J. Japan. Assoc. Min. Petr. Jcon. Geol. 80, 515-525, 1985

Margarite-paragonite-muscovite assemblages from the low-grade metapelites of the Tono metamorphic aureole, Kitakami Mountains, Northeast Japan

YASUKOOKUYAMA-KUSUNOSE Fuel GeologyDepartment, Geological Survey of Japan Ibaraki,305, Japan.

Margarite occurs in the low-grade zone of the contact aureole of the Tono granitic mass, Kitakami Mountains, Northeast Japan, coexisting with muscovite, paragonite, chlorite and quartz, but without carbonates and feldspars. The margarite is produced by the reaction between pyrophyllite and zoisite in the progressive metamorphism at a temperature near the upper stability limit of the assemblage margarite +quartz. The solubility of muscovite component in paragonite in the three white micas is suppressed as compared to that in the muscovite-paragonite assem blage, whereas the paragonite solubility in muscovite of the three-phase assemblage is similar to that of the two-phase assemblage. Paragonite component in margarite in three-phase assem blage of this area is about 17.3-22.5 mol% ; this value is the smallest for the margarite in three phase assemblages so far reported. The low paragonite solubility of margarite in this area is probably due to the low pressure condition of the contact metamorphism of the Tono area.

ragonite (Ackermand and Morteani, 1973; Introduction Enami, 1980; Selverstone et al., 1984) and with Margarite, which usually occurs in emery muscovite (Jones, 1971; Guidotti and Cheney, deposit (Aoki and Shimada, 1965) and marble 1976; Frey, 1978). However, the occurrence (Jones, 1971 ; Okrush et al., 1976), has recently of the assemblage of three white micas, which been described from various metamorphic envi is the low temperature analogue of plagioclase ronments as reviewed by Frey et al. (1982). alkali feldspar-A12SiO5-quartz assemblage, is Many of margarite are of retrograde origin scarce (Hock, 1974 ; Guidotti et al., 1979). such as pseudomorphs after A12SiO5 minerals Compositions of coexisting two white micas (Guidotti and Cheney, 1976; Guidotti et al., suggest that there is an extensive solid solution 1979; Atsumi, 1984). However, the formation on the margarite-paragonite join whereas mar of margarite clearly defines an isograd in some garite and muscovite are practically immisci regional metamorphic terrain (Frey et al., ble. However, little is known on the mis 1982). The occurrence of margarite is impor cibility relations in the three-phase system. tant because its stability field is restricted to This paper reports the mode of occurrence low temperature and relatively high aH:o, espe of margarite in the contact aureole of the Tono cially in silica-saturated compositions (Storre granitic mass, the Kitakami Mountains, North and Nitsch, 1974 ; Chatterjee, 1974a, 1976; Per east Japan, and discusses the miscibility rela kins et al., 1980). tions in coexisting margarite, paragonite and Margarite frequently occurs with pa- muscovite.

(Manuscript received, August 1, 1985; accepted for publication, October 24, 1985) 516 Yasuko Okuyama-Kusunose

Geological outline The South Kitakami Mountains is mainly occupied by the late Paleozoic sedimentary sequence, which was intruded by numerous granitic bodies of Cretaceous age. The Tono granitic mass, measuring about 40km across, is chiefly composed of hornblende-biotite granodiorite with a biotite K-Ar age of 117-120 m.y. (Kawano and Ueda, 1965). The contact aureole of the Tono mass is 3-7 km wide. In the Miyamori-Ohazama district situated in the northwest of the aureole, Okuyama (1979, 1980) has recognized three mineral zones of progressive metamorphism, namely, the chlo rite, andalusite and sillimanite zones (Fig. 1). The chlorite zone, which develops widely in the Fig. 1. Mineral zone map of progressive contact metamorphism in the Miyamori-Ohazama study area, is further divided into two subzones district (after Okuyama, 1980) with local by the presence or absence of chloritoid in the ities of margarite-bearing rocks. Al-rich metapelites with molar XFeo of bulk compositions more than 0.79 (Okuyama, 1980). 09-05 and 14-01) (Fig. 1). The assemblage pyrophyllite+Fe-rich chlorite Sample 14-01 is a fine-grained metapelite is stable instead of chloritoid+Fe-poor chlorite with greenish gray color and shows weak slaty in the lower chlorite zone; cleavage. It has the following assemblage; margarite+paragonite+muscovite+chlorite+ 3Fe7.2Mg1.8Al6Si5020(OH)16+3A14Si8020(OH), Fe-rich chlorite pyrophyllite zoisite+quartz+pyrrhotite+sphene+ =9FeA12Si05(OH)2+2Fe6.3Mg2.7Al6Si5O20(OH)16 apatite+carbonaceous matter. Under the chloritoid Fe-poor chlorite microscope, margarite, paragonite, muscovite +20 Si02+5 H2O (1) and chlorite occurs as lath-shaped crystals of The andalusite zone is defined by the appear 5-30 gin in width. The sample shows micro scopic banding structure in which quartzose ance of andalusite formed by the decomposition of paragonite+quartz in the chlorite zone; and micaceous thin laminae are interbedded with each other. Although the proportion of paragonite+quartz=andalusite+albite+H20 the constituent minerals are different in individ (2) ual laminae, three white micas form crowded The transition of andalusite to sillimanite intergrowths with each other. White micas defines the sillimanite zone. The mineral para and chlorite generally show weak preferred geneses show the low pressure condition of orientation. Margarite frequently shows a metamorphism at most 2 kbar (PH2O=Ptotal). characteristic sheaf-like habit (Fig. 2A). Chlorite associated with white micas is pale Petrography gray ripidolite (Hey, 1954) (Table 1). Zoisite Two margarite-bearing rocks were dis occurs as fine-grained granular crystals disper covered from the higher chlorite zone (samples sed among white micas and chlorite. It has Margarite-paragonite-muscovite assemblages 517

Fig. 2. Photomicrographs of margarite and associated minerals. A) Sheaf-like margarite (high relief) showing weak preferred orientation in sample 14-01. Plane-polarized. B) The association of margarite (Ma), paragonite (Pa) and muscovite (Ms) in chloritoid (Ctd)-bearing sample 09-05. Some analyzed points are also shown. Plane-polarized.

Table 1. Microprobe analyses of chlorite, chlori Table 2. Unit cell parameters of margarite in toid and zoisite in margarite-bearing ample 09-05 meptanplifps

gonite+muscovite+chlorite+chloritoid+

quartz+pyrrhotite+sphene+apatite+tourma line+carbonaceous matter. Its matrix mainly

consists of fine-grained margarite, paragonite,

muscovite and chlorite. These minerals some

times form platy crystals about 50-100ƒÊm (Fig.

2B). Sheaf-like margarite is also observed.

Both fine-grained and platy chlorites are

brownish gray ripidolite (Table 1). Chloritoid

occurs as isolated prismatic crystals or as

rosettes of radiating crystals. It is poor in Mn

and Mg and lacks noticeable chemical zoning

(Table 1). The presence of margarite was also confirmed by the X-ray powder diffraction.

Numbers in the parentheses are those of grains Cell parameters were determined for a marga analyzed. rite condensate from sample 09-05 (Table 2). The values agree well with those of Guggen low Mn content and Fe/(Fe+Al) ratio (0.02 heim and Baily (1975) that were obtained for -0.03) (Table 1). well-characterized sample containing compa Sample 09-05 is a black spotted slate with rable paragonite component. numerous chloritoid porphyroblasts. It has the following assemblage ; margarite+para- 518 Yasuko Okuyama-Kusunose

Table 3. Microprobe analyses of coexist Chemistry of white micas ing white micas

White micas as well as chlorite, chloritoid

and zoisite were chemically analyzed using

Hitachi electron microprobe X-560S of Tohoku

University with an energy dispersive X-ray

analyzing system. The analytical method fol

lows Fujimaki and Aoki (1980). White micas

were frequently too fine-grained to obtain reli

able data. Compositions listed in Table 3 are

the average of spot analyses of relatively large

platy grains of margarite, paragonite and mus covite.

Although minor Fe, Mg and Ti are neglect

ed, all the mica analyses show slight deviations

from the ideal white micas. Step analyses of

margarite and paragonite were performed at

5ƒÊm interval, but no interlayering was detect

ed.

The amounts of Na in margarites are

equivalent to 17.3-22.5 mol% of paragonite

solid solution. Total numbers of cations in the

octahedral site show small deviation from the

ideal ones. This, and the amount of excess Si Numbers in the parentheses are those of as well as the minor contents of Fe and Mg grains analyzed. indicate that the contribution of other Na mica

components (e.g., Frey et al., 1982; Grew and

Sandiford, 1984) is negligible to the Na content Bulk rock chemistry

of margarite in this locality. K in margarite Wet chemical analyses of the two rock and Ca in muscovite are very low (less than 0.01 specimens described above are given in Table 4 atom, on the basis of 22 oxygens), showing together with the average chemical composi negligible solid solution with each other. tion of chlorite zone metapelites containing

Margarite and muscovite components in pa only muscovite and paragonite as white micas ragonite are 3.2-5.0 mol% and 3.8-6.9 mol%, (Okuyama, 1980). Both types of metapelites respectively. Paragonite solid solution in mus show following chemical characteristics as covite reaches 11.8-14.3 mol%. Small chemi compared to the averaged compositions of cal variation in the compositions of these micas geosynclinal pelitic rocks (Shaw, 1956 Miya suggests that the chemical heterogeneity is not shiro and Haramura, 1962) ; high in Al203 con conspicuous. tent, ignition loss, molar Na20/(Na20+K20)

The amounts of the other elements are ratio, and molar XFeo, and low in Si02, Na20, very low. Although Mg and Fe tend to concen K20 contents. The chloritoid-bearing sample trate in muscovite (Table 3), the amount of 05-09 is especially high in A1203 content and celadonite molecule does not exceed 15 mol%. XFeo. The principal characteristic of marga Margarite-paragonite-muscovite assemblages 519 rite-bearing rocks is their high CaO content. not a crystalline graphite in the chlorite zone Alkali oxides contents in margarite-bearing (Okuyama, 1980) and probably includes hydro rocks are lower than those in metapelites of the gen and oxygen. Therefore, the margarite average chlorite zone, which may also be bearing rocks can be treated in the system favourable in the formation of margarite. A1203-FeO-MgO-CaO-Na2O-C-H,0-02 with excess of quartz, muscovite, sphene, carbona Discussion ceous matter and fluid. We further make Formation of margarite in the Tono contact following two assumptions that FeO and MgO aureole are treated as a single component (Fe, Mg) 0 Margarite-bearing rocks in the Tono con and that the fluid in calcic metapelites are H2O tact aureole do not show textures that suggest dominant. The above eight-component vol retrograde mineralization. The margarite in ume can be projected to the A1203-(Fe, Mg)O- the study area is a prograde mineral in the CaO-Na2O tetrahedron under these assump higher chlorite zone. tions. The phases considered here in this In the lower chlorite zone of the study area, system are albite NaA1Si3O3, calcite CaCO3, calcite and zoisite occur sporadically with chlorite (F e, M g)3 AL Si, 0,0 (0,H)10, chlo quartz, muscovite, pyrite or pyrrhotite and ritoid (Fe, M g) A126SiO5(OH)2, margarite carbonaceous matter in the following assem Ca2Al8Si,020(OH),, paragonite Na2Al0Si6020(OH)4, blages; pyrophyllite Al,Si8020(OH),, and zoisite chlorite+albite+calcite+zoisite Ca2A13S13O12(OH). The treatment of FeO and chlorite+paragonite+calcite MgO as a single component can be justified chlorite+paragonite+calcite+albite because we need not consider the effect of Fe chlorite+paragonite+zoisite Mg substitution to the paragenetic relation of chlorite+paragonite+zoisite+albite white micas. The other assumption is not chlorite+paragonite+zoisite+ adequate to the calcite-bearing assemblages pyrophyllite because relatively high C02/H2O ratios are Probably, they are the candidates of the low expected in the coexisting fluid. However, this temperature equivalents of the margarite-bear does not influence the present discussion ing assemblages. because, as is shown below, the margarite in the These metapelites always contain quartz, study is not introduced through reactions sphene, muscovite and carbonaceous matter. involving calcite. The presence of carbonaceous matter and The mineral parageneses of calcic pyrrhotite suggests a low f o, condition of metapelites in the chlorite zone of the study metamorphism. The Fe2O3 in zoisite, and area is schematically illustrated in Fig. 3. The MnO in zoisite and chloritoid are very low and margarite-forming devolatilization reactions can be ignored. The coexistence of hydrous that are possible within the parageneses of the silicates with calcite and carbonaceous matter lower chlorite zone (Fig. 3A) are as follows; suggests the presence of mixed fluid in which H2O, CO, and CH, are the principal constitu 5 pyrophyllite+4 zoisite ents. CH, probably behave as an inert dilutant =4 margarite+36 quartz+4 H2O (3) and are not important in the discussion of 2 paragonite+2 calcite+4 quartz =margarite+4 albite+2 H20+2 C02 (4) paragenetic relations. Carbonaceous matter is assumed here to be a pure carbon, although it is 5 paragonite+4 zoisite+4 quartz 520 Yasuko Okuyama-Kusunose

Fig. 3. Paragenetic relations in the calcic metapelites in the lower chlorite zone of the study area (A) and in the margarite-bearing rocks of the higher chlorite zone (B). Mineral abbreviations: Ab, albite; Cal, calcite; Chl, chlorite; Ctd, chloritoid; Pyr, pyrophyllite; Zo, zoisite.

Table 4. Chemical analyses of margarite =4 margarite+10 albite+4 HO (5) bearing metapelites (1 and 2) and average chemical composition of Comparing the right-hand sides of these reac margarite-free metapelites in the tions with the mineral parageneses in Fig. 3B, chlorite zone (3) we see that only reaction (3) involves reactant and product assemblages consistent with the natural occurrences in the Tono aureole. Con sequently, the margarite-bearing assemblages are formed from the chlorite+paragonite+ zoisite+pyrophyllite assemblase. Fig. 3 shows that this assemblage is isochemical with two margarite-bearing assemblages. The figure also shows that the assemblage marga rite+paragonite+chlorite+chloritoid is more aluminous than the assemblage margarite+ paragonite+chlorite+zoisite, which is consis tent with the bulk chemistry of the specimens * Total Fe as FeO. containing each paragenesis (Table 4). The Al ** XF eO=FeO*, mol%/(MgO, mol% + FeO*, mol%). content of chlorite in sample 09-05 is higher than that in sample 14-01 reflecting the aluminous bulk composition. However, the compositions of margarite, paragonite and muscovite (cf. Table 3) are not affected by the Temperature of equilibrium difference in coexisting phases. Note that the The reaction (4) has not yet been examined high XFeo as well as the high A1203 content is experimentally. Frey (1978) indicated graphi also essential for the calcic metapelites to have cally that in the system Al203- chloritoid, as in the case of Ca-poor metapelites FeO-CaO-Na20-CO2-H20 (plus excess mus (Okuyama, 1980). covite, quartz and fluid) reaction (4) defines a Margarite-paragonite-muscovite assemblages 521 low temperature stability limit of margarite rite zone of the Tono contact aureole . Natural at a very low Xco2 condition in the low pres margarite is a solid solution containing other sure greenschist facies. He also showed that mica components, especially paragonite com this reaction takes place at slightly lower ponent (Table 3), and it probably affects the temperature than the reaction (1)' that produces stability of margarite. However, the extent of chloritoid; paragonite solid solution in margarite is limited in the study area as is discussed below . 3 pyrophyllite+2 Fe-chlorite =9 chloritoid+20 quartz+5 H20 (1)' Miscibility relations in the three white micas

Hoschek (1969) investigated this Fe-endmem Coexisting three white micas has also been

ber reaction and indicated that the temperature reported from the Tauern Window, Austria of equilibrium is about 4201C at 2 kbar PH2O. (Hock, 1974) and from the Georgetown area, The chloritoid-producing reaction in the study California (Guidotti et al., 1979). Three white area (reaction (1)) probably occurs at higher micas in the Tauern Window occur in a pelitic temperature than the reaction (1) due to the schist of the greenschist facies of the Alpine contribution of the Mg-chlorite molecule in metamorphism in the following assemblage; chlorite. Therefore, the temperature of equi margarite+paragonite+phengitic muscovite+ librium of the assemblage chloritoid+ chlo chlorite+chloritoid+zoisite+calcite+ rite+margarite+paragonite (sample 09-05) is dolomite+quartz. Those in the Georgetown slightly higher than 4201C. A margarite-free area occur in the matrix of retrograded metapelite (sample Nak 154) occuring close to metapelites and pseudomorphs after andalusite. the locality of sample 14-01 contains coexisting Chemical compositions of coexisting three

muscovite and paragonite with XK(=K/(K+ white micas, including those in the Tono aure Na)) 0.86 and 0.06, respectively (Okuyama ole, are compiled in Fig. 4.

Kusunose, in prep.). These compositions give

temperature about 4501C at PH,o=2 kbar (Eug

ster et al., 1972). Therefore, the recrys

tallization temperature of the assemblage mar

garite+paragonite+muscovite+chlorite+ zoisite (sample 14-01) is presumed to be around

450•Ž. The temperature suggests that marga rite in the Tono aureole was recrystallized near the upper stability limit of margarite+quartz at 2 kbar PH2O (Chatterjee, 1976; Perkins et al.,

1980). Chatterjee (1976) and Perkins et al. •\•\ T ono ltnts sway, (1980) have also discussed that the breakdown •\•\ Tauern Window (Hock, 1974) of the assemblage of margarite+quartz takes -¥-¥- Georgetown area(Guidotti et al., 1979) place at lower temperatures than that of para Fig. 4. Compositions of coexisting margarite-pa gonite+quartz. The latter reaction defines the ragonite-muscovite in terms of end-mem andalusite isograd in the Tono aureole as noted ber white micas. Bars on the muscovite before. These stability relations in end paragonite join indicate compositions of coexisting muscovite and paragonite in member white micas are in harmony with the margarite-free metapelites of the higher occurrence of prograde margarite in the chlo hlorite zone of the Tono contact aureole. 522 Yasuko Okuyama-Kusunose

All the three-phase assemblages show among three localities. Three white micas in almost immiscible relation on the margarite the Georgetown area show slightly larger muscovite join and limited extent of solid solu extents of solid solution as compared to those tion on the margarite-paragonite and mus in the Tono aureole. In the Tauern Window, covite-paragonite joins. Paragonite in three however, the amount of paragonite component phase assemblages tends to have lesser amount in margarite is larger (about 29 mol%) whereas of margarite component than that of muscovite that in muscovite is smaller (about 9 mol%) component. Chemical compositions of than those in the Tono aureole and Georgetown coexisting muscovite and paragonite in the area. The low paragonite content in Alpine higher chlorite zone of the Tono aureole are muscovite may be attributed to its high also shown in Fig. 4. The muscovite compo celadonite content (Enami, 1983; Grambling, nent in paragonite is slightly low in the three 1984). However, compositions of the other phase assemblages as compared to those in white micas considered here are relatively pure muscovite-paragonite assemblages. Enami with respect to the components other than the (1980) also showed in the Iratsu amphibolite end-member muscovite, paragonite and mar mass of the high pressure type Sanbagawa garite. It is highly probable that the mis metamorphic belt that the muscovite compo cibility relation in Fig. 4 manifests the nent in paragonite is lower in margarite-bear difference in intensive parameter of metamor ing rocks than in margarite-free ones. The phism amomg three localities. paragonite solubility of muscovites in the study Enami(1980) compiled the compositions of area is not so different between the three-phase coexisting margarite and paragonite from the and muscovite-paragonite assemblages (Fig. 4). Iratsu amphibolite mass, Tauern Window The extent of solid solution is different (Hock, 1974) and Greiner Schiefer series (Ack

Fig. 5. Irregular cell volume-composition relation of natural margarites. Mixing line joins the synthetic margarite (1, Chatterjee, 1974a) and paragonite (Chatterjee, 1974b). Volume data are from: 2, Aoki and Shimada (1965); 3, sample H-148/70 of Hock (1974); 4, Guggenheim and Baily (1975); 5, Guidotti and Cheney (1976); 6, Guidotti et al. (1979); 7, Enami (1980) and 8, present study. Chemical compositions of these margarites indicates that they contain very little amounts of mica components other than margarite, paragonite and muscovite. Margarite-paragonite-muscovite assemblages 523

ermand and Morteani, 1973). He pointed out complex volume-composition relation on the that the compositional gap on the margarite margarite-paragonite join. paragonite join lies between 0.51 and 0.84XNa(= Na/(K+Na+Ca)) in the low grade amphibolite Acknowledgements: I am greatly indebted facies and between 0.27-0.29 and 0.90-0.91 XNa to Professor K. Aoki of Tohoku University for in the greenschist facies. The paragonite con allowing me to use the electron microprobe tent of margarite in the Tono contact aureole is facility. Assistance from Dr. T. Yoshida and low in comparison with the value inferred from Mr. M. Nakagawa of Tohoku University, and this solvus, if we consider its temperature of Dr. M. Murata of Rigaku Denki Co. Ltd. is also equilibrium discussed before. Note that the grateful. Thanks are due to the staff of the miscibility relation proposed by Enami (1980)is workshop section of the Geological Survey of based on the coexisting margarite and pa Japan for the quality of polished thin sections. ragonite from the high pressure metamorphic Valuable suggestions by Dr. T. Nakajima of the terrains. This strongly suggests that the high Geological Survey of Japan for the early draft pressure condition is favourable in enlarging of the manuscript is appreciated. I also grate the solubility limit of margarite toward pa fully acknowledge Professor S. Banno of Kyoto ragonite. Although the pressure effect on pa University and Professor H. Onuki of Hiro ragonite composition is not clear, the low pa saki University for their constructive criticism ragonite content of Tono margarite is probably which greatly improved a more final version of due to the low pressure condition at this local the manuscript. ity. The pressure dependence of the margarite References limb of margarite-paragonite solvus inferred Ackermand, D., and Morteani, G. (1973), Occurrence from the natural assemblages is consistent with and breakdown of paragonite and margarite in the occurrence of strongly sodic margarite Greiner Schiefer series (Zillerthal Alps, Tyrol). Contrib. Mineral. Petrol., 40, 293-304. formed in the retrograde stage of some high Aoki, T. and Shimada, N. (1965), Margarite from pressure metamorphic terrains (Grew and San the Shin-kiura mine, Oita Prefecture. J. Min diford, 1984 ; Selverstone et al., 1984). Mar eral. Soc. Japan, 7, 87-93. (in Japanese) Atsumi, H. (1984), Finding of margarite in the garite composition can be an important clue to Ryoke metamorphic rocks from the Kansa understand the detailed P-T paths of metamor gawa area, Aichi Prefecture. J. Geol. Soc. phism. Japan, 90, 505-508. (in Japanese) The pressure effect on margarite-parago Chatterjee, N. D. (1974a), Synthesis and upper ther mal stability limit of 2M-margarite. Schweiz. nite solid solution can be assessed from the Mineral. Petrogr. Mitt., 54, 753-767. volume - composition relation (cf. Waldbaum Chatterjee, N. D. (1974b), X-ray powder pattern and and Thompson, 1968). However, the available molar volume of synthetic 2M-paragonite. volume data of sodic margarites scatter around Contrib. Mineral. Petrol., 43, 25-78. Chatterjee, N.D. (1976), Margarite stability and the ideal mixing line between synthetic marga compatibility relation in the system CaO- rite and paragonite (Fig. 5). Guggenheim and A1203-Si02-H2O as a pressure-temperature Baily (1975) found that some of the natural indication. Amer. Mineral., 61, 699-709. sodic margarite has tetrahedral layers with Al Enami, M. (1980), Notes on petrography and rock forming mineralogy (8) Margarite-bearing Si ordering contrary to the other 2M, white metagabbro from the Iratsu mass in the San micas. The possible volume change due to the bagawa belt, central Shikoku. J. Japan. Assoc. Al-Si ordering should be noted in relation to the Min. Petr. Econ. Geol. 75, 245-253. 524 Yasuko Okuyama-Kusunose

Enami, M. (1983), Petrology of pelitic schists of the rol., 22, 208-232. oligoclase-biotite zone of the Sanbagawa Jones, J.W. (1971),Zoned margarite from the Bad metamorphic terrain, Japan; phase equilibria shot formation (Cambria) near Kaslo, British in the highest grade zone of a high-pressure Columbia. Can. J. Earth Sci., 8, 1145-1147. intermediate type of metamorphism. J. Kawano, Y. and Ueda, Y. (1965),K-Ar dating on Metamor. Geol., 1, 141-161. he igneous rocks in Japan II. Granitic rocks in Eugster, H.P., Albee, A.L., Bence, A.E., Thompson, Kitakami Massif. Sci. Rep., Tohoku Univ., J. B., Jr. and Waldbaum, D.R. (1972), The two Sera III, 9, 179-215. phase region and excess mixing properties of Miyashiro, A. and Haramura, H. (1962),Chemical paragonite-muscovite crystalline solutions. J. composition of Paleozoic slates IV. Zonal dis Petrol., 13, 147-179. tribution of geosynclinal sediments and the Frey, M. (1978), Progressive low-grade metamor position of regional metamorphic belts. J. phism of a black slate formation, central Swiss eol. Soc. Japan, 68, 75-82. (in Japanese with Alps, with special reference to pyrophyllite and English abstract) margarite bearing assemblages. J. Petrol., 19, Okrush, M., Bunch, T.E. and Bank, H. (1976),Para 95-135. genesis and petrogenesis of a corundum-bear Frey, M., Bucher, K., Frank, E. and Schwander, H. ing marble at Hunza (Kashimir). Mineral. (1982), Margarite in the Central Alps. Schweiz. Deposita,11, 278-297. Mineral. Petrogr. Mitt., 62, 21-45. Okuyama, Y. (1979), Tono contact metamorphic Fujimaki, H. and Aoki, K. (1980), Quantitative ureole in the northwestern Miyamori district, microanalyses of silicates, oxides and sulfides Iwate Prefecture, with special reference to using an energy-dispersive type electron probe. low-grade metapelites. M.Sc. thesis, Tohoku Sci. Rep., Tohoku Univ., Ser. III, 14, 261-268. Univ., Sendai. (in Japanese) Grambling, J. A. (1984), Coexisting paragonite and Okuyama, Y. (1980),Low-grade metapelites in the quartz in sillimanitic rocks from New Mexico. contact metamorphic aureole around the Tono Amer. Mineral., 69, 79-87. granodiorite pluton, Miyamori-Ohazama dis Grew, E. S. and Sandiford, M. (1984), A staurolite trict, Kitakami Mountains. J. Japan. Assoc. talc assemblage in tourmaline-phlogopite Min. Petr. Econ. Geol., 75,359-371.(in Japanese chlorite schist from northern Victoria Land, with English abstract) Antarctica, and its petrologic significance. Perkins, D. III, Westrum, E. F. and Essene, E.J. Contrib. Mineral. Petrol., 87, 337-350. (1980), The thermodynamic properties and Guggenheim, S. and Baily, S.W. (1975), Refinement phase relations of some minerals in the system of margarite structure in subgroup symmetry. CaO-A12O3-SiO2-H2O.Geochim. Cosmochim. Amer. Mineral., 60, 1023-1029. Acta, 44, 61-84. Guidotti, C.V. and Cheney, J. (1976), Margarite Selverstone, J., Spear, F.S., Franz, G. and Morteani, pseudomorphs after chiastolite in the Rangeley G. (1984),High-pressure metamorphism in the area, Maine. Amer. Mineral., 61, 431-434. SW Tauern Window, Austria; P-T paths from Guidotti, C.V., Post, J. L. and Cheney, J. T. (1979), hornblende-kyanite-staurolite schists. J. Pet Margarite pseudomorph after chiastolite in the rol., 25, 501-531. Georgetown area, California. Amer. Mineral., Shaw, D.M. (1956),Geochemistry of pelitic rocks 64, 728-732. III. Major elements and general geochemistry. Hey, M. H. (1954), A new review of the chlorites. Bull. Geol. Soc. Amer., 67, 919-934. Mineral. Magazine, 30, 277-292. Storre, B. and Nitch, K.H. (1974),Zur stabilit2t von Hock, V. (1974), Coexisting phengite, paragonite Margarit im System CaO-A12O3-SiO2-H2O. and margarite in metasediments of the Mittlere Contrib. Mineral. Petrol., 43, 1-24. Hohe Tauern, Austria. Contrib. Mineral. Pet Waldbaum, D.R. and Thompson, J. B., Jr. (1968), rol., 43, 261-273. Mixing properties of sanidine crystallineesolu Hoschek, G. (1969), The stability of staurolite and tions II. Calculations based on volume data. chloritoid and their significance in metamor Amer. Mineral., 53, 2000-2017. phism of pelitic rocks. Contrib. Mineral. Pet- Margarite-paragonite-muscovite assemblages 525

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北 上 山 地,遠 野 花 山 質 岩 体 の接 触 変 成 帯 の 低 温 部 で は,マ ー ガ ラ イ トが,マ ス コバ イ ト・パラ ゴ ナ イ ト・ 緑 泥 石 ・石英 と共 存 して 産 出 す る 。 こ こで は マ ー ガ ラ イ トと炭 酸 塩 鉱 物 及 び長 石 類 と の共 存 は 認 め られ な い 。 マ ー ガ ラ イ トは パ イロ フイ ラ イ トと ゾ イ サ イ トの反 応 に よ り累 進 的 に 生 成 し,生 成 温 度 は マ ー ガ ラ イ ト+石 英 組 合 せ の 高 温 限 界 に 近 か っ た 。 白 色 雲 母3相 共 存 系 で の パ ラ ゴ ナ イ トのマ ス コバ イ ト成分 固 溶 量 は,類 似 の条 件 に お い て マ ス コバ イ ト との み 共 存 す る パ ラ ゴ ナ イ トでの そ れ よ り低 い。 しか し,マ ス コバ イ ト中の パ ラ ゴ ナ イ ト成 分 固 溶 量 は 両 者 の 間 で 著 し くは 異 な らな い。 この 地 域 の 白色 雲 母3相 共 存 系 で の マ ー ガ ラ イ トのパ ラ ゴ ナ イ ト成分 固 溶 量 は17.3～22.5mol%で,こ れ まで 他 地 域 か ら報 告 さ れ た もの よ り 小 さ い。 接 触 変 成 作 用 の 低 圧 条 件 が マ ー ガ ラ イ トの パ ラ ゴ ナ イ ト成分 固 溶 量 を 規 制 して い る可 能 性 が あ る。