Magnesioferrite-Olivine Rock and Monticellite-Bearing Dunite from the Iwanai-Dake Alpine-Type Peridotite Mass in the Kamuikotan Structural Belt, Hokkaido, Japan

J. Japan. Assoc. Min, Petr. Econ. Geol. 77, 23-31. 1982

Magnesioferrite-olivine rock and monticellite-bearing dunite from the Iwanai-dake alpine-type peridotite mass in the Kamuikotan structural belt, Hokkaido, Japan

JITSUYA NAGATA Department of Earth Sciences, Kanazawa University, Kanazawa 920, Japan

Magnesioferrite, monticellite, perovskite and calcium brittle mica occur in the alpine- type Iwanai-dake peridotite mass in the Kamuikotan structural belt, Hokkaido. These minerals were not primary phases, but were produced through metasomatic processes at a later stage of the granulite facies equilibration.

Introduction

Magnesioferrite, monticellite, perovskite and calcium brittle mica were found in the Iwanai-dake peridotite mass, an alpine-type intrusion in Hokkaido. As these minerals have not been described in alpine-type peri dotite, their modes of occurrence, chemis tries and paragenetic relations will be described in this paper. The Iwani-dake mass (Fig. 1) is intrud- ed in the Kamuikotan structural belt which is a melange zone consisting of high-pressure metamorphic rocks, low-pressure metamor Fig. 1. Locality of the Iwanai-dake peridotite phic rocks derived from an ophiolitic suite, mass, Hokkaido. and ultramafic rocks (Banno et al., 1978; Asahina and Komatsu, 1979). The Iwanai- ing to Arai (1978), equilibrium temperature dake mass is composed largely of dunite and of ultramafic rocks is estimated to 600 to harzburgite and is almost free from ser 700°C using the olivine-Ca-rich clinopyroxene pentinization (Bamba, 1955; Kato, 1978; geothermometer and olivine-spinel geother Niida and Kato, 1978; Arai, 1978). Gener mometer. ally, dunite and harzburgite form layered structures. Small amounts of chromitite Mode of occurrence, petrography and are also present. Ultramafic rocks of the mineral chemistry Iwanai-dake mass are intensely deformed, A large quarry in the center of the and translation lamellae of olivine and Iwanai-dake peridotite mass contains orthopyroxene are very common. Accord- exposures of remarkably fresh dunite and

(Manuscript received September 21, 1981) 24 Jitsuya Nagata

harzburgite. mode of occurrence in the outcrop is shown Magnesioferrite occurs in one magnesio- in Fig. 2. Magnesioferrite-olivine rock ferrite-olivine rock, which consists of single consists of magnesioferrite and olivine in layer concordant with well-developed rhyth- equal amounts, and minor amounts of mic layering of dunite and harzburgite. calcium brittle mica (clintonite or xantho- Its thickness does not exceed 20 cm. The phyllite) and serpentine. Fine calcium

Table 1. Selected EPMA analyses of minerals in magnesioferrite-olivien rock and IW 328 dunite from the Iwanai-dake peridotite mass

Fig. 2. Mode of occurrence of th e magnesioferrite -olivine rock i n outcrop Magnesioferrite-olivine rock and others from the peridotite mass 25

Fig. 3. Range of spinet solid solution. Al-Cr-Fe3+ (A) and Mg-Fez+ (B) atomic ratios of magneisoferrite (solid circle) in magnesioferrite-olivine rock, and of spinel core (solid diamond) and rim (open diamond) in dunite, specimen IW 328, from the Iwanai dake mass. Tie lines connect core-rim pairs.

brittle mica (10 X 30 ƒÊm) sometimes occurs olivine. Specimen IW 328 studied in detail in monticellite-bearing dunite. Table 1 will be described below.

shows the representative analyses, and Fig. 3 Dunite, specimen IW 328, is free from

illustrates the range of spinet solid solu- severe serpentinization and usually exhibits tions. Olivine is very magnesian with the equigranular texture as shown in Fig. 4, Fo molecule ranging from 96.7 to 98.7 per though olivine porphyroclasts do occur. cent and has high CaO contents, 0.1 to 0.2 Selected microprobe analyses of the min

weight percent. erals in this sample are listed in Table 1. Monticellite was detected in 2 samples Olivine, Fo91 .1, is mostly 0.05 to 0.4 mm of 400 peridotite sections from the Iwanai dake quarry examined by electron probe. One is a dunite, specimen I W 328, and contains monticellite, Ca-rich diopside, perovskite, ferripargasite, Fe3+-rich spinel, calcium brittle mica, andradite, magnetite, ilmenite and calcite as well as olivine, brucite and serpentine. The other is a dunite, specimen IW 104, and contains olivine, diopside, chromian spinel, brucite and serpentine. It occurs about 20 m above the magnesioferrite-olivine rocks Fig. 4. Photomicrograph of IW 328 dunite, showing equigranular texture. Crossed mentioned previously. Very fine monti- nicols. In this photo, all grains are cellite grains are attached to the rim of olivine. 6 jitsuyia Nagata

in diameter. Fig. 5 shows the Cs.U contents CaO content of olivine in grain A _'- eases in olivine of this sample and those in towards the monticellite-olivine iiterface monticellite-free dunite and harzburgite (Fig. 6). However, this relationship iay be from the Iwanai-dake mass. The CaO reversed as in grain B, in which the Ca content of the olivine from monticellite- content of olivine increases towards monti- bearing dunite ranges from 0.1 to 0.4 weight cellite. In Fig. 7, parallel lamellae of mon percent, and is distinctly higher than that of ticellite in olivine are seen using a back- olivine from monticellite-free dunite, less scattered electron image and a CaKa X-ray than 0.1 weight percent. image of an olivine grain. Even though: Monticellite has several modes of oc the crystallographic orientation of the currence, (1) irregular-shaped and 100 to monticellite in the olivine could not be

200ƒÊm across at olivine grain boundary , (2) determined (we could find only one olivine as composite grains with diopside about porphyroclast to contain such lamellae), 50ƒÊm across, and (3) as lamellae in porphyro- it is possible that they are exsolution

clastic olivine (3 mm in diameter) (Fig . 7). Monticellite of mode (3) is too minute to

analyze with electron probe , and chemical data for modes (1) and (2) are presented . The Ca/(Ca+Mg-f-Fe*) atomic ratio are around 0.49, and the Mg/(Mg-r-Fe*) ratio are 0.79, which are lower than the value of coexisting olivine , 0.91 (Fe* is total Fe). Mode (1) is illustrated in Fig . 6-a which shows a Ca profile across the interface of monticellite and neighboring olivine . The

Fig. 5. Histograms of CaO conte Fig. 6. Compositional nts of olivine profiles across the int erface of monticellit in harzburgite (A), dunite (B) and e and neighboring monticellite-bearing dunite (IW 328) olivine in regard to C a. Ca ntent of olivine decreases (a) or i (C) from the Iwanai-dake mass . N, nci s (b) t number of analyses . owards monticellite . Mo; ?? cellite, O1; olivine .

2 Magnesioferrite-olivine rock and others from the peridotite mass 27

Fig. 8. Al-Cr-(Mg+Fe*) atomic ratios of diopside from the Iwanai-dake mass. Fe*: total iron.

from chemistry alone. Perovskite occurs as isolated rounded grain and 50,um across at most. Andradite is included in calcite, and minute magnetite and ilmenite grains Fig. 7. Backscattered electron image (A) and CaKa X-ray image (B) of an olivine are scattered all around the thin section. porphyroclast. Discussion lamellae. Most of accessory minerals of the du Chemical analyses of diopsides show nites described above are rare minerals, and that the (Ca-0.5 Al)/(Ca-0.5A1+Mg+Fe*) their known modes of occurrence will be ratio, i.e. Ca/(Ca+Mg+Fe*) ratio corrected briefly reviewed below. for tschermakite substitution, is 0.49 to 0.50 Magnesioferrite has been known notably and is higher than those of diopside from from fumaroles of Vesuvius (Deer, et al., the ordinary dunite and harzburgite of the 1962a) and dolomitic marble at Lhngban, same mass, 0.44 to 0.47 (Fig. 8). Sweden (Von de Pijpekamp et al., 1969). Pargasite has a composition : In Japan, magnesioferrite was identified in a Na0,9Ca2.0 (Mg3.9Fe2+0.1)(Fe3}0.4T10.1Cr0.1A10.4) magnetite-bearing pyrite ore of Bessi and (S16.0A12.0)O22 (OH) 2 Shingu mines by Curie point measurement The range of solid solutions of the and ore microscopy (Yamaoka, 1962). spinel is shown in Fig. 3. Fe3+/(Al+Cr+Fe3+) Monticellite is riot rare in skarns and ratio at the rim is about 0.35 and is higher carbonatites, but from mafic and ultramafic than that of the core, 0.15. rocks it has been described only from alonite At the margin of spinel, very fine-grain- in the Oka area, Canada (Gold, 1967), and ed calcium brittle mica is often attached. monticellite peridotite in the Highwood Its small grain size prevents us from deter- Mountain alkaline intrusive complex, Montana mining whether it is clintonite or xanth- (Buie, 1941) . ophyllite which cannot be distinguished In addition to the occurrences in skarns 28 Jitsuya Nagata

and carbonatites, perovskite has been ratios of olivine and spinel, respecti ?? of the described in alnoite (Dawson et al., 1978) and lower temperature assemblage are t ?? values kimberlite (Mitchell and Clarke, 1976; actually observed in the Iwanai-da ??e mass. Gogineni et al., 1978). In Japan, monti The Mg/(Mg+Fe2+) ratio of the assumed cellite and perovskite were described from primary magnesioferrite should be 0.84. It skarn (Kusachi et al., 1973, 1979). follows that Mg-Fe2+ partition coefficient, The most common occurrence of calcium defined by (Mg/Fe*)o1ivine/(Mg/Fe2+)spinel brittle mica, clintonite and xanthophyllite, changed from 1.9 to 27.0 with falling tem is with talc in chlorite schist, and in metas perature. On the other hand, thermodynami- omatically altered limestones (Deer et al., cal data of relevant solid solutions given 1962b). In Japan, the known occurrence by Robie et al. (1978) present the data of xanthophyllite is limited to skarn (Sekino of Mg-Fe2+ partitioning on the following et al., 1975). reaction : Mg2SiO4+Fe2+Fe3+204 Genesis of ina,anesioferrite (forsterite) (magnetite) Two hypotheses may be considered for =Fe2+2SiO4+MgFe3+2O4 (2) the gensis of the magnesioferrite equilibrated with olivine to form the magnesioferrite- (fayalite) (magnesioferrite) which gives the coefficient to be 2.1 at olivine rock in question. 1200°C and 6.2 at 800°C. Thus, the par The first hypothesis that there was tition coefficients calculated from Robie magnesioferrite-olivine rock coexisted with et al. (1978) and from equation (1) are con basaltic magma has the difficulty that sistent at high temperature but significantly magnesioferrite has been unknown in alpine- inconsistent at low temperature type peridotite. Its formation requires . The second hypothesis that metaso- quite a high f(02), about 10-2 atm at 1300•Ž matism is responsible also had difficulties . (Ulmer, 1969), usually not realized in basaltic The most important is that even though magmas. It appears quite unlikely that chromian spines of the Iwanai such an oxidized environment is realized -dake peridotite is often zoned owing to retrograde when alpine-type peridotite coexisted with metamorphism basaltic magma . , magnetite-rich chromian If magnesioferrite was a primary phase spinel but not magnesioferrite -rich spinel is , formed at the rim we should accept that the following exchange . However , magnesioferrite-olivine rock has some feat reaction proceeded from the left to right ures hand side with falling temperature : in common with monticellite -bearing dunite such that calcium brittle mica Mg1•E82Fe2+0 .18SiO4+Mg0.84Fe2+0.16Fe3+2O4 and highly oxidized minerals occur (Fo9l olivine) . In the monticel- =Mg1.96Fe2+0.04SiO lite-bearing dunite 4+( , ferripargasite, andradite Fogs olivine) and also magnetite -rich spinel do occur . T Mg0.70Fe2+0 .30Fe3+2O4 (1) he fact that the Iwanai -dake peridotite We assume that Fo component in olivine of was oxidized after the prima the primary assemblage is 0 ry assemblage .91, the value had been formed normal for olivine from alpine-type peridotite , is also possibly supported , i by the occurrence of ch ncluding the majority of the Iwanai romite lam ??tae in -dake olivine as described by Arai (1978) al mass. Mg/(Mg+Fe*) and Mg/(Mg+Fe2+) one of these features offers direct evi ?? to Magnesioferrite-olivine rock and others from the peridotite mass 29 deduce the genesis of magnesioferrite, but showed that olivine from an intrusive per- we think at present that the second hypothe- idotite body contains less than 0.15 weight sis is a little more likely. percent CaO. Thus, it is quite unlikely that Genesis of monticellite monticellite-bearing dunite, occupying a The stability of monticellite + olivine quite small portion of the complex, co- assemblage is one thing and that of mon- existed with basalt equilibrated with the majority of the complex. ticellite-bearing dunite another. Fig. 9 shows the paragenetic relations of a part of We also do not believe that monticellite the system CaO-MgO-Si02. Monticellite could have formed under subsolidus condi- and enstatite are incompatible and hence if tions in a closed system because monticellite- monticeUite of the Iwanai-dake peridotite bearing dunite is a rare rock in the Iwanai- mass was a primary mineral, i.e. a mineral dake peridotite mass. Dunite and harz- which coexisted with basaltic magma, the burgite containing olivine with CaO less monticellite-free dunite and harzburgite than 0.1 weight percent cannot produce should have equilibrated with quite different monticellite and also associated olivine, the magmas. The monticellite-bearing dunites latter containing more than 0.15 weight contain olivine with CaO weight percent percent CaO. larger than 0.15, but monticellite-free dunite Fig. 7 indicates that monticellite and harzburgite contain olivine with CaO lamellae were exsolved from relatively Ca- less than 0.1 weight percent. Therefore, enriched olivine. A compositional profile the majority of dunite of the Iwanai-dake of Fig. 6-a suggests that some Ca from mass forms an ordinary dunite-harzburgite olivine was supplied to form monticellite. complex, without anomalous CaO contents, It is suggested, therefore, that the in which monticellite could not have been monticellite was formed by a two-step a primary phase. Simkin and Smith (1970) process ; the first, enrichment of Ca in olivine, and the second, exsolution and precipitation of monticellite from Ca-rich olivine : we do not know if these two processes were continuous. Monticellite exsolution lamellae are not observed in equigranular olivine but only in an olivine porphyroclast preserved monticellite exsolution lamellae. The temperatures at which these steps of process took place may be estimated from the solvus of monticellite-forsterite system as determined by Warner and Luth (1973). As monticellite and olivine of the specimen we examined have high Mg/(Mg+ Fe*) ratios, multicomponent correction was not applied. The minimum Fig. 9. Phase relation for a portion of the temperature of the formation of the most system CaO-MgO-Si02. Di ; diop Ca-rich olivine (Ca/(Ca+Mg+Fe*)=0.006) side, En; enstatite, Mo ; monti cellite, Fo ; forsterite. is 815°C and that of monticellite exsolution 30 Jitsuya Nagata

(Ca/(Ca+Mg+Fe*): monticellite=0.490, equilibrated with basaltic magma and olivine = 0.003) is 720°C. These values were before the cooling of it to lower temperature

read on a diagram, Fig. 10 in which NMO. than the granulite facies, i.e. 600 to 700•Ž (mole percent of CaMgSiO4 in olivine) was according to geothermometer by Arai (1978),

plotted against 1 /T. The error of temperature Genesis of Ca-rich diopside estimation is about ±30°C. The uncertainty The monticellite-bearing dunite IW of geothermometers makes it impossible to 328 contains diopside, but as is seen in Fig. 8, determine whether or not the formation of its composition is distinctly different from monticellite from olivine took place at higher those in monticellite-free dunite and temperature that the granulite facies equili- harzburgite in the Iwanai-dake peridotite bration, during which homogeneous olivine mass. It is natural, though circumstantial, and pyroxenes of the Iwanai-dake peridotie to consider that such diopside with unusu- mass were formed. ally high (Ca-0.5A1)/(Ca-0.5A1+ Mg + Fe*) Thus, the enrichment of Ca in olivine ratio was formed by the same process as the should have taken place before the final formation of Ca-rich olivine. stage of the granulite facies equilibration. Although we could not reach at a clear-cut Acknowledgements: This paper is a part of conclusion, it is possible that the Ca was my Master thesis work that was carried out supplied to the dunite-harzburgite com- at the Department of Earth Sciences , Kana- plex after the stage when the complex zawa University. I am deeply indebted to Dr. S. Arai of the Tsukuba University for his supervision and encouragement

throught the work and critical review . I wish to express my hearty appreciation to Prof. S. Banno of the Kyoto University , P rof. M. Yamasaki and Dr . H. Sato of the Kanazawa University and Dr . K. Yokoyama of the University of Auckland for their constructive discussions and c ritical reading of the manuscript .

References

Arai, S. (1978), Chromian spinel lamellae in olivine from the Iwanai -dake peridotite mass , Hokkaido , Japan. Earth Planet . Sci. Lett., 39 267-273 . Asahina, T. and Komatsu , M. (1979), The ophiolitic complex in the Kamuik Hokkaido otan tectonic belt , , Japan. J. Geol. Soc. Japan . , 85, 317-337 Bamba, T. (1955), Petrological study on the Iwa naidakeperid otite mass Hokkaid . Bull. Geol. Commit. Fig. 10. Relationship of 1/T t o, 29, 7-14 (in Japanes o the NMo (mole e).Banno , S., Ishizuka percent CaMgSiO4 in olivine) based , H., Gouchi, N . and Imaizumi on the results of Warner a , Kamuikotan belt in Hokkaid , M. (1978) nd Luth (1973) tectonic co o: The , ntact of high-pressure meta morphicand l belt ow-pressure ophiolite su ccession . Magnesioferrite-olivine rock and others from the peridotite mass 31

Abst. Int. Geodyn. Conf., Tokyo, 1978, 14-15. Japan, 14, 124-130 (in Japanese). Buie, B.F. (1941), Igneous rocks of the Highwood Mitchell, R.H. and Clarke, D.B. (1976), Oxide and Mountains, Montana. Part III. Dikes and sulphide mineralogy of the Peuyuk kimberlite, related intrusives. Geol. Soc. Am. Bull., 52 Somerset Island, N.W.T., Canada. Contrib. , 1753-1808. Mineral. Petrol., 56. 157-172. Dawson, J.B., Delany, J.S. and Smith, J.V. (1978), Niida, K. and Kato, T. (1978), Ultramafic rocks in Aspects of the mineralogy of alnoitic breccia, Hokkaido. Assoc. Geol. Collb. Japan, Monogr. Malaita, Solomon Islands; comparison with 21, 61-81 (in Japanese). continental kimberlites. Contrib. Mineral. Van de Pijpekamp, B., Burke, E.A.J. and Petrol., 67, 189-193. Maaskant, P. (1969), Magnesioferrite, a mineral Deer, W.A., Howie, R.A. and Zussman, J. (1962a), new for LAngban, Sweden. Arkiv. Min. Geol. Rock-Forming Minerals. 3, Sheet Silicates. Stockholm, 5, 1-10. 99-102. Longmans, Green and Co. Ltd., London. Robie, R.A., Hemingway, B.S. and Fisher, J.R. Deer, W.A. Howie, R.A. and Zussman, J. (1962b), (1978), Thermodynamic properties of minerals Rock-Forming Minerals. 5, Non-Silicates. 75. and related substances at 298.15°K and 1 bar Longmans, Green and Co. Ltd., London. (105 pascals) pressure and at higher tem Gold, D.P. (1967), Alkaline ultrabasic rocks in the peratures. U.S. Geol. Surv. Bull., 1452, 1-456. Montreal area, Quebec. in P.J. Whyllie, Ed., Sekino, H., Kanisawa, S., Harada, K. and Ishikawa, Ultramafic and Related Rock. 288-297. Y. (1975), Aluminian xanthophyllite and John Wiley and Sons, New York. aragonite from Japan. Mineral. Mag., 40, Gogineni, S.V., Melton, C.E. and Giardini, A.A. 421-423. (1978), Some petrological aspects of the Simkin, T. and Smith, J.V. (1970), Minor-element Prairie Creek diamond-bearing kimberlite distribution in olivine. J. Geol., 78, 304-325. diatreme, Arkansas. Contrib. Mineral. Petrol., Ulmer, G.C. (1969), Experimental investigations of 66, 251-261. chromite spinels. in H.D.B. Wilson, Ed., Kato, T. (1978), The Saru-gawa ultrabasic massif Magmatic Ore Deposits. 114-122. Econ. Geol. in Kamuikotan belt, Central Axial Zone of Monogr. 4. Hokkaido. Chikyu Kagaku, 32, 273-279 (in Warner, R.D. and Luth, W.C. (1973), Two-phase Japanese with English abstract). data for the join monticellite (CaMgSiO4)- Kusachi, I., Henmi, C, and Henmi, K. (1973), forsterite (Mg2SiO4): Experimental results and Perovskite from Fuka, the Town of Bitchu, numerical analysis. Amer. Mineral., 58, 998- Okayama Prefecture. J. Mineral. Soc. Japan, 1008. 11. 219-226 (in Japanese). Yamaoka, K. (1962), Studies on the bedded cupri Kusachi, I., Henmi, C. and Henmi, K. (1979), ferous iron sulfide deposits occurring in Contant minerals from Kushiro, Hiroshima the Sambagawa metamorphic belt. Sci. Rep. Prefecture (6) Monticellite. J. Mineral. Soc. Tohoku Univ. Ser. III, 8, 1-68.

北海道・神居古潭構造帯・岩内岳力ンラン岩体に産するマゲネシオフェライトーカンラン石岩とモンチセリカンラン石を含むダナイト

長田実也

北海道・神居古潭構造帯・岩内岳カンラン岩体から,アルプス型超苦鉄質岩類としては世界で初めての,マグネシオフェライトに富んだ岩石と,モンチセリカンラン石を含むダナイトが見出きれた。こうした鉱物の形成

は,酸化作用や岩石中のCa/Si比を増加きせるような交代作用が局所的に起こった結果であると考えられる。

地名

Iwanai-dake 岩内岳