J. Japan. Assoc. Min. Petr. Econ. Geol. 73, 300-310, 1978. 208-240 M. Y. OLD JADEITE-GLAUCOPHANE SCHISTS 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 mineral paragenesis: first, low P metamorphism 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 lawsonite+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+quartz 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, garnet amphibolite and basic large amount of serpentinite and blocks granulite. Basic and pelitic schists of albite exotic to the neighbouring Chichibu forma epidote 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 minerals, 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 amphibole 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 amphiboles. Actinolite is partly replaced (12) lawsonite+pumpellyite+glaucophane by magnesioriebeckite along cleavage 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