumpellyite in pelitic schists in the Asemi - gawa area. (b) Pumpellyites in basic schists in the Asemi - gawa area (Nakajima, 1982) plotted for comparison. eral assemblages. In the CaO - NaAlO 2 -Al 2 O 3 system, the characteristic four - phase assemblage of calcite + albite + muscovite + epidote commonly occurs not only in most pelitic schists, but also in basic and some calcareous schists in the Sanbagawa metamorphic belt (Goto et al., 2007). They also suggested that calcite + albite + muscovite + lawsonite is the characteristic four - phase assemblage of pelitic schists in the chlorite zone, on the basis of the reports of the sporadic appearance of lawsonite from chlorite zone (Watanabe and Kobayashi, 1984; Ueno, 1999, 2001). The reaction between lawsonite - bearing and epidote - bearing assemblage is written as following (Goto et al., 2007); 3CaAl 2 Si 2 O 8 2H 2 O + CaCO 3 Lawsonite Calcite = 2Ca 2 Al 3 Si 3 O 12 (OH) + CO 2 + 5H 2 O (1). Clinozoisite Fluid For the stability field of the calcite - albite - muscovite - lawsonite assemblage, pressure, temperature and XCO 2 should lie between the following two reactions (Goto et al, 2007); Figure 6. Histograms showing the frequency of epidote Y Fe [=Fe 3+ / (Fe 3+ + Al)] of the individual specimens. Histograms were arranged with increasing metamorphic grade from the bottom to the top. K 2 Al 4 Si 6 Al 2 O 20 (OH) 4 + 2CaCO 3 + 4SiO 2 + 2H 2 O Muscovite Calcite Quartz Fluid = 2KAlSi 3 O 8 + 2CaAl 2 Si 2 O 8 2H 2 O + 2CO 2 (2), K - feldspar Lawsonite Fluid Na 2 Al 4 Si 6 Al 2 O 20 (OH) 4 + 2CaCO 3 + 4SiO 2 + 2H 2 O Paragonite Calcite Quartz Fluid = 2NaAlSi 3 O 8 + 2CaAl 2 Si 2 O 8 2H 2 O + 2CO 2 (3). Albite Lawsonite Fluid The reaction 2 limits low pressure, high temperature and low XCO 2 side of the stability field, and the reaction 3 limits vice versa. For stability field of the calcite + albite + muscovite + epidote assemblage, the following reac-

8 270 H. Shinjoe, A. Goto, M. Kagitani and C. Sakai = 6NaAlSi 3 O 8 + 4Ca 2 Al 3 Si 3 O 12 (OH) Albite Clinozoisite + 8CO 2 + 4H 2 O (5). Fluid The reaction 4 limits low pressure, high temperature and low XCO 2 side of the stability field, and the reaction 5 limits vice versa. The phase relation between lawsonite and pumpellyite for the actinolite - free pelitic schists can be written as, Mg 10 Al 4 Si 6 O 20 (OH) CaAl 2 Si 2 O 8 2H 2 O + 17CaCO 3 Chlorite Lawsonite Calcite + 8SiO 2 Quartz = 10Ca 4 MgAl 5 Si 6 O 21 (OH) CO 2 Mg Pumpellyite H 2 O (6). Fluid The phase relation between pumpellyite and epidote can be also expressed as, 30Ca 4 MgAl 5 Si 6 O 21 (OH) CO 2 Mg Pumpellyite Fluid - = 3Mg 10 Al 4 Si 6 O 20 (OH) Ca 2 Al 3 Si 3 O 12 (OH) Chlorite Clinozoisite + 28CaCO SiO H 2 O (7). Calcite Quartz Fluid These two reactions demonstrate that pumpellyite forms from the lawsonite - bearing assemblage, and decomposes to the epidote - bearing assemblage. The reactions 6 and 7 radiate from an invariant point on the reaction 1. Concerning the pumpellyite stability filed, the following reaction also generates in the connection of reactions 2 and 4, Figure 7. (a) Diagram for Si (pfu) versus Al (pfu) showing the phengite compositions. (b) Diagram for Si (pfu) versus Mg/ (Mg+Fe 2+ +Mn) showing the chlorite compositions. tions (Goto et al., 2007) are written as, 3K 2 Al 4 Si 6 Al 2 O 20 (OH) 4 + 8CaCO SiO 2 Muscovite Calcite Quartz = 6KAlSi 3 O 8 + 4Ca 2 Al 3 Si 3 O 12 (OH) K feldspar Clinozoisite - + 8CO 2 + 4H 2 O (4), Fluid 3Na 2 Al 4 Si 6 Al 2 O 20 (OH) 4 + 8CaCO SiO 2 Paragonite Calcite Quartz 23K 2 Al 4 Si 6 Al 2 O 20 (OH) 4 + 2Mg 10 Al 4 Si 6 O 20 (OH) 16 Muscovite Chlorite + 80CaCO SiO 2 + 8H 2 O Calcite Quartz Fluid = 46KAlSi 3 O Ca 4 MgAl 5 Si 6 O 21 (OH) 7 K feldspar Mg Pumpellyite CO 2 (8). Fluid The reaction 8 demonstrates the higher XCO 2 limit for the stability of pumpellyite - K feldspar assemblage. Taking account of above reactions and the presence of lawsonite + pumpellyite + epidote coexistence in the specimen ( ), we schematically illustrate an isobaric T - XCO 2 diagram (Fig. 8). The invariant point (IP1), allow-

9 Ca - Al hydrous silicates in the chlorite - grade pelitic schists 271 XCO 2 of the IP1 are constrained between IP2 and IP3 under P = GPa based on the estimation for the chlorite zone conditions by Goto et al. (2007). In the chlorite zone pelitic schists, muscovite is common mineral. On the other hand, K - feldspar, which is rarely found, is considered to be detrital origin (Sakai, 1985). Thus, the shaded area in Figure 8, bounded by the reactions 6, 7 and 8, is the stability range of pumpellyite in equilibrium with minerals found commonly in the chlorite zone pelitic schists. The stability field of pumpellyite is lower in XCO 2 than that of IP1 and IP3. Therefore the fluid phase released by the chlorite zone metamorphism is extremely high in H 2 O in the presence of CaCO 3. Implications to the occurrence of hydrous Ca-Al silicates in the chlorite zone of the Sanbagawa belt Figure 8. An idealized geometry in an isobaric T - XCO 2 diagram in the system K 2 O - Na 2 O - CaO - MgO - Al 2 O 3 -SiO 2 -CO 2 -H 2 O with excess quartz, calcite and fluid phase. Pumpellyite, lawsonite, and epidote coexists at the invariant point IP1. P - T - XCO 2 conditions of the invariant point formed by the reactions 1, 2 and 4 (IP2) and that formed by the reactions 1, 3 and 5 (IP3) obtained by Goto et al. (2007) are described in the text. The shaded area shows the stability range of the pumpellyite + CaCO 3 + muscovite + albite + chlorite assemblage. Sanbagawa metamorphic field gradient path presumed by the presence of pumpellyite + CaCO 3 + muscovite + albite + chlorite assemblage was shown by heavy arrow. Abbreviations are Pmp, pumpellyite; Lws, lawsonite; Ep, epidote; Chl, chlorite; Ms, muscovite; Pg, paragonite; Kfs, K - feldspar; Ab, albite; Qtz, quartz; Cal, calcite; Fld, fluid. ing the presence of lawsonite + pumpellyite + epidote coexistence, should be restricted within the range between the reactions 3 and 8 and that between the reactions 5 and 8. T - XCO 2 stability field for the calcite + albite + muscovite + clinozoisite assemblage was constrained by the thermodynamic calculation of the reactions 1 to 5 by Goto et al. (2007). Based on the presence of calcite + albite + muscovite + lawsonite assemblage and absence of aragonite, they estimated the range of pressure condition of the chlorite zone as GPa. They calculated that the invariant point formed by the reactions 1, 2 and 4 (IP2) is at 312 C in equilibrium with a fluid of composition XCO 2 = , and that formed by the reactions 1, 3 and 5 (IP3) is at 322 C in equilibrium with a fluid of composition XCO 2 = at 0.6 GPa. At 0.8 GPa, the stability condition of IP2 is at 344 C in equilibrium with a fluid of composition XCO 2 = , and that of IP3 is at 355 C in equilibrium with a fluid of composition XCO 2 = (Goto et al., 2007). Hence, temperature, and The problem of local versus external control of fluid composition during metamorphism has been usually a subject of debate. Previous studies on the fluid composition of the Sanbagawa schists suggest a progressive increase in XCO 2 of the fluid phase (e.g., Itaya and Banno, 1980; Goto et al., 2007), which imply that the fluid composition was basically controlled by the mineral assemblages. Under the condition of internally buffered fluid, as temperature increase and reach the reaction 6, both CO 2 and H 2 O are added and fluid composition moves along the reaction 6 towards IP1 in Figure 8. We found the three phase assemblage, pumpellyite + lawsonite + epidote, coexisting in pelitic schist ( ) from the chlorite zone of the Sanbagawa metamorphic belt, in addition to two - phase assemblages, pumpellyite + epidote and lawsonite + epidote. Further, upper limit of both pumpellyite - bearing (60204) and lawsonite - bearing (122611) samples occur close to each other near the garnet - in isograd. That is, the metamorphic conditions of the studied area, the high - grade part of the chlorite zone, are close to conditions of the invariant point IP1 in Figure 8. Distribution of pumpellyite and lawsonite widely overlap and both of them disappear near the garnet - in isograd. These lines of evidence suggest that coexisting fluid was close to the internally buffered condition. An idealized geometry of an isobaric T - XCO 2 diagram shown in Figure 8 predicts that the stable hydrous Ca - Al mineral changes from lawsonite through pumpellyite to epidote for pelitic schists in the chlorite zone if metamorphic conditions pass through the shaded area as Sanbagawa path did. The mineral sequence from lawsonite through pumpellyite to epidote is attained by the progressive increase in both temperature and XCO 2 of the fluid phase. In the low - grade part of the chlorite zone, therefore, lawsonite is expected to be predominant hy-

10 272 H. Shinjoe, A. Goto, M. Kagitani and C. Sakai drous Ca - Al mineral. The chlorite zone can be divided into lawsonite and epidote zones because the region of appearance of pumpellyite and lawsonite widely overlap and both of them disappear near the garnet - in isograd. Lawsonite is, however, inadequate as index mineral, because the lawsonite - bearing sample (122611) occurs near the garnet - in isograd and hence most of the chlorite zone is covered by the lawsonite zone. Additionally, sporadic occurrence and low modal abundance of lawsonite make hard to identify it, the work on the revealing regional extent of the zone requires much effort. Index minerals defining mineral zones should be more commonly present. For example, the frequency of occurrence of garnet is as high as 90% in the garnet zone pelitic schist, and that of biotite exceeds 60% in the biotite zone pelitic schist (Goto et al., 2002). The low modal abundance and sporadic occurrence of three Ca - Al silicate minerals might be due to the low CaO content of pelitic schists. Goto et al. (1996) reported whole rock compositions of 198 Sanbagawa pelitic schists including 35 samples from the chlorite zone. As shown in Figure 4, the range of CaO contents of our samples lies within those of 35 samples of Goto et al. (1996), they however occupy relatively high CaO part. Most of samples reported by Goto et al. (1996) plotted in lower CaO part, which might explain sporadic occurrence of hydrous Ca - Al silicates. Epidote occurs throughout the chlorite zone, although the frequency of occurrence of epidote is about 65% in the chlorite zone samples (Goto et al., 2002), which is clearly lower than those in the higher - grade zones ( >90%). Since analysis of mineral assemblages were carried out in model system K 2 O - Na 2 O - CaO - MgO - Al 2 O 3 -SiO 2 -CO 2 -H 2 O, effect of the ferric iron cannot be accounted in our model. Appearance of epidote in chlorite zone seems to be due to ferric iron expanding its stability to the lower - grade side. Indeed most of the epidotes in the chlorite zone are ferric iron - rich in composition. However, epidote in a sample (122611) close to the disappearance of pumpellyite and lawsonite has clearly low ferric iron content (Fig. 6). An anomalous interference color of ferric iron - rich epidotes leading easier identification under the microscope may bias the frequency of occurrence against those of pumpellyite and lawsonite in the chlorite zone. Pumpellyite stability also might be affected by MnO, since pumpellyite in a specimen (60204) contains considerable amount of MnO ranging from 1.0 to 2.1 wt%. MnO content of the bulk rock of the specimen (= 0.22 wt%) is much higher than those of the other pumpellyite - bearing samples (= wt%). The high MnO contents of the pumpellyite in the specimen (60204) might be simply reflecting bulk rock composition. In the CaO - NaAlO 2 -Al 2 O 3 system, most Sanbagawa basic schists are characterized by the assemblage of calcite + albite + muscovite + epidote (Goto et al., 2007). Thus, our proposed pumpellyite decomposition mixed volatile the reaction 7 can be applied to the basic schists instead of pure dehydration reaction of, 25Ca 4 MgAl 5 Si 6 O 21 (OH) 7 + Mg 10 Al 4 Si 6 O 20 (OH) 16 Mg Pumpellyite Chlorite SiO 2 Quartz = 7Ca 2 Mg 5 Si 8 O 22 (OH) Ca 2 Al 3 Si 3 O 12 (OH) Tremolite Clinozoisite + 67H 2 O (9), Fluid which was proposed by Nakajima et al. (1977). In the study area, lawsonite has not been found from basic schists so far, and hence, the pumpellyite formation reaction is still unclear. This might be partly due to the rare occurrence of basic schists in the lower grade part of the studied area. If lawsonite occurs in the lower grade area of the chlorite zone, reaction 6 is most plausible for the pumpellyite formation reaction. At extremely lower XCO 2, the metamorphic reaction should be represented by the mixed volatile reactions instead of pure dehydration reactions in the presence of CaCO 3. Lawsonite - bearing metabasites were described by Suzuki and Ishizuka (1998) from northern part of the Northern Chichibu belt in further lower grade part of the chlorite zone. Suzuki (1995) reported the occurrence of metamorphic aragonite in metabasites from the Mikabu greenstone belt and the northern part of the Northern Chichibu belt (MB - NNC complex; Suzuki and Ishizuka, 1998). In classical view, the MB - NNC complex is the weakly metamorphosed part of the Sanbagawa belt (Banno, 1964). The structural continuity between the MB - NNC complex and Sanbagawa belt has not yet been confirmed. Also it has not been clear whether MB - NNC complex grades into low - grade part of the Sanbagawa belt. Radiometric ages (Dallmeyer et al., 1995) imply both MB - NNC complex and Sanbagawa belt were suffered metamorphism coevally within a same subduction complex, even if some structural discontinuity between the MB - NNC complex and Sanbagawa belt is present. The progressive hydrous Ca - Al mineral sequence from lawsonite through pumpellyite to epidote in the chlorite zone suggested by isobaric T - XCO 2 diagram (Fig. 8) may imply that general continuity of metamorphic condition between MB - NNC complex and chlorite zone of the Sanbagawa belt.

11 Ca - Al hydrous silicates in the chlorite - grade pelitic schists 273 Application to other subduction zone metamorphic rocks Pumpellyite, lawsonite and epidote are widely used as index minerals for low grade zones in high P/T metamorphic belt (Brown, 1977). The coexistence of pumpellyite + lawsonite + epidote has also been reported from several high P/T metamorphic belt (e.g., Fuscaldo area in Calabria; de Roever, 1972; Black Butte and Ball Rock area in northern California Coast Range; Brown and Ghent, 1983; Horokanai - Kamietanbetsu area in Kamuikotan belt; Shibakusa, 1989). In these areas, three hydrous Ca - Al silicates - bearing rocks also contain albite, phengite, and calcite/aragonite. Consequently, metamorphic conditions in the low grade region of some high P/T metamorphic belt might be also close to conditions of the invariant point (IP1) in Figure 8. If the metamorphic conditions pass through the lower XCO 2 area than IP1 as in the case of Sanbagawa belt, the stable hydrous Ca - Al silicate changes from lawsonite through pumpellyite to epidote for both CaCO 3 -bearing pelitic and basic schists with increasing metamorphic grade, while solid solution effect may overlap the mineral sequence. The progressive mineral sequence from lawsonite thorough pumpellyite to epidote has indeed been reported in the metasediment (Takayama, 1988) and metabasite (Shibakusa, 1989; Sakakibara and Ota, 1994) from the Kamuikotan metamorphic belt, although the progressive hydrous Ca - Al silicates sequences are unclear for the Fuscaldo area and the Black Butte/Ball Rock areas (de Roever, 1972; Brown and Ghent, 1983). For the Horokanai - Kimietanbetsu area of the Kamuikotan metamorphic belt, Shibakusa (1989) described regional distribution of mineral assemblages of metabasites in detail, and divided the area into three mineral zones based on the assemblages of hydrous Ca - Al silicates; zone I is characterized by lawsonite + pumpellyite without epidote, zone II by lawsonite + pumpellyite + epidote, and zone III by pumpellyite + epidote without lawsonite. The hydrous Ca - Al silicates - bearing rocks also contain phengite, albite, and aragonite, hence they are characterized by the occurrence of aragonite + albite + muscovite + lawsonite and aragonite + albite + muscovite + clinozoisite assemblages in the CaO - NaAlO 2 -Al 2 O 3 system of Goto et al. (2007). The occurrence of jadeite + quartz - bearing rocks was also reported from Kamuikotan belt, however they were found only as exotic blocks in serpentinite (Takayama, 1986). Although the coexistence of pumpellyite + lawsonite + epidote has not been reported, the progressive mineral sequence from lawsonite thorough pumpellyite to epidote was also known in the metabasites from the Ward Creek blueschist sequence of Cazadero, Central Franciscan belt (Maruyama and Liou, 1988), and Tokoro belt in eastern Hokkaido, Japan (Sakakibara, 1991). Maruyama and Liou (1988) divided the Ward Creek metabasites into three mineral zones; lawsonite, pumpellyite, and epidote zones. In the lawsonite zone, lawsonite is the only hydrous Ca - Al silicate, and lawsonite + pumpellyite coexistence is observed in pumpellyite zone. Epidote zone was defined by the appearance of epidote, and lawsonite and pumpellyite persist in epidote zone as minor phase (Maruyama and Liou, 1988). Progressive mineral sequence of Ward Creek metabasites are characterized by aragonite + albite + muscovite + lawsonite, aragonite + jadeite + muscovite + lawsonite, and aragonite + jadeite + muscovite + clinozoisite assemblages in the CaO - NaAlO 2 -Al 2 O 3 system of Goto et al. (2007). Sakakibara (1991) proposed six mineral zones for Tokoro metabasites. Among them, three mineral zones, lawsonite - Na pyroxene, epidote - Na pyroxene (2), and epidote - actinolite zones, suffered high P/T metamorphism. In the lawsonite - Na pyroxene zone, coexistence of lawsonite + pumpellyite without epidote is observed. The epidote - Na pyroxene (2) and epidote - actinolite zones are characterized by epidote + pumpellyite coexistence, and lawsonite is absent in the latter zone (Sakakibara, 1991). The high P/T mineral zones of Tokoro metabasites are characterized by aragonite - albite - muscovite - lawsonite and calcite - albite - muscovite - clinozoisite assemblages in the CaO - NaAlO 2 -Al 2 O 3 system of Goto et al. (2007). As described above, mineral sequence from lawsonite through pumpellyite to epidote has been reported from several subduction - related metamorphic belts. Metamorphic conditions of these areas may be close to conditions of the invariant point (IP1) in Figure 8. Hence the mixed volatile reactions of 6 and 7 proposed in the present study can be applicable to explain the mineral sequence found in CaCO 3 -bearing metabasites and metasediments and the fluid phase released by the metamorphic reactions are persistently high in H 2 O even in the presence of CaCO 3 - phase. ACKNOWLEDGMENTS We thank Dr. Kazumi Yokoyama for providing access to the electron probe micro analyzer, Ms. Masako Shigeoka for assistance of the microprobe observation and analyses and Professor Keitaro Kunugiza and Professor Shigenori Maruyama for their critical comments. Critical comments by Dr. Tatsuki Tsujimori, an anonymous reviewer, and the associated editor Dr. M. Satish - Kumar were helpful to improve the manuscript. This work was started from 1985 by one of us (MK) at Kyoto University, and his sincere thanks are also due to late Emeritus Professor Shohei

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