Tanakamiyama a Classic Japanese Pegmatite District
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VRAM 84(6) #428695 Tanakamiyama A Classic Japanese Pegmatite District anakamiyama, 太神山, sometimes written 田上山 2002). The literature of the Tanakami-fudo temple (Shu- (Mt. Tanakami; roughly 600 meters in elevation), gendo sect), which was built in 859 on the top of Tanakami- T lies just east of the city of Kyoto, on the southern yama, suggests that the name of this mountain, 太神 (tana end of Lake Biwa, in Shiga Prefecture (fig. 3). It is part of + kami = great + god), is related to Amaterasu-omikami, a large area of eroded hills that is underlain by Tanakami the sun goddess of Japanese mythology (Ashkenazi 2008). A Granite (figs. 1 and 2). Known as the Tanakami region or contrary etymology simply Tanakami, it is one of the most important specimen- JOHN RAKOVAN is found in Omi Yochi producing pegmatite districts in Japan, along with the Hiru- Department of Geology Shiryaku (1734) by kawa area in Gifu Prefecture and the Fukushima district, Miami University Tokikiyo Samukawa, Ishikawa Prefecture (Nambu 1970). Tanakami is probably Oxford, Ohio 45056 a comprehensive ge- most noteworthy for its production of fine topaz crystals [email protected] ography of Omi and as the type locality for masutomilite (a manganese- Province. This states lithium mica). MIEKO ONO that tanakami, 田上 Recognized as a classic Japanese mineral locality, Tanaka- Department of German, Russian, (= rice field + above), miyama is also noted for a long history of cultural and envi- and East Asian Languages was originally tani- ronmental significance. In a waka poem (the classical style of Miami University kami, 谷上(valley + Japanese verse that predates haiku) in the book Minamoto no Oxford, Ohio 45056 above), as is writ- Toshiyori Kashu about Tanakamiyama (ca. 1128), Kumorina- ten in the Nihongi ki yuzukuhiomo mitsurukana, kore kaminoyamano shirushi- CARL A. FRANCIS (AD 720 the earli- naruran translates as “Glorious sunset! This must be a sign of Mineralogical Museum est official history a holy mountain.” This reference alludes to the sacred stature Harvard University of Japan). Thus, of the mountain to the Shugendo religion, which incor- Cambridge, Massachusetts 02138 the pronunciation porates an ascetic, mountain-dwelling lifestyle (Kasahara [email protected] changed over time 520 ROCKS & MINERALS VRAM 84(6) #428695 Figure 1 (left page). Northwest view from Tanakamiyama (May Figure 2 (top). Geological map of the Tanakamiyama area (red 2005). The southern end of Lake Biwa and the city of Otsu can box in fig. 5). The partial ring of Cretaceous granites (in red), be seen in the background. Note the highly weathered granite including the Tanakami Granite, indicates the location of the exposure in foreground. Biwa Caldera. Figure 3 (left). Map of Japan, prepared by William Besse. from tani-kami, 谷上, through ta-no-kami, 田上, to ta-na- kami, 田上.* For more worldly matters, the Tanakami Mineral Muse- um, located in Tanakamieda-cho, Otsu (by appointment only, phone 077-5460-163 or 077-546-1921), houses a small collection of local minerals, but many finer specimens can be seen in larger museum collections around Japan and in older mineral collections abroad. *A widely held view, however, is that tanakami, 田上, actually means 田神 (rice field + god) (Yanagida 1975; Wada 1994). Dr. John Rakovan, an executive editor of Rocks & Minerals, is a professor of mineralogy and geochemistry at Miami Univer- sity, Oxford, Ohio. Dr. Mieko Ono is a professor of Japanese linguistics at Miami University. Dr. Carl Francis, a consulting editor of Rocks & Minerals, is the curator of the Harvard Mineralogical Museum. Volume 84, November/December 2009 521 VRAM 84(6) #428695 Japanese geology (Kunz 1920). He was a major force in the modernization of mining in Japan, and he amassed what still remains the finest collection of Japanese minerals in existence. After his death, this collection of more than 3,700 specimens was acquired by Mitsubishi Mining Corpora- tion and is currently on display at the Ikuno Mine Mineral Museum (Rakovan 2005a). Many of these specimens are depicted in Wada’s Minerals of Japan (1904). Geology Tanakamiyama is located on the main Japanese island of Honshu (fig. 3). The Inner (northern) Zone of southwest Honshu is divided geologically into the Ryoke, Sanyo, and Sanin belts (fig. 5), which differ in age of deposition, degree of metamorphism, and nature of mineralization (Yuhara, Kagami, and Nagao 2000). Tanakamiyama lies in the Sanyo Belt (Kohno, Nakano, and Shimobayasi 2009), near the boundary of the metamorphic Ryoke Belt, both of which experienced extensive felsic igneous activity during the Cretaceous Period (Hayama and Tainosho 2000; Yuhara, Kagami, and Nagao 2000). The exposure of the Tanakami Granite pluton measures 20 kilometers east-west and 8 kilometers north-south. It is the largest and southernmost of several igneous intrusions that form a geographic ring of granitic exposures associ- ated with the formation of the Biwa-ko Caldera (Sawada et al. 1994). Most of these intrusions are hosted by Juras- sic sedimentary rocks of the Tamba accretionary complex Figure 4. Prof. Tsunashiro Wada. (Hashimoto et al. 2000). The pluton is composed of biotite granite with high silica and alkali contents (73.3–76.7% SiO ; 8.0–8.9% Na O+K O) and Rb/Sr ratios. The granite Harvard Collection 2 2 2 In the United States one collection that is rich in speci- is rich in rare-earth elements (REE), with increasingly high mens from this location is that of the Harvard Mineral- concentrations of fluorine and lithium toward the core of ogical Museum (figs. 6–18). Notable specimens include the pluton (Ishihara, Nakano, and Terashima 2005). several exceptional topaz crystals, numerous fine orthoclase Interestingly, the Tanakami Granite, and its treasure trove crystals—many in association with quartz and albite—and of fine minerals, was formed long before the Japan archi- an outstanding gem crystal of aquamarine, one of the finest Figure 5. Regional geological map of southwest Japan. MTD = that we have seen from Japan. Most of Harvard’s specimens Median Tectonic Line; ISTL = Itoigawa-Shizuoka Tectonic Line; were obtained at the turn of the last century, including the red rectangle around Kyoto and Tanakamiyama indicates several specimens sent to Harvard by Dr. Tsunashiro Wada the area shown in figure 2 (modified from Yuhara 2000). (fig. 4). This collection is on dis- play at the Karl E. Limper Geol- ogy Museum at Miami University in Oxford, Ohio, until December 2010. Wada (1856–1920) is consid- ered the father of modern min- eralogy and geology in Japan. He was the first Western-trained mineralogist (in part by German geologist Heinrich Edmund Nau- mann), the first Japanese professor of mineralogy at Tokyo Univer- sity, and the first director of the Geological Survey of Japan; these are but a few of the seminal posi- tions that Wada held in the field of 522 ROCKS & MINERALS VRAM 84(6) #428695 Figure 6 (left). Beryl (var. aquamarine), 6.2 cm high; highly etched. Harvard specimen 88294, Terry Huizing photo. Figure 7 (center). Topaz and orthoclase, 7 cm high. It has been suggested that the large red inclusion is a garnet; however, the authors are skeptical. Harvard specimen 103077, Terry Huizing photo. Figure 8 (right). Microcline, 8 cm high, exhibiting the Baveno twin law. Harvard specimen 84699, Terry Huizing photo. pelago existed. Japan is a tectonic arc that has been evolving meters at its widest part. The pocket yielded numerous crys- throughout the Phanerozoic. It is the result of subduction of tals including a very large topaz (weighing 6 kilograms) and the ancient Pacific seafloor underneath the eastern edge of a doubly terminated crystal of smoky quartz just over 30 cm the Eurasian Plate (Taira 2001). The complex tectonic geom- in length (Obayoshi 2000). etry of the Japanese islands involves the convergence of four distinct tectonic plates, including the North American Plate Mineralogy (locally called the Okhotsk Plate) on which the United States The mineralogy of the Tanakamiyama pegmatites is sim- is also located. The island arc separated from the mainland ple, as is common in NYF-type pegmatites. Dominant relatively recently, with the opening of the Japan Sea back minerals include microcline, albite, quartz, beryl (aquama- arc basin, about 15 million years ago. Throughout the rine), topaz, muscovite, zinnwaldite, and masutomilite. A Cretaceous Period, oblique subduction of the northward- complete list of minerals from this location is given in the moving Kula-Pacific Ridge along the eastern margin of the table below. Eurasian Plate generated a large number of granitic intru- Tanakamiyama is most famous for its superb topaz speci- sions (Kinoshita 1999), including the granites and granitic mens. The first description of what was probably topaz pegmatites of Tanakamiyama and the Hirukawa district of from Tanakamiyama was made by Sekitei Kinouchi (birth Gifu Prefecture. A rubidium-strontium whole rock isochron age of 79.5 Minerals reported from Tanakamiyama. Compilation ± 9.0 million years with an initial 87Sr/86Sr ratio of 0.70951 from Obayoshi (2002) and Mindat (2008). ± 0.00037 (Sawada et al. 1994) provides the time of intru- Albite Hübnerite Quartz (smoky sion of the Tanakami Granite (Morioka, Tainosho, and Allanite-(Ce) Lazulite quartz, agate) Kagami 2000). Almandine Masutomilite* Ralstonite? Beryl (var. Microcline Rutile Syngenetic, miarolitic NYF*-type pegmatites are associ- aquamarine) Milarite† Samarskite-(Y) ated with the Tanakami Granite as irregular, crystal-lined Biotite Molybdenite Schorl pockets with thin rinds of graphic granite. This is a similar Cassiterite Monazite-(Ce) Siderite Columbite Montmorillonite Sphalerite style of occurrence to other miarolitic NYF-type pegmatites Euxenite-(Y) Muscovite Topaz such as those found in the Conway Granite in New Hamp- Fergusonite-(Y) Opal (var. hyalite) Vivianite shire and the Pikes Peak Batholith in Colorado (London Fluorite Orthoclase Wolframite Gadolinite-(Y) Pyrite Yttrotantalite-(Y) 2008; Rakovan 2008).