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 con- tains monticellite, Ca-rich diopside, pero- vskite, 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.