J. Japan. Assoc. Min. Petr. Econ. Geol. 73, 300-310, 1978.

208-240 M. Y. OLD - IN THE KUROSEGAWA TECTONIC ZONE NEAR KOCHI CITY, SHIKOKU

SHIGENORI MARUYAMA

Department of Earth Sciences, Faculty of Science, Nagoya University*

YOSHIO UEDA Institute of Mineralogy, Petrology and Economic Geology, Tohoku University, Sendai 986

and SHOHEI BANNO Department of Earth Sciences, Faculty of Science, Kanazawa University, Kanazawa 920

A new member of the Kurosegawa tectonic zone was found in the serpentinite near Kochi city. They are high P and low T schists derived from basalt and chert. Three metamorphic events can be deciphered in the high-pressure schists, based upon the texture and paragenesis: first, low P at intermediate- to high grade, second, high P and low T metamorphism of the jadeite-glaucophane facies and the thrid, retrograde crystallization of the second stage high P schists within the stability field of +pumpellyite+glaucophane. Further, the formation of analcime replacing jadeite took place. The second and third metamorphism can be distinguished on pyroxene mineralogy that j adeite+ was stable in the second, but albite+quartz+aegirinej adeite in the third stage of metamorphism. Not all of the high P and low T schists had suffered low P metamorphism before they were metamorphosed by the high P one. Some basaltic rocks directly changed to high P and low T schists. Two muscovites in the schists give K-Ar ages of 208-240 m.y., and a relic igneous biotite, being partly replaced by chlorite, gives 225 m.y. of K-Ar age. These values are different not only from those of the Sanbagawa schists, but also from the other members of the Kurosegawa zone.

shaped and surrounded by serpentinite, but INTRODUCTION small blocks are xenoliths in serpentinite. A The Kurosegawa tectonic zone is a variety of rock-types have been recognized fault zone in the Chichibu terrain in south among exotic blocks: unmetamorphosed west Japan. It runs in EW direction in Silurian and Devonian, granite and gneiss the middle subbelt of the Chichibu terrain (426 m.y., Rb-Sr, Nohda, 1973), amphi (Yamashita, 1957) and accompanied by a bolite, amphibolite and basic large amount of serpentinite and blocks granulite. Basic and pelitic schists of albite exotic to the neighbouring Chichibu forma amphibolite facies (429 m.y., K-Ar, tion, measuring from a few meters to Maruyama and Ueda, 1975) also occur. several kilometers. Large blocks are lense Several years ago, we found that

* Present address , Department of Earth Science, Faculty of Education, Toyama University, Toyama 930 (Manuscript received July 25, 1978) Fig. 1. Geological map of the Ino-Engyoji area. 208-240 m.y. old jadeite-glaucophane schists in the Kurosegawa tectonic zone 301

another type of exotic blocks occur in westwards for more than 18 kilometers is in serpentinite of the Kurosegawa zone of contact with mudstone, accompanied by a Shikoku. It includes high P and low T small amount of chert, sandstone and green schists among which are jadeite-glaucophane stone, which is probably a part of the Ino schists. In the following, we intend to formation (Katto and Kawasawa, 1958). describe the mode of occurrence , petrography The contact between the serpentinite and the and K-Ar ages of the high P and low T sedimentary sequence is concordant at some schists xenoliths in detail. places, but discordant at others. Several roof pendants of the Ino formation, one of MODE OF OCCURRENCE them measuring about 0.04km2, were A geologic map of the Kurosegawa zone mapped in the serpentinite.

near Kochi city in Shikoku, is shown in Fig . I The xenoliths of the high P and low T Members constituting the tectonic zone are schists occur in the northern part of the divided into two groups on the basis of large serpentinite mass referred above. The their scale; one is called as tectonic unit and largest of the blocks measures 100m3, but is usually larger than 1.5•~0.3 square they are usually about 0.5m3 (Fig. 6). kilometer in size, and the other smaller than

this size is called as tectonic block herein. PETROGRAPHY Tectonic units consist of the Triassic Kochi The high P and low T schists so far

gatani, Permian Takaoka, Silurian Yoko collected are metabasites and metachert. kurayama formations, and the Ino forma Although all the schists examined contain tion of unknown age as shown in Fig. 1 . high-pressure , such as jadeite, The former four are weakly metamorphosed glaucophane, chloromelanite and lawsonite, in some places, but practically unmetamor a few types of original rocks could be

phosed in others. The Ino formation deciphered. They are pillow lava, hyalo suffered glaucophanitic metamorphism clastite and massive lava on the one hand,

(Banno, 1964, Nakajima et al., in prepara and greenschist and amphibolite on the tion). Serpentinite is one of the major other. constitutent rock-types of the zone in this The schists derived from volcanics pre area. In some places it forms large masses, serve augite and biotite as relic minerals, but in others, it forms separate belts and hyaloclastitic texture can be seen under intruded into fault and are several meters the microscope. These metavolcanics wide, continuing for more than several suffered high P and low T metamorphism kilometers. The faults bordering tectonic without having intermediate stages of low units or large tectonic blocks are very often pressure metamorphism to be mentioned intruded by serpentinite. Small tectonic later. blocks, measuring less than 1m3 of am High P and low T metamorphics derived phibolite, albite-epidote amphibolite, basic from amphibolite is medium- to coarse granulite, garnet amphibolite, gneiss and grained and melanocratic with glaucophane schists are always enclosed by as dominant mafic phase. Fresh rock is serpentinite. commonly bluish or greenish black. A

In the area north of the cities of Ino continuous series of texture is observed and Kochi, large serpentinite mass, extending between the two extreme cases: one being 302 S. Maruyama, Y. Ueda, S. Banno

poor in high-pressure minerals and the (4) lawsonite+pumpellyite+glaucophane other mostly consisting of them. +jadeite+aegirinejadeite+salite+ Pale-green actinolite coexisting with chlorite+albite+quartz hornblende, green and brown, and Ca-rich (5) lawsonite+pumpellyite+crossite+ clinopyroxene are on the way being epidote+albite+quartz replaced by alkali amphibole (Fig. 2), but (6) lawsonite+pumpellyite+chloromelanite plagioclase is exclusively albite (less than +chlorite+albite+quartz XAn=0.01) that sometimes includes lawsonite, (7) lawsonite+pumpellyite+glaucophane suggesting its derivation from Ca-bearing +albite+quartz plagioclase. (8) lawsonite+glaucophane+stilpnomelane +albite+quartz (9) lawsonite+pumpellyite+glaucophane +chioromelanite+microcline+albite +quarz (10) lawsonite+glaucophane+albite (11) lawsonite+pumpellyite+glaucophane

Fig. 2. Replacing texture of actinolite by alkali +albite . Actinolite is partly replaced (12) lawsonite+pumpellyite+glaucophane by magnesioriebeckite along or +albite+aegirinejadeite crack, which in turn is thoroughly mantled by parallel-symmteric glaucophane, thereby (13) lawsonite+glaucophane+albite suggesting that magnesioriebeckite is not +aegirinejadeite equilibrated with matrix minerals. When ever magnesioriebeckite appears, it shows (14) pumpellyite+chlorite+albite nonequilibrated texture with matrix min Partial assemblages of (7) are common. erals as in this sample. Assemblages (10)-(14) are free of quartz. Sphene, apatite, white mica (phengite so far Thus, at least two stages of metamor identified), calcite and opaque minerals phic recrystallization are recognized in may be added to the above assemblages. some of the high P and low T schists. The Opaque minerals were identified for as mineral assemblagesformed by high P and semblages (4), (5), (6) and (9). Pyrite, low T metamorphismwill be describedfirst, chalcopyrite and sphalerite are present, and the relationships between these two but hematite and rutile have not been stages of metamorphism will be discussed observed. Analcime is sometimes observed later. around jadeitic pyroxene. Mineral assemblages of metabasites In the metamorphosed chert, following

formed by high P and low T metamorphism assemblages were observed:

are as follows: (15) lawsonite+glaucophane+graphite (1) lawsonite+glaucophane+stilpnomelane +quartz (mode 50% •})+albite

+jadeite+aegirinejadeite+albite+ (16) glaucophane+graphite+quartz (mode quartz 90% •})+albite (2) lawsonite+pumpellyite+jadeite+ Aragonite was sought by staining with aegirinejadeite+albite+quartz Feigl's solution, but not detected except as (3) lawsonite+pumpellyite+glaucophane veinlet minerals in serpentinite. +aegirinejadeite+albite+quartz The assemblages listed above include 208-240 m.y. old jadeite-glaucophane schists in the Kurosegawa tectonic zone 303 some metastable ones such as (1), (2) and the list of mineral assemblages. A group (4), and it is apparent that not all of them of pyroxenes has compositions covering both are isofacial. These problems will be dis the impure jadeite and aegirinejadeite fields cussed below from the viewpoint of pyro as defined by Essene and Fyfe (1967), but xene mineralogy. they are jointly called aegirinejadeite, be There are five types of metamorphic cause they were formed by the same process, pyroxenes in the xenoliths. The first group and their majority belong to aegirinejadeite, is augite relics derived from higher as well as it is too artifiical to separate them temperature metamorphism preceded the into two groups. Chloromelanite and high P and low T one and it is partly salite form distinct groups, both chemically replaced by alkali amphibole. Metabasites and genetically. containing the metamorphic augite relics Jadeite appears in the matrix or vein. always contain brown hornblende which is It is in direct contact with quartz in some also replaced by alkali amphibole at its cases, but they usually reacted with periphery. Therefore, such augite may be quartz resulting in the formation of albite neither of igneous nor high P and low T or aegirinejadeite. Two types of reaction metamorphism, but of the preceding low texture can be distinguished. First type is pressure one in origin. The pyroxenes shown in Fig. 4, in which we see that quartz formed by high P and low T metamorphism and jadeite occur 0.5 mm apart, but not in are salite, jadeite and impure jadeite, direct contact. Between them are the chloromelanite, and aegirinejadeite. Fig. 3 pseudomorph after pyroxene consisting of shows their compositions. Jadeite and aggregates of albite and less jadeitic impure jadeite are jointly called jadeite in pyroxene, which are again armoured by fresh albite without inclusions. The less jadeitic pyroxene is too fine-grained to determine the exact composition. How ever, it was confirmed that it has some amounts of Ca and Fe by an electron-probe microanalyzer. Jadeite sometimes occurs in the aggregates of pyroxene pseudomorph as irregular crystals being replaced partly by mesh-like veinlets of albite, and partly by high-refracting mineral, probably less jadeitic pyroxene. This texture can be explained by the reaction between jadeite and quartz outside their stability field. Another example of reaction is illustarted in Fig. 3. Composition of the pyroxenes from Engyoji Fig. 5, where jadeite is mantled by aegirine schists xenoliths in terms of the end member components jadeite (Jd), acmite jadeite. The boundary between them is chemically sharp. This texture can also be (Ac) and diopside+hedenbergite (Di+Hd). Circle and star show EPMA and wet explained by the reaction mentioned above. chemical analyses, respectively. The tie In some places, however, jadeite is in lines connect the core and rim of retrograde contact with quartz. Lawsonite, pum jadeite. 304 S. Maruyama, Y . Ueda, S. Banno

3 Fig. 5. Reaction texture between jadeite and quartz. Jadeite is generally mantled by aegirinejadeite, but it is in direct contact with quartz at some places. Fig. 4. Reaction texture between jadeite and quartz. Abbreviationsare as follow; Ps. and TiO2, and high CaO contents (Table 1). Jd: pseudomorphafter jadeite, Rem, Jd: remnant jadeite, Qz: quartz, Ab: albite. It occurs in the thin section containing jadeite-aegirinejadeite overgrowth, but pellyite and glaucophane never show retro salite occur only in the isolated pool which graded textures. Therefore, we think consists of salite, lawsonite, pumpellyite and that the breakdown of jadeitic pyroxene took glaucophane, and probably not equilibrated place within the stability field of the with jadeite and glaucophane in the matrix, assemblage lawsonite+pumpellyite+ although it is a metamorphic pyroxene. glaucophane, in other words, probably in As discussed above, jadeitic pyroxene the aragonite field (Brown, 1977). shows texture of reaction with quartz. Chloromelanite occurs in the matrix or Then, by eliminating the assemblage of in the vein, the latter composed mainly of later reaction, following three groups of albite and quartz with lesser amount of mineral assemblages are distinguished. glaucophane, pumpellyite and lawsonite. Lawsonite, pumpellyite and glaucophane Chloromelanite is strongly zoned with the appears, from the texture, to coexist in all core richer in augite and poorer in acmite, these groups of assemblage. and the rim with low augite and high acmite (1) jadeitic pyroxene (jadeite mol. contents, i.e., jadeite content does not vary more than 70%)+quartz in the zoned chloromelanite. (2) albite+quartz without Napyro Salite coexists with lawsonite, pum xene pellyite and glaucophane. It has low Al2O (3) albite+quartz+Na-pyroxene 208-240 m.y. old jadeite-glaucophane schists in the Kurosegawa tectonic zone 305

Table 1. Representative microprobe and wet chemical analyses of metamorphic pyroxenes

* Total Fe as FeO ** H2O (+)=0 .75, CraO3=0.018, analyst: H. Haramura

(jadeite mol. less than 30%) albite (XAU•…0.01) often contain lawsonite It is, therefore, concluded that high P and inclusions. Thus, the An content of alleged low T schists formed at different P-T condi plagioclase is uncertain. Representative che tions occur as xenoliths in serpentinite. The mineral assemblages probably Table 2. Representative microprobe analyses formed before high P and low T metamor of calciferous amphiboles of the pre ceding metamorphism phism are as follows; (17) actinolite+calcic plagioclase (18) actinolite+hornblende (blue green)+calcic plagioclase (19) hornblende (green)+calcic plagio clase (20) hornblende (brownish green)+ calcic plagioclase (21) hornblende (brownish green)+ augite+calcic plagioclase with or without ilmenite and magnetite. These assemblages are observed in rocks having high-pressure minerals, and an ambi guity remains as to the stability of the as semblage. The presence of calcic plagioclase is inferredfor the texture that coarse-grained * Total Fe as FeO 306 S. Maruyama, Y. Ueda, S. Banno

mical analyses of calciferous amphiboles of and they are cut by pumpellyite vein con assemblages (1) and (2) are listed in Table taining small amounts of quartz, albite and

2. Hornblende contain 1.8 Ca and 0.2 Na glaucophane. The lawsonite-poor part for 0=23, which is not of high-pressure meta mainly consists of brownish-green hornblende morphism, and should be of low-pressure and augite, both of which are more or less one. Assemblages (1) and (2) indicate that replaced by glaucophane at the periphery. the preceding metamorphism belongs to Plagioclase is albite with XAn=0.01 and lowpressure type. includes many fresh lawsonite tablets. The

On these observations, we may sum lawsonite-rich part mainly consists of law marize the metamorphic history of the high sonite, pumpellyite, glaucophane with pressure xenoliths as follow. First, low small amounts of impure jadeite, aegirine pressure type metamorphism of inter jadeite, albite, quartz, muscovite, analcime mediate- to high-grade took place, and its and a high-refracting mineral, probably products as well as basaltic pillow lava and sphene. Muscovite is richer in lawsonite hyaloclastite, and chert having been intact rich layer than the other. Three pyroxene by the low-pressure metamorphism, suffered exist, augite formed by the preceding low the jadeite-glaucophane type metamorphism. P and high T metamorphism, and jadeite and

Some of the schists suffered again the retro aegirinejadeite, the latter replacing the grade metamorphism within high P and former by high P and low T metamorphism. low T regime. Finally, they were brought SM74103119: Pumpellyite-bearing dolerite up to the ground surface being enclosed by from the road side of Engyoji Pass and is a serpentinite. Analcime around jadeitic xenolith 3•~2m2 in size in serpentinite pyroxene was probably formed on the way (Fig. 6). Igneous mineral and subophitic to the surface, or on the ground surface by texture are well preserved. Augite with weathering. TiO2=0.69 wt% (average of 37 analyses)

and Al2O2=2.30 wt% (same as before) K-Ar DATING contents, sericitized plagioclase, TiO2 rich

Mode of mica is usually very small. (5.5 wt%) and partly chloritized biotite Among 105 samples examined, two samples are the relic igneous minerals. Existence of rich in muscovite, and one containing Ti- and Al-rich augite and Ti-rich biotite biotite were selected. indicates that this rock belongs to alkali

SM74110202: A probably metasomatized basalt series. Metamorphic minerals are, metabasite with the assemblage, jadeite+ albite, chlorite, pumpellyite, sericite and aegirinejadeite+albite+lawsonite+ sphene. pumpellyite+quartz+muscovite+sphene+ K-Ar dates: The K-Ar dates of the micas carbonate located from 0.3 kilometer east of from those specimens are shown in Table 3.

Engyoji temple. Nearly pure jadeite (jadeite Muscovite from SM75011116 and SM74 mol. about 99.9%) is often enclosed by 110202 are 208 and 240 m.y., respectively. aegirinejadeite as shown in Fig. 5. The latter sample has no relict mineral of

SM75011116: Metabasite derived from the preceding metamorphism and its mus the same locality as above. The rock is covite has not any replacing texture there composed of altenation of lawsonite-rich and by suggesting that it has been equilibrated lawsonite-poor layers (each layer: 1cm •}) with high-pressure minerals. Although the 208-240 m.y. old jadeite-glaucophane schists in the Kurosegawa tectonic zone 307

Table 3. K-Ar ages of muscovite in metabasites and biotite in metadolerite from the Kurosegawa tectonic zone, central Shikoku

Analyst: Y. Ueda 40Ar R: radiometric argon 40, ƒÉe=0.585•~10-10yr-1, Ap=4.72•~ 10-10yr-1, 40K/K=1.19•~10-2 atom.%. sample SM75011116 has relic minerals, its characteristic in low temperature facies of muscovite occur in the domains accompany the jadeite-glaucophane type facies series, ing high-pressure minerals, lawsonite, pum such as Franciscan and New Caledonia. We pellyite, glaucophane and jadeite as such. may add the Kamuikotan belt in Hokkaido Any replacing texture of muscovite has not (Gouchi, in preparation) as another example been observed. of the typical high-pressure schists. Consequently, radiometric ages of musco In the Sanbagawa belt, although alkali vite from them are the possible minimum amphibole including glaucophane, pum ages of the high-pressure metamorphism. pellyite and lawsonite occur, the assemblage Biotite in SM74103119 gave 225 m.y. lawsonite+pumpellyite+glaucophane has It is presumably a relic igneous mineral, as not been observed by Miyashiro and Seki discussed above, and gives the lowest (1958) and even pumpellyite+glaucophane estimate of the igneous crystallization age. is rare and occurs only in some unusual rock However, the biotite is sometimes chloritiz types (Nakajima personal communication.) ed along the cleavage and crack (Fig. 6), Thus, even though some ambiguties remains so that the age could give the time of chlori to be solved as to the paragenetic relations tization, that could have enhanced material in the Kanto Mountains, where the migration within biotite. presence of jadeite+quartz (Seki, 1960) and absence of lawsonite+pumpellyite+ glaucophane (Seki, 1958) are reported, the DISCUSSION Sanbagawa metamorphic belt is generally The presence of jadeite+quartz as avoid of jadeite+quartz and lawsonite+ semblage in some of the schists xenoliths pumpellyite+glaucophane. described here undoubtedly show that It follows that the schists xenoliths they belong to the typical jadeite-glauco described here belong to a different meta phane facies. Some xenoliths contain morphic facies from those of the Sanbagawa albite+ quartz+ chloromelanite assemblage, metamorphics so far exposed on the surface but they also accompany lawsonite+pum of the earth. pellyite+glaucophane. Brown (1977), Radiometric ages of the Sanbagawa in his review of the paragenetic relations of schists are less than 138 m.y. (Miller et al., calc-alumino silicates of low-temperature 1961, Banno and Miller, 1965, Yamaguchi and medium- to high-pressure metamor and Yanagi, 1970, Ueda et al., 1977), while those of the schists xenoliths reported here phism, has demonstrated that lawsonite+ are 208-240 m.y. Some geologists con pumpellyite+glaucophane assemblage is 308 S. Maruyama, Y. Ueda, S. Banno

Fig. 6 Photographs and microphotographs. ‡@ A jadeite-glaucophane xenolith in serpentinite. ‡A A pumpellyite-bearing metadolerite in serpentinite. ‡B Aegirinejadeite being in direct contact with quartz. With one nichol. ‡C Relic igneous biotite in pumpellyite-bearing meadolerite. With one nicol. Abbreviations: Ser; serpentinite, Jd-gl schist; jadeite-glaucophane schist, Aegjd; aegirinejadeite, Qz; quartz, Biot: biotite, Chl; chlorite. sidered that the Sanbagawa metamorphism The Kurosegawa tectonic zone is a began before Triassic, probably at Permian fault zone accompanying high-grade meta based on the unconformity in the Kurose morphic rocks such as gneiss and amphi gawa tectonic zone (cf. Yamashita, 1957), bolite, acidic plutonic rocks, the supracrustal but recently Sato et al. (1977) and Matsuda rocks of Silurian to Devonian, all of which (1978) found middle to upper Triassic are exotic to the surrounding Chichibu Conodonts from the weakly metamorphosed System (Ichikawa et al., 1956), but the Chichibu formations that suffered the abundance of serpentinite is another im Sanbagawa metamorphism, such as the portant feature of this zone. We may add pumpellyite-actinolite zone of Kanto Moun the high-pressure schists xenoliths described tains (Zone IIb of Toriumi, 1975) and the here as an additional member of the Kuro southern marginal belt in Shikoku (Naka segawa zone. jima et al, 1977), respectively. It has As a member of the Kurosegawa tec become clear that the Sanbagawa metamor tonic zone, the schists xenoliths have signi phism began after middle Triassic at least in ficantly younger radiometric ages than the some places. From the geochronological other constitutents, which are, so far as evidences we consider that the schists radiometric dates are measured, always xenoliths in this district are not of the about 400 m.y. old. Chronological rela Sanbagawa metamorphism but of another tions of the schists xenoliths to the other older one. constitutent members are not understood. 208-240 m.y. old jadeite-glaucophane schists in the Kurosegawa tectonic zone 309

It may be related to a 240 m .y. event Gouchi, N. (1978), On the Kamuikotan metamorphic reported by Nohda (1973), as the age of rocks in the western part of the Asahikawa city, Hokkaido. M. Sc. Thesis, Kanazawa University rejuvenation of K-Ar age of K-feldspar in (in Japanese with English abstract). 400 m.y. gneiss, but the possibility that Ichikawa, K., Ishii, K., Nakagawa, C., Suyari, K.

they were formed by different geological and Yamashita N. (1956), The Kurosegawa tectonic zone. Jour. Geol. Soc. Japan, 62, 82- events cannot be disproved now . 103 (in Japanese with German abstract). In any case, the presence of typical Katto, J. and Kawasawa, K. (1958), Paleozoic System of the northern environs of Ino, Kochi jadeite-glaucophane schists xenoliths in the Prefecture. Rept. Kochi Univ., no. 19, 1-8 (in Kurosegawa zone, situated on the Pacific Japanese). ocean side of the Sanbagawa belt requires Maruyama, S. and Ueda, Y. (1976) , Schist xenoliths further consideration on the development in ultrabasic body accompanied with Kurose gawa tectonic zone in eastern Shikoku, Japan. of the Japanese Islands in southwest Japan. Jour. Japan. Assoc. Min. Pet. Econ. Geol., 70, Our trial view on this problem will be dis 47-52 (in Japanese with English abstract).

cussed elsewhere in near future. Matsuda, T. (1978), Discovery of the Middle-Late Triassic conodont genus Metapolygnathus from calcareous schist of the Sambagawa southern ACKNOWLEDGEMENTS marginal belt in central Shikoku. Jour. Geol. Soc. Japan, 84, 331-333 (in Japanese). We wish to express our sincere thanks to Miller, J. A., Shido, F., Banno, S. and Uyeda, S.

Mr. H. Haramura for chemical analysis of (1961), New data on the age of orogeny and metamorphism in Japan. Japan. Jour. Geol. jadeite, and to Mr. T. Nakajima for coll Geogr., 32, 145-151. aboration in the study of metamorphic Miyashiro, A. and Seki, Y. (1958), Mineral rocks of this area and for oral communcia assemblages and subfacies of the glaucophane

tions of his experience in the Kanto Moun schist facies. Japan. Jour. Geol. Geogr., 29, 199-208. tains. Messrs. I. Hiraiwa and S. Yogo of Nakajima, T., Banno, S. and Suzuki, T. (1977), Nagoya University made some of thin Reactions leading to the disappearance of

sections examined, to whom we are grateful. pumpellyite in low-grade metamorphic rocks of the Sanbagawa metamorphic belt in central One of us (S. M.) is indebted to Prof. K. Shiku, Japan. Jour. Petrol., 18, 263-284. Ishioka, Dr. K. Suwa and Dr. Y. Tsuzuki Nohda, S. (1973), Rb-Sr dating of the Yatsushiro

of the same University for their continuous granite and gneiss, Kyushu, Japan. Earth Planet. Sci. Lett., 20, 140-144. encouragement. Sato, T., Takizawa, S. and Kodato, T. (1977), Revision of stratigraphy and structure of the

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English abstract). Yamashita, N. (1957), Mesozoic era. The Associa Yamaguchi, M. and Yanagi, T. (1970), Geochrono tion for Geological Collaboration, Tokyo, the logy of some metamorphic rocks in Japan. first and second volumes, pp. 94, pp. 116 (in Eclog. Geol. Helv., 63, 371-388. Japanese).

四国高 知市付近の黒瀬川構造帯に産する208-240 m.y.の 放射性年代 を示す ヒスイ輝石-藍 閃石片岩

丸山 茂徳 ・植田 良夫 ・坂野 昇平

四 国 中央 部 高 知 市 北 方 の黒 瀬 川 構 造 帯 か ら ヒ ス イ輝石-藍 閃 石 型 相 系列 に属 す る変成 岩 が 発 見 され た。 それ ら は蛇 紋 岩 中 に ゼ ノ リス と して 産 す る。変 成 岩 の源 岩 は緑 色 片 岩 ・角閃岩 ・塊状溶 岩 ・枕状 溶 岩 ・ハイ ア ロ ク ラス タ イ ト・チ ャー トであ る。組 織 の観 察 か ら3回 の 変成 作 用 が 識別 きれ る。 それ ら は初 め に低 圧 型 相 系 列 に属 す る 中 ～ 高 変成 度 の 変成 作 用,次 に ヒス イ輝 石 線 閃石 型 低 温 高 圧 変成 作 用 お よび高 圧 下 で の 後 退 変成 作 用 で あ る。 全 て の高 圧 変成 岩 が最 初 の低 圧 変 成 作 用 を受 け た訳 では ない 。 高 圧 鉱 物 と平 衝 で あ った と思 われ る自雲 母 のK- Ar年 代 は208-240 m.y.を 示 す 。従 っ て,こ れ らの 高 圧 変成 岩 は三 波川 変成 作用 よ り古 い変 成 作 用 の 産 物 で あ る。

Ino:伊 野, Kochigatani:川 内 ケ 谷, Engyoji:円 行 寺, Takaoka:高 岡, Yokokurayama:横 倉 山