J. Japan. Assoc. Min. Petr. Econ. Geol. 70, 377-387, 1975 SOME PETROLOGICAL ASPECTS ON STILPNOMELANE IN GLAUCOPHANITIC METAMORPHIC ROCKS MITSUOHASHIMOTO NationalScience Museum, Tokyo KEIICHIROKANEHIRA Departmentof Earth Sciences, Chiba University, Chiba Under the glaucophaniticmetamorphic conditions, stilpnomelane appears in schists whose grade rangesfrom the pumpellyitezone to the biotitezone. Theparageneses of stil pnomelanein these schistsare rather simple,and the mineralis associatedcommonly with quartz, chloriteand calcite and rarely with epidote, pumpellyiteand actinolite. The apparentwide range of chemicaland mineralogicalcompositions of stilpnomelane-bearing schistsis due to small-scaleheterogeneity of rocks, but not due to their suitablebulk chemicalcomposition. INTRODUCTION the Sangun (Miyakawa, 1961, 1964; Nureki, Stilpnomelane is a common mineral in 1967; Hashimoto, 1968a, 1968b; Nishimura crystalline schists of the glaucophanitic 1971), Sambagawa (Kojima, 1944; Seki, metamorphic terranes. The mineral has 1958; 1961, Hide, 1961; Iwasaki, 1956, been reported to occur in various kinds of 1963; Banno, 1964; Kanehira, 1967; Ernst rocks of pelitic, psammitic, quartzofeldspa et al. 1970; Toriumi, 1971; Tanaka & thic, basic, quartzose and calcic composi Fukuda, 1974), Kamuikotan (Banno & tions, but has not been found in highly Hatano, 1963; Tazaki, 1964; Shibakusa, 1974; Herve, 1975) and other (for example, magnesian rocks. In spite of its wide oc Kanisawa, 1964) glaucophanitic metamor currence, however, most papers describe merely the presence or absence of this phic terranes of Japan (see also Miyashiro, mineral in rocks and do not give details of 1961, 1967; Hashimoto et al., 1970). The mineral has been found also in Paleozoic and the parageneses. Only a few excellent Mesozoic greenstones and associated rocks studies by Zen (1960) and Brown (1971) of the Tamba (Hashimoto & Saito, 1970), have discussed the mineral associations of Chichibu (Hashimoto & Kashima, 1970; stilpnomelane-bearing schists. This paper Suzuki 1972, Ishimoto 1974), Kitakami deals briefly with the modes of occurrence (Moriya, 1972) and other areas (Seki et al., and paragenetic relations of stilpnomelane 1964). Katada & Sumi (1966) reported in the glaucophanitic and allied low-grade stilpnomelane in association with biotite in metamorphic rocks mostly of Japan. a basic tuff of the non-glaucophanitic Ryoke STABILITY RANGE OF STILPNOMELANE belt. IN GLAUCOPHANITIC METAMORPHIC In these Japanese metamorphic dis ROCKS tricts the stability range of stilpnomelane A number of papers have reported extends from the pumpellyite zone to the occurrences of stilpnomelane in schists of biotite zone as shown in Fig. 1. Most (Manuscript received, October 3, 1975) 378 M, Hashimoto and K. Kanehira Fig. 1. Appearance and disappearance of stilpnomelane in various localities of glaucophanitic metamorphism in Japan. Numbers or names of zones of stilpnomelane occurrence are in rectangles. authors state that stilpnomelane becomes stability of pumpellyite in basic rocks, in the stable first in the pumpellyite zone where Kanto Mountains Sambagawa terrane. The pumpellyite appears in basic rocks but acti low temperature limit of the stability of nolite does not. Seki (1961),however, reports stilpnomelane in the rocks now considered, the occurrence of stilpnomelane in a zone therefore, may be in the pumpellyite zone. whose metamorphic grade is so low that The frequency of occurrence of stilp even pumpellyite does not crystallize in nomelane in schists seems to increase in basic rocks. But it is not indisputable that zones of the pumpellyite-actinolite and the mineral so identified under the micro epidote-glaucophane facies. The mineral scope is truly stilpnomelane, because the has been reported to occur in nearly all identification of such fine-grained and kinds of rocks of these facies zones excluding micaceous minerals in poorly recrystallized magnesian rocks such as talc schist and rocks is not easy. On the other hand, highly chloritic schist. In much higher Toriumi (1971) has not found stilpnomelane grade zones stilpnomelane becomes less in rocks of his Zone I, which is a zone of the common again. But it persists further into zeolite facies and defined by the associa zones whose grades are higher than the tion of laumontite and quartz in sandstones garnet isograd of the glaucophanitic meta and further characterized by the in morphism, and in some areas it is found Some petrological aspects on stilpnomelane in glaucophanitic metamorphic rocks 379 even in rocks of the biotite zone, particularly in the Alpine metamorphic belt of Switzer in rocks of psammitic and quartzose com land is located in the anchimetamorphic positions (Miyakawa, 1964; Ernst et al., zone, in the lower grade part of which the 1970). Thus the higher temperature limit mineral is sometimes associated with of the stilpnomelane stability range in the glauconite. The mineral facies of this Japanese glaucophanitic belts seems to anchimetamorphic zone is assigned by the exceed the biotite isograd. authors to the prehnite-pumpellyite facies. Stilpnomelanehas been found widely in Niggli (1960, 1970) states that stilpnomelane rocks of the chlorite and biotite zones of the is found in rocks of his stilpnomelane zone Alpine and Otago Schist belts of New Zea and of the lower part of the chloritoid zone. land (Hutton, 1940; Mason, 1962; Brown, Although the mineral facial characteristics 1967, 1971; Kawachi, 1970; Bishop, 1972). of the Swiss Alpine belt are unusual as The mineral appears first in greywackesof the compared with any other metamorphic Chl. 1 zone (e.g., Hutton, 1940; Kawachi, terranes of the world, the above-mentioned 1970), which is a zone of the prehnite two zones may be of the glaucophane schist pumpellyite facies. The stability of stil facies in a broad sense. Stilpnomelane pnomelane increases in the higher grade becomes unstable in the higher grade part zones, and in the Chl. 3, Chl. 4 and Biotite of the chloritoid zone and in much higher zonesin the eastern and western Otago dis zones it disappears. tricts at least a third of rocks studied by Meisl (1970) described in detail the low Brown (1971) carry this mineral. The temperature metamorphic rocks of the above-mentioned three zones are all con Soonwald and Taunus districts, southwest sidered to be of the greenschist facies. Germany. Stilpnomelane, pumpellyite and Finally, stilpnomelane disappears in the actinolite along with subordinate crossite, garnet zone of the epidote amphibolite all being the characteristic minerals of the facies. In contrast with the Japanese pumpellyite-actinolite facies, are found glaucophanitic metamorphic terranes, the over the whole area. Furthermore, stil garnet isograd is located at the higher tem pnomelane in rocks of the quartz-albite perature side of the biotite isograd in the muscovite-chlorite subfacies or the glauco New Zealand schist belts, as in the case of phane schist facies are reported to occur the classical Barrovian zones of Scotland. also from the Turkish glaucophanitic ter The pressure of metamorphism would have ranes by van der Kaaden (1966, 1969) and been lower in the New Zealand than in the cogulu (1967). It occurs, according to the Japanese belts. authors, in quartzose metamorphic rocks Similar wide occurrence of stilpnome associated with metavolcanic and meta lane is reported from the outer zones of the sedimentary rocks. Occurrences of stilp metamorphic belt of the Swiss Alps (Niggli, nomelane in similarly low temperature and 1960, 1970; Martini & Vuagnat, 1970;Frey moderately high pressure metamorphic et al., 1973; Frey et al., 1974 and many rocks have been reproted from the Carpa other papers). According to Frey et al. thians by Streckeisen (1968), Krautner & Medesan (1969) and Maier (1969), from (1973)and Frey et al. (1974),the lowertem Spanish Cordillera by Nijhuis (1964) and perature limit of the stilpnomelanestability 380 M. Hashimoto and K. Kanehira from Calabria by de Roever (1972). The pressure condition of metamorphism is In the Franciscan blueschist belt of considered to have been higher in the latter western United States, metaclastic rocks than in the Japanese and probably in the and metachert are the principal types of European Alpine metamorphic belts (Ernst rocks in which stilpnomelane is a stable et al., 1970; see also Hashimoto, 1972). mineral (e.g. Coleman & Lee, 1963; Ernst, Wide occurrence of stilpnomelane in 1965; Ghent, 1965; Ernst et al., 1970). argillaceous phyllites are reported by Zen According to Ernst & Seki (1967), Coleman (1960) from the Castleton area, which is in (1967), Ernst et al. (1970) and others, the a greenschist facies zone of the Barrovian selected areas of the Franciscan terranes type metamorphic belt of the Appalachians. are classified into two or three parts or Except this study, however, the occurrence zones, which are considered to represent of stilpnomelane in the Barrovian kyanite different grades of metamorphism. Pum sillimanite type terranes has not been pellyite is the characteristic mineral in documented well (see also Mather & Ather metavolcanic rocks in the seemingly lower ton, 1965; Mather, 1970). grade zone, which is said to be a"high In conclusion, it can be said that the pressure" pumpellyite zone, because it is a stability field of the mineral stilpnomelane part of the exceedingly high-pressure jade in the regional metamorphic rocks extends ite-glaucophane type of fades series of this over the physical conditions of the higher belt.