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Preliminary Notes on the Basic Inclusions Found in the Migmatite Area of the Hidaka Mountains, Hokkaido

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Preliminary Notes on the Basic Inclusions Found in the Migmatite Area of the Hidaka Mountains, Hokkaido 岩石鉱 物鉱床 学会 誌 60巻5号,1968年 研 究 短 報 文 PRELIMINARY NOTES ON THE BASIC INCLUSIONS FOUND IN THE MIGMATITE AREA OF THE HIDAKA MOUNTAINS, HOKKAIDO YONO TONOSAKI Institute of Earth Science,Hakodate Branch, Hokkaido Universityof Education Petrology of the metamorphosed calcareous concretions in hornfels of the southern part of the Hidaka Mountains was already investigated by Suzuki (1934). Similar studies were made in detail by Hunahashi et al. (1956), Asai (1956, 1958, 1959) and Hirota (1963) on various inclusions in gneisses and migmatites along the Horoman River, Hidaka Province. The writer discovered some basic inclusions consisting chiefly of biotite and amphibole in the migmatite area of the southern Hidaka Mountains. This is a very rare type of inclusions found in metamorphic rocks and migmatites in Hidaka. This paper briefly describes the petrographical nature of the basic inclusions. The various inclusions deriving from argillaceous sediments and calcareous concretions in Hidaka show usually a schistose or gneissose structure. Such inclusions occur as eye-shaped or dark ovoidal small patches intercalated between gneiss and hornfels fragments enclosed by metamorphic rocks and migmatites. The basic inclusions in question occur in the upper reaches of the Saruru River near Meguro, Hidaka Province. The area belongs to the eastern part of the regional Hidaka metamorphic complex. The basic inclusions are found only in tonalite (Hidaka research group, Hunahashi et al., 1967) and aplitic migmatite (Hunahashi and Igi, 1956; Igi and Hata, 1956), and always have lenticular forms of various sizes. A specimen of the basic inclusions was obtained from tonalite (Fig. 2). It was found as a xenolith-like rock in the coarse-grained tonalite. Macroscopically it is dark grey to black in (Manuscriptreceived, July 19, 1968) 206 Yono Tonosaki Fig. 1. Index map showing the location of the basic inclusion described in this paper. Topographical map (scale 1 : 50,000) of the Horoizumidistrict, Hidaka Province. colour, presenting a contrast to the light colour of his host rock (Fig. 2). That basic inclusion is fine-grained, exhibiting granoblastic texture in thin section. It is composed mainly of biotite, phlogopite, plagioclase and quartz. Amphibole is present as an essential mineral only in the central part of the basic inclusion. Modal com position (volume per cent) of the marginal and central parts of a basic inclusion chemically analyzed are shown in Table 1. The modal compositions of the central part are characterized more by a notable increase of biotite and by a decrease of the salic minerals than in the marginal part. Biotite is the predominant mineral of the basic inclusion; in the central part it amounts to 80-90% of the whole constituents. Quartz occurs as irregular minute grains with rounded or rugged out lines in both the central and the marginal parts of the basic inclusion. These grains show microgranoblastic aggregation, commonly associated with various amounts of plagioclase and other minerals. The principal minerals of the marginal and central parts of the Basic in clusions found in the migmatite area 207 Fig. 2. Ovoidal dark basic inclusion found in tonalite from Meguro, Hidaka Province. Table 1. Modal compositions (volume per cent) of the marginal and central parts of a basic inclusion in tonalite from Meguro, Hidaka Province. above-mentioned specimens are microscopically as follows. Marginal part: The marginal part is medium-grained, light greyish, with marked schistosity. The rock is characterized by a porphyroblastic 208 Yono Tonosaki Table 2. Chemical compositions and norms of the marginal and central parts of a basic inclusion in tonalite from Meguro, Hidaka Province. Analyst: N. Nakano texture owing to the presence of large well-formed biotite. Biotite is commonly the most important mineral in the marginal part of the basic inclusion. The mineral is subhedral or euhedral. 0.8•~0.4mm. The largest flake reaches 3mm or more in size. It shows distinct pleochroism with X=light yellow, Y•àdeep brown, Z= reddish brown. Refractive index of the mineral determined in sodium light by the immersion method is ƒÀ•àƒÁ=1.640, indicating a composition with a comparatively high percentage of iron. Phlogopite is found around the largest biotite. It shows rugged svmplektitic margin, especially when associated with biotite. It shows marked pleochroism, X=colourless, Y•àZ=light yellow •` greenish yellow. The optic-axial angle 2Vƒ¿ of the mineral is nearly 0•‹, and the refractive index is ƒÀ•àƒÁ=1.575. Plagioclase is commonly prismatic in habit, 0.3mm in length. Polysynthetic twinning after the albite law is common, while that after the Carlsbad law is comparatively rare. The optical properties of plagioclase are as follows: a=1.556, ƒÀ=1.560, ƒÁ=1.565, ƒÁ-ƒ¿= 0.009, 2VƒÁ=88•‹-86•‹. It may be labradorite of An55. Basic inclusions found in the migmatite area 209 Central part : The central part is black or dark greyish, fine-grained massive or weakly foliated. The rock shows a granoblastic texture and is characterized by the presence of amphibole. Biotite is flaky, 0.2mm or more in size. The pleochroism is remarkable, with X=yellowish brown, Y•àZ=dark brown. The refractive index of the crystal is ƒÀ•àƒÁ=1.638. Muscovite occurs as irregular flakes. The optic-axial angle ranges from 2V,=42•‹ to 4•‹. Amphibole is important for its usual presence. It occurs as rounded prismatic or short prismatic crystals, less than 0.2mm in length. The crystals show distinct cleavages parallel to (110) and (110) making an angle of about 124•‹ in the basal sections. Simple or multiple twinning on (100) is rarely observed. The pleochroism and absorption of the mineral are as follows : X=yellowish green, Y=light green, Z=bluish green, and Z>Y>X. The optical properties are ; ƒ¿=1.645, ƒÀ=1.657, ƒÁ=1.665, ƒÁ-ƒ¿=0.020, 2Vƒ¿=83•‹- 80•‹, cƒ©Z=22•‹. It may belong to the group of actinolitic common hornblende. Plagioclase is a shrot prismatic crystal with a grain-size 0.1mm. Its optical properties are as follows : ƒ¿=1.560, ƒÀ=1.564, ƒÁ=1.569, ƒÀ-ƒÁ=0 .009, 2VƒÁ=85•‹-82•‹. In composition it corresponds to labradorite, with anorthite percentage An62. Chemical compositions of the marginal and central parts of a basic inclusion are shown in Table 2, together with normative com positions. Chemical constituents of the basic inclusion reveal that the central part is richer in MgO, total FeO and K2O but poorer in SiO2 than in the marginal part. It is hardly acceptable that the original rock of this inclusion had essentially different compositions which resulted in the above mentioned variation of the chemical composition. The writer considers that the metasomatism in relation to the migmatization seems to have played an important role in producing the different chemical compositions between the marginal and central parts of the inclusion. The petrochemistry and genesis of the basic inclusions will be fully discussed in another paper. In conclusion, the writer wishes to express his thanks to Professor Hiroshi Asai of the Hokkaido University of Education, for his kind guidance during the course of this study. Thanks are also due to Mr. Nobuo Nakano of the Hakodate NissanKagaku Company for his assistance in the chemical analyses. 210 Yono Tonosaki REFERENCES Asai, H. (1956), The basic xenolith at the midstream of the river Horoman, Hidaka Province (in Japanese with English abstract). Jour. Jap. Assoc. Min. Pet. Econ. Geol., 40, 93•`103. •\ (1953), On the so-called metamorphosed calcareous concretions in felsic metamorphic rocks which lie in the basic rocks at the upstream of Horoman River, Hidaka Province (in Japanese with English abstract). Ibid., 42, 215•`227. •\ (1959), Study of felsic metamorphic rock and its associated inclus ion in gabbro-diorite of Horoman River, Hidaka Province, Hokkaido (in Japanese with English abstract). Ibid., 43, 26•`35; 73•`84. Hidaka research group, Hunahashi, M., Hashimoto, S. and Kim, C.W. (1967), Granitic rocks of the axial zone of Hokkaido (in Japanese with English abstract). Memorial volume of Prof. H. Shibata, 1•`13. Hirota, S. (1963), Granitization in the Horoman district, southern part of the Hidaka metamorphic zone (in Japanese with English abstract). Jour. Geol. Soc. Japan, 69, 82•`98; 119•`129. Hunahashi, M. and Igi, S. (1956), Explanatory text of the geological map of Japan ••Horoizuimi••(scale 1:50,000) (in Japanese with English abstract). Geol. Surv. Japan. Hunahashi, M., Hashimoto, S., Asai, H., Igi, S., Tonosaki, Y., Kizaki, K., Hirota, S. and Kasugai, A. (1956), Gneisses and migmatites in the southern extreme region of the Hidaka metamorphic zone, Hokkaido (in Japanese with English abstract). Jour. Geol. Soc. Japan, 62, 401 •` 408; 464•`471; 541•`549. Igi, S. and Hata, M. (1956), Explanatory text of the geological map of Japan •• Saruru•• (scale 1:50,000) (in Japanese with English abstract). Geol. Surv. Japan. Suzuki, J. (1934), Metamorphosed calcareous concretions in the hornfels at the southern coast of Tokati Province, Hokkaido. Jour. Fac. Sci., Hokkaido Imp. Univ., Ser. IV, 2, 323•`338. 日 高 山 賑 の ミ グ マ タ イ ト域 に 発 見 し た 塩 基 性 包 有 物(予 報) 外 崎 与 之 摘 要 日高 国 目黒地方 の トナル岩(従 来 の花 嵩 岩質 ミグマ タイ トに当た る)か ら,著 し く黒雲 母 に富 み,陽 起 石質 普通 角閃 石 を伴 う若子 の塩 基性 包 有物 を見出 した。本 岩 は黒 雲 母 岩類 似 の特 異 な岩質 を有 し,日 高変 成 帯で は未 記載 の包 有 物 で ある。 これ らの塩基性 包 有物 の 産状 を注意 し,次 いで,代 表 的試 料の 周縁 部 と中核 部 との鉱物 組 成,化 学 組成 を示 し,そ の精 徴 を述べ た。 な お,本 塩基 性 包有 物 に つ き,ミ グマ タ イ ト 化 作用 に関 連 あ る交代 作 用が,そ の生 成 に重 要 な役割 をに な うこ とを指摘 した。.
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