Eur. J. Mineral. 2014, 26, 279–291 Published online February 2014 Magmatic zoisite and epidote in tonalite of the Ryoke belt, central Japan 1 1,2, 3 1 YOSUKE MASUMOTO ,MASAKI ENAMI *,MOTOHIRO TSUBOI and MEI HONG 1 Department of Earth and Planetary Sciences, Nagoya University, Nagoya 464–8601, Japan 2 Present address: Center for Chronological Research, Nagoya University, Nagoya 464–8602, Japan *Corresponding author, e-mail: [email protected] 3 Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Sanda, Hyogo 669–1337, Japan Abstract: Magmatic epidote and zoisite commonly occur in Cretaceous tonalite of the Hazu area in the Ryoke belt, central Japan. The tonalite is mainly composed of amphibole, biotite, plagioclase, quartz, and epidote/zoisite with minor ilmenite, magnetite, pyrite, zircon, and apatite. Small amounts of K-feldspar occur as an interstitial phase between other felsic phases or perthitic patches in plagioclase. Epidote occurs as inclusions in plagioclase, as interstitial phase in the matrix, and as 3þ 3þ secondary phase in chlorite pseudomorphs after biotite, and in saussuritized plagioclase. The XFe [¼ Fe /(Al þ Fe )] value of the secondary epidote ranges from 0.27 to 0.39. Epidote inclusions in plagioclase and interstitial grains contain less Fe3þ 3þ (XFe ¼ 0.08–0.29), Fe -poor epidote with XFe , 0.18 occurs only as inclusion. Zoisite with XFe value of 0.01–0.07 occurs 3þ only as inclusions in plagioclase, and usually has thin lamella-like layers of Fe -poor epidote with XFe ¼ 0.09–0.14. The 3þ Fe -poor epidote with XFe , 0.20 and zoisite included in plagioclase occasionally form aggregates with K-feldspar and quartz. A thin sodic plagioclase zone develops at the boundary between the Fe3þ-poor epidote and zoisite inclusions and their host plagioclase. Such a reaction texture is not observed at the boundary between Fe3þ-richer epidote inclusions with XFe . 0.20 and their host plagioclase. Epidote grains with XFe . 0.20 in plagioclase and the matrix are a magmatic phase 3þ that crystallized directly from the tonalite magma. The Fe -poor epidote (XFe , 0.20) and zoisite were probably formed by a local reaction between the trapped melt and its host plagioclase, and these are considered not to have been in equilibrium with the tonalite magma. Compositions of amphibole-plagioclase assemblages allowed for temperature estimates in the range of 730–770 C and minimum pressures of 0.47–0.57 GPa for the epidote/zoisite-bearing tonalites of the Hazu area. Epidote/ zoisite-free tonalites occur in other areas of the Ryoke belt. There may be several tonalite bodies that record different intrusion processes and solidification depths in the Ryoke belt. Key-words: magmatic zoisite, magmatic epidote, tonalite, Ryoke belt, Japan. 1. Introduction magma was fed from a magma chamber at a depth corre- sponding to 0.8–1.3 GPa. Subsequently, magmatic epidote The petrological significance of magmatic epidote was was also described from monzogranite and diorite, as experimentally and petrographically demonstrated in the reviewed by Schmidt & Poli (2004); Schmidt & Thompson 1980s (Schmidt & Poli, 2004). Naney (1983) showed that (1996) experimentally showed that the epidote-in curve is epidote can coexist with a melt phase at 0.8 GPa in granitic positioned on the slightly higher temperature side for tonalite and granodioritic systems, and suggested that the presence of compositions than for granodiorite compositions, and it shifts magmatic epidote is almost-certain evidence for high- towards the lower pressure (P)/temperature (T)sidewith pressure crystallization of silicate magma. Zen & increasing oxygen fugacity. Furthermore, zoisite was reported Hammarstrom (1984) identified epidote as an important mag- from high-P migmatites and pegmatites derived from eclo- matic constituent of tonalite and granodiorite within the gites (Nicollet et al., 1979; Franz & Smelik, 1995). These mobile belt extending from northern California to southern petrographical and experimental studies clearly suggest that Alaska, and suggested that epidote indicates a minimum epidote-group minerals are an index phase that implies rela- intrusive pressure of about 0.5–0.6 GPa. Evans & Vance tively high-P solidification of a silicic melt, and the stability (1987) reported elongate phenocrysts of epidote, which tex- conditions for epidote-group minerals depend slightly on the turally ensures the magmatic origin, from rhyodacitic dikes in melt composition and increase towards the low P/T side with 3þ 3þ Colorado, and considered that the epidote-bearing dike an increase in their XFe [¼ Fe /(Al þ Fe )] values. 0935-1221/14/0026-2360 $ 5.85 # Downloaded fromDOI: http://pubs.geoscienceworld.org/eurjmin/article-pdf/26/2/279/3977483/279_ejm26_2_279_291_masumoto_gsw.pdf 10.1127/0935-1221/2014/0026-2360 2014 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart by guest on 25 September 2021 280 Y. Masumoto et al. Magmatic zoisite and clinozoisite-epidote (referred to as mainly composed of pelitic, psammitic, and siliceous litholo- epidote hereafter) commonly occur in a tonalitic pluton, a gies, with minor amounts of metabasite. The metamorphic member of the Ryoke belt, from the Hazu area, central grade generally increases from the chlorite–biotite zone in the Japan. The purpose of this paper is to document the mineral northwest, through the K-feldspar–cordierite zone, to the chemistry and petrological characteristics of the zoisite/ sillimanite–K-feldspar/garnet–cordierite zones in the south- epidote-bearing samples and to interpret their depths of east, and then locally decreases towards the MTL (Ikeda, crystallization based on the stabilities of zoisite/epidote 1998; Miyazaki, 2010). The low-grade part of the Ryoke and pressure estimations using the hornblende metamorphic rocks passes into the unmetamorphosed geobarometer. Jurassic accretionary complex of the Mino terrane. Chemical U-Th Total-Pb Isochron Method (CHIME) mona- zite ages from 98.0 Æ 3.2to100.7Æ 3.2 Ma were reported as 2. Outline of Geology peak metamorphic ages of the Ryoke metamorphism (Suzuki et al., 1996a, 1996; Suzuki & Adachi, 1998). Granitoids in the The Ryoke belt, which consists of low-P/T metamorphic Ryoke belt of central Japan are categorized into fifteen plu- rocks originating from the Mesozoic accretionary complexes tons from the oldest Kamihara tonalite (94.5 Æ 3.1 – 94.9 and Cretaceous granitoid plutons, stretches throughout the Æ 4.9 Ma CHIME monazite age: Nakai & Suzuki, 1996) to Inner Zone (the Japan Sea side) of southwest Japan over a the youngest Naegi granite (67.2 Æ 3.2 – 68.3 Æ 1.8 Ma: length of roughly 600 km (Fig. 1). The south of the Ryoke belt Suzuki et al., 1994b; Suzuki & Adachi, 1998), based on their is bounded by the Sanbagawa belt, which is a high-P/T sub- intrusive relations (Ryoke Research Group, 1972) and radio- duction metamorphic belt of Cretaceous age. These two belts metric ages (Suzuki & Adachi, 1998). The Kamihara tonalites form perhaps the best-known example of paired metamorphic defined by the Ryoke Research Group (1972) occur as three belts (Miyashiro, 1961). The boundary between the Ryoke major bodies in the Hazu, Shimoyama, and Tenryu areas and Sanbagawa belts is a major strike-slip fault, the Median (e.g., Suzuki & Adachi, 1998). Tectonic Line (MTL). Members of the Ryoke belt are dis- The Hazu area, from which the tonalite samples were tributed widely in central Japan, in which the Hazu area collected, is situated at the southwestern margin of the studied in this paper is located. Ryoke belt in central Japan (Fig. 1a). In the northern part of The geological configuration of the Ryoke belt in central the Hazu area, the Ryoke metamorphic rocks are distributed Japan is summarized in Suzuki & Adachi (1998), including widely, whereas tonalite occurs along the southern coastline the distributions of the Ryoke metamorphic rocks and a series (Fig. 1b). The Ryoke metamorphic rocks in the Hazu area are of granitoid plutons. The Ryoke metamorphic rocks are divided into sillimanite and sillimanite–K-feldspar zones in (a) 36°N Japan Sea (b) R247 Nagoya ISTL Honshu Ryoke belt Osaka MTL Fig. 1b Shikoku Pacific Ocean Kyushu 132°E 134°E 136°E 200 km MT2503 MT1703 YM2910A MT1704 YM2907B YM2906 Mt. Hara YM2911 Mikawa Bay Quaternary sediments 34°46’ N YM2912 Kamihara tonalite Ryoke metamorphic rocks 1 km 137°10’ N Fig. 1. (a) Distribution of the Ryoke belt in southwest Japan and (b) geological sketch map of the Hazu area, central Japan (simplified part of Geological Survey of Japan AIST, 2010). Abbreviations used are as follows: MTL, Median Tectonic Line; ISTL, Itoigawa-Shizuoka Tectonic Line. Downloaded from http://pubs.geoscienceworld.org/eurjmin/article-pdf/26/2/279/3977483/279_ejm26_2_279_291_masumoto_gsw.pdf by guest on 25 September 2021 Magmatic zoisite and epidote in tonalite of the Ryoke belt, central Japan 281 the northern and southern parts, respectively (Asami, 1977). (Fig. 2a and b). The host plagioclase around the zoisite The typical mineral assemblage of metapelite in the sillima- inclusions is locally modified in composition, and a thin nite zone is biotite þ muscovite þ sillimanite þ plagioclase zone with less calcic plagioclase characteristically devel- þ quartz. Andalusite occurs in the lower-grade part of the ops between the inclusions and the host phase (Fig. 2a). sillimanite zone. Staurolite is reported to occur sporadically Epidote in plagioclase also occurs as a single grain. A less in this zone (Asami, 1977). Metapelite in the sillimanite–K- calcic plagioclase zone is usually observed also around feldspar zone is mainly composed of biotite, muscovite, epidote inclusions with XFe, 0.2. On the other hand, sillimanite, K-feldspar, plagioclase, and quartz. The modal there is no obvious compositional modification of plagio- amount of muscovite in the sillimanite–K-feldspar zone is clase around most of the epidote inclusions with XFe. 0.2 lower than that in the sillimanite zone and tends to decrease (Fig.