J. Min. Petr. Econ. Geol. 88, 141-156, 1993 Mineralogy of piemontite-bearing schist in the Yamagami metamorphic rocks of northeastern Abukuma Plateau Masahide AKASAKA*, Jun WATANABE**, Kenji TOGARI** and Makoto KAWAMURA** * Department of Geology , Faculty of Science, Shimane University, 1060 Nishikawatsu, Matsue 690, Japan ** Department of Geology and Mineralogy , Faculty of Science, Hokkaido University, Sapporo 060, Japan Piemontite-bearing schist (Pm-schist) in the Yamagami metamorphic rocks, northeastern Abukuma Plateau, was studied in terms of bulk chemical composition, mineral assemblage and mineral chemistry. The Pm-schist occurs as intercalating member of the albite-porphyroblastic muscovite quartz schist and epidote-amphibolite. Petrographic features suggest that the primary mineral assemblage is quartz+muscovite+albite+chlorite+piemontite+tourmaline+hematite+rutile. Apatite, epidote and carbonate also occur as minor minerals. Veins formed of euhedral quartz and anhedral K-feldspar intruded into microgranoblastic quartz aggregation. The compositions of piemontite range between Pm11Ps22Cz67 and Pm17Ps18Cz65. Muscovite is phengitic (Si 6.54-6.64 atoms per 22 oxygens) and contains moderate amount of hydromuscovite component. Chlorites are clinochlore with Fe/(Mg+Fe)=0.02 and pycnochlorite with Fe/(Mg+ Fe)=0.38. Hematite contains Mn2O3 component. The bulk chemical composition of the Pm-schist is characterized by the high silica content, 82.41 wt.%, and high oxidation ratio, (Fe3+•~100)/(Fe3++Fe2+)=93. MnO content is very low at 0. 02 wt.%. Appearance of piemontite, Mg-chlorite and hematite containing Mn2O3 component suggests that fo2 condition during the metamorphism was very high, which is consistent with the extremely high oxidation ratio of the bulk composition. The extremely high fo2 condition during metamor phism may be explained by the dissociation equilibrium of H2O with H2 and O2. Keywords: Piemontite, Piemontite-bearing schist, Spotted schist, Yamagami metamorphic rock, Glaucophane schist facies and structure of the YMR and found the . Introduction piemontite-bearing schist (Pm-schist) in the Yamagami metamorphic rocks (YMR) are YMR, although the locality and detail on mine distributed along a shear zone of northeastern rals of the Pm-schist were not described. In Abukuma plateau together with Wareyama, 1975, one of the authors (M. K.) and co-workers Matsugadaira and Yaguki metamorphic rocks. also found the Pm-schist from the northern Kuroda and Ogura (1960, 1963) revealed that the part of the River Udagawa during their study of YMR are composed of epidote-amphibolite, Paleozoic formations. Another occurrence of epidote-actinolite schists and spotted schists the Pm-schist was confirmed in the southern and that they belong to glaucophane schist branch of the River Udagawa by another of facies. Iwamatsu (1975) studied stratigraphy authors (J. W.) and co-workers. Preliminary (Manuscript received, July 10, 1992; accepted for publication, February 18, 1993) I 142 Masahide AKASAKA,Jun WATANABE, Kenji TOGARI and Makoto KAWAMURA description of the Pm-schist was reported by Sheared Granite). In the River Manogawa Togari et at. (1986). Maekawa (1988) district, very small masses of epidote-am summarized the main mineral assemblage of phibolite and sheared granite are also distribut metapelites, metapsammites and metachert in ed. the YMR to be muscovite +garnet+quartz+ The Matsugadaira metamorphic rocks albite•}piemontite, but the occurrence of (MMR), composed of pelitic, siliceous, psam piemontite was not shown. mitic and basic semischists, are overlain by the In this paper, we report the occurrence, Upper Devonian Ainosawa Formation with bulk chemical composition and mineralogy of supposed unconformity (Sato, 1974). Thus the Yamagami Pm-schist. MMR are generally interpreted to be pre - Devonian in age as well as the YMR. Geologi II. Geological setting cal relationship between the MMR and the The study area is situated in the northern YMR is not clear, but it may be considered to most part of the Abukuma Eastern Marginal be in tectonic contact. Tectonic Belt (Hunahashi, 1979). This is Paleozoic strata of this area consist mainly bounded on both sides by NNW-SSE trending of clastic facies. The Paleozoic and older Futaba and Hatagawa sheared zones. This rocks are intruded by Early Cretaceous granitic belt consists of metamorphic rocks, un plutonics. metamorphosed Paleozoic strata (Devonian- Permian) and Early Cretaceous igneous rocks. III. Occurrence of piemontite-bearing schist Formation of the belt is generally assigned to The geologic map around the studied area Early Cretaceous. However, NE-SW trending is shown in Fig. 1. The Pm-schist occurs at older tectonic components are recognized in the two localities; (1) upper stream of the River belt (Watanabe et at., 1983a, 1983b), suggesting Maebarazawa, a southern branch of the River that the tectonics of the belt is more compli Udagawa, and (2) middle stream of the River cated than previously considered. Togamorizawa, a northern branch of the River Although the stratigraphic relation of the Udagawa. In the first locality, the Pm-schist YMR is not clear because of the sporadic distri occurs as intercalated member of muscovite bution of outcrops, Kuroda and Ogura (1963) quartz schist around a quarry and the Fu stated that epidote-amphibolites, spotted green kuyamagami mine (cupriferous iron sulfide ore schists and spotted pelitic schists can be consid deposit). The muscovite-quartz schist in this ered to be arranged from the lower to the area contains albite-porphyroblast, and is so upper. According to Iwamatsu (1975), the called spotted schist. The epidote-amphibolite lower member of the YMR is mainly composed near the muscovite-quartz schist also contains of fine-grained epidote-amphibolite intercalat plagioclase porphyroblasts. The occurrence of ing with thin layers of quartz-schist, the middle the plagioclase porphyloblast-epidote am member coarse-grained epidote-amphibolite phibolite is described in Watanabe et at. (1979). alternated with muscovite-quartz schist, and Relations between epidote-amphibolite, mus the upper member muscovite-quartz schist covite-quartz schist and Pm-schist, are not accompanied with Pm-schist. In the eastern clear because of strong microfolding and part of the River Udagawa district, the epidote ptygmatic folding. In the locality (2), the Pm amphibolites are typically exposed accompany -schist occurs as intercalated member of the ing small masses of sheared granite (Yamagami spotted muscovite-quartz schist. These Mineralogy of piemontite-bearing schist in the Yamagami metamorphic rocks 143 Fig. 1. Geological map of the Soma district. *Area marked by ellipse represents the region of spotted schist and plagioclase porphyroblastic amphibolite. schists in the locality (2) occur as very thin sis.IV layers between epidote-amphibolite and Yamagami Sheared Granite. Petrography In the present study, only the sample from The Pm-schist is pale green to dark green the locality (1) was available for the miner ish in color, and contains an amount of quartz, alogical investigation and bulk chemical analy muscovite and albite prophyroblasts. Along 144 Masahide AKASAKA,Jun WATANABE, Kenji TOGARI and Makoto KAWAMURA Fig. 2. Backscattered electron image of Yamagami Pm-schist. Bar represents 100 ƒÊm. Ab, albite; Ap, apatite; Chl, chlorite; Hm, hematite; Mu, muscovite; Pm, piemontite; Qz, quartz; Ru, rutile. A, Albite-porphyroblast B, Anhedral K-feldspar C, D, Prismatic piemontite showing nematoblastic texture. Mineralogy of piemontite-bearing schist in the Yamagami metamorphic rocks 145 the schistosity, dark reddish brown parts rich in is traversed by cracks perpendicular to the piemontite are developed. Ptygmatic quartz elongate direction and separated into numerous veins are found. fragments. It shows a marked pleochroism; The main minerals of the Pm-schist are X=lemon- to orange-yellow, Y=amethyst quartz, muscovite, albite, chlorite, piemontite, to pink, Z=red to violet. 2Vx=78 to 84•‹, tourmaline, hematite and rutile in decreasing c•ÈX=-4 to -6•‹. Cleavage; (100) perfect. amounts, and minor minerals are apatite, car Twinning; (100). Unit-cell parameters calcu bonate and epidote. Along the schistosity, lated from the indexed X-ray powder quartzofeldspathic seams, which are composed diffraction pattern by using least-squares of sutured xenomorphic lenticular quartz method is ao=8.91 (1), bo=5.667 (8), co=10.17 grains up to 2mm in size and sutured xenomor (1) A, ƒÀ=115.55 (4)•K and VO=463 (1) A3 (stan phic albite porphyroblasts up to 2mm in size, dard deviations shown in parentheses). The are formed in a microgranoblastic aggregation compositions and structural formulae of of quartz granules (Fig. 2A). Muscovite and piemontite are listed in Table 1. The composi chlorite show lepidoblastic texture. tions of piemontite range between Pm11Ps22Cz67 Piemontite and epidote form nematoblastic and Pm17Ps18Cz65, where Pm, Ps and Cz are texture (Fig. 2C). Hematite, rutile and tour Ca2Mn3Si3O12(OH), Ca2Fe3Si3O12(OH) and maline are associated with piemontite, mus Ca2Al3Si3O12(OH) components, respectively. covite and chlorite (Fig. 2D). Piemontite, MnO and Mn2O3 are recalculated on the basis epidote, hematite, rutile, tourmaline and apatite of stoichiometry to make octahedral occupancy show cataclastic feature (Fig. 2C, D). The 3. The occupancy of Al, Mn3+ and Fe3+ in the veins formed of quartz and K-feldspar occur in octahedral sites is shown in the Al-Mn3+-Fe3+ a microgranoblastic