. Japan. Assoc. Min. Petr. Econ. Geol. 78, 11-20, 1983

K-Ar ages of Sangun metamorphic rocks in Yamaguchi Prefecture and their geologic significance

YUJIRO NISHIMURA

Institute of Earth Sciences, Faculty of Liberal Arts , Yamaguchi University, Yamaguchi 753, Japan.

Eizo NAKAMURA Department of Geology, University of Toronto, Toronto, Ontario M5S 1A1, Canada.

IKUO HARA Institute of Geology and Mineralogy, Faculty of Science, Hiroshima University, Hiroshima 730, Japan.

K-Ar ages have been determined on white micas from four Sangun pelitic schists in the Nishiki-cho and Toyoga-dake areas, Yamaguchi Prefecture. The dates from the Nishiki-cho area are 172 and 175 Ma, while those from the Toyoga-dake area are 264 and 274 Ma. They represent a wide range of time, 172 to 274 Ma, which is nearly identical with that of eight ages, 165 to 264 Ma, reported previously from the Sangun metamorphic rocks. The twelve available dates on the Sangun metamorphic rocks can be divided into two age clusters, an older age cluster around 260 Ma (middle Permian) and a younger cluster around 170 Ma (middle Jurassic). It is suggested that the two age clusters may coincide with the two stages of uplift for the Sangun metamorphic rocks. Namely, the age cluster around 260 Ma may indicate the event in which the Sangun metamorphic rocks were emplaced as nappes from the deep tectonic position of glaucophanitic metamorphism into a shallow tectonic position and the metamorphism had ceased. On the other hand, the age cluster around 170 Ma may represent an event related to the beginning of the formation of the pile nappe structure in which the Sangun metamorphic rocks tectonically overlie the Jurassic non-metamorphic equivalents of the Tamba Group.

(regional metamorphic) belt in the Inner Introduction Zone of Southwest Japan, called the Sangun The Sangun metamorphic rocks com- (metamorphic) belt or the Sangun-Chugoku posed of schists and phyllites are exposed belt (Nishimura et al., 1977; Fig. 4). It in scattered small areas from north Kyushu has been demonstrated that the original to northwest Kinki. The Paleozoic forma sediments of the belt were probably deposit tions and the Yakuno ophiolitic complex ed in Devonian to Permian time and associated with them are also weakly underwent the Sangun metamorphism. The metamorphosed. They were all affected by metamorphosed rocks in some areas were one and the same cycle of metamorphism of unconformably covered by Lower Triassic the glaucophane-schist facies series, called sediments of neritic facies (Nakazawa and the Sangun metamorphism, and constitute a Shiki, 1954) or Upper Triassic sediments

(Manuscript received October 7, 1982)

J 12 Yujiro Nishimura, Eizo Nakamura and Ikuo Hara

of molasse facies (Hase, 1950). Thus, a younger limit on the time of the Sangun metamorphism has been considered on the basis of geologic evidence to be late Permian to early Triassic (e.g. Nishimura, 1977). On the other hand, Permian to Triassic conodont-bearing strata of chert facies (mainly Upper Triassic) have been found recently from several areas in and around the Sangun-Chugoku belt (Toyohara, 1977;

Kimura, 1977; Tanaka, 1980; Hayasaka and Fig, 1. Locations of the Nishiki-cho and Toyoga Hara, 1981). More recently, radiolarian dake areas in Yamaguchi Prefecture. fossils of Early to Middle Jurassic age have also been discovered from some parts of results and discuss their geologic signif the strata (Hayasaka and Hara, 1982; icance. Isozaki and Matsuda, 1982). These strata are lithologically characterized by a dom Geological outline inance of banded cherts and pebbly mud 1. Nishiki-cho area stones and are correlated with the Tamba Nishimura (1971) has described the Group which is widespread in the Tamba geology and metamorphism of the Nishiki zone on the east of the Sangun-Chugoku cho area in the east of Yamaguchi Pre belt. The Sangun metamorphic rocks fecture. The area is mainly occupied by the always overlie these strata (Hara et al., Sangun metamorphic rocks (Tsuno Group) 1980). The Paleozoic formation and the and overlying weakly metamorphosed rocks Yakuno ophiolitic complex in the Maizuru (Nishiki Group) (Fig. 2). The Tsuno Group zone are also thrust over the Tamba Group consists mostly of pelitic, psammitic and of the Tamba zone (Hayasaka and Hara, basic schists being accompanied by siliceous 1980). Therefore, it can be pointed out and calcareous schists. Serpentinties also that the Sangun-Chugoku belt is made up occur subordinately. The total thickness of the nappe group and its underlying non- is estimated to be 1,700 meters or more. metamorphic strata (= equivalents of the The Nishiki Group is typified by a dom Tamba Group). inance of alternating beds of psammite, These new geologic observations de pelite and acidic tuff, but contains inter mand reexamination of the commonly ac calated chert, limestone and conglomerate. cepted theory of the geologic development The total thickness amounts to 2,900 to of the Sangun-Chugoku belt. Geochrono 3,900 meters. The latter Group yields oc logical data may provide a clue to solve this casional Permian fusulinid fossils, whereas problem. However, available radiometric the Tsuno Group has not yet been dated by dates on the belt are very meagre. A K-Ar fossil evidence. dating program was on the Sangun meta The metamorphic terrain is divided into morphic rocks from the Nishiki-cho and the following three zones on the basis of the Toyoga-dake areas, Yamaguchi Prefecture metamorphic mineral assemblages of basic (Fig. 1). In this paper we present the rocks: zone A (pumpellyite-chlorite) , zone K-Ar ages of Sangun metamrphic rocks 13

and represents the typical facies series of the high-pressure intermediate group (Miya shiro, 1961). The rocks were folded to form anticlinal and synclinal structures, the axes of which are roughly horizontal with nearly E-W trend through the Tsuno and Nishiki Groups. Thus, rocks of the Tsuno Group are exposed in the anticlinal zones and those of the Nishiki Group in the synclinal zones. Along the boundary be tween the two Groups is found a kind of tectonic slide. On the south of this area, the Sangun metamorphic rocks are in thrust fault con tact with the Permian Mikawa Group (= equivalents of the Tamba Group), and further south the Mikawa Group is thrust over the Triassic to Jurassic Kuga Group (=equivalents of the Tamba Group) (Kojima, 1953; Toyohara, 1977; Hara et al., 1979). The vergence of the thrusting is southward. Therefore, it must be concluded that the Nishiki-cho-Kuga area is characterized by the prevalence of pile nappe structures.

Fig. 2. Geological map of the eastern part of 2. Toyoga-dake area the Nishiki-cho area (Extracted from Nishimura, 1971). The Nagato. tectonic zone is a narrow Dots represent the localities of the fault zone trending from NNE to SSW in dated specimens. D: Deal, K: Kokabe, M: Miyanokushi. the western part of Yamaguchi Prefecture and is accompanied by various kinds of B (pumpellyite-actinolite) and zone C tectonic blocks of old basement rocks. (epidote-glaucophane). The boundary Tectonic blocks of orthogneisses, trondhje surfaces between the zones are nearly mites and amphibolites of about 400 Ma in parallel to bedding planes, and the grade of radiometric age mainly occur in the metamorphism increases downwards in the northern areas of the zone, e.g. Dai and stratigraphic succession. Zone A comprises Misumi, whereas blocks of schists, meta the whole of the Nishiki Group, zone B gabbroic rocks and Paleozoic rocks are coincides with the uppermost horizons of widespread in the southern areas, e.g. the Tsuno Group and zone C corresponds Toyoga-dake and Ozuki (Murakami and to the lower horizons of the Tsuno Group. Nishimura, 1979). The progressive metamorphism from zone A Fig. 3 shows the geological map of the to zone C is called the Sangun metamorphism Toyoga-dake area, The schists are exposed 14 Yujiro Nishimura Eizo Nakamura and Ikuo Hara

metamorphic conditions grading from the pumpellyite-actinolite facies through the glaucophane-schist facies to the barroisite zone, corresponding to the generalzied zona tion of the Sangun metamorphic rocks (Hashimoto, 1972; Nishimura et al., 1977). Therefore, it has been proposed that the schists in the present area may be tectonic blocks derived from the Sangun meta morohic rocks (Nishimura, 1976). The main part of the Toyoga-dake area is composed of weakly metamorphosed pelites, psammites, greenstones and lime stone conglomerates, called the Toyohigashi Group. The limestone pebbles in the con glomerates yield foraminifera fossils rang ing from Carboniferous to Middle Permian in age. Thus, the Group may be correlated with the Upper Permian. However, it is difficult to establish the stratigraphy and the geologic structure for the Group in detail, as it has been faulted and subjected to intense cataclasis. Metagabbros, metadiabases and ser

Fig. 3. Geological map of the Toyoga-dake area pentinites also occur as small lenticular (Modified from Nishimura, 1976). bodies intercalated in the Toyohigashi Dots represent the localities of the dated Group. A K-Ar age of hornblende from specimens. the metagabbro has been reported as 233 H: Higashinagano, N: Nakano, T: To yoga-dake. Ma (Shibata et al., 1979). The schists and the Paleozoic rocks of along the eastern and western margins of this area are unconformably covered by the the tectonic zone and made up of pelitic, Lower Jurassic Higashinagano Formation of basic and siliceous schists. Because they the Toyora Group on the west of the area, are extremely variable in degree of and are in fault contact with the Upper recrystallization and deformation, the Triassic Aso Formation of the Mine Group metamorphic terrain can not be zonally clas on the east. In the Asa area situated to sified. Representative mineral assemblages the southwest, some outcrops of the of all the basic schists observed are as Sangun metamorphic rocks are found. It follows: chlorite-epidote-actinolite-pum- is well known that one of them at Nawazi pellyite-albite, chlorite-epidote-actinolite- bana, Onoda City, is unconformably over crossite-albite, chlorite-epidote-albite and lain by the unmetamorphosed Upper Trias chlorite-epidote-actinolite-albite (porphy sic Tsubuta Formation of the Mine Group roblast). These assemblages suggest the (Hase, 1950; Kimura, 1977). K-Ar ages of Sangun metamrphic rocks 15

Toyoga-dake, Toyota-cho, Toyoura-gun Description of the dated specimens (Fig. 3). The rock is composed largely of Four specimens used for dating were chlorite, white mica, calcite, albite and collected from two outcrops in the Nishiki quartz with minor amounts of sphene, cho and Toyoga-dake areas, respectively apatite, tourmaline, carbonaceous material (Figs. 2 and 3). They are dark-colored and opaque mineral. Some albite crystals pelitic schists, and white mica flakes glisten, are porphyroblastic and helicitic. They are silver-colored, on the schistosity planes. as large as one millimeter in diameter and The white micas are phengitic in composi include chlorite, white mica, sphene and tion, with the b. -values measured as 9.041 carbonaceous material. This rock is the to 9.025 by X-ray diffraction. The dated highest in metamorphic grade of the dated specimens did not undergo any metamor specimens. phism resulting from contact with younger plutonic intrusions. Experimental procedure and results Specimen 791011-1: Pelitic schists from K-Ar ages of four white micas from the an exposure on the riverbed west of Deai, pelitic schists described in the preceeding Nishiki-cho, Kuga-gun (Fig. 2). The rock is section were determined at Tohoku Univer placed in the lower horizon of the upper sity (by E.N.). Their host rocks were finely formation of the Tsuno Group, belonging crushed and sieved, and then white micas to zone C (Nishimura, 1971). The con were concentrated by hydraulic flotation stituents are white mica, chlorite, garnet, and tapping on filter papers and purified by albite and quartz, with accessories calcite, means of an isodynarnic separator. The apatite and carbonaceous material. Garnet purification of these samples was practically occurs as idiomorphic to hypidiomorphic complete, excepting only minor inclusions of crystals of diameter less than 0.1 millimeter. carbonaceous material. The size fraction of Specimen K1011-14: Pelitic schist 80 to 150 mesh was used. The analytical from a cutting on the road at Kokabe, Mika- method is essentially the same as that wa-cho, Kuga-gun (Fig. 2). The rock is described by Kawano and Ueda (1964). from nearly the same horizon and meta- Decay constants used in this study are: morphic grade as specimen 791011-1. It is A,=0.581•~10-10yr-1, A =4.962•~10-10 yr-1, composed mainly of white mica, chlorite, 40K/K=1 .167•~10-4, 4OArR: radiogenic calcite, albite and quartz. Small amounts argon 40. All ages cited in this paper are of sphene, apatite, tourmaline and carbo also recalculated using the standard set of naceous material are also found. decay constants recommended by the

Specimen 80306-6: Pelitic schist from JUGS Subcommission on Geochronology an exposure on the small valley south of (Steiger and Jager, 1977). Higashinagano, Toyota-cho, Toyoura-gun K-Ar analytical data are given in Table (Fig. 3). This rock is composed mainly of 1. The white mica ages obtained show a white mica, chlorite, albite and quartz, with wide range of time overall, from 172 to 274 subordinate amounts of sphene, apatite and Ma, but can be shown to fall into two distinct carbonaceous material. groups. That is, two ages from the Nishiki Specimen 80306-8: Pelitic schist from cho area are 172 and 175 Ma, while those an exposure on the small valley southwest of from the Toyoga-dake area 264 and 274 16 Yujiro Nishimura, Eizo Nakamura and Ikuo Hara

Table 1. K-Ar analytical data for the Sangun metamorphic rocks from the Nishiki-cho and Tovoga-dake areas in Yamaguchi Prefecture

ƒÉ e=0.581•~10-10yr-1, ƒÉƒÀ=4.962•~10-10yr-1, 40K/K=1.167•~10-4, 40ArR: radiogenic argon 40

Ma, respectively. The latter corresponds to (Fig. 5). The older one groups around 260 middle Permian time and the former to Ma (middle Permian) and the younger one middle Jurassic time. Since the analyzed is that around 170 Ma (middle Jurassic). white micas have not been subjected to any All 260 Ma ages are in the western part contact metamorphism or weathering, the of the Sangun-Chugoku belt, but all of the age gap between the two areas, which is ages measured in the eastern part are very large and outside the error ranges, around 170 Ma. must have important geologic implication. Generally, it has been stated that minerals in a rock tend to lose radiogenic Discussion argon and strontium through even a mild Eight age determinations on the Sangun thermal event. Suppe and Armstrong metamorphic rocks have been reported (1972) summarized as follows: "K-Ar and previously from the Tari (Shibata and Igi, Rb-Sr mineral dates from high-temperature 1969; Shibata et al., 1972), Kurume (Ueda metamorphic terrains indicate the ap and Onuki, 1969) and Yame (Yanagi, 1967) proximate time of post-metamorphic cooling areas. Although some of them are obtained below a temperature of 200•Ž to 300•Ž and by different analytical methods and on thus yield information concerning the different samples, they show a time range uplift and denudation or cooling history of a from 165 to 264 Ma, which is consistent region." with that of the ages newly described in this Recent progress on the tectonics of the paper. In the Tari area, two white micas Sangun-Chugoku belt has revealed that from one specimen are dated at 173 and 179 there were two stages of the uplift of Ma by the K-Ar method (Shibata and Igi, the metamorphic rocks (Hara et al., 1980; 1969), while a white mica-whole rock Rb-Sr Hara, 1981, 1982; Nishimura, 1981). age from the same specimen yields 165 Ma The first uplift represents the event in (Shibata et al., 1972). These ages deter which the metamorphic rocks of the Sangun- mined by different methods appear to be Chugoku belt were emplaced as nappes from concordant within the error ranges. There a deep tectonic position of glaucophanitic fore, the four newly determined ages may be metamorphism into a shallow tectonic posi examined together with the previously tion. This is confirmed by the Carnian reported eight ages. sediments unconformably overlying the

All of the available data are shown on Sangun metamorphic rocks in the Asa area, the simplified geological map (Fig. 4). The described in the foregoing section (Hale, age data can be divided into two age clusters 1950; Kimura, 1977), and by the Scytian K-Ar ages of Sangun metamrphic rocks

Fig. 4. Simplifiedgeological map of the Sangun-Chugoku belt,showing the areal distributionof availableradiometric dates for the Sangun metamorphic rocks. Numerals denote the radiometricdates in Ma and those in squaresarenewly determined. Abbreviations: Sib=San-in branch of the Sangun belt,Syb=San-yo branch of the Sangun belt,N-zone=North zone,M-zone=Middle zone,Ry=Ryoke belt,Mz=Maizuru zone,Kz=Kamigori zone,Tm=Tamba zone,Nz=Nagato tectonic zone.

1 7

18 Yujiro Nishimura, Eizo Nakamura and Ikuo Hara

Hayasaka and Hara, 1982; Isozaki and Matsuda, 1982), Triassic strata in Tsuyama (Kimura, 1977), Tsunodani Formation in Wakasa (Hayasaka and Hara, 1981, 1982) and Tamba Group in Asako (Hayasaka and Hara, 1980). The second uplift appears to have begun after the deposition of the Lower Jurassic sediments. Fig. 5. Frequency distribution of available It is significant that the two age radiometric dates for the Sangun me clusters distinguished from the radiometric tamorphic rocks. ages of the Sangun metamorphic rocks in sediments unconformably overlying the disputably coincide with the two stages of Permian rocks and the Yakuno ophiolitic the uplift of the metamorphic rocks in the complex in the Maizuru zone (Nakazawa and Sangun-Chugoku belt. Therefore, our Shiki, 1954; Nakazawa, 1961). Because preliminary conclusion is that the age the metamorphic rocks of the Sangun cluster around 260 Ma corresponds to the Chugoku belt are at least in part derived phase when the Sangun metamorphic rocks from sediments of Late Permian age (Seki, were emplaced as nappes from the deep 1964; Hashimoto, 1968; Nishimura, 1971; tectonic position of the Sangun metamor Hase et al., 1975), the time of the first phism into a shallow tectonic position and uplift must be considered to be late the metamorphism had ceased, and that the Permian. age cluster around 170 Ma corresponds to The second uplift corresponds to the the beginning of the formation of the pile event in which the metamorphic rocks and nappe structure in which the Sangun their unconformably overlying Triassic sedi metamorphic rocks tectonically overlie the ments had begun to thrust over the Tamba Jurassic non-metamorphic equivalents of Group and its equivalents. This is clearly the Tamba Group. shown in the Nishiki-cho-Kuga area describ ed in the foregoing section (Hara et al., Acknowledgements: The K-Ar ages were 1979) as well as in the Masuda (Okamura determined at the Institute of Mineralogy, et al., 1975; Nishimura and Okamoto, 1976), Petrology and Economic Geology, Tohoku Tsuyama (Tsukaguchi and Nakashima, University by E.N. under the supervision 1978), Wakasa (Uemura et al., 1979; Hara of Prof. Y. Ueda. The authors would like et al., 1980) and Asako (Hayasaka and to thank Emeritus Prof. Y. Ueda of Tohoku Hara, 1980) areas. The Tamba Group and University for helpful advice and valuable its equivalents are of Permian to Jurassic suggestions on K-Ar dating. We would age and consist mainly of banded cherts like to express our appreciation to Dr. John and pebbly mudstones. They are as M. Allen of University of Toronto for critical follows: Mikawa and Kuga Groups in reading of the manuscript. We are also Kuga (Toyohara, 1977; Hayasaka and indebted to Mr. Y. Matsubara of Yama Hara, 1982), Triassic strata in Yoshiwa guchi University for assistance in separating (Toyohara, 1977), Kanoashi Group in some white micas. The present study was Nichihara (Toyohara, 1977; Tanaka, 1980; partly supported by the Grant in Aid for K-Ar ages of Sangun metamrphic rocks 19

Scientific Research from the Ministry of Annual Meet. Geol. Soc. Japan, 213, (abstract).* Education, Science and Culture of Japan Kawano, Y. and Ueda, Y. (1964), K-Ar dating on the igneous rocks in Japan (I). Sci. Rep. (No. 454282). Tohoku Univ., Ser. III, 9, 99-122. Kimura, T. (1977), Japanese Islands -the geologic References history- [I]. Kokin Shoin, Tokyo.* Kojima, G. (1953), Contributions to the knowledge Hara, I. (1981), Tectonic framework of paired of mutual relations between three metamorphic metamorphic belts-Hida belt and Sangun zones of Chugoku and Shikoku, southwestern belt. In Tectonics of Paired Metamorphic Japan, with special reference to the meta Belts (I. Hara. Ed.), 147-153. morphic and structural features of each meta Hara, I. (1982),,Evolutional processes of paired morphic zone. J. Sci. Hiroshima Univ., Ser. metamorphic belts-Hida belt and Sangun C, 1, 17-46. belt-. Mem. Geol. Soc. Japan, 21, 71-89. Miyashiro, A. (1961), Evolution of metamorphic Hara, I., Hide, K. and Nishimura, Y. (1980), Some belts. J. Petrol., 2, 277-311. problems on Paleozoic-Mesozoic tectonism in Murakami, N. and Nishimura, Y. (1979), Nagato Southwest Japan. J. Assoc. Struc. Geol., 25, tectonic zone. In The Basement of the 1-12.* Japanese Islands (Memorial Volume Prof. H. Hara, I., Higashimoto,. S., Mikami, T., Nishimura, Kano), Akita Univ., 153-181.** Y., Okimura, Y., Sawada, T., Takeda, K., Nakazawa, K. (1961), On the so-called Yakuno Yokoyama, S. and Yokoyama, T. (1979), intrusive rocks in the Yakuno district, South Palaeozoic-Mesozoic Group of the Yasaka west Japan. (Studies on the stratigraphy gorge district. The Yasaka Gorge: Landscape and the geologic structure of the Maizuru and Environment, Hiroshima and Yamaguchi, zone, Part 9). In Memorial Volume Prof. J. 283-324.** Makiyama, Kyoto Univ., 149-161.** Hashimoto, M. (1968), Glaucophanitic metamor Nakazawa, K. and Shiki, T. (1954), Geology of phism of the Katsuyama district, Okayama the Miharaiyama district, Yabu-gun, Hyogo Prefecture, Japan. J. Fac. Sci., Univ. Tokyo, Prefecture, Japan, with special reference to the Sec. II, 17, 99-162. Triassic Miharaiyama Group-A study on Hashimoto, M. (1972), Mineral facies of the Sangun the stratigraphy and geologic structure of the metamorphic rocks of the Chugoku Provinces. "Maizuru zone" (Part 2) - . J. Geol. Soc. Japan, Bull. Nat. Sci. Mus., 15, 767-775.** 60, 192-201.** Hase, A. (1950), The stratigraphy of the Triassic Nishimura, Y. (1971), Regional metamorphism of System in the southwestern part of Yamagu the Nishiki-cho district, Southwst Japan. chi Prefecture. Sci. Rep. Fac. Sci., Kyushu J. Sci. Hiroshima Univ., Ser. C, 6, 203-268. Univ., Geol., 2, 101-128.** Nishimura, Y. (1976), Metamorphic rocks and Hase, A., Mondo, M. and Hide, K. (1975), The gabbros of the Toyogadake area in the Nagato weakly metamorphosed Paleozoic formations tectonic zone. The Basement of the Island Are near Ibara City, western Okayama, Prefecture. (Toko-hiban), 3, 33-35.* Geol. Rep.. Hiroshima Univ., 20, 1-20.** Nishimura, Y. (1977), Some problems on the geo Hayasaka, Y. and Hara, I. (1980), Structural logic developments of the SangunChugoku relationship between the Yakuno complex belt. Assoc. Geol. Collaboration Japan, and the Paleozoic-Mesozoic formations of the Monograph, 20, 1-7.** Tamba belt in the Asako district, Hyogo Pre Nishimura, Y. (1981), Tectonics of the Sangun belt. fecture. Studies on Late Mesozoic Tectonism In Tectonics of Paired Metamorphic Belts (I. in Japan, 2, 77-81.* Hara, ed.), 143-145. Hayasaka, Y. and Hara, I. (1981), Geologic signif Nishimura, Y. and Okamoto, T. (1976), Lawsonite icance of Triassic formation of the Wakasa albite schist from the Masuda district, South district, Tottori Prefecture. 88th Annual west Japan. In Memorial Volume Prof. G. Meet. Geol. Soc. Japan, 484 (abstract).* Kojima, Hiroshima Univ. Press, 144-153.** Hayasaka, Y. and Hara, I. (1982), Discovery of Nishimura, Y., Inoue, T. and Yamamoto, H. (1977), Jurassic radiolarian fossils from the Chugoku Sangun belt, with special reference to the belt and its geologic significance. 89th Annual stratigraphy and metamorphism. In The Meet. Geol. Soc. Japan, 556, (abstract).* Sambagawa Belt (K. Hide ed.), Hiroshima Isozaki, Y. and Matsuda, T. (1982), Biostratigraphy Univ. Press, 257-282.** of Triassic conodonts from bedded chert Okamura, Y., Nishimura, Y., Hase, A., Soeda, A., sequences in the Mino and Tamba zones. 89th Okimura, Y., Hirowatari, F., Motomura, Y., 20 Yujiro Nishimura, Eizo Nakamura and Ikuo Hara

Akatsuka, M., Inoue, T., Aguin, H., Otani, Suppe, J. and Armstrong, R. L. (1972), Potassium K. and Kojima, S. (1975), Sangun metamorphic argon dating of Franciscan metamorphic rocks. rocks of the Masuda district, Sanin Provinces, Am. J. Sci., 272, 217-233. Japan. Bull. Fac. Educ. Yamaguchi Univ., Tanaka, K. (1980), Kanoashi Group, an olisto 25, 19-36.** strome, in the Nichihara area, Shimane Seki, Y. (1964), Low-grade alteration of Permian Prefecture. J. Geol. Soc. Japan, 86, 613- pyroclastic rocks and intrusive and meta 628.** morphic complex in the Maizuru zone. J. Toyohara, F. (1977), Early Mesozoic tectonic Japan. Assoc. Min. Petr. Econ. Geol., 51, 158- development of the northwestern Chichibu 161.** geosyncline in west Chugoku, Japan. J. Fac. Shibata, K. and Igi, S. (1969), K-Ar ages of Sci., Univ. Tokyo, Sec. II, 19, 253-334. muscovite from the muscovite-quartz schist Tsukaguchi, H. and Nakashima, N. (1978), Geology of the Sangun metamorphic terrain in the Tari of the western part of Tsuyama (MS). Hiro district, Tottori Prefecture, Japan. Bull. Geol. shima University.** Surv. Japan, 20, 707-709.** Ueda, Y. and Onuki H. (1969), K-Ar dating on the Shibata, K., Wanless, R.K., Kano, H., Yoshida, T., metamorphic rocks in Japan (I)-Yatsushiro, Nozawa, T., Igi, S. and Konishi, K. (1972) Kiyama, Chikugo, Higo and Sonogi metamor Rb-Sr ages of several so-called basement rocks phic rocks in Kyushu-. Sci. Rep. Tohohu in the Japanese Islands. Bull. Geol. Surv. Univ., Ser. III, 10, 313-321. Japan, 23, 505-510.** Uemura, F., Sakamoto, T. and Yamada, N. (1979), Shibata, K., Uchiumi, S. and Nakagawa, T. (1979), Geology of the Wakasa district, Quadrangle K-Ar age results-1. Bull. Geol. Sum. Japan, series, scale 1:50,000, Geol. Surv. Japan, 91p.** 30, 675-686.** Yanagi, T. (1967), On the Chikugo metamorphic Steiger, R. H. and Jager, E. (1977), Subcommission rocks. J. Geol. Soc. Japan, 73, 113, (abstract).* on geochronology: convention on the use of decay constants in geo- and cosmochronology. * in Japanese Earth Plant. Sci. Lett., 36, 359-362. ** in Japanese with English abstract

山 口 県 に お け る 三 郡 変 成 岩 のK-Ar年 代 と そ の 地 質 学 的 意 義

西 村 祐 二 郎,中 村 栄 三 ・原 郁 夫

由 口県錦 町地 域 と豊 ケ岳 地 域 に 産 す る四 つ の三 郡 泥 質 片 岩 に 含 まれ る 白雲 母 に つ い て, K-Ar年 代 を 測 定 し た。 錦 町地 域 の年 代 は172Maと175 Maで あ るが,豊 ケ岳 地 域 のそ れ は264Ma 7と274 Maを 示 す0そ れ らは 広 い 年 代 範 囲 を示 し,既 報 の8個 の年 代 値 が 示 す165Maか ら264 Maに わ た る年 代 範 囲にほ ぼ一 致してい る。 三 郡 変 成 岩 に関し て 得 られてい るすべ て の年 代値は, 260Ma付近の 古 い年 代 群(課 紀 中期)と170Ma付 近 の 若 い年 代 群(ジ ュラ紀 中期)に 区 分 され る。 そ 為 らは 三 郡 変 成 岩 め 二 つの 上 昇 時 期 に 十 致 す る こ とを 示唆 し て い る。 す な わ ち, 280Ma付 近 の 年代 群 は,三 郡 変成 岩 が 藍閃 変 成 作 用 の構 造 場 か ら よ り浅 い構 造 場 へ ナ ップ と し て上 昇 し,そ の 変 成 作 用 が終 結 した 出来 事 示 す で あ ろ う。 と こ ろが, 170碗 付 近 の年 代 群 は,三 郡 変 成 岩 類 が丹波層群相当層の非変成ジュラ系の上に構造的に重なるパイルナップ構造の形成の初期に関係した出来事を表 わ す で あ ろ う。 Nishiki-cho 錦 町 Toyoga-dake 豊 ケ 岳 Kuga 玖 珂 Mikawa 美 川 Yoshiwa 吉 和 Kanoashi 鹿 足 Tsunodani 角 谷 Wakasa 若 桜 Asako 朝 来 Tamba 舟 波 Minee 美弥 Tsubuta 津 布 田