Mg-rich chloritoid in a corundum-bearing zoisite rock from the Sanbagawa belt, central Shikoku, Japan

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1 148 Journal of Mineralogical and Petrological Y. Banno Sciences, Volume 104, page , 2009 Mg-rich chloritoid in a corundum-bearing zoisite rock from the Sanbagawa belt, central Shikoku, Japan Yasuyuki Banno Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Higashi, Tsukuba , Japan Mg - rich chloritoid with the ratio Mg/(Mg + Fe 2+ ) = 0.37 to 0.49 was found in a corundum - bearing zoisite rock from the Besshi area in the Sanbagawa metamorphic belt, Japan. The zoisite rock consists mainly of zoisite, corundum, garnet, and amphibole. Corundum was retrogressively replaced by secondary chloritoid, zoisite, clinozoisite, chlorite, and paragonite. Staurolite rarely occurs in aggregates of the secondary phases (chloritoid, chlorite, and paragonite), which occur along the cracks in corundum, and is surrounded by chloritoid. This fact indicates that chloritoid was formed by the following retrogressive reaction: 7chlorite + 8corundum + 4staruolite + 15H 2 O 51chloritoid. The estimated temperature for the above univariant reaction in the MASH system is 637 ± 30 C at 10 kbar and 744 ± 34 C at 20 kbar. The textural evidence and the estimated temperature suggest that chloritoid formed during the exhumation stage from eclogite to epidote - amphibolite facies conditions. Keywords: Chloritoid, Staurolite, Corundum, Garnet, Zoisite rock, Retrograde metamorphism, Besshi area, Sanbagawa belt INTRODUCTION Chloritoid [(Fe 2+,Mg,Mn) 2 Al 4 Si 2 O 10 (OH) 4 ] is a relatively common mineral that is a constituent of low - to - medium grade regionally metamorphosed pelitic rocks and is commonly used as a metamorphic index mineral in orogenic belts (Deer et al., 1997). Chloritoid with Mg# [= Mg/(Mg + Fe 2+ )] less than 0.3 commonly occurs in low - to - intermediate - pressure metamorphic areas (cf., Fig. 1 of Chopin and Schreyer, 1983). In contrast, magnesiochloritoid [(Mg,Fe 2+,Mn) 2 Al 4 Si 2 O 10 (OH) 4 ], which is the Mg - analogue of chloritoid (Chopin et al., 1992), is a key mineral in metapelites in blueschist facies. The formation of magnesiochloritoid occurs only under metamorphic conditions characterized by very high P/T (Chopin and Schreyer, 1983). Chloritoid is a rare mineral that occurs in the Sanbagawa belt, which is a high - P/T metamorphic belt. Recently, several studies have reported on the occurrence of chloritoids (Mg# = 0.06 to 0.33) as inclusions in garnets of pelitic schists (Takasu, 1986; Sakurai, 2000; Wallis and Aoya, 2000; Zaw Win Ko et al., 2005a) and basic schists (Zaw Win Ko et al., 2005b). The present author found Mg - rich chloritoid (Mg# = ~ 0.4) associated with doi: /jmps a Y. Banno, y - banno@aist.go.jp Corresponding author staurolite in a corundum - bearing zoisite rock from the Besshi area of the Sanbagawa belt. The occurrence of chloritoid - staurolite assemblage in the Sanbagawa belt is reported for the first time in this paper. Although the corundum - bearing zoisite rocks in the Besshi area have been petrographically described by Minakawa and Momoi (1982), no occurrence of chloritoid and staurolite was reported. Yokoyama and Goto (1987) found magnesiostaurolite and staurolite from the corundum - bearing zoisite rocks in the Besshi area, but they did not report the occurrence of chloritoid. Mg - rich chloritoid with Mg# = 0.25 to 0.50 is commonly found in metapelitic lithologies of blueschist facies (Chopin and Schreyer, 1983; Azañón et al., 1998). Therefore, the corundum - bearing zoisite rock from the Besshi area provides a rare example of a mineral association containing Mg - rich chloritoid. In this paper, the mode of occurrence, chemistry, and formation of the Mg - rich chloritoid are described. The studied specimen has been stored at the Geological Museum, Geological Survey of Japan, AIST, Tsukuba, and its registration number is GSJ R GEOLOGIC SETTING The Sanbagawa metamorphic belt belongs to the high -

2 Mg - rich chloritoid in a corundum - bearing zoisite rock 149 Figure 1. Metamorphic zonation map of the Sanbagawa belt in the Besshi area [adapted from Higashino (1990)]. The boundary shown between the Eastern and Western Iratsu masses according to that proposed by Kugimiya and Takasu (2002). The areal extent of the Seba mass is obtained from Aoya (2001). The location of the corundum - bearing zoisite block (GSJ R87393) is shown by an open star. Abbreviations: HA, Higashi - akaishi mass; TN, Tonaru mass; WI, Western Iratsu mass; EI, Eastern Iratsu mass; SB, Seba mass. pressure intermediate group of metamorphism (Miyashiro, 1961) and represents the deeply subducted part of the Mesozoic accretionary complex along the eastern margin of Eurasia. A considerable part of the Sanbagawa belt mainly comprises pelitic, basic, and quartz schists. In central Shikoku, the higher - grade part of the Sanbagawa belt is divided into four mineral zones chlorite, garnet, albite - biotite, and oligoclase - biotite zones in the increasing order of their metamorphic grade on the basis of mineral parageneses of pelitic schists (e.g., Higashino, 1975, 1990; Enami, 1983: Fig.1). High - grade albite - biotite and oligoclase - biotite zones are widely distributed in the Besshi area of central Shikoku. The metamorphic grade of these mineral zones is equivalent to that of epidote - amphibolite facies (Enami et al., 1994). Within the region of epidote - amphibolite metamorphism, there are numerous ultramafic and mafic masses such as the Higashi - akaishi peridotite mass and the Iratsu, Tonaru, and Seba metabasite masses (Kunugiza et al., 1986). The Iratsu mass is divided into two independent complexes (the Eastern and Western Iratsu masses) on the basis of its lithology. The protolith of the Eastern Iratsu mass is considered to be layered gabbro, whereas that of the Western Iratsu mass consists of basic volcaniclastics with minor pelitic, siliceous, and carbonaceous sediments (Takasu and Kohsaka, 1987; Takasu, 1989). These ultramafic and mafic masses in the Besshi area underwent extensive recrystallization under epidote - amphibolite facies conditions; however, these masses also locally preserve evidence of eclogite facies metamorphism, which occurred prior to the epidote - amphibolite facies stage (e.g., Kunugiza et al., 1986; Takasu, 1989; Aoya, 2001). The metamorphic history of the Iratsu mass is controversial. Takasu (1989) put forward a proposal of its metamorphic history in terms of the metamorphic facies associated with the mass as follows: (1) Eastern Iratsu mass: granulite eclogite blueschist eclogite epidote - amphibolite; (2) Western Iratsu mass: epidote - amphibolite eclogite epidote - amphibolite. In contrast, Ota et al. (2004) concluded that the metamorphic history of the Eastern and Western Iratsu masses is characterized by a simple, clockwise P - T path with its peak associated with eclogite facies. They further suggested that the granulite facies is a product of pre - subduction metamorphism. PETROGRAPHY A block of corundum - bearing zoisite rock was found as river float in the Hodono - dani (valley), a small tributary of the Dozan - gawa (river) in the Besshi area, Ehime Prefecture (Fig. 1). After studying the block, the present author could conclude that it was almost certainly derived from the Iratsu mass because of the following reasons: (1) zoisite rock, which is considered to form a layer of metamorphosed anorthosite in the metagabbro, is one of the characteristic rock types of the Iratsu mass (Banno et al., 1976; Kunugiza et al., 1986) and (2) the Iratsu mass is distributed in the uppermost stream of the Hodono valley (Fig. 1). Corundum - bearing zoisite rock mainly comprises coarse - grained zoisite, corundum, and garnet with a relatively small amount of amphibole. Zoisite occurs in a subhedral and prismatic form and can have a length of up to 1 cm. Both corundum and garnet occur as anhedral crystals and their width can reach up to 1.5 cm. Amphi-

3 150 Y. Banno bole is pale bluish - green and occurs as subhedral crystals with lengths of up to 4 mm. Corundum is usually surrounded by aggregates of fine - grained chloritoid, zoisite, clinozoisite, chlorite, and paragonite (length <1.3 mm; Fig. 2a). The aggregate also occurs along fractures in corundum. Sometimes, the anhedral garnet crystals occur in the aggregates. Chloritoid occurs on the corundum side of the aggregates (Fig. 2a). Chloritoid is pale green with a bluish tint and occurs as a prismatic crystal with a length less than 0.5 mm. It is partly replaced by chlorite and paragonite. Staurolite rarely occurs in aggregates of chloritoid, chlorite, and paragonite, which occur along the cracks in corundum. Figure 2b clearly shows that chloritoid grows into staurolite. It is also rare for staurolite to occur in the form of inclusions in corundum. Along the fractures, coarse - grained garnet is extensively replaced by fine - grained minerals (length <1.2 mm). These fine - grained minerals mainly comprise zoisite, clinozoisite, chlorite, and amphibole with relatively small amounts of paragonite. No inclusions have been observed in garnet. Fine - grained amphibole occurs only along fractures in garnet outside the aggregate of the fine - grained minerals that enclose corundum, and it is not associated with the garnet within the aggregate. MINERAL CHEMISTRY Mineral analyses were performed using a JEOL JXA R electron microprobe. The accelerating voltage, specimen current, and beam diameter were maintained at 15 kv, 12 na on the Faraday cup, and 2 µm, respectively. Synthetic quartz (for Si), rutile (Ti), corundum (Al), Cr 2 O 3 (Cr), MnO (Mn), hematite (Fe), periclase (Mg), wollastonite (Ca), natural albite (Na), and adularia (K) were used as standards. The Bence and Albee (1968) method was used for matrix corrections. The chemical compositions of chloritoid, garnet, corundum, staurolite, and chlorite are given in Table 1. Coarse-grained zoisite, corundum, garnet, and amphibole Figure 2. Backscattered electron images of chloritoid in the corundum - bearing zoisite rock from the Besshi area. (a) Chloritoid and other constituents enclosing corundum. (b) Chloritoid growing into staurolite. Abbreviations: Cld, chloritoid; Crn, corundum; Grt, garnet; Chl, chlorite; Pg, paragonite; Czo, clinozoisite; Zo, zoisite; St, staurolite. Zoisite is chemically uniform and its Y Fe [= Fe 3+ /(Fe 3+ + Al)] value is Corundum is light purplish - pink. The Fe 2 O 3 content of corundum can reach up to 0.51 wt% and its Cr 2 O 3 content is less than 0.17 wt%. All the iron in garnet was assumed to be ferrous, and the end - member proportion (X i ) was calculated as i/(fe + Mn + Mg + Ca). Garnets belong to the almandine - pyrope series and are poor in Mn (less than 0.07 X Sps ) and rich in Ca (up to 0.26 X Grs ). Backscattered electron images of garnet indicate that it is composed of dark and light parts that correspond to the earlier and later garnets, respectively (Fig. 3a). The light part is formed along the cracks in the dark part. The brightness of the backscattered electron image is mainly due to the variation in the Fe and Mg contents. The garnet in the dark part is characterized by increasing X Prp and decreasing X Alm from the core to the rim (Fig. 3b). This garnet is considered to preserve a prograde Fe - Mg zonation pattern and is hereafter referred to as prograde garnet. The garnet in the light part has a composition that is poor in pyrope and rich in almandine in comparison to the prograde garnet (Fig. 3a). Textural and chemical evidence shows that this garnet is a retrograde product (hereafter referred to as retrograde garnet). The compositional range of the prograde garnet is X Alm : , X Prp : , X Grs : , X Sps : 0.01 whereas that of the retrograde garnet is X Alm : ,

4 Mg - rich chloritoid in a corundum - bearing zoisite rock 151 Table 1. Chemical compositions of chloritoid, garnet, corundum, staurolite, and chlorite in the corundum - bearing zoisite rock from the Besshi area * Total Fe as FeO. ** Total Fe as Fe 2 O 3. Number of analytical points. # Chemical composition used for obtaining the reaction curves in Figure 5. Abbreviations: Cld, chloritoid; PGrt, prograde garnet; RGrt, retrograde garnet; Crn, corundum; St, staurolite; Chl, chlorite; mmg, most Mg - rich composition; mfe, most Fe - rich composition; n.d., not detected. X Prp : , X Grs : , X Sps : (Fig. 4). The prograde garnet has higher Mg# values ( ) than the retrograde garnet ( ). The amphibole nomenclature used in this paper is that proposed by Leake et al. (1997) and the Fe 3+ /Fe 2+ values were calculated for a total of 13 cations, excluding Ca, Na, and K (O = 23). Abbreviations for the element - site of amphibole are [4]: tetrahedral T - sites; [B]: B - sites and [A]: A - sites. Isolated matrix amphibole is pargasite with [4] Al = 1.75 to 2.12, [B] Na = 0.20 to 0.34 and [A] (Na + K) = 0.61 to 0.83 atoms per formula unit (apfu). Backscattered electron images show the chemical heterogeneity of this mineral, which is mainly due to the variation of the Mg/(Mg + total Fe) values ( ). The amphibole does not show a clear zoning pattern. Fine-grained minerals surrounding corundum The Mg# values of chloritoid range from 0.37 to 0.49, and its composition is intermediate in a chloritoid - magnesiochloritoid solid solution. The manganese content is very low, and the amount of Mn - chloritoid component [= Mn/(Fe + Mg + Mn)] is less than Chloritoid is generally homogeneous, but the zoning pattern of larger grains shows a slight increase in the Fe content toward the rim. The Y Fe values of zoisite and clinozoisite are in the ranges and , respectively. Chlorite has a narrow compositional range with Mg# = 0.76 to 0.78 apfu and Si = 2.59 to 2.79 apfu, and its MnO content can reach up to 0.13 wt%. Paragonite has a compositional range in which Si = 5.22 to 5.94 apfu and X Na [= Na/(Na + K + Ca)] = 0.73 to It contains up to 3.61 wt% CaO, and its margarite component [= Ca/(Na + K + Ca)] is

5 152 Y. Banno Figure 4. Compositional range of garnet in the corundum - bearing zoisite rock from the Besshi area. Abbreviations: Grs, grossular; Sps, spessartine. The other abbreviations are as defined in Figure 3. whereas that with [4] Al < 1.68 apfu has a barroisitic composition with [A] (Na + K) = 0.24 to 0.59 apfu and [B] Na = 0.42 to 0.69 apfu. Staurolite Figure 3. (a) and (b). Backscattered electron images of garnet in the corundum - bearing zoisite rock from the Besshi area. The dark and light parts correspond to earlier and later garnets, respectively. Numbers indicate X Prp (first line) and X Alm (in italics) (on the second line). Abbreviations: Prp, pyrope; Alm, almandine Fine-grained minerals replacing coarse-grained garnet The Y Fe values of zoisite and clinozoisite are and , respectively. Chlorite has a slightly higher Mg# ( ) than that surrounding the corundum grains ( ). Paragonite compositions are close to the composition of the end - member (Si = 5.92 apfu, X Na = 0.95). Amphibole varies in composition from flake to flake and does not show a clear zonal structure. Its composition is intermediate between that of barroisite and pargasite with [4] Al = 1.08 to 1.99 apfu and [B] Na = 0.22 to 0.69 apfu. An amphibole with [4] Al > 1.68 apfu tends to be rich in [A] (Na + K) ( apfu) and poor in [B] Na ( apfu) and has a pargasitic composition, The chemical formula of staurolite was calculated on the basis of O 44 (OH) 4. Water and Li 2 O are important constituents of staurolite (Holdaway et al., 1986); however, no attempt was made to analyze these elements because of a paucity of the material. Qualitative analyses of staurolite show negligible amounts of ZnO. Staurolite occurs as (1) inclusions in corundum and (2) a constituent mineral in aggregates of fine - grained minerals along fractures in corundum. The former is chemically homogeneous, with Mg# = 0.36 to 0.37, Si = 7.66 to 7.69 apfu and the cation sum = to apfu. In contrast, the latter has a lower Mg# ( ) and cation sum ( ) and a higher Si content ( apfu). DISCUSSION The studied zoisite rock mainly comprises coarse - grained zoisite, corundum, garnet, and amphibole. Thus, the mineral assemblage of the peak stage is considered to be zoisite + corundum + Mg - rich garnet + pargasite. Unfortunately, this mineral assemblage does not constrain the P - T condition on the basis of net - transfer reactions in the CFMASH system. Ota et al. (2004) concluded that the P - T history of the Iratsu mass is characterized by a clockwise P - T path with its peak associated with the eclogite

6 Mg - rich chloritoid in a corundum - bearing zoisite rock 153 Figure 5. Possible retrograde P - T path of the corundum - bearing zoisite rock from the Besshi area. Abbreviations: FASH, chemical system SiO 2 -Al 2 O 3 -FeO - H 2 O; MASH, chemical system SiO 2 -Al 2 O 3 -MgO - H 2 O; V, H 2 O. The other abbreviations are as defined in Figure 2. facies. The P - T path in their study suggests that the peak metamorphic stage of the zoisite rock probably corresponds to the eclogite facies stage. Aggregates of fine - grained minerals enclose coarse - grained corundum or garnet. This indicates that these fine - grained minerals are products of the retrograde stage. Staurolite rarely occurs in aggregates of fine - grained minerals that occur along the fractures in corundum. Figure 2b clearly shows that chloritoid is in contact with staurolite, chlorite, and corundum. This textural relation indicates that chloritoid coexists with staurolite, chlorite, and corundum. The following reaction represents the formation of chloritoid: 7(Mg,Fe) 5 Al 2 Si 3 O 10 (OH) 8 + 8Al 2 O 3 Chlorite Corundum + 4(Fe,Mg) 4 Al 18 Si 7.5 O 44 (OH) H 2 O Staurolite 51(Fe,Mg)Al 2 SiO 5 (OH) 2. Chloritoid The P - T condition during the retrograde stage can be determined using the assemblage containing chloritoid + staurolite + chlorite + corundum. The mineral equilibrium is calculated using the THERMOCALC program (ver. 3.21) (Holland and Powell, 1998) with coexisting compositions of chloritoid, staurolite, and chlorite (cf., Table 1). The activities of these minerals are obtained using the AX2 program (ver. 2.2). Calculations are made taking pure corundum into consideration because of very small Fe 2 O 3 and Cr 2 O 3 contents. The metamorphic fluid is assumed to be pure H 2 O (X H2 O = 1). The effect of water activity on the P - T estimations is small; the difference in the estimated T conditions at constant pressure when X H2 O is between 1.0 and 0.8 is 7-13 C. The estimated temperatures for the above univariant reaction in the MASH system are 637 ± 30 C at 10 kbar and 744 ± 34 C at 20 kbar. The temperature obtained from the calculation using the FASH system is approximately 130 C lower than that obtained from the MASH system at kbar (Fig. 5). The process of formation of staurolite in the aggregate along the fractures in corundum is not clear. Yokoyama and Goto (1987) reported the presence of magnesiostaurolite (Mg# = 0.5 to 0.6) in corundum - bearing zoisite rocks from the Besshi area. Magnesiostaurolite apparently coexists with coarse - grained zoisite, amphibole, corundum, and garnet (Mg# = ~ 0.6) and is considered to be stable at the peak condition (Yokoyama and Goto, 1987). Moreover, they also found staurolite (Mg# = 0.2 to 0.3) that was partly altered to an aggregate of chlorite, paragonite, and margarite and concluded that the staurolite was formed at the retrograde stage. Their interpretation implies that the staurolite (Mg# = ~ 0.3) that occurs as a constituent mineral in the aggregate of fine - grained minerals in the studied zoisite rock could also be a retrograde product. One of the possible reactions for the formation of staurolite at an early stage of retrograde metamorphism is as follows: garnet + corundum + H 2 O staurolite + chlorite. The equilibrium conditions at the peak metamorphic stage for the eclogite assemblage from the Eastern and Western Iratsu masses are estimated to be P = 14 to 25 kbar and T = 500 to 800 C, which implies large P - T variations (Ota et al., 2004). Recently, Miyamoto et al. (2007) pointed out that P - T estimations of the eclogite facies stage in the Sanbagawa belt should be carefully re - examined on the basis of textural and compositional heterogeneities of constituent minerals. They studied kyanite - bearing quartz eclogites, which occur as a small unit along the northeastern margin of the Higashi - akaishi mass, and observed the following: (1) most garnet grains in the kyanite - quartz eclogite consist of inner and outer segments that correspond to the products of eclogite facies and epidote - amphibolite facies stages, respectively and (2) the Mg - rich outermost rim of garnet does not always represent the composition at the peak eclogite stage but can form at the subsequent epidote - amphibolite facies stage. Because the P - T conditions of the Eastern and Western Iratsu masses reported by Ota et al. (2004)

7 154 Y. Banno have been estimated on the basis of the assumption that the pyrope - rich outermost rim of garnet represents the climax of the eclogite facies stage, the P - T estimates ranging towards the lower P - T side (cf., Fig. 5) may be caused by the dataset including inappropriate garnet compositions (Miyamoto et al., 2007). Therefore, only the higher P - T conditions estimated by Ota et al. (2004) can represent the climax P - T condition of the Iratsu mass. The eclogitic masses and the surrounding Sanbagawa schists suffered the epidote - amphibolite facies metamorphism (Takasu, 1989; Enami et al., 1994). The retrograde P - T conditions of the epidote - amphibolite facies stage for the zoisite rock could be assumed to be similar to the climax P - T condition of the Sanbagawa schist from the albite - biotite zone in the Besshi area (8-11 kbar, C; Enami et al., 1994). The temperatures estimated from the assemblage containing chloritoid + staurolite + chlorite + corundum are roughly within those reported from the eclogite assemblage of the Iratsu mass (Ota et al., 2004) and higher than those of the Sanbagawa schist from the albite - biotite zone. It is assumed that the coarse - grained minerals in the zoisite rock were formed at the peak stage under the eclogite facies condition prior to the epidote - amphibolite facies stage. The temperatures estimated from the secondary mineral assemblage are not contradictory to the assumed peak condition of eclogite facies metamorphism. Figure 5 shows a possible retrograde P - T trajectory for the zoisite rock. Textural evidence and the estimated temperature suggest that chloritoid formed during the exhumation stage from the eclogite to epidote - amphibolite facies conditions. ACKNOWLEDGMENTS The author thanks Dr. Y. 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