J. Japan. Assoc. Min. Petr. Econ. Geol.73 , 359-379, 1978.

STABILITY AND PARAGENESIS OF Fe-Ti OXIDE MINERALS AND SPHENE IN THE BASIC SCHISTS OF THE SANBAGAWA METAMORPHIC BELT IN CENTRAL SHIKOKU, JAPAN

TETSUMARU ITAYA Institute of Mineralogy, Petrology and Economic Geology, Tohoku University, Sendai, 980 Japan

and

MASAYUKI OTSUKI* Department of Earth Science, Kanazawa University, Kanazawa 920, Japan

The Sanbagawa basic schists in the Shiragayama area, central Shikoku contain hematite, ilmenite, rutile, sphene and some magnetite. Mineral assemblages of Fe-Ti oxide and silicate minerals in hematite-bearing ones are significantly different from those in hematite-free variety. Excluding albite, quartz, epidote, chlorite and phengite, the mineral assemblages of hematite-bearing ones are hematite+riebeckitic actinolite+sphene or hematite+ crossite+sphene in the lower grade zone and hematite+ilmenite+rutile+ hornblender magnetite in the higher grade zone. In the other variety, the mineral as semblages are actinolite+sphenefrutile in the lower grade zone, and ilmenite+hornblende in the higher grade zone. Combining the mode of occurrence and chemistry of Fe-Ti oxides and sphene as well as the Mn-Fe2+partitioning among magnetite, hematite and ilmenite, stability and paragenesis were determined. Magnetite, ilmenite and rutile in the hematite-bearing basic schists occur in the garnet and biotite zones, whereas, in the hematite-free ones, rutile is restricted to the garnet zone at the prograde stage, and ilmenite occurs in the biotite zone. Sphene is widespread in all the zones, but its occurrence is restricted to the lower grade zone at the prograde stage of metamorphism; its occurrence in the higher grade zone postdates major mineralization. The stable oxide mineral assemblages are magnetite- hematite, hematite+rutile, magnetite +hematite+ilmenite, magnetite+hematite+rutile, hematite+ilmenite+rutile and magnetite-hematite+ilmenite+rutile. Ilmenite and hematite contain significant amounts of MnO; the maximum MnO content of ilmenite and hematite are 24.1 and 2.1 wt. per cent, respectively. Therefore, the paragenesis of Fe-Ti oxide minerals can be determined only in the FeO-Fe2O3-TiO2MnO system. The stability of Fe-Ti oxide minerals and sphene is controlled by the bulk-rock chemistry as well as pressure, temperature and oxygen fugacity.

1970; Rumble, 1973, 76; Mariko et al., 1975).

INTRODUCTION Their stabilities in the FeO-Fe2O3-TiO2 Magnetite, hematitie, ilmenite and system at high temperature (500-1300•Ž) rutile are common Fe-Ti oxide minerals in have been studied experimentally and their metamorphic rocks (Buddington and Lind chemical compositions: and parageneses are sley, 1964; Kanehira et al., 1964; Westra, known to be sensitive to temperature and

* Present address: Geological Institute, Faculty of Science, University of Tokyo, Tokyo (Manuscript received October 17, 1978) 360 Tetsumaru Itaya and Masayuki Otsuki oxygen fugacity (Carmichael, 1961; Lindsley, 1962, 1963, 1965, 1973; Taylor, 1964; OUTLINE OF GEOLOGYAND PETRO Katsura et al., 1976). However, their GRAPHY stabilities in multi-component system such The study area is a part of the San as metamorphic rocks may be controlled by bagawa metamorphic belt in central the chemistry of the system as well as Shikoku (Fig. 1), being east of Shirataki pressure, temperature and oxygen fugacity. (Ernst et al., 1970)and south of the Sazare The paragenesis and stability of oxide (Kurata and Banno, 1974)areas, having been minerals and sphene in the Sanbagawa pelitic called Shiragayama area by Higashino schists of the Shiragayama area in central (1975). This area is underlain by the Shikoku, were discussed in a previous paper Minawa and Ojoin (Ozyoin) formations as (Itaya and Banno, in prep.). In this paper, defined by Kojima and coworkers (Kojima, we intend to discuss the paragenesis and 1951 and others). Higashino (1975) has stability of these minerals in basic schists divided this area into three mineral zones: of the same area. In contrast with the the chlorite zone, garnet zone and biotite Sanbagawa pelitic schists, which are in zone, in the ascendingorder of metamorphic reduced state because of the presence of temperature. Although the pelitic schists carbonaceous matters, the oxidation state of the Sanbagawa belt is in extremely of basic schists varies considerably, from reduced state, being characterized by the hematite-bearing schists to pyrrhotite-bear presence of carbonaceous matters and ing ones, making it possible to obtain more pyrrhotite (Itaya, 1975), the oxidation information about the paragenesis. Thus, state of basic schists varies considerably. not only the relationships among ilmenite, It ranges from hematite-bearing to pyr rutile and sphene, but also those including rhotite-bearing. The relationships among hematite and magnetite can be shown. The the silicate mineral assemblages,the oxida low temperature paragenetic relations in the tion state of the host rocks and the meta FeO-Fe2O3-TiO2 and FeO-Fe2O3-TiO2-MnO morphic grade are shown in Table 1. The systems can be established from the observa present area can be divided into three zones, tion on natural paragenesis. solelybased upon the mineral assemblagesof As in the pelitic schists, the oxide the hematite-bearing basic schists, namely: minerals and sphene in the basic schists riebeckitic actinolite zone, crossite zone commonly reacted at the retrograde stage and subcalichornblende zone in the order of of metamorphism, giving rise to the forma ascending metamorphic temperature. The tion of various composite grains postdating location of isograds definedby the hematite major silicate assemblages. Therefore, the bearing basic schists are compared with paragenetic relation is determined only those defined by the pelitic schists in Fig. after the retrograde assemblages were 3. However,in the followingsections, the eliminated. metamorphic grade will be defined only by The stability of oxides and sphene, and the isograds for the pelitic schists, partly their role in estimating the fugacities of because we have to define different mineral CO2 and O2 are discussed in relation to the zones for hematite-bearing and pyrrhotite temperature, silicate assemblage and oxida bearing basic schists, and partly because the tion state of host rocks. accuracy locating isograds is better for Stability and paragenesis of Fe -Ti oxide minerals and sphene in the basic schists 361 362 Tetsumaru Itaya and Masayuki Otsuki

Fig. 2. Sample localities and the distribution of mineral zones along the Asemi valley. The location of this route is shown by a quadrilateral in Fig. 1; The mineral zone is after Higashino (1975); Numerals show the specimens studied in detail. the pelitic schists, which are the most and Banno, silicate minerals formed by predominant rock-type of the present area. progressive metamorphism will be called Although the majority of mineralogy major silicate minerals, herein. The rocks of the Sanbagawaschists is attributable to a used in the present work are basic schists, single event of progressive metamorphism, mainly derived from pyroclastics, collected disequilibriumparageneses occur not uncom along the Asemi river. Some quartz and

monly in small domains in thin section psammitic schists with greenish color and (Kurata, 1972; Higashino, 1975; Itaya containing sphene were also examined for and Banno, in prep; Otsuki and Banno, in comparison. Localities of specimens are prep.). Therefore, as proposed by Itaya shown in Fig. 2, where numerals refer to samples studied in detail. Table 1 Frequency of silicate mineral The petrography of basic schists is

assemblages of the basic quoted wherever necessary, but the details schists in the chlorite, garnet and biotite zones. are not described here. Interested readers may refer to another paper to be published

elswhere (Otsuki and Banno, in prep.).

ANALYTICAL PROCEDURE

Minerals in 121 polished thin sections

examined in this study were identified with

both petrographic and ore microscopes.

Especially, fine-grained sphene and rutile

(less than tens of microns) were possible to be confirmed only with ore microscope, and,

if necessary, by qualitative analysis with Epidote, chlorite, phengite, albite and an electron-probe microanalyzer (EPMA), quartz always occur. Minor minerals are aegirinaugite, stilpnomelane, tour model Hitachi XMA 5A with 3 channel malline, apatite and calcite. detection systems and a 38•‹ take off angle. Stability and paragenesis of Fe-Ti oxide minerals and sphene in the basic schists 363

Fig. 3 Mineralogical data plotted on a line parallel to the Asemi valley, showing modes of occurrence of magnetite, hematite, ilmenite, rutile and sphene (texture not taken into account); modes of occurrence of various composite grains; modes of occurrence of mineral species as inclusions in garnet; partition coefficient between garnet and chlorite for Mg-Fe of Higashino (1975); Mn/Fe2+ ratios of magnetite, hematite and ilmenite; partition coefficient for Mn-Fe2+ among magnetite, hematite and ilmenite. 364 Tetsumaru Itaya and Masayuki Otsuki

In quantitative analyses of magnetite, Modes of the minerals were measured by the rutile and fine-grained ilmenite and hematite, method by Itaya and Banno (in prep.).

the accelerating voltage, specimen current and beam diameter were kept at 15kv, 0.03 MODE OF OCCURRENCEAND CHEMICAL ƒÊA and 2-3ƒÊm, respectively. Ilmenite COMPOSITIONS OF Fe-Ti OXIDE MIN and hematite grains often contain lamellae ERALS AND SPHENE of hematite and ilmenite, respectively, The Fe-Ti oxide minerals in the basic which are too fine to be resolved for in schists are magnetite, hematite, ilmenite dividual analysis. In order to determine and rutile. Fig. 3 shows the relationships the bulk compositions of single mineral between mode of occurrence of these mine grains with such lamellae, a beam diameter rals and sphene, and metamorphic grade. of 5-20ƒÊm was used, and 5-10 measure The basic schists are divided into two groups, ments were made per grain. As standards, hematite-bearing and-free schists. They synthetic SiO2, Al2O3, TiO2, MnO, MgO, do not differ only in terms of presence or CaSiO3, Cr2O3 and Ni2SiO4, and natural absence of hematite; their silicate mineralogy hematite were used. The background intensities were measured by shifting the Table 2 Frequency of Fe-Ti oxide minerals wavelength of detection for all elements. in the chlorite, garnet and biotite

The correction of observed intensity was zones; denominator is total num bers of examined samples. made using a computer program written by

Yui and Shoji (1976). In order to estimate

the amount of ferric iron in the magnetite,

hematite and ilmenite, microprobe analyses

were recalculated by the method described

by Carmichael (1967) or Rumble (1973).

Table 3 Frequency of oxide mineral assemblages in the chlorite, garnet and biotite zones. Relationships between these mineral assemblages and frequency of composite grains are shown. Stability and paragenesis of Fe-Ti oxide minerals and sphene in the basic schists 365 and the composition of the ilmenite are of occurrence of composite grains of TiO2- distinctly different. In the chlorite zone, minerals. Hematite-bearing schists con all the hematite-bearing basic schists tain composite grains of rutile-sphene only. contain alkali-amphibole, but only one of But, hematite-free ones contain various ten hematite-free ones contain it (Table 1). kinds of composite grains, which are Also the two groups differ in terms of mode described later. In both groups of basic

Table 4 Chemical compositions of magnetite , hematite, ilmenite and rutile; "core" and "rim" are shown for zoned hematite; "c.g." means ilmenite in the composite grains; FeO* is FeO as total Fe; "*" shows that some radiation from Ca content of sphene may be detected in microprobe analysis because of fine-grained rutile which forms composite grains with sphene; quantitative analysis for rutile in Nos. 38 and 40 could not be done because of very fine-grained rutile in composite grains with sphene; FeO and Fe2OB contents on lower column show recalculated microprobe analysis (see text). 366 Tetsumaru Itaya and Masayuki Otsuki

schists, rutile is common in the garnet and Specimen No. 52, which is a spessartine- biotite zones. Ilmenite and magnetite hematite-bearing quartz schist, contain the occur in the schists from the middle garnet highest MnO so far reported in the zone to the biotite zone, ilmenite being more literatures. common. Sphene and hematite occur in all Magnetite: All magnetite grains are three zones. The frequencies of oxide euhedral. The mode is 0.01-0.3 in volume minerals and their mineral associations in per cent. MnO, TiO2, CaO, Al2O3 are the chlorite, garnet and biotite zones are present in small amount, but SiO2 is quite shown in Tables 2 and 3, respectively. significant (Table 4). Among possible combinations of Fe-Ti Hematite: Hematite generally occurs as oxides, magnetite alone, magnetite+ilmenite, individual grains with euhedral or subhedral magnetite + ruffle and hematite + ilmenite form. Its mode is 0.01-0.5 in volume per are not yet observed. Mineral assemblages cent. In the garnet and biotite zones, including magnetite+rutile, i.e., magnetite hematite grains often contain fine ilmenite +hematite+ihnenite+rutile, magnetite+he lamellae (about 1 X 50 ,um2 on polished matite+rutile and magnetite+ilmenite+ section). This type of hematite grain rutile, which are rare in natural occurrence, (ilmeno-hematite after Buddington et al., do occur in this area. 1963) is sometimes intergrown with ilmenite Representative chemical compositions containing fine hematite-lamellae (hemo of the oxide minerals are shown in Table 4. ilmenite) as shown in Fig. 8a. Hematite Ni and Cr were not detected with the EPMA and ilmenite with such a texture will be analysis. Ilmenite and hematite in called hematite-ilmenite intergrowth here

Fig. 4 Hematite with ilmenite-lamellae in the core. Numerals show the points analyzed by EPMA whose beam diameter was kept at about 20 micrometers. Stability and paragenesis of Fe-Ti oxide minerals and sphene in the basic schists 367 after. Coarse-grained (hundreds of microns to a few millimeters) and fine-grained (tens of microns) hematite often coexist in one polished section; the former generally con tains fine ilmenite-lamellae only in the core (Fig. 4), whereas the latter usually does not contain them and is always euhed Fig. 5 Zoned hematite. Broken line shows about contour for TiOQ content. ral. Hematite-rutile intergrowth was observed in a few specimens from the lower chemically homogeneous not only with part of the garnet zone (Fig. 8b). respect to those present in each grain but Hematite contains some TiO2 and MnO , also with respect to all grains in each and its minor components are CaO, Al2O3 polished section (Fig. 6). and SiO2. It is often chemically zoned, Ilmenite : Ilmenite is common in the with a core rich in TiO2 and MnO and a rim garnet and biotite zones. Its mode is 0.01- poor in them (Fig. 5). This is true in the 0.5 in volume per cent. In the hematite-

Sanbagawa quartz schists of the Nagatoro bearing schists, it is generally intergrown area, Kanto mountains, described by Mariko with ilmeno-hematite bringing about the et al. (1975). The core of coarse-grained texture shown in Fig. 8a. It often contains hematite with ilmenite-lamellae (Fig. 4) fine hematite-lamellae (about 1•~50ƒÊm2 have similar chemical compositions with on polished section). Independent euhedral respect to all grains in each polished section, ilmenite grains also occur. In this type of and that of zoned hematite without ilmenite schist, ilmenite-rutile, ilmenite-sphene and lamellae (Fig. 5) also have similar chemical ilmenite-rutile-sphene composite grains do composition. Fine-grained hematite is not occur. All ilmenites in the hematite similar in chemical composition to the rim free schists from composite grains with coarse-grained ones. Ilmeno-hematite in sphene or rutile, or both. the hematite-ilmenite intergrowth is Ilmenite is manganoan and homo

Fig. 6 Distribution frequencies of MnO component of equilibrium hematite and ilmenite, and ilmenite in the composite grains. 368 Tetsumaru Rays and Masayuki Otsuki

geneous when occurring as independent In the garnet and biotite zones, however, grains in one polished section. Minor com euhedral fine crystals and anhedral or ponents are CaO, Al2O3and MgO. Ilmenite subhedral coarse-grained ones (hundreds of in hematite-ilmenite intergrowth is also microns to a few millimeters) coexist in one homogeneous within one polished section polished section. In the hematite-free (Fig. 6). On the other hand, ilmenite occur schists of the garnet and biotite zones, ring as composite grains vary in composi sphene commonly forms composite grains tion, significantly from grain to grain as with ilmenite or rutile, or both, but in the shown in Fig. 6, in which the variation of hematite-bearing portions of the same zones, MnO content of ilmenite in ilmenite-rutile it forms composite grains with only rutile. composite grain is shown. Composite grains of TiO2-minerals: All Rutile: Rutile is widespread in the garnet possible types of composite grains of TiO2- and biotite zones. Its mode is 0.01-0.5 in minerals, ilmenite-rutile, ilmenite-sphene, volume per cent. In the hematite-bearing rutile-sphene and ilmenite-rutile-sphene are schists, it is often surrounded by sphene. commonly observed in the basic schists as in In the hematite-free schists, however, it the pelitic ones (Itaya and Banno, in prep.). commonly forms composite grains with Hematite-bearing portions, however, contain ilmenite or sphene, or both. It also com only sphene-rutile composite grains. The monly occurs as independent euhedral pillar modes of occurrence of these composite shaped grains in the hematite-bearing grains are summarized in Fig. 3, and the schists. SiO2, Al2O3, MnO, CaO and FeO relationships between their modes of (as total Fe) were detected with EPMA but occurrence and oxide mineral assemblages they are very low. are shown in Table 3. The chemical com Sphene: Sphene is common in all the zones, position of ilmenite in the composite grains although sometimes, it is absent in the vary significantly not only among the com hematite-bearing schists of the garnet and posite grains but also within one com biotite zones. In the chlorite zone, sphene posite grain. As an example, such a is euhedral and fine-grained (tens of microns). chemical heterogeneity of ilmenite in regard to MnO content is shown in ilmenite-rutile composite grain in specimen No. 9 in Figs. 6 and 7.

As earlier mentioned, in contrast with the local chemical heterogeneity of ilmenite in the composite grains, independent euhedral ilmenite grains in hematite-bearing schists are chemically homogeneous within one polished section. Hence, equilibrium domain during crystallization of the in dependent euhedral ilmenite grains (reaching at least a few centimeters) was distinctly Fig. 7 Variation of MnO content of ilmenite in one composite grain. "x" shows the point larger than that during crystallization of analyzed by EPMA. ilmenite in the composite grains (0.1 milli Stability and paragenesis of Fe-Ti oxide minerals and sphene in the basic schists 369

meters). In the pelitic schists, it was stage of metamorphism. Based upon these shown by Itaya and Banno (in prep.) that facts, the hematite-ilmenite intergrowth, the exchange equilibrium of Mn and Fe2+ core of zoned hematite and independent have been maintained between the rim of euhedral ilmenite will be called, hereafter, zoned garnet and coarse-grained indepen as "equilibrium hematite and ilmenite". dent euhedral ilmenite and that the latter In Table 4, the chemical compositions of was in equilibrium with the silicate phases this type of hematite and ilmenite are shown. during progressive stage of metamorphism. Those of some ilmenite in the composite This was not so in the case of ilmenite in grains and the rim of zoned hematite are the composite grains. Since the chemical also shown in order to compare with equi features of euhedral ilmenite and composite librium hematite and ilmenite. The Mn/ grains in the basic schists are the same as Fe2+ atomic ratios of the hematite, ilmenite those in the pelitic schists, we consider that and magnetite are plotted on a line parallel euhedral ilmenite in the basic schists was in to the Asemi valley (Fig. 3). We see in this equilibrium with garnet and other silicate figure that the Mn/Fe2+ratios of equilibrium phases during progressive metamorphism, hematite and ilmenite tend to decrease with and that ilmenite in the composite grains increasing grade, although each ratio varies was formed at a retrogressive stage of meta on large scale owing to the effects of the morphism. The fact that minerals formed bulk rock compositions. As the hematite at the retrograde stage of metamorphism free schists do not contain independent are locally heterogeneous is seen not only in euhedral ilmenite (equilibrium ilmenite), oxides, but also in silicates such as garnet, the Mn/Fez} ratios of ilmenite in the com epidote and amphibole. posite grains can not be compared with that The chemical compositions of ilmenite of the equilibrium ilmenite, but these ratios hematite intergrowth and the core of zoned are distinctly larger than that extrapolated hematite are homogeneous with respect to from the trend of Mn/Fe2+ ratios of the grains in one polished section. This fact equilibrium ilmenite in the hematite-bearing suggests that their equilibrium domains schists (Fig. 3). In the pelitic schists of the were almost the same in size as that of present area, ilmenites in the ilmenite-rutile euhedral i menite. Therefore, they might composite grains are distinctly richer in MnO have been in equilibrium with other oxides than the equilibrium ilmenite which was in and major silicates during progressive equilibrium with major silicate phases during metamorphism as euhedral ilmenite does. progressive metamorphism. Similar process This is supported by the systematic may have operated in hematite-free basic partitioning of Mn and Fez} between oxide schists, but also the equilibrium ilmenite minerals, as will be described later. The were consumed by the formation of the rim of zoned hematite is heterogeneous even composite grains. in one grain and its composition is similar As rutile and sphene have limited range to that of hematite occurring in low-grade of solid solutions, chemical informations sug zone. As the chemical composition of fine gestive of the origin of the composite grains grained hematite is similar to that of the composed only of rutile and sphene are not rim of zoned hematite, both have been available. We, however, consider that formed at the same stage, i.e., retrograde rutile armoured by sphene in the biotite zone 370 Tetsumaru Itaya and Masayuki Otsuki

Fig. 8 Microphotographs and Sketches of Fe-Ti oxide minerals a Ilmeno-hematite - hemo-ilmenite intergrowth b Hematite-rutile intergrowth c Coexisting magnetite, hematite, ilmenite and rutile in specimen No. TH 17. Magnetite and rutile occur 0.2mm apart. was formed at the retrograde stage by the inations of inclusions in garnet suggest that decomposition of rutile, because the exam sphene was stable only in the lower-grade Stability and paragenesis of Fe -Ti oxide minerals and sphene in the basic schists 371

zone of the present area , as will be described 0.45•}0.15, 0.038•}0.030 and 0.0097•} later. Thus, the minerals in composite 0.0036, respectively. Among these minerals,

grains have to be distinguished from those the tendency to incorporate Mn decrease in

in equilibrium with major silicate phases in the following order, ilmenite, hematite and

considering the parageneses. magnetite. Thus, the systematic distri

bution of Mn among magnetite, hematite

Mn-Fe2+ PARTITIONING BETWEEN and ilmenite in the hematite-bearing schists

MAGNETITE, HEMATITE AND ILME suggests that they attained chemical

equilibrium during the Sanbagawa meta NITE morphism. If magnetite and the "equilibrium

hematite and ilmenite" mentioned above STABILITY OF Fe-Ti OXIDE MINERALS

were in equilibrium with one another, they AND SPHENE BASED UPON INCLUSIONS

should also have been in exchange equi IN GARNET

librium. These minerals have some ranges We can obtain the information of the of Mn-Fe2+ substitution, and their parti stabilities of Fe-Ti oxides and sphene from tioning are examined below. the observations of mineral species as The partitioning for Mn and Fe2+ inclusions in garnet (Itaya and Banno, in between magnetite, hematite and ilmenite prep.). is described in terms of apparent partition Garnet is not common but is observed coefficient K'Mn-Fe2+ defined as: in the hematite-free basic schists (12 among

K•Lhm-ilMn-Fe2+=(XMn/XFe2+)hm/(XMn/XFe2+)il 52 specimens in the biotite zone), and it (1) is very rare in the hematite-bearing ones (4

among 52 specimens in the zone). Garnet

in basic schists is distinctly zoned, having

MnO-rich core and -poor rim (Fig. 9), as

does the garnet in the pelitic schists of the

present area (Itaya and Banno, in prep.). K•Lmt-ilMn-Fe2+=(XMn/XFe2+)mt/(XMn/XFe2+)il (3) where X denote mole fractions of relevant Partition coefficient between the garnet rim

end members. These partition coefficients and chlorite for Mg-Fe in the basic schists

correspond to the following exchange reac of the biotite zone is nearly constant, 0.1

tions, respectively: •} 0.05 (Otsuki, 1977), and this value is similar

(MnTiO3)hm+(FeTiO3)il =( FeTiO3)hm+(MnTiO3)il ...... (4)

(MnFe23+O4)mt+(FeTiO3)hm =(Fe2+Fe23+O4)mt+(MnTiO3)hm..(5)

(MnFe23+O4)mt+(FeTiO3)il

=(Fe2+Fe23+O4)mt+(MnTiO3) (6) The relationships between the values K•LMn-

F2+ in each pair and the metamorphic

grade are shown in Fig. 3. The average values

of K•Lhm-ilMn-Fe2+' K•Lmt-hmMn-Fe2+ and K•Lmt-ilMn-Fe2+ are Fig. 9 Typical zoning pattern of garnet

K•Lmt-hmMn-Fe2+=(XMn/XFe2+)mt/(XMn/XFe2+)hm (2) 372 Tetsumaru Rays and Masayuki Otsuki to that of garnet-chlorite pair in the pelitic in the same manner as in the pelitic schists schists as reported by Kurata and Banno of the present area. These facts suggest (1974) and Higashino (1975). Therefore, it that the stability fields of Fe-Ti oxides and is considered that the garnet rim was in sphene in the hematite-bearing schists are equilibrium with other major silicate distinctly different from those in the he phases and that the core with high MnO matite-free ones. The fact that garnet content has been formed at lower tem commonly includes sphene as well as Fe-Ti perature than the rim. oxides in the basic and pelitic schists, shows The modes of occurrence of the mineral that when these minerals coexisted with the species present as inclusions in garnet are growing garnet, they occur as inclusions in summarized in Fig. 3. Frequencies of garnet. And this also suggests that, among oxide mineral assemblages found in each of these minerals, those which are not included garnet grain are seen in Table 5. Garnet in garnet did not coexist with the garnet. in the hematite-bearing schists commonly The mineral species included in the Mn-rich contains hematite, ilmenite and rutile, and core were captured at lower temperatures sometimes magnetite, but not sphene. In this group of schists, hematite, ilmenite and Table 5 Frequency of finding assemblages rutile crystals are included in all parts from of Fe-Ti oxide minerals and the Mn-rich core to the Mn-poor rim of sphene included in garnet. garnet (Fig. 10). On the other hand, garnet in the hematite-free schists include commonly rutile and sphene, and sometimes ihnenite, but not magnetite. Garnet porphyroblasts in this group contain sphene and rutile crystals only in the Mn-rich core, and ilmenite only in the Mn-poor rim (Fig. 10),

Fig. 10 Location of hematite, ilmenite, rutile and sphene included in garnet. Hema tite, ilmenite and rutile in hematite-bearing basic schists (No. 23) are included in all part from the core to the rim of garnet; in hematite-free ones (No. 19), sphene and rutile are included only in Mn-rich core, and ilmenite, only in Mn-poor rim. Stability and, paragenesis of Fe-Ti oxide minerals and sphene in the basic schists 373 than those in the Mn-poor rim. Therefore, hematite and ilmenite. A continuous solid in the hematite-bearing schists, hematite, solution at least between (100%FeTiO3 ilmenite, rutile and magnetite were stable 0%MnTiO3) and (50%FeTiO3 50%MnTiO3) in the garnet and biotite zones, but sphene exists at the temperature range of the San which is not included in garnet coexists with bagawa metamorphism. Nos. 52, 55 and only hematite in the chlorite zone. In the TH32 come from the middle garnet zone, hematite-free ones, it is considered that Nos. 38 and 40 were collected near the ilmenite was stable only in the higher grade biotite isograd and Nos. 24 and TH17, in zone, and sphene and rutile which are in the middle biotite zone. Among these cluded only in the Mn-rich core coexisted in samples, Nos. 52, 38 and TH32 are the garnet zone, but not in the biotite spessartine- and hematite-bearing quartz zone. In the chlorite zone, sphene was schists. It is seen in Fig. 12 that the stable. The schematical stability fields of miscibility gap between hematite and these minerals are shown in Fig. 11. The ilmenite on the FeTiO3-MnTiO3-Fe2O3 stability relations of Fe-Ti oxides and sphene system is slightly dependent on the MnTiO3 in the hematite-free basic schists are similar content and different between the pairs to those of the pelitic schists of the present from the garnet zone and from the biotite area as described by Itaya and Banno (in zone. prep.). By extrapolating the gap on the join FeTiO3-Fe2O3from Fig. 11, average chemical compositions of coexisting hematite and ilmenite in the middle garnet zone, near the biotite isograd and in the middle biotite zone are estimated to be about Il25Hm75- Il93Hm7, Il31Hm69- Il89Hm11and Il36Hm64- Il83Hm17, respectively. Fig. 13 shows schematical miscibility gap between FeTiO3 and Fe2O3 as the function of metamorphic grade. We see in this figure that the misci Fig. 11 Schematical stability fields of magne bility gap of the binary system FeTiOa Fee tite, hematite, ilmenite, rutile and O3 are distinctly asymmetric. This is in sphene of Sanbagawa basic schists in harmony with the petrographic observations progressive metamorphism. in metaquartzite of Western New Hamp shire (Rumble, 1973) and in metamorphic MISCIBILITY GAP BETWEEN HEMATITE rocks of Southeastern Sierra de los Filagr'es, AND ILMENITE SE Spain (Westra, 1970) as well as experi As seen in Table 4, hematite and ments of Lindsley (1973). Although Fig. ilmenite can be treated as the ternary solid 13 is semi-quantitative, it clearly shows solution with the following end members: that the miscibility gap between hematite FeTiO3, MnTiO3 and Fe2O3. Their chemical and ilmenite narrows with increase of compositions are plotted on the FeTiO3 metamorphic temperature. Rumble (1973) MnTiOs Fe2O3 diagram (Fig. 12). Tie lines has shown that coexisting hematite and in this figure show coexisting equilibrium ilmenite in metaquart7ite. of the garnet and 374 Tetsumaru Itaya and Masayuki Otsuki

Fig. 12 Chemical compositions of- ilmenite and hematite plotted on FeTiOa MnTiOa Fe2O3 diagram. Tie line shows coexisting "equilibrium hematite and ilmenite"; specimen Nos.52, 55 and TH 32(solid line) are from the middle garnet zone, Nos. 38 and 40 (broken line), collected near the biotite isograd, and Nos, 24 and TH17 (dotted line), collected in the middle biotite zone; No. 11 is quartz schists with heamtite+rutile assemblage in the biotite zone; Open circles show ilmenite or hematite coexisting with magnetite or rutile, or both. staurolite zone in Western New Hampshire tion coefficient for Mn-Fe2+ between coexist are Il30Hm70 and Il80Hm10, respectively. ing hematite and ilmenite of the metaquart This gap is similar to that of coexisting zite of Western New Hampshire (Rumble, hematite and ilmenite near the biotite 1973) lies between those of the garnet and isograd of the present area. This is in biotite zones of the present area. The concordance with the fact that the parti- view that the subsolidus equilibrium of the garnet and staurolite zone of Western New Hampshire and biotite isograd of the Sanbagawa are similar to each other is rather natural, because the biotite isograd in the latter corresponds to the temperature at which the assemblage chlorite+almandine (Ca-bearing)+biotite is stable.

PARAGENESIS OF Fe-Ti OXIDE MINE RALS ON THE FeO-Fe2O3-TiO2-MnO SYSTEM The frequencies of Fe-Ti oxide mineral assemblages are shown in Table 3. In the Fig. 13 Extrapolated miscibility gap be tween ilmenite and hematite on previous sections, it was mentioned that in FeTiOe Fe2O2 system. hematite-free schists, ilmenite and rutile Stability and paragenesis of Fe-Ti oxide minerals and sphene in the basic schists 375 were commonly observed in the biotite zone, nificant amounts of MnO component though but they were considered not to have coexist magnetite and rutile are practically free of ed during progressive metamorphism . On MnO. Therefore, the paragenesis of these the other hand, in the hematite-bearing ones, oxide minerals must be treated in terms of the stability ranges of magnetite, hematite, the FeO-Fe2O3-TiO2-MnOsystem. ilmenite and rutile overlap in the garnet and In the biotite zone, magnetite+rutile is biotite zones. Inclusions in garnet and observed in 6 samples, three of which are partitioning for Mn-Fe2+ among magnetite, in four phase assemblage (TH17, 24 and TH hematite and ilmenite show that the as 16). The four phase assemblage was semblages hematite+rutile, magnetite+ observed in every part of a polished section, hematite, magnetite+hematite+ilmenite and magnetite and rutile sometimes occur and hematite+ihnenite+rutile were stable 0.2mm apart (Fig. 8c). The MnTiO3: during progressive metamorphisn. How FeTiO3: Fe2O3ratios of ilmenite and hematite ever, our observations are still insufficient in four phase assemblage are 3.9:79.0:17.1 to decipher the stability of assemblages and 1.2:36.7:62.1, respectively. In Fig. 12, which include magnetite+rutile, i.e., hematite and ilmenite in magnetite+he magnetite+hematite+ihnenite+rutile, mag matite+ilmenite or rutile+hematite+ilme netite+hematite+rutile and magnetite+ nite assemblages are plotted above the ilmenite+rutile. hematite-ilmenite join of TH17 and 24, and In the FeO-Fe2O3-TiO2 system, mag this is consistent with the presence of four netite+rutile and hematite+ilmenite as phase tetrahedron in the system in question. semblages are incompatible. Based upon Furthermore the MnTiO3:FeTiO3:Fe2O3 thermodynamic claculations, Lindsley (1962) ratios of hematite in two magnetite+rutile considered that magnetite+rutile was stable +hematite assemblages in the garnet zone at lower temperature instead of hematite+ are 0.3:28.6:71.1 and 1.2:28.2:70.6, i.e., ilmenite. Kanehira et al. (1964) observed they are poorer in MnTiO3 than those of hematite +ilmenite association in the San the four phase assemblage. Therefore, bagawa schists, and considered: that he the presence of magnetite+rutile and he matite+ilmenite assemblage was stable matite+ilmenite in the biotite zone of the at least in the albite-epidote amphibolite present area can be explained in terms of facies. Mielk and Schreyer (1972) reported four component system. In the Mn-free that, in regionally metamorphosed pelites system, hematite and ilmenite are in of Fichtelegebirge, Germany, magnetite and compatible. In this sense, the miscibility rutile from detrital ilmenite grains occured in gap between ilmenite and hematite estimat a lower grade zone, but, in a higher grade ed in the previous section is a metastable zone, ilmenite and hematite occured instead. extension of the miscibility gap. Rumble (1973) observed magnetite+rutile Two samples of hematite-free schists of assemblage in the garnet and staurolite the biotite zone contain magnetite, rutile zone of Western New Hampshire, and and ilmenite, but ilmenite and rutile occur considered that the assemblage was formed only as composite grains. Since we con from ilmeno-hematite by cooling. As sider that rutile in the hematite-free schists mentioned before, however, in the present did not coexist with ilmenite in a prograde area, ilmenite and hematite contain sig stage, these two samples might have had 376 Tetsumaru Itaya and Masayuki Otsuki magnetite+ilmenite assemblage during the garnet zone are not contradictory with progressive metamorphism. In the garnet the four phase tetrahedron determined in zone, three samples (Nos. 52, 38 and 40) the biotite zone. The schematical para contain four phases. However, in No. 52, geneses of these oxide minerals are shown in the four phase assemblage is actually Fig. 14. estimated from the inclusions in garnet. Magnetite, hematite and ilmenite occur as DISCUSSION inclusions in one garnet grain, while hema The discussion on the paragenesis among tite, ilmenite and rutile, in another. There magnetite, hematite, ilmenite and rutile fore, it is not decisive whether the four" in a temperature range of low- to medium phases were in equilibrium with each other. grade metamorphism was presented in the In No. 38, magnetite, hematite and ilmenite previous sections, let us then consider here occur as independent grains, but only one the stability of these minerals as well as rutile grain being a few micrometer in sphene in multi-component metamorphic diameter was observed. In No. 40, in rocks. dependent hematite and ilmenite, and In spite of the possibility of strong bulk fine-grained rutile, which forms composite composition dependence of the appearence grain with sphene, occur, but fine grained of these accessory minerals, there are some magnetites concentrate forming one layer relationships between their occurrences and elongated to a direction of schistosity on the nature of host rocks. One of them is polished section, and, in this layer, hematite, the rarity of rutile in the hematite-free ilmenite and rutile are not observed. schists, both in basic schists described in Hence we do not dare to conclude that the this paper and in pelitic schists described four phase assemblage was stable in these elsewhere. In general, higher oxygen specimens. The compositions of coexist fugacity favors rutile+hematite by the ing hematite and ilmenite in other samples in following reaction 2FeTiO3+1/2O2=Fe2O3+2TiO2 (7) Thus, in the hematite-bearing schists, rutile appears even if TiO2 content is low, while in the hematite-free schists, rutile occurs only in rocks with high Ti/Fe ratio. In the hematite-bearing schists, minerals on both side of equation (7) coexist and buffer the fugacity of oxygen at a value higher than magnetite+hematite buffer (Mariko et al., 1975). In the hematite-bearing basic schists, rutile and hornblende began to occur from the middle garnet zone. Therefore, the following schematized chemical reaction Fig. 14 Schematical paragenesis of Fe-Ti oxide may explain the formation of rutile at the minerals on the FeO-Fe2O2-TiO2-MnO system. expense of sphene as discussed by Banno Stability and paragenesis of Fe-Ti oxide minerals and sphene in the basic schists 377 and Kanehira (1961). 6. On the contrary, we consider that the sphene+chlorite+quartz= chemical reactions among the silicate and hornblende+rutile+water (8) oxide minerals, such as reaction (8), deter Although hornblende and rutile appear mine the parageneses of Fe-Ti oxide minerals. almost at the same grade, it is required However, the assemblage of sphene+rutile+ that reaction (8) takes place only after horn calcite+quartz observed in some hematite blende becomes a stable phase by the free basic schists as well as the pelitic reaction among glaucophane, chlorite and schists of the garnet zone (Itaya and Banno, epidote. Because the stability of rutile is in prep.) buffers the fugacity of CO2 and in part estimated from its occurrences as helps us to estimate it in the same manner inclusions in garnet, it is at present im as Ernst (1972). On the other hand, the possible to determine whether or not rutile hematite-bearing basic and quartz schists, was formed at higher temperature, even in which rutile did not coexist with sphene, only slightly higher than hornblende. We are in higher CO2 fugacity than the pelitic observed that in the Kotu district in eastern schists and hematite-free basic schists with Shikoku, rutile occurs in glacuophane minerals in both sides of equation (9). Both schists, and hence equation (8) may explain groups of schists are sometimes observed the general formative pattern of rutile but in the same outcrop of the garnet zone. cannot explain the details. In the hematite- This suggests that CO2 component in fluid bearing and hematite-free basic schists and phases was internally controlled at least pelitic schists of the present area, rutile in a scale on the order of an outcrop during and ilmenite must be formed at the expense the Sanbagawa progressive metamorphism. of sphene. This may be in parageneses of sphene and other silicate minerals (e.g. ACKNOWLEDGEMENTS chlorite). However, since the influence The authors sincerely thank Dr. S. of the decomposition of sphene to other Banno for his continuous encouragement and silicates is very small, it is at present the critical comments on this manuscript. impossible to clarify the mechanism. Their thanks are also due to Dr. H. The formation of rutile from sphene can Onuki for his warm encouragement to T. also be explained by the following reaction Itaya and Dr. G.R. Balce for the critical CaTiSiO5+CO2=TiO2+CaCO2+SiO2 reading of this manuscript. ( 9) Qualitative analyses of some minerals However, the frequency of finding calcite in discussed in this manuscript were done the basic schists does not appear to be related with a microprobe (model JEOL 50A) of to the metamorphic grade as seen in Table Waseda University. The authors thank Prof. N. Imai for giving T. Itaya an oppor Table 6 Frequency of calcite-bearing speci mens in the chlorite, garnet and tunity to use the instrument, and Mr. Y. biotite zones; denominator is total Ogasawara for assisting T. Itaya in operat numbers of examined samples. ing it.

REFERENCES Banno, S. and Kanehira, K. (1961) Sulfide and 378 Tetsumaru Itaya and Masayuki Otsuki

oxide minerals in the schists of the Sanbagawa Soc. Japan, 57, 177-190. and central Abukuma metamorphic terranes. Kurata, H. (1972) Local chemical heterogeneity Japan. Jour. Geol. Geogr., 32, 331-348. of chlorite in Sanbagawa pelitic schists from Banno, S. and Kurata, S. (1972) Distribution of Sazare area, central Shikoku. Jour. Geol. Ca in zoned garnet of low-grade pelitic schists. Soc. Japan, 78, 653-657. Jour. Geol. Soc. Japan, 78, 507-512. Kurata, H. and Banno, S. (1974) Low-grade Buddington, A.F., Fahey, J. and Vlisidis, A. (1963) progressive metamorphism of pelitic schists of Degree of oxidation of Adirondack iron oxide the Sazare area, Sanbagawa metamorphic ter and iron titanium oxide minerals in relation rain in central Shikoku, Japan. Jour. Petrol., to petrogeny. Jour. Petrol., 4, 138-169. 15, 361-382. Buddington, A. F. and Lindsley, D.H. (1964) Lindsley, D. H. (1962) Investigations in the Iron-titanium oxide minerals and synthetic system FeO-Fe2O3-TiO2. Carnegie Inst. Wash. equivalents. Jour. Petrol., 5, 310-357. Year Book, 61, 100-106.

Carmichael, I. S.E. (1967) The iron-titanium •\(1963) Fe-Ti oxides in rocks as thermo oxides of salic volcanic rocks and their as meters and oxygen barometers. Carnegie sociated ferro-magnesian silicates. Contr. Inst. Wash. Year Book, 62, 60-66.

Mineral. Petrol., 14, 36-64. •\ (1965) Iron-titanium oxides. Carnegie Ernst, W. G., Seki, Y., Onuki, H. and Gilbert, M.C. Inst. Wash. Year Book, 64, 144-148. (1970) Comparative study of low grade •\ (1973) Delimitation of the hematite metamorphism in the California Coast Range ilmenite miscibility gap. Geol. Soe. Am. Bull., and the outer metamorphic belt of Japan. 84, 657-661. Geol. Soc. Amer. Memoir 124. Mariko, T., Tanaka, K. and Itaya, T. (1975) Oxide Ernst, W. G. (1972) CO2-poor composition of the and sulfide minerals in pelitic and psammitic fluid attending Franciscan and Sanbagawa schists from the Nagatoro district, Saitama low-grade metamorphism. Geochim. Cosmo Prefecture, Japan. Jour. Japan Assoc. Min. chim. Acta, 36, 497-504. Pet. Econ. Geol., 70, 413-424. Higashino, T. (1975) Biotite zone of Sanbagawa Mielke, H. and Schreyer, W. (1972) Magnetite metamorphic terrain in the Shiragayama area, rutile assemblages in metapelites of the central Shioku, Japan (in Japanese). Jour. Fichtelgebirge, Germany. Earth Planet. Sci. Geol. Soc. Japan, 81, 653-670. Lett., 16, 423-428. Itaya, T. (1975) Pyrrhotite from the Sanbagawa Otsuki, M. (1977) Progressive metamorphism of pelitic schists of the Shiragayama area, central basic schists of the Asemi river area, San Shikoku, Japan.. Mineral. Jour. Japan, 8, bagawa metamorphic terrain in central Shikoku. 25-37. MSc. thesis. Kanazawa University. Itaya, T. and Banno, S. (in prep.) Stability of TiO2- Rumble, D. (1973) Fe-Ti oxide minerals from minerals in the pelitic schists of the Sanbagawa regionally metamorphosed quartzites of metamorphic belt in central Shikoku, Japan. Western New Hampshire. Contr. Mineral. Kanehira, K., Banno, S. and Nishida, K. (1964) Petrol., 42, 181-195.

Sulfide and oxide minerals in some metamor •\ (1976) Oxide minerals in metamorphic phic terrains in Japan. Japan, Jour. Geol. rocks. Short course notes, volume 3, Oxide Geogr., 35, 175-191. Minerals, edited by Douglas Rumble, III. Katsura, T., Kitayama, K., Aoyagi, R. and Mineral. Soc. Am., R1-R20. Sasajima, S. (1976) High temperature experi Taylor, R. W. (1964) Phase equilibria in the ments related to Fe-Ti oxide minerals in system FeO-Fe8O8 TiO2 at 1300•Ž. Am.

volcanic rocks (in Japanese). Bull. volcan. Soc. Mineral., 49, 1916-1030. Japan, 21, 31-56. Westra, L. (1970) The role of Fe-Ti oxides in Kawachi, Y. (1968) Large-scale overturned plurifacial metamorphism of Alpine age in structure in the Sanbagawa metamorphic zone the South eastern Sierra de los Filabrrs, SE in central Shikoku. Jour. Geol. Soc. Japan, Spain. Ph. D. Thesis. Amsterdam University. 74, 607-616. Yui, S. and Shoji, T. (1976) Computer programs Kojima, G. (1951) Stratigraphy and geological used in the ZAF correction (in Japanese).

structure of the crystalline schists region in Jour. Mineral. Soc. Japan, Spec. Issue., 12, central Shikoku (in Japanese). Jour. Geol. 70-81. Stability and paragenesis of Fe-Ti oxide minerals and sphene in the basic schists 379

四 国 中 央 部 三 波 川 塩 基 性 片 岩 中 のFe-Ti酸 化 鉱 物 及 びSpheneの 安 定 関 係 と 共 生 関 係

板 谷 徹 丸 ・大 槻 正 行

四 国 中 央 部 白 髪 山 地 域 の 三 波 川 塩 基 性 片 岩 中 に は, hematite, ilmenite, rutile spheneが 普 遍 的 に 生 じ て お り, magnetiteも と き ど き観 察 さ れ る 。 酸 化 鉱 物 と珪 酸 塩 鉱 物 の 鉱 物 組 合 せ はhematiteの 存 否 で 明 ら か に 異 な る 。 hematiteを 含 む 塩 基 性 片 岩 の 場 合,低 変 成 度 地 域 で は, hematite+riebeckitic actinoltie+sphene又 はhematite

+crossite+spheneで あ り,高 変 成 度 地 域 で は, hematite+ilmenite+rutile+hornblende±magnetiteで あ る。 hematiteを 含 ま な い 塩 基 性 片 岩 の 場 合,低 変 成 度 地 域 で は, actinolite+sphene±rutileで あ り,高 変 成 度 地 域 で

は, hornblende+ilmeniteで あ る(albite, quartz. epidote. chlorite. phengiteは 除 い て い る)。 Fe-Ti酸 化 鉱 物 及 びsphereの 安 定 関 係 と 共 生 関 係 を,こ れ ら の 鉱 物 の 産 状,化 学 組 成 及 びMn-Fe2+の 分 配

関 係 を 使 っ て 明 ら か に し た 。hematiteを 含 む 塩 基 性 片 岩 で は, magnetite, i㎞enite及 びrutileがgametと biotite帯 で 安 定 で あ っ た 。 対 し て, hematiteを 含 ま な い 塩 基 性 片 岩 で は, ilmeniteはbiotite帯 で の み 安 定 で

あ り, rutileはgarnet帯 で の み 安 定 で あ っ た 。spheneは 低 変 成 度 地 域 に の み 安 定 で あ っ た 。 高 変 度 地 域 に 生 じ て い るsphereは 後 退 変 成 作 用 期 に 形 成 さ れ た 。 Fe-Ti酸 化 鉱 物 の 安 定 な鉱 物 組 合 せ は, magnetite+hematite, hematite+rutile, magnetite+hematite+ilmenite,

hematite+ilmenite+rutile, magnetite+hematite+ruble及 びmagnetite+hematite+ilmenite+rutileで あ る 。 ilmeniteとhematiteはMnOを 相 当 量 含 む の で,こ れ ら の 鉱 物 の 共 生 関 係 は, FeO-Fe2O3-TiO2-MnO系 で の み

記 述 さ れ る 。 Fe-Ti酸 化 鉱 物 及 びspheneの 安 定 関 係 は 圧 九 温 度,酸 素 フ ュ ガ シ テ ー に よ っ て の み 支 配 さ れ る の で な く,

母岩の化学組成によって も支配 される。三波川変成作用における流体相中のCO2成 分は内部で支配 されていた。