J. Japan. Assoc. Min.P etr. Econ. Geol. 76. 245-253, 1980.

NOTES ON PETROGRAPHY AND ROCK-FORMING

MINERALOGY (8) MARGARITE-BEARING

METAGABBRO FROM THE IRATSU

MASS IN THE SANBAGAWA BELT,

CENTRAL SHIKOKU

MASAKI ENAMI

Department of Earth Sciences, Nagoya University, Nagoya 464, Japan

Margarite-bearing metagabbro was found from the Iratsu epidote amphibolite mass in the Sanbagawa metamorphic belt. The sample is a metamorphosed anorthosite layer and consists of zoisite, paragonite, kyanite, margarite, chlorite, quartz and clay mineral (kaolinite or dickite). The unit cell parameter of margarite is as follows: a0=5.11 A, b0=8.79 A, c0=19.15 A. ƒÀ=95.3•‹. Muscovite component in margarite is almost negligible, but paragonite content in it reaches about 27 mole%. Paragonite is divided into Ca-rich (CaO=0.4-1.1 wt%) and Ca-poor (CaO=0.2-0.4 wt%) types. Within the constitutent minerals, zoisite, kyanite and Ca-poor paragonite were formed with the decomposition of plagioclase in original anorthosite, and represent the equilibrium assemblage at the highest temperature stage during the Sanbagawa metamorphism. Margarite and Ca-rich paragonite were formed during the retrogressive metamorphism within the Sanbagawa stage.

INTRODUCTION MODE OF OCCURRENCEAND PETROGRAPHY Margarite, a calcic dioctahedral mica, has been reported from numerous rock The Iratsu epidote amphibolite mass is types: namely ore deposits (Aoki and Shima a metamorphosed layered gabbro complex da, 1965), pelitic schists (Chinner, 1974; that occurs in the epidote amphibolite facies Frey and Orville, 1974; Hock, 1974; Guidotti area of the Sanbagawa metamorphic belt and Cheney, 1976; Frey, 1978), marbles in central Shikoku (Banno et al., 1976; 1978). (Jones, 1971; Okrush et al., 1976) and The mass consists mainly of epidote amphi amphibolites (Ackermand and Morteani, bolite, hornblende eclogite, quartz eclogite 1973). and zoisite rock. Recently, margarite was found from a Zoisite rock is a metamorphosed anorth metagabbro of the Iratsu epidote amphibolite osite layer in the metagabbro and consists mass in the Sanbagawa metamorphic belt of wide (7-30cm in width) zoisite-rich band in central Shikoku. This note is the first and thin (0.5-3cm) amphibole-rich band. report of margarite from the metamorphic The former consists mainly of zoisite, rocks in Japan, and will discuss the paragonite, kyanite and quartz and the petrogenesis of the margarite-bearing rock. latter is composed of hornblende, clinozo

(Manuscript received March 13, 1980) 246 Masaki Enami

the zoisite rock which was collected from a river bed on the Nikubuchi stream (Fig. 1). The sample is subdivided into two parts: paragonite-rich and zoisite-rich parts. The former consists mainly of paragonite and kyanite with subordinate amounts of zoisite, quartz and margarite, on which detailed chemical works have been done, and the latter consists of zoisite and quartz with subordinate amounts of chlorite and para gonite. Clay mineral is also found as a Fig. 1: Geological sketch map of the Iratsu mass minor constituent, and is probably kaolinite and the surrounding peridotites and or dickites judged from its chemical com schists (after Banno et al., 1976) with location of margarite-bearing rock. position (cf. Table 1). Coexistenceof marg arite and paragonite was confirmed ?? isite, chlorite, garnet, albite and quartz. the X-ray powder method. Two different Margarite is found in a zoisite-rich band of ways of distinguishing margarite from

Table 1: Chemical compositions of minerals. Abbreviations for minerals: Ma: margarite, Pa(I): type I paragonite, Pa (II): type II paragonite, Zo: zoisite, Ky: kyanite, Chl: chlorite, Clay: clay mineral.

* Total Fe as FeO . ?? Total Fe as Fe2O3. Margarite-bearing metagabbro from the Iratsu mass 247

paragonitehave been used in literatures alyses are shown in Table 1. (d (060)spacing, Velde, 1971; and d (0.0. 10) Margarite: Three varieties of mode of spacing, Chatterjee, 1971). The d (060) occurrence are observed: (1). Most margarites and d(0.0. 10) spacingsof margarite in the built up rims around kyanite and measure

sample studied are 1.465 and 1.908A, 10-50 ƒÊm in width (Fig. 2a), (2). It is respectively,and showgood agreementwith observed as veinlets cutting kyanite grains. the publisheddata (cf. Aoki and Shimada, The veinlets consist of medium-grained (50-

1965). Althoughcoexistence of paragonite 100 ƒÊm) margarite and paragonite and

and muscoviteis common in the Iratsu aggregates of very fine-grained (2-3 ƒÊm)

metagabbros(Iwata, 1975; Enami, 1978), silicate minerals (Fig. 2b). Minerals in the

muscovitewas not found in the sample aggregates are too fine to be identified studied. microscopically and to be analyzed with

Chemicalanalyses were carriedout by EPMA. With the EPMA analyses of the

means of JXA-5A electronprobe micro aggregates, Si, Al, Ca and Na are detected - ?? analyzer (EPMA) of J. E.O. L. The ac as major components and Fig. 3 shows the celerating voltage, specimen current and relative concentrations of CaO and Na2O.

beam diameter were kept at 15 kV, 0.02ƒÊA CaO and Na2O contents vary 7.5-11.5 wt%

and 5 ƒÊm, respectively. Representative an and 1.1-3.5 wt%, respectively. CaO and

Fig. 2: Photomicrographs of margarite and associated minerals. a) Margarite being observed as rims around kyanite. Crossed nicols. b) (Maragarite+paragonite) veinlets cutting kyanite. Crossed nicols. c) Margarite being observed at the boundary between zoisite inclusion and kyanite host. Open nicol. Abbreviations for minerals: Ma: margarite, Pa: paragonite, Ky: kyanite, Zo: zoisite, Mx: mixtures of margarite and paragonite. 248 Masaki Enami

Fig. 3: CaO-Na2O relation of fine aggregates in veinlet. Open circles: compositions of fine aggregates. Solid circles: average compositions of independent margarite and type II paragonite.

Na2O contents of numerous analyzed spots Table 2 shows the unit cell parameter of correspond to those of margarite , however, margarite obtained with the X-ray powder some analyzed spots are richer in Na2O and diffractometer. These values agree well poorer in CaO than independent margarites. with those given in literatures. The com The compositions of such Na-rich spots are positional range of margarite on the Na plotted on the paragonite-margarite join in -Ca-K diagram is shown in Fig. 4a. the present system, showing the presence of Muscovite component in margarite is almost mixture of margarite and subordinate negligible but paragonite solid solution amounts of paragonite. (3). Some margarites reaches about 27 mole%. Only minor con occur at the boundary between zoisite in tents of Fe and Mg were recorded, while clusion and kyanite host (Fig. 2c). Textural Mn and Ti were below the detection limit. relations described above strongly indicate Paragonite: Paragonite is divided in that margarite postdated kyanite and to two types, types I and II, by the dif zoisite. ference of mode of occurrence. Type I

Table 2: Unit cell parameters of margarita. Notes: (1): Aoki and Shimada (1965), (2): Takeuchi (1965), (3): Hock (1974), (4): Guggenheim and Bailey (1975). Margarite-bearing metagabbro from the Iratsu ma ss 249

Type II paragonite shows prismatic form

measuring about 2 ƒÊm-2.0mm in length

and occurs as veinlets in kyanite or rims

around kyanite (Fig. 5b). Chemical com

position is also different between the two types of paragonites. Fig. 4b shows the

compositional range of paragonite on the

Na-Ca-K diagram. Muscovite components

of types I and II paragonites vary 2.4-5.6 Fig. 4: Compositionsof the analyzed margarites and 2.2-5.1 mole%, respectively, and are and paragonitesin the diagram Na-Ca-K. similar to each other but are smaller than those of paragonites in the margaritefree

paragonite occurs as independent and samples from the same mass. On the other prismatic form being parallel to the weak hand, margarite components of types I and foliation which is represented by the ar II paragonites vary 1.8-3.8 and 2.8-7.1 mole rangement of zoisite grains (Fig. 5a). Grain %, respectively, and type II paragonite is size varies from 0.1 to 1.5mm in length. richer in margarite component than type I

Fig. 5: Photomicrographs of paragonite and associated minerals. a) Type I paragonite. b) Type II paragonite. Abbreviations for minerals: Pa (I): type I paragonite, Pa (II): type II paragonite, Zo: zoisite, Ky: kyanite, Qz: quartz. 250 Masaki Enami

paragonite. Type I paragonite is in equilibrium and two stages of equilibration equilibrium with zoisite and kyanite because are recognized. any textures of replacing kyanite by type I 1) The first stage: Yokoyama (1976a)

paragonite were not observed. Type II has shown that the zoisite+kyanite+

paragonite postdated kyanite because it quartz assemblage was formed with the occurs as psuedomorph after kyanite. decomposition of original plagioclase. Type Ca-rich composition of type II paragonite I paragonite is considered to be also formed indicates that type II paragonite is saturated with the decomposition of the plagioclase in margarite component and is in equilib and to be in equilibrium with zoisite, kyanite rium with margarite. and quartz on the following reasons. First, Zoisite: Zoisite occurs as rounded and type I paragonite does not show any subhedral form (50-500 ƒÊm in size) forming textures of replacing kyanite, and secondly,

small domains within the paragonite-rich albite component in the plagioclase is the

parts. It also occurs as inclusions in only candidate to supply Na2O for the forma kyanite. Any zonal structures were not tion of type I paragonite. observed with the microscopic observations. An example for the reaction to form

XPs (=Fe3+/Fe3++Al) content of zoisite is the assemblage of the first stage from the about 0.026-0.030 and is fairly constant original plagioclase is as follows: within a thin section. 40(Na0.1,Ca0.9)(Al1.9,Si2.1)O8+13H2O Kyanite: Kyanite occurs as sub

hedral grain measuring 0.5-3.0mm in size , =18Ca2Al3Si3O12(OH)+2Na2Al6Si6O20(OH)4 and contains zoisite and quartz as inclusions . Kyanite is always surrounded by margarite +5Al2SiO5+13SiO2•c•c•c(1)

and type II paragonite. With EPMA

analyses, small amount of Fe (0.16-0.21 wt% Comparing these assemblage with the as Fe2O3) is detected. thermal history of the Iratsu mass, the left Chlorite: Chlorite is observed as vein hand of Eq. (1) has been considered to lets in the zoisite-rich domains and its represent the equilibration of the granulite

chemical composition is characterized with facies stage and the right hand to represent

low XFe (=Fe/Fe+Mg) content (0 .19-0.20). that of the epidote amphibolite facies stage of the Sanbagawa metamorphism (Yoko

DISCUSSION yama, 1976a; Banno et al., 1976). 2) The second stage: The second The margarite-bearing zoisite rock stage is represented by the mineral as discussed here is a metamorphosed anorth semblage of margarite+type II paragonite . osite layer, in which original plagioclase Margarite is considered to be formed with contained about 10 mole% albite com the decomposition of zoisite and kyanite on ponent but contained little orthoclase com the following reasons. (1): Most margarites ponent (Yokoyama, 1976a; Banno et al. , occur as rims around kyanite or veinlets in 1976). So, we may discuss the mineral kyanite. (2): Some margarites are observ paragenesis of the rock in the system of ed at the boundary between zoisite inclus SiO2Al2O3-CaO-Na2O. Apparently , not all ion and kyanite host . Type II paragonite the minerals in the sample studied are in was also formed with the decomposition of Margarite-bearing metagabbro from the Iratsu mass 251

kyanite and in equilibrium with margarite, Tauern has the assemblage paragonite+ because aggregates of type II paragonite margarite+muscovite+ chlorite+chloritoid

and margarite are observed as pseudomorphs +calcite+dolomite+zoisite+quartz, and after kyanite. its equilibrium temperature can be estimat An example of the reaction to form ed lower than 490•Ž by using calcite-dolo

margarite and type II paragonite from mite geothermometry (cf. Hock, 1974; zoisite and kyanite is as follows: Bickel and Powell, 1977). Further, Ackerm

2Ca2Al3Si3O12(OH)+7Al2SiO5+2NaAlSi3O8 and and Morteani (1973) studied a amphi

bolite from the low-grade amphibolite fades

+3H2O=2Ca2Al8Si4O20(OH)4 area in the Greiner Schiefe Series, in which

plagioclase, hornblende, paragonite, marga +Na2Al6Si6O20(OH)4+4SiO2 •c•c•c•c•c(2) rite, garnet, calcite, epidote, biotite and

kyanite occur. In this sample, the composi

Eq. (2) requires the decomposition of tional gap lies between 0.51 and 0.84 XNa.

albite, which is absent in the sample studied. The compositional ranges of the gap report

It is possible that albite component was ed from the two areas indicate that the

derivedfrom relic albite in thefirst stage and compositional gap between margarite and all albite was consumed with the formation paragonite closes rapidly with increasing of margarite and type II paragonite. temperature. In the zoisite rock discussed

Another possible explanation is that albite here, the compositional gap lies between

component was derived from a amphibole 0.27 and 0.91 XNa, and is similar to that for

rich layer, which is intercalated as a thin the greenschifst facies condition. The wide

band (0.5-3.0cm) into the zoisite rock and range of the gap probably indicates that

consists mainly of hornblende, clinozoisite, margarite+type II paragonite assemblage

chlorite, garnet, albite and quartz. Even was formed under lower temperature than

though the source of Na2O for the reaction the epidote amphibolite facies condition of

has not been defined as yet, there is little the first stage. Further, Chatterjee (1976)

doubt that margarite and type II paragonite and Perkins et al. (1980) have shown that were formed with the decomposition of the with decreasing temperature, kyanite + mineral assemblage in the first stage. quartz assemblage breaks down into Chatterjee (1976) and Perkins et al. pyrophyllite at 380-430•Ž at the pressures (1980) have indicated that the margarite+ of 4-8 kb. In the sample studied, pyrophy quartz assemblage is stable at lower tem llite was not observed and this reaction may perature condition than the zoisite+kyanite define the minimum temperature of the assemblage. The equilibrium condition of second stage. the second stage may be estimated with the In regard to the thermal history of the compositional range of miscibility gap Iratsu mass, three stages of equilibration between margarite and paragonite. Hock have been recognzied so far, and they cor (1974) studied a pelitic schist from the respond to the igneous stage (Yokoyama, greenschist facies area in the Hohe Tauern, 1976b; Banno and Yokoyama, 1977), the and showed that the compositional gap granulite facies metamorphic stage (Yoko exists between 0.29 and 0.90 XNa (=Na/ yama and Mori, 1975; Yokoyama, 1976a) Na+Ca+K). The sample from the Hohe and the Sanbagawa metamorphic stage 252 Masaki Enami

(Banno et al., 1976; 1978), respectively. Japanese with English abstract). in Hide, K ed. Sanbagawa belt,, Hiroshima Univ. Press. Further, it is pointed out in this paper that a 57-68. retrogressive reaction took place locally •\ Higashino, T., Otsuki, M., Itaya, T. and

postdating the formation of major minerals. Nakajima, T. (1978). Thermal structure of the Sanbagawa metamorphic belt in central Combining available data, comparison of Shikoku. Jour. Phys. Earth, 26, supplement, mineral assemblage at each stage of 345-356. equilibration is as follows: Bickel, M. J. and Powell, R. (1977), Calcite-dolomite

1): Igneous stage•c•c•c•c(plagioclase). geothermometry for iron-bearing carbonates. The Glockner area of the Tauem window, 2): Granulite facies stage Austria. Contr. Mineral, and Petrol., 59, 281-

•c•c•c•c•c plagioclase (An=90). 292. Chatterjee, N. D. (1971), Phase equilibria in the 3): Sanbagawa stage Alpine metamorphic rocks of the environs of (a). Progressive epidote amphibolite the Dora-Maria massif, western Italian Alps. facies stage•c•c•c•c•c•c Neues Jb. Miner. Abb., 114, 181-245.

zoisite+kyanite+type •\ (1976), Margarite stability and com patibility relations in the system CaO-Al2O3- I paragonite+quartz. SiO2-H2O as a pressure-temperature indicater.

(b). Retrogressive stage•c•c•c•c•c•c Amer. Mineral., 61, 699-709.

margarite+type II paragonite Chinner, G. A. (1974), Dalradian margarite: a preliminary note. Geol. Mag, 111, 75-78. +quartz. Enami, M. (1978), Petrological study of epidote amphibolites and basic schists in the Besshi

ACKNOWLEDGEMENTS area, Sanbagawa metamorphic terrain in central Shikoku. M. Sci. Thesis, Kanazawa The author would like to express his Univ (MS).

sincere thanks to Dr. S. Banno and Prof. H. Frey, M. (1978), Progressive low-grade metamor

Onuki for the critical comments on this phism of a black shale formation, central Swiss Alps, with special reference to pyrophyllite manuscript, Critical reading of Dr. K . and margarite bearing assemblages. Jour.

Shiraki is much appreciative. His thanks Petrol., 19, 95-135. •\ and Orville, P. M. (1974), Plagioclase in are due to Prof. K. Ishioka, Dr. K. Suwa and margarite-bearing rocks. Amer. Jour. Sci., Dr. K. Suzuki for their warm encourage 274, 31-47.

ment. Guggenheim, S. and Bailey, S. W. (1975), Refine ment of margarite structure in subgroup sym metry. Amer. Mineral., 60, 1023-1029. REFERENCES Guidotti, C. V. and Cheney, J. (1976), Margarite

Ackermand, D. and Morteani pseudomorphs after chiastolite in the Rangeley , G. (1973), Occur area, Maine. Amer. Mineral., 61, 431-434 rence and breakdown of paragonite and . Hock, V. (1974), Cexisting phengite, paragonite and margarite in the Greiner Schiefer series (Ziller margarite in metasediments of the Mittlere thai Alps, Tyrol). Contr. Mineral . and Petrol., Hohe Tauern, Austria. Contr. Mineral. and 40, 293-304. Petrol., 43, 261-273. Aoki, Y. and Shimada, N. (1965) , Margarite from Iwata, O. (1975), The petrogic study of Iratsu the Shin-kiura mine, Oita Prefecture (in epidote amphibolite mass, central Shikoku, Japanese). Jour. Min. Soc. Japan, 7, 87-93. Banno, S., Yokoyama, K., Iwata Japan (in Japanese with English abstract). , O. and Ter M. Sci. Thesis, Kanazawa Univ. (MS) ashima, S. (1976), Genesis of epidote amphi . Jones, J. W. (1971), Zoned margarite from the bolite masses in the Sanbagawa metamorphic Badshot formation (Cambrian) near Kaslo belt in central Shikoku (in Japanese with , British Columbia. Canadian Jour . Earth Sci., English abstract) . Jour. Geol. Soc. Japan, 82 , 199 8, 1145-1147. -210 . Okrusch, M., Bunch. T. E. and Bank , H. (1976), •\ and Yokoyama, K. (1977) , Peridotite- Paragenesis and petrogenesis of a corundum metagabbro complex in central Shikoku (in bearing marble at Hunza (Kashmir) . Mineral, Margarite-bearing metagabbro from the Iratsu mass 253

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Velde, B. (1971), The stability and natural occur pyroxenes rock from the Iratsu epidote am rence of margarite. Mineral. Mag., 38, 317- phibolite mass in central Shikoku. Geol. Soc. 323. Japan, 81, 29-37.

四国中央部三波川 帯 ・五良津岩体 中のマーガ ライ トを含む岩石

榎 並 正 樹

四国 中央 部 三波 川 帯 中 に 位 置 す る五 良 津 緑 レ ン石 角 閃 岩 体 か ら マ ーガ ラ イ トを含 む岩 石 が見 い だ され た 。この マ ー ガ ラ イ ト は,パ ラ ゴ ナ イ ト成 分 を 最 大27mole%含 み,そ の 格 子 定 数 はa0=5.11 A, b0=8.79 A, c0=19.15

A, β=95.3° で あ る 。 マー ガ ラ イ ト は ゾ イ サ イ ト,パ ラ ゴ ナ イ ト,藍晶 石,緑 泥 石,石 英,粘 土鉱 物(カ オ リ ナ イ トま た は デ ィ ツ カ イ 卦)と 共 存 し て い る 。 こ れ ら の 鉱 物 の す べ て が 互 い に 平 衡 で あ る わ け で は な く,少 な く と

も次 の2つ の 平 衡 時 期 が 識 別 で き る 。 1):ゾ ィ サ イ ト+藍 晶 石+Caに 乏 し い パ ラ ゴ ナ イ ト 2): Caに 富 む パ ラ ゴ ナ イ ト+マ ー ガ ラ イ ト 1)の 鉱 物 組 合 せ は,三 波 川 累 進 変 成 作 用 に と も な う緑 レ ン 石 角 閃 岩 相 の 条 件 下 で,原 岩 の 斜 長 石 の 分 解 に

よ っ て 形 成 さ れ た 。 他 方, 2)の 鉱 物組 合 せ は 後 退 変 成 作 用 時 に, 1)の 鉱 物 組 合 せ の 分 解 に よ っ て 形 成 さ れ た 。