J. Min. Petr. Econ. Geol. 88, 141-156, 1993

Mineralogy of piemontite-bearing schist in the Yamagami

metamorphic rocks of northeastern Abukuma Plateau

Masahide AKASAKA*, Jun WATANABE**,

Kenji TOGARI** and Makoto KAWAMURA**

* Department of Geology , Faculty of Science, Shimane University,

1060 Nishikawatsu, Matsue 690, Japan ** Department of Geology and Mineralogy , Faculty of Science,

Hokkaido University, Sapporo 060, Japan

Piemontite-bearing schist (Pm-schist) in the Yamagami metamorphic rocks, northeastern Abukuma Plateau, was studied in terms of bulk chemical composition, mineral assemblage and

mineral chemistry.

The Pm-schist occurs as intercalating member of the albite-porphyroblastic muscovite

quartz schist and epidote-amphibolite. Petrographic features suggest that the primary mineral assemblage is quartz+muscovite+albite+chlorite+piemontite+tourmaline+hematite+rutile. Apatite, epidote and carbonate also occur as minor minerals. Veins formed of euhedral quartz

and anhedral K-feldspar intruded into microgranoblastic quartz aggregation.

The compositions of piemontite range between Pm11Ps22Cz67 and Pm17Ps18Cz65. Muscovite is

phengitic (Si 6.54-6.64 atoms per 22 oxygens) and contains moderate amount of hydromuscovite component. Chlorites are clinochlore with Fe/(Mg+Fe)=0.02 and pycnochlorite with Fe/(Mg+

Fe)=0.38. Hematite contains Mn2O3 component. The bulk chemical composition of the Pm-schist is characterized by the high silica content,

82.41 wt.%, and high oxidation ratio, (Fe3+•~100)/(Fe3++Fe2+)=93. MnO content is very low at 0.

02 wt.%.

Appearance of piemontite, Mg-chlorite and hematite containing Mn2O3 component suggests

that fo2 condition during the metamorphism was very high, which is consistent with the extremely high oxidation ratio of the bulk composition. The extremely high fo2 condition during metamor

phism may be explained by the dissociation equilibrium of H2O with H2 and O2.

Keywords: Piemontite, Piemontite-bearing schist, Spotted schist, Yamagami metamorphic

rock, Glaucophane schist facies

and structure of the YMR and found the . Introduction piemontite-bearing schist (Pm-schist) in the Yamagami metamorphic rocks (YMR) are YMR, although the locality and detail on mine distributed along a shear zone of northeastern rals of the Pm-schist were not described. In Abukuma plateau together with Wareyama, 1975, one of the authors (M. K.) and co-workers Matsugadaira and Yaguki metamorphic rocks. also found the Pm-schist from the northern Kuroda and Ogura (1960, 1963) revealed that the part of the River Udagawa during their study of YMR are composed of epidote-amphibolite, Paleozoic formations. Another occurrence of epidote-actinolite schists and spotted schists the Pm-schist was confirmed in the southern and that they belong to glaucophane schist branch of the River Udagawa by another of facies. Iwamatsu (1975) studied stratigraphy authors (J. W.) and co-workers. Preliminary

(Manuscript received, July 10, 1992; accepted for publication, February 18, 1993)

I 142 Masahide AKASAKA,Jun WATANABE, Kenji TOGARI and Makoto KAWAMURA description of the Pm-schist was reported by Sheared Granite). In the River Manogawa

Togari et at. (1986). Maekawa (1988) district, very small masses of epidote-am summarized the main mineral assemblage of phibolite and sheared granite are also distribut

metapelites, metapsammites and metachert in ed.

the YMR to be muscovite +garnet+quartz+ The Matsugadaira metamorphic rocks albite•}piemontite, but the occurrence of (MMR), composed of pelitic, siliceous, psam

piemontite was not shown. mitic and basic semischists, are overlain by the In this paper, we report the occurrence, Upper Devonian Ainosawa Formation with

bulk chemical composition and mineralogy of supposed unconformity (Sato, 1974). Thus

the Yamagami Pm-schist. MMR are generally interpreted to be pre - Devonian in age as well as the YMR. Geologi II. Geological setting cal relationship between the MMR and the

The study area is situated in the northern YMR is not clear, but it may be considered to

most part of the Abukuma Eastern Marginal be in tectonic contact.

Tectonic Belt (Hunahashi, 1979). This is Paleozoic strata of this area consist mainly

bounded on both sides by NNW-SSE trending of clastic facies. The Paleozoic and older

Futaba and Hatagawa sheared zones. This rocks are intruded by Early Cretaceous granitic

belt consists of metamorphic rocks, un plutonics. metamorphosed Paleozoic strata (Devonian-

Permian) and Early Cretaceous igneous rocks. III. Occurrence of piemontite-bearing schist

Formation of the belt is generally assigned to The geologic map around the studied area

Early Cretaceous. However, NE-SW trending is shown in Fig. 1. The Pm-schist occurs at

older tectonic components are recognized in the two localities; (1) upper stream of the River

belt (Watanabe et at., 1983a, 1983b), suggesting Maebarazawa, a southern branch of the River

that the tectonics of the belt is more compli Udagawa, and (2) middle stream of the River

cated than previously considered. Togamorizawa, a northern branch of the River

Although the stratigraphic relation of the Udagawa. In the first locality, the Pm-schist

YMR is not clear because of the sporadic distri occurs as intercalated member of muscovite bution of outcrops, Kuroda and Ogura (1963) quartz schist around a quarry and the Fu stated that epidote-amphibolites, spotted green kuyamagami mine (cupriferous iron sulfide ore

schists and spotted pelitic schists can be consid deposit). The muscovite-quartz schist in this

ered to be arranged from the lower to the area contains albite-porphyroblast, and is so

upper. According to Iwamatsu (1975), the called spotted schist. The epidote-amphibolite

lower member of the YMR is mainly composed near the muscovite-quartz schist also contains of fine-grained epidote-amphibolite intercalat plagioclase porphyroblasts. The occurrence of ing with thin layers of quartz-schist, the middle the plagioclase porphyloblast-epidote am member coarse-grained epidote-amphibolite phibolite is described in Watanabe et at. (1979).

alternated with muscovite-quartz schist, and Relations between epidote-amphibolite, mus

the upper member muscovite-quartz schist covite-quartz schist and Pm-schist, are not

accompanied with Pm-schist. In the eastern clear because of strong microfolding and

part of the River Udagawa district, the epidote ptygmatic folding. In the locality (2), the Pm amphibolites are typically exposed accompany -schist occurs as intercalated member of the

ing small masses of sheared granite (Yamagami spotted muscovite-quartz schist. These Mineralogy of piemontite-bearing schist in the Yamagami metamorphic rocks 143

Fig. 1. Geological map of the Soma district. *Area marked by ellipse represents the region of spotted schist and plagioclase porphyroblastic amphibolite.

schists in the locality (2) occur as very thin sis.IV layers between epidote-amphibolite and Yamagami Sheared Granite. . Petrography In the present study, only the sample from The Pm-schist is pale green to dark green the locality (1) was available for the miner ish in color, and contains an amount of quartz, alogical investigation and bulk chemical analy muscovite and albite prophyroblasts. Along 144 Masahide AKASAKA,Jun WATANABE, Kenji TOGARI and Makoto KAWAMURA

Fig. 2. Backscattered electron image of Yamagami Pm-schist. Bar represents 100 ƒÊm. Ab, albite; Ap, apatite; Chl, chlorite; Hm, hematite; Mu, muscovite; Pm, piemontite;

Qz, quartz; Ru, rutile. A, Albite-porphyroblast B, Anhedral K-feldspar

C, D, Prismatic piemontite showing nematoblastic texture. Mineralogy of piemontite-bearing schist in the Yamagami metamorphic rocks 145 the schistosity, dark reddish brown parts rich in is traversed by cracks perpendicular to the piemontite are developed. Ptygmatic quartz elongate direction and separated into numerous veins are found. fragments. It shows a marked pleochroism;

The main minerals of the Pm-schist are X=lemon- to orange-yellow, Y=amethyst quartz, muscovite, albite, chlorite, piemontite, to pink, Z=red to violet. 2Vx=78 to 84•‹, tourmaline, hematite and rutile in decreasing c•ÈX=-4 to -6•‹. Cleavage; (100) perfect. amounts, and minor minerals are apatite, car Twinning; (100). Unit-cell parameters calcu bonate and epidote. Along the schistosity, lated from the indexed X-ray powder quartzofeldspathic seams, which are composed diffraction pattern by using least-squares of sutured xenomorphic lenticular quartz method is ao=8.91 (1), bo=5.667 (8), co=10.17 grains up to 2mm in size and sutured xenomor (1) A, ƒÀ=115.55 (4)•K and VO=463 (1) A3 (stan phic albite porphyroblasts up to 2mm in size, dard deviations shown in parentheses). The are formed in a microgranoblastic aggregation compositions and structural formulae of of quartz granules (Fig. 2A). Muscovite and piemontite are listed in Table 1. The composi chlorite show lepidoblastic texture. tions of piemontite range between Pm11Ps22Cz67

Piemontite and epidote form nematoblastic and Pm17Ps18Cz65, where Pm, Ps and Cz are texture (Fig. 2C). Hematite, rutile and tour Ca2Mn3Si3O12(OH), Ca2Fe3Si3O12(OH) and maline are associated with piemontite, mus Ca2Al3Si3O12(OH) components, respectively.

covite and chlorite (Fig. 2D). Piemontite, MnO and Mn2O3 are recalculated on the basis

epidote, hematite, rutile, tourmaline and apatite of stoichiometry to make octahedral occupancy

show cataclastic feature (Fig. 2C, D). The 3. The occupancy of Al, Mn3+ and Fe3+ in the

veins formed of quartz and K-feldspar occur in octahedral sites is shown in the Al-Mn3+-Fe3+

a microgranoblastic quartz aggregation. In diagram (Fig. 3). The structural formula show

the veins, quartz forms saccharoidal aggre that there is an excess of Mn over the octahe

gates, and K-feldspar occurs as xenomorphic dral occupancy of 3.00 so that Mn cations

grains (Fig. 2B). partly substitute Ca as Mn2+. Rarely epidote occurs as prisms up to 0.2

V. Mineralogy mm long or rim around piemontite. It is sepa

Mineral analyses were performed with rated into fragments by cracks perpendicular to

electron microprobe analyzers, a JEOL JCMA- the elongate direction. It has weak pleo

733 of Hokkaido University and a JEOL JXA- chroism; X=colorless, Y=pale lemon-yellow

733 of Shimane University. Acceleration volt and Z=colorless. This mineral is rich in MnO,

age is 15 kV and beam current 0.02 ƒÊA. Both up to 3.23 wt.% (Table 1). MnO contents are

of ZAF correction and empirical correction by higher in rims than in core, by the substitution

Bence and Albee (1968) were employed. Ca ?? Mn2+. Mn is largely present as Mn2+ and

1. Piemontite and epidote the degree of substitution of Mn2+ for Ca

Piemontite occurs within quartzofeldspath attains up to 21%. It contains 2.7% Pm and ic seams associated with chlorite, muscovite, 27% Ps at maximum. hematite, rutile, and epidote, and is occasion 2. Associated minerals ally included as minute crystals within albite Muscovite is relatively abundant and the porphyroblasts and xenomorphic quartz grains. size varies from 0.2 to 1mm. Muscovite flakes

The mineral is long prisms up to 0.2mm long. with twisted and warped appearance show

Preferred orientation is clearly developed. It parallel arrangement and are entangled to form 146 Masahide AKASAKA,Jun WATANABE, Kenji TOGARI and Makoto KAWAMURA

Table 1. Representative analyses of piernontite and epidote

*Total Fe as Fe 2O3 ** Recalculated value a seam. The composition of muscovite is tance of the hydromuscovite ((H3O)Al2AlSi3O10 listed in Table 2. Total Fe is represented as (OH)2) and hydropyrophyllite ((H2O)Al2Si4O10 Fe2O3, which leads the octahedral site occu (OH)2) components. The muscovite composi pancies to ideal dioctahedral stoichiometry. tions in the present study are represented The mineral is phengitic (Si 6.54-6.64 atoms per mainly by the muscovite, celadonite and 22 oxygens). Generally muscovite composi hydromuscovite components with other minor tions are represented mainly by the muscovite, components, but not by the muscovite, celadonite and pyrophyllite components. celadonite and pyrophyllite components (Table However, Loucks (1991) emphasized the impor 2 and Fig. 4). Mineralogy of piemontite-bearing schist in the Yamagami metamorphic rocks 147

Fig. 3. Al-Mn3+-Fe3+ diagram for piemontites and epidotes in Yamagami Pm schist. Fig. 4. Plots of chemical compositions of mus Filled circle, piemontite; filled triangle, covite from Yamagami Pm schist (filled epidote; dashed line, compositional range circle) on Muscovite (Mus)-Celadonite of piemontite and epidote from the Arrow (Cel)-Hydromuscovite (HMu) diagram. Junction pink Pm-schist (Kawachi et al., 1983) tourmaline rocks.

Composition of porphyroblastic albite is Hematite is up to 0.2mm with hypidiomor listed in Table 3. Compositional variations are phic to xenomorphic forms with cracks. Gen Ab97-99An1-2Or0-1. Although most of albites erally hematite coexists with rutile. End mem do not show twinnings, part of them show ber composition of the hematite is MnTiO3 1.0,

carlsbad twinning. Rarely albite twinning is Mn2O3 0.5 and Fe2O3 98.5% (Table 6), which is found at the marginal part. recalculated on the assumption that Mn3+ and

Chlorite is up to 0.8mm in size and shows Fe2+ do not exist simultaneously in the distorted-entangled aggregate forming seams hematite. Reineche (1986) showed that the along schistosity. There are two types of Mn2O3 content recalculated by the method chlorite; pale greenish and brownish in color. mentioned above is in good agreement with the

According to the nomenclature after Hey actual presence of Mn3+ given by the

(1954), the former with Fe/(Mg+Fe)=0.02 and oxidimetric analysis.

Si=5.72 is clinochlore, and the latter with Fe/ Rutile forms idiomorphic crystals, brown

(Mg+Fe)=0.381 and Si=5.78 is pycnochlorite ish in color, smaller than 0.2mm in size, or

(Table 4). The compositional variation is shows intergrowth texture with hematite. shown in Fig. 5. Refractive indices and birefringence are charac

Tourmaline forms prismatic or hexagonal teristically high. The composition is listed in crystals, up to 0.1mm. It is often traversed by Table 7. cracks and separated into fragments. The Fluor-apatite is broken into xenomorphic mineral is nearly colorless. The tourmaline is grains with very low birefringence. The com close to dravite (Table 5, Fig. 6). In Al-‡”Fe- position is listed in Table 8. Mg diagram after Henry and Guidotti (1985), Hexagonal carbonate occurs rarely. This the tourmalines fall in the field of those formed is almost pure CaCO3, with 0.3 wt.% MnO. in metapsammites, metapelites and quartz Xenomorphic K-feldspar grains, up to 1 148 Masahide AKASAKA,Jun WATANABE, Kenji TOGARI and Makoto KAWAMURA

Table 2. Representative analyses of muscovite

*1 Total Fe as Fe2O3

*2 Calculated H2O wt. % based on the assumption that a deficit in the interlayer charge contributed alkali cation is satisfied by H3O+ (after Loucks (1991)) *3 Muscovite: K(Al , Fe3+)3Si3O10(OH)2 Hydromuscovite: (H2O)(Al, Fe3+)3Si3O10(OH)2 Celadonite: K(Mg, Mn)(Fe3+, Al)Si4O10(OH)2 Pyrophyllite: • Al2Si4O10(OH)2 Paragonite: Na(Al, Fe3+)3Si3O10(OH)2 Margarite: Ca(Al, Fe3+)4Si2O10(OH)2 Ti-mica: KMgTiAlSi3O10(OH)2 Mineralogy of piemontite-bearing schist in the Yamagami metamorphic rocks 149

Table 3. Composition of Table 4. Representative analyses of albite chlorites*

*Total Fe as Fe2O3

mm in diameter, occurring in quartz veins do not show twinning and, thus, the identification of this mineral is very difficult by microscope.

Compositional variation are small: Ab 2-6 *1: brownish chlorite

mol%. 2: Pale greenish chlorite ** Total Fe as FeO and total Mn as MnO VI. Bulk chemical composition

Bulk chemical composition of the Pm The silica content of the Pm-schist is as

-schist collected from the upper stream of the high as 82.41 wt.%. MnO content is very low

River Maebarazawa is listed in Table 9. To at 0.02 wt.%. Oxidation ratio, defined as

estimate the original rock, we removed atomic percentage (Fe3+•~100)/(Fe3++Fe2+), is ptygmatic quartz veins from the analyzed sam high attaining 93. ple as possible as we can. Analytical methods are as follows; gravimetric for SiO2, H2O(+) VII. Discussion

and H2O(-), spectrophotometric for TiO2, Appearance of piemontite indicates that fO2

MnO and P2O5, chelate titration for Al2O3, Fe2 condition during metamorphism was very high,

O3, MgO and CaO, volumetric KMnO4 titration because the critical factor for production of

for FeO, and flamephotometric for Na2O and piemontite in a rock is not high Mn content but K2O. unusually high oxidation state at which Mn is 150 Masahide AKASAKA,Jun WATANABE, Kenji TOGARI and Makoto KAWAMURA

Table 5. Representative analy sis of tourmaline

Fig. 5. Mg/(Mg+Fe)-Al/(Mg+Fe+Al) diagram for muscovite (filled triangle) and chlorite (filled circle) in Yamagami Pm-schist. Dotted areas represent compositions of muscovite and chlorite from Arrow Junc tion pink Pm-schist (Kawachi et al., 1983). oxidized to a trivalent state (Smith and Albee, 1967; Grapes and Hashimoto, 1978; Keskinen *Total Fe as FeO and Liou, 1979, 1987; Keskinen, 1981; Kawachi **Total Mn as MnO et al., 1983; Akasaka et al., 1988). In the Yamagami Pm-schist, chemical compositions of the minerals associated with gested that normally Fe2+-bearing solid solu piemontite are consistent with the very high fO2 tion would tend to be enriched in the Mg end metamorphic condition. For example, member under extremely high fO2 condition, hematite is regarded to contain Mn2O3 compo because, in the highly oxides rocks, most Fe is nent by the end member calculation, which present as Fe, which prefers hematite and suggests that hematite has been formed at very epidote-group minerals. Kawachi et al. (1983) high fO2 metamorphic condition as well as confirmed that the chlorites with lowest Fe piemontite. The hematite containing Mn2O3 have been found in the most highly oxidized component has been reported from highly oxid metamorphic rocks, irrespective of bulk Fe ized low grade metamorphic rocks at Vitali, content. The result in the present study that Andros Island, Greece (Reineche, 1986). Mg-chlorite coexists with piemontite is consis The pale greenish chlorites in the present tent with the previous works referred above. study are extremely rich in Mg, the Mg/(Mg+ Thus, the mineralogical aspects of Fe) ratio ranging up to 0.98. Coexistence of piemontite, hematite and Mg-chlorite in the Mg-chlorite with piemontite has been reported Yamagami Pm-schist suggest that these min from the Shadow Lake piemontite zone (Kes erals have been formed at very high fO2 condi kinen, 1981) and the Arrow Junction Pm-schist tion, which is consistent with the high oxidation (Kawachi et al., 1983). Keskinen (1981) sug ratio of the bulk composition. Mineralogy of piemontite-bearing schist in the Yamagami metamorphic rocks 151

Fig. 6. Plot of chemical compositions of tourmaline from Yamagami Pm-schist (filled square) on Al- Fe(total)-Mg diagram (after Henry and Guidotti, 1985). 1, Li-rich granitoid pegmatites and aplites; 2, Li-poor granitoids and their associated peg matites; 3, Fe3+-rich quartz-tourmaline rocks; 4, metapelites and metapsammites coexisting with an Al-saturating phase; 5, metapelites and metapsammites not coexisting with an Al saturating phase; 6, Fe3+-rich quartz-tourmaline rocks, calc-silicate rocks, and metapelites; 7, low-Ca metaultramafics and Cr, V-rich metasediments; 8, metacarbonates and metapyrox enites. Area surrounded by dashed line represents compositional range of tourmalines from the Arrow junction pink Pm-schist (Kawachi et al, 1983)

It is apparent that epidote has been also metamorphism. Grapes and Hashimoto (1978) formed at high fO2 condition, because epidote stated that viridine-piemontite bearing seam in contains Mn3+. However, Mn2+/Mn3+ ratios of the manganiferous schists, Hidaka Mountains, the epidote are higher than those of the Hokkaido, is the metamorphosed equivalent of piemontite. The existence of the piemontite an oceanic oxidized zone in a pelagic ocean rimmed with epidote in the YMR suggests that core. Kawachi et al. (1983) also discussed that the fO2 has decreased slightly at the last stage of high fO2 condition which produced highly oxid the metamorphism. ized minerals in the Arrow Junction Pm-schists Generally, origin of the high fO2 is attribut is believed to be inherited from MnOx in the ed to the existence of highly oxidized Mn parent pelagite. Mottana (1986) summarized minerals in the protoliths. For example, that the apparent abundance in oxidized Mn Brown et al. (1978) suggested that highly oxid minerals in the chert suffered blueschist-facies ized Mn minerals, such as found in Mn-nodules metamorphism as the results of maintenance of or some hydrothermal vein, in protoliths of the the initial high diagenetic fO2 in the individual Pm-schists from St. Marcel, Piemonte, Italy, layers due to the rapid development of the must have maintained high fO2 throughout metamorphism in the subduction zone. The 152 Masahide AKASAKA,Jun WATANABE, Kenji TOGARI and Makoto KAWAMURA

Table 6. Analysis of hematite Table 8. Analyses of apatite

*Total Fe as Fe 2O3 ** Total Mn as MnO

Table 7. Analysis of rutile

*Total Fe as FeO *Total Fe as Fe2O3 **Total Mn as MnO **Total Mn as MnO

(Kawachi et al., 1983), and the viridine piemontite bearing schists in the Chiroro River interpretation that highly oxidized Mn-min area, Hidaka Mountains, Hokkaido, contain erals in the initial sediments produced high fO2 total MnO over 1 wt.% (Grapes and Hashimoto, condition during metamorphism also holds for 1978). However, the Yamagami Pm-schist is the case of metamorphosed manganiferous iron very low in total MnO content (0.02 wt. %), and ore deposits (Togari et al., 1988). the origin of very high fO2 seems not to be The Pm-schists, which formed at high fO2 attributed to the highly oxidized Mn-minerals condition caused by highly oxidized Mn min in the initial sediments. erals, tend to be relatively rich in total MnO; Although we do not have enough evidence for example, total MnO contents of the Arrow to explain the origin of high fO2 an idea that the Junction pink Pm-schist and brownish-pink high fO2 condition of the Yamagami Pm-schist Pm-schist are 1.38 and 2.28 wt. %, respectively has been caused by the dissociation equilibrium Mineralogy of piemontite-bearing schist in the Yamagami metamorphic rocks 153

Table 9. Chemical composi ing agent, they mentioned that the high fO2

tion of Yamagami condition is considered to have been produced Pm-schist by more complicated chemical environment.

The occurrence of the Yamagami Pm

schist, which is associated with the spotted

schist of the glaucophane schist facies and with

the cupriferous iron ore deposit, is similar to

that of the Sambagawa Pm-schists, because the

Pm-schists of the Sambagawa belt are gener

ally restricted to the zone of green schists

containing spotted schists, glaucophane schist

and pyritic bedded deposits (Suzuki, 1924).

Kuroda and Ogura (1960, 1963) pointed out that

mineral assemblage and texture of the spotted

schists in the YMR are very similar to those of

the spotted schists from the Sambagawa

metamorphic zone. Based on the description

and classification on the Sambagawa Pm of H2O with H2 and O2 in the source rock may - schists by Suzuki (1924), assemblage of the be possible. Miyashiro (1958, 1973) suggested main minerals in the Yamagami Pm-schist is that pure water can produce high fO2condition more similar to that of the 'piemontite at which hematite is to be stable for almost the hematite-sericite-quartz-schist' than the entire temperature range of metamorphism so 'piemontite -sericite-quartz-schist' and long as PH2O is higher than 1 bar. He also ' piemontite-sericite-calcite-quartz-schist'. discussed that high fO2 condition produced by However, the quantitative relation of the main dissociation of H2O is changed to reduced con minerals in the 'piemontite-hematite-sericite- dition by organic materials and graphite. If quartz-schist' is quartz>hernatite> organic material and graphite are absent in the piemontite>epidote>sericite>magnetite>cal initial sediments, high fo, condition would be cite (Suzuki, 1924), and it is different from that maintained during metamorphism because the of the Yamagami Pm-schist. mobility of O2 in rocks undergoing metamor So far as bulk chemical composition, min phism is extremely small (Chinner, 1960; Smith eral assemblage and mineral compositions are and Albee, 1967; Miyashiro, 1973). As graph concerned, the Yamagami Pm-schist is very ite and organic material are not found in the similar to the pinkish Pm-schists of Arrow

Yamagami Pm-schist, above explanation on Junction, Otago, New Zealand. The bulk com the origin of highly oxidized condition seems to position of the Arrow junction pinkish Pm be reasonable. On the other hand, Kuroda and schist is SiO2 80.31, TiO2 0.33, Al2O3 7.58, Fe2O3 Ogura (1963) suggested that metamorphic oxi 4.06, FeO 0.38, MnO 1.38, MgO 0.89, CaO 1.54, dation state of the Yamagami epidote-am Na2O 0.82, K2O 1.30, P2O5 0.03, L. O. I. 1.24 wt. % phibolites was high because of appearance of and the oxidation ratio is 91•}5 (Kawachi et al., muscovite and disappearance of biotite. 1983; Coombs et al., 1985). Thus the contents Although they recognized the significance of O2 of major elements of the Yamagami Pm-schist produced by the dissociation of H2O as oxidiz are comparable to those of the Arrow Junction 154 Masahide AKASAKA,Jun WATANABE, Kenji TOGARI and Makoto KAWAMURA

Pm-schist. The mineral assemblage of the of silicates and oxides. J. Geol., 76, 382-403. Brown, P., Essen, E. J. and Peacor, D. R. (1978),The Arrow Junction pink Pm-schists is quartz+ mineralogy and petrology of manganese-rich albite+phengite+chlorite+piemontite+spes rocks from St. Marcel, Piedmont, Italy. sartine+hematite+rutile+tourmaline•}cal Contrib. Mineral. Petrol., 67, 227-232. cite. Only difference of the mineral assem Chinner, G.A. (1960),Pelitic gneisses with varying ferrous/ferric ratios from Glen Clova, Angus, blages between the Yamagami Pm-schist and Scotland. J. Petrol., 1, 178-217. the Arrow Junction pink Pm-schists is that Coombs, D.S., Dowse, M., Grapes, R., Kawachi, Y. garnet occurs in the latter but not in the former. and Roser, B. (1985),Geochemistry and origin The similarity of the mineral compositions is of piemontite-bearing and associated man ganiferous schists from Arrow Junction, West shown in Figs. 3, 5 and 6 for piemontite, epidote, ern Otago, New Zealand. Chem. Geol., 48, 57- muscovite, chlorite and tourmaline. The 78. metamorphic pressure and temperature of the Grapes, R.H. and Hashimoto, S. (1978), Man

Arrow Junction Pm-schist have been estimated ganiferous schists and their origin, Hidaka Mountains, Hokkaido, Japan. Contrib. Mine to be about 6.4 kb and 400•Ž (Kawachi et al., ral. Petrol., 68, 23-35. 1983). The metamorphic conditions of the Henry, D.J. and Guidotti, C.V. (1985), Tourmaline Yamagami Pm-schist seem to be close to those as a petrologic indicator mineral: an example of the Arrow Junction Pm-schist. from the staurolite-grade metapelites of NW Maine. Am. Mineral., 70, 1-15. Hey, M.H. (1954),A new review of the chlorites. Acknowledgements: We thank Drs Y. Mineral. Mag., 30, 277-292. Kuroda of Shinshu University, Y. Kawachi of Hunahashi, M. (1979),Abean metamorphics of the

University of Otago, A. Takasu of Shimane eastern marginal belt of the Abukuma Moun tains. In The Abean Orogeny (Minato, M., University, Prof. Emeritus K. Yagi of Hok Hunahashi, M., Watanabe, J. and Kato, M. kaido University and referees for their critical Ed.). pp. 427, Tokai Univ. Press, Tokyo, 284- reading of the manuscript. We also thank to 291.

Drs T. Watanabe of Hokkaido University, Y. Iwamatsu, A. (1975), Folding-styles and their tectonic levels in the Kitakami and Abukuma Sawada of Shimane University and M. Saka mountainous lands, northeast Japan. Jour, kibara of Ehime University for their valuable Fac. Sci., Univ. Tokyo., Sec.II, 19, 95-131. advices, Mr. S. Terada of Hokkaido University Kawachi, Y., Grapes, R., Coombs, D.S. and Dowse, M. (1983), Mineralogy and petrology of a and Drs T. Furuno and T. Uehara of Shimane piemontite-bearing schist, western Otago, University for their help in the operation of New Zealand. J. Metamorphic. Geol., 1, 353- EPMA. 372. The present study was supported by a Keskinen, M. (1981), Petrochemical investigation of the Shadow lake piemontite zone, eastern Grant-in-aid for Scientific Research from the Sierra Nevada, California. Am. J. Sci., 281, Ministry of Education, Science and Culture of 896-921. Japan (03640675). Keskinen, M. and Liou, J. G. (1979),Synthesis and stability relations of Mn-Al piemontite, Ca2MnAl2Si3O12(OH).Am. Mineral., 64,317- References 328. Akasaka, M., Sakakibara, M. and Togari K. Keskinen, M. and Liou, J. G. (1987), Stability rela (1988), Piemontite from the manganiferous tions of Mn-Fe-Al piemontite. J. Metamor hematite ore deposits in the Tokoro belt, phic Geol., 5, 495-507. Hokkaido, Japan. Mineral. Petrol., 38, 105- Kuroda, Y. and Ogura, Y. (1960), Discovery of 116. spotted schists from the northern Abukuma Bence, A. E. and Albee, A. L. (1968), Empirical cor plateau and its significance. J. Min.Petr. rection factors for the electron microanalysis Econ, Geol., 4, 287-291 (in Japanese with Eng Mineralogy of piemontite-bearing schist in the Yamagami metamorphic rocks 155

lish abstract). California. Contrib. Mineral. Petrol., 16,198- Kuroda, Y. and Ogura, Y. (1963), Epidote-am 203. phibolites from the northeastern Abukuma Suzuki, J. (1924), On the piedmontite schists of plateau, Japan. Sci. Rep., Tokyo Kyoiku Japan. Japan Jour. Geol. Geogr., 3, 135-149. Daigaku, Sec. C, 80, 246-268. Togari, K., Watanabe, J. and Akasaka, M. (1986), Loucks, R.R. (1991), The bound interlayer H2O Piemontite from the Abukuma metamorphic content of potassic white micas: Muscovite- belt. Abstr. Autumn Meeting of Japan Assoc. hydromuscovite-hydropyrophyllite solutions. Petro. Miner. Econ. Geol., Miner. Soc. Japan Am. Mineral., 76,1563-1579. and Soc. Mining Geol. Japan, Mito, 11 (in Maekawa, H. (1988), High P/T metamorphic rocks Japanese) in north east Japan -Motai-Matsugadaira Togari, K., Akasaka, M., Sakakibara, M. and zone-. Earth Sci. (Chikyu Kagaku), 42, 212- Watanabe. T. (1988), Mineralogy of man 219 (in Japanese with English abstract). ganiferous iron ore deposits and chert from Miyashiro, A. (1958), Regional metamorphism of the Tokoro belt, Hokkaido. Mining Geol., the Gosaisho-Takanuki district in the central Special Issue, No. 12, 115-126. Abukuma Plateau. Jour. Fac. Sci. Univ. Watanabe, J., Shimaoka, H., Uchiyama, K., Inoku Tokyo, Sec. II, 11, 217-272. chi, T., Takahata, H., Kawamura, M. and Miyashiro, A. (1973), Metamorphism and metamor Ogata, T. (1979), The Yamagami metamorphic phic belt. pp. 492, George Allen and Unwin, complex. In The Abean Orogeny (Minato, London. M., Hunahashi, M., Watanabe, J. and Kato, M. Mottana, A. (1986), Blueschist-facies metamor Ed.). pp. 427, Tokai Univ. Press, Tokyo, 41-50. phism of manganiferous chert: A review of Watanabe, J., Takahata, H., Uchiyama, K. and the Alpine occurrences. In Blueschists and Tsuchiya, T. (1983a), A proposal of the newly eclogites (Bernard, W. and Brown, E. H. Ed.), established "Idegawa Structural Belt" in the Geol. Soc. Amer. Mem., 164, 267-299. Abukuma eastern marginal belt, Fukushima Reineche, T. (1986), Phase relationships of sursas Prefecture I. Geological bases for setting of site and other Mn-silicates in highly oxidized the belt and its significance. J. Geol. Soc. low-grade, high-pressure metamorphic rocks Japan, 89, 331-346 (in Japanese with English from Evvia and Andros Islands, Greece. abstract). Contrib. Mineral. Petrol., 94, 110-126. Watanabe, J., Takahata, H., Uchiyama, K. and Sato, T. (1974), The geological map and stratigra Tsuchiya, T. (1983b), A proposal of the newly phic relationship of the Upper Devonian established "Idegawa Structural Belt" in the Ainosawa Formation in the Soma District, Abukuma eastern marginal belt, Fukushima Fukushima Prefecture, Japan. Jour. Fac. Lib. Prefecture-II. Petrogenesis of "felsite" dyke Art., Shinshu Univ., Nat. Sci., 8, 5-14. swarms. J. Geol. Soc. Japan, 89, 455-468 (in Smith, D. and Albee, A. L. (1967), Petrology of a Japanese with English abstract). piemontite-bearing gneiss, San Gorgonio Pass,

阿武隈山地北東部山上変成岩中の含紅簾石片岩に関する鉱物学的研究

赤坂 正秀 ・渡辺 順 ・戸苅 賢二 ・川村 信人

阿武隈山地北東部 山上変成岩を構成する含紅簾石片岩 の全岩化学組成,鉱 物組 み合わせ,構 成鉱物の化 学組成を検討した。 含紅簾石片岩は,ア ルバ イ ト斑状変晶を含む白雲母石英片岩 と緑簾石角閃岩の間に挟 まれて産出す る。片 状組織が発達 しているが,圧 砕組織 も認め られる。含紅簾石片岩の鉱物組み合わせは,石 英+白 雲母+ア ルバイ ト+緑 泥石+紅 簾石+電 気石+赤 鉄鉱+ル チルである。少量の緑簾石,燐 灰石,微 量の炭酸塩鉱物 が伴われる。 また,石 英 とカ リ長石か らなる脈 が入 っている。 紅簾石の化学組成はPm11PS22CZ67か らPm17PS18CZ65の 範囲にある。 白雲母 は, Siが6.54-6.64 atoms 156 Masahide AKASAKA,Jun WATANABE, Kenji TOGARI and Makoto KAWAMURA

per 22 oxygensの フ ェ ン ジ ャ イ トで あ る が,ハ イ ド ロマ ス コバ イ ト成 分 を 含 む 。緑泥 石 は, Fe/(Mg+Fe)= 0.02の ク リ ノ ク ロ ア と, Fe/(Mg+Fe)=0.38の ピ ク ノ緑 泥 石 か ら な る 。 赤 鉄 鉱 は, Mn2O3成 分 を 含 む｡ 全 岩 化 学 組 成 は, SiO2が82.41 wt. %, (Fe3+×100)/(Fe3++Fe2+)値 が93で あ る。MnO含 有 量 は0.02 wt. %で,き わ め て 低 い 。 紅 簾 石, Mg緑 泥 石, Mn2O3成 分 を 含 む 赤 鉄 鉱 が 産 出 す る こ と か ら,高 酸 素 分 圧 下 で 変 成 作 用 が 進 行 し た と考 え ら れ る 。 こ の こ と は,全 岩 化 学 組 成 の(Fe3+×100)/(Fe3++Fe2+)値 が きわ め て 高 い こ と と調 和 的 で あ る 。 変 成 作 用 時 の 高 酸 素 分 圧 条 件 は,H2OがH2+O2/2に 解 離 す る こ と に よ っ て 発 生 し た,と 考 え られ る。 Ainosawa,合 の 沢; Fukuyamagami,福 山 上; Futaba,双 葉; Hatagawa,畑 川; Maebarazawa,前 原

沢; Mano,真 野; Manogawa,真 野 川; Matsugadaira,松 ケ 平; Motal,母 体; Oashi,大 芦; Ryozen,

霊 山; Shiode,塩 手; Somanakamura,相 馬 中 村; Tateishi,立 石; Togamorizawa,戸 ケ 森 沢; Uda gawa,宇 田川; Uwano,上 野; Wareyama,割 山; Yaguki,八 茎; Yamagami,山 上; Yumiorezawa,弓

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