岩石鉱 物鉱 床学 会誌 63 巻 6 号, 1990 年 ON THE ASSOCIATION OF POTASSIUM FELDSPAR AND CORUNDUM FOUND IN THE HIDA METAMORPHIC BELT MORIHISA SUZUKI and GEORGE KoJIMA Institute of Geology and Mineralogy, Faculty of Science, Hiroshima University Abstract: The association of corundum and potassium feldspar is found in a feldspathic gneiss in the Hida Metamorphic Belt. The equilibrium assemblage of the gneiss is: corundum-potassium feldspar plagioclase-biotite-rutile-graphite-apatite-zircon-ore. The potassium feldspar -corundum association, which represents the higher temperature side of the equilibrium curve of muscovite dehydration without quartz, would suggest the possibility of the granulitic facies metamorphism in this Belt. INTRODUCTION The association of potassium feldspar and corundum is significant for the determination of the pressure-temperature condition of metamorphism. The breakdown of micas has been experimentally realized under high temperature and high pressure of a few kilobars, to produce anhydrous minerals and water. For example, KAl3Si3 •Z10 (OH)2•¨ KAISi3 08 + A1203 + H20 muscovite potassium corundum water feldspar Certain experiments on the breakdown of micas afford important bases for the interpretation of upper stability limit of temperature of natural assemblages. The presence of quartz in the reactant assemblage reduces the upper stability temperature of mica at about 100•Ž, because of the coupled reaction between quartz and corundum as shown below. KAl3 Si3 •Z10 (OH) 2 + Si02 •© KA1 Si3•Z8 + Ale Si05 + H2O muscovite quartz potassium alumino water feldspar silicate Therefore, the presence of corundum associated by potassium feldspar suggests relatively high temperature of metamorphism. The association of potassium feldspar and corundum in metamorphic rocks (Manuscript received, February 23, 1970) On the association of potassium feldspar and corundum 267 has hitherto been recorded from some places, most of them being related to the contact metamorphism. For example, in the vicinity of Comrie, the assemblage of potassium feldspar-corundum-cordierite-biotite (•}andalusite, spinel and plagioclase) was reported as formed from silica-deficient, aluminous hornfels (Tilley, 1924). In Japan, there are a few reports on the coexistence of corundum with potassium feldspar. Hasegawa (1955) found in the Kitakami Mountains the assemblage of corundum-orthoclase-sillimanite-plagioclase cordierite-spinel derived from aluminous shale in a contact aureole of granodiorite. In the Abukuma metamorphic terrain, the association of •@corundum-potassium feldspar-plagioclase-andalusite- muscovite-biotite is detected in aluminous shale in a contact zone of gabbro (Shido, 1958). Judged from geological situations of these examples, they may represent the metamorphic condition of the pyroxene hornfels facies. The junior author, M. Suzuki, has found a corundum-bearing feldspathic gneissin the Hida Metamorphic Belt, which is characterized by regional occur rence of gneisses.* In this paper, the authors intend to stress the significance of the potassium feldspar-corundum association as suggesting the presence of metamorphic rocks of the granulitic facies, which has not been ascertained in the metamorphic belts in Japan. GEOLOGICAL SETTING AND OCCURRENCE The potassium feldspar-corundum gneiss in question is located in the gneiss area of the Odori River**, Gifu Prefecture, which represents the SW part of the Hida Metamorphic Belt (Fig. 1). Along the Odori River runs the Atotsugawa fault dividing the mapped area into two parts. In the southern half of the area, biotite gneiss (biotite almandine-potassium feldspar-plagioclase-quartz), basic migmatite (hornblende biotite-potassium feldspar-plagioclase-quartz), pyroxene gneiss (orthopyroxene clinopyroxene - brown hornblende - almandine - potassium feldspar - plagioclase quartz) and the so-called "syenitic rock" of the Inishi-type are distributed with the trend of EW, dipping to the south. The northern half consists mainly of calcareous gneisses (crystalline limestone and cale-silicate gneiss with scapolite, wollastonite and diopside) and hornblende-clinopyroxene gneiss (clinopyroxene-hornblende-potassium feldspar-plagioclase-quartz) with the trend of EW, dipping to the north. * Hattori(1957) reported an occurrenceof corundum and spinelin biotite gneisses in the vicinity of the sennotani graphite deposit, Toyama Prefecture, which is situated in the NE part of the Hida Metamorphic Belt. ** 岐 阜 県 吉 城 郡 河 合 村 小 鳥川 268 Marihisa Suzuki and George Kojima Fig. 1. Geological map along the Odori River, Gifu Prefecture. Younger rocks: 1. Cenozoic volcanic rocks, 2. Tetori series (Mesozoic) 3. Amo granite, 4. potassium feldspar porphyritic granite. Hida metamorphic rocks : 5. potassium feldspar-corundum gneiss, 6. basic migmatite, 7. hornblende-clinopyroxene gneiss, 8. pyroxene gneiss, 9. biotite gneiss, 10. "syenitic rock" of the Inishi-type, 11. calcareous gneiss, 12. migmatitic granite, 13. fault. On the association of potassium feldspar and corundum 269 Table 1, Petrographic data for potassium feldspar-corundum gneiss . hysterogene minerals : titanite, epidote, chlorite and prehnite The potassium feldspar-corundum gneiss in question is found in the Hane valley in the northern part of the area, about 1000 m above the sea level. The gneiss is enclosed in the hornblende-clinopyroxene gneiss, forming a large fusiform body of 20 •~ 5 m. The contact between this gneiss and the horn- blende-clinopyroxene gneiss cannot be observed. The gneissosity of these gneisses is harmonic with each other and the long axis of the fusiform body seems parallel to the gneissosity of the surrounding gneiss. There exists no igneous intrusion in the vicinity, which might presumably have effected contact metamorphism. PETROGRAPHY The mineral assemblage of the rock in question is a composite one, which should be divided into separate mineral assemblages, each representing independent metamorphic condition of different metamorphic episode or phase. The essential metamorphic assemblage including the potassium feldspar corundum association may be: corundum-potassium feldspar-plagioclase-biotite rutile-graphite-apatite-zircon-ore. Chlorite, epidote, prehnite and titanite must be the products of later metamorphic episode. Some petrographic data are shown in Table 1. The molecular percent of orthoclase in potassium feldspar was determined by the 2ƒÆ-value of (201) after Orville (1963). The molecular percent of anorthite in plagioclase was determined by the value of 20(131)-2ƒÆ 270 Marihisa Suzuki and George Kojima (131) and 2ƒÆ(241)-2ƒÆ(241) after Smith and Yorder, JR. (1956) and Bambauer et al. (1967), respectively. The paragonite molecular percent in muscovite was determined by the value of the basal spacing (002) after Evans and Guidotti (1966). Corundum has a pinky tint (so-called ruby) and forms spindle-shaped porphyroblasts. The largest crystal reaches to 4cm in the long diameter. The long axes of corundum crystals lie on the gneissosity plane. Some corund- um crystals include rutile. The corundum crystal has been altered to muscovite from the margin, forming a kelyphitic rim between corundum and potassium feldspar. This feature suggests the direct contact between these minerals in the original assemblage (Fig. 2 and 3). Rutile changes to titanite along its fracture. The gneiss shows distinct lamination. The original rock of the gneiss may chemically correspond to a trachytic rock. SIGNIFICANCE OF THE ASSOCIATION OF CORUNDUM AND POTASSIUM FELDSPAR Several reaction equilibrium curves have been presented for the breakdown of muscovite, namely, KA13 Si3 010 (OH) 2 •¨ KA13 Si3 08 + A12 03 + H20 muscovite potassium corundum water •@feldspar According to Turner (1968), the value of dP/dT at the point of 700•Ž, 2kb, calculated from thermochemical data, is 28 bars/deg.. The values of inclina- . Fig. 2. Corundum porphyroblasts (viewed on the long axis of corundum crystal). On the association of potassium feldspar and corundum 271 Fig. 3. The kelyphitic rim of muscovite between corundum and potassium feldspar (cor: corundum, kf: potassium feldspar, mu: muscovite, ru: rutile). Upper: crossed nicols, Lower: open nicol tion of the experimentally detcrmined curves near this point on the P-T diagram are as follows: Yorder and Eugsters' curve (1955) : 20 bars/deg. Veldes' curve (1966): 50 bars/deg. Evans' curve (1965) 25 bars/deg. (Fig. 4) Consequently, the Evans' curve is most concordant to that calculated above. According to the Evans' curve, if PH20=Psolid=3kb, the decomposition temperature is about 710•Ž if PH20=Psolid=lkb, the decomposition temperature is about 640•Ž 272 Marihisa Suzuki and George Kojima - - HS , Yorder & H P Eugster ( 1955) - - - - BVNelde (1966) 8W Evans (1965) Fig. 4. Equilibrium diagram for the decomposition of muscovite. Therefore, muscovite will be dehydrated to potassium feldspar and corundum within the temperature range of 640•‹•`710•Ž under the pressure condition of 1 •` 3 kb, which may correspond to the intermediate depth-zone of 3•`11 km. In certain metamorphic terrains of the granulite facies metamorphism, the temperature during metamorphism has been estimated as follows. In Adirondack massif, New York, the temperature deduced from the composition of iron oxide pairs is 550625•KC (Buddington, 1963). In Broken Hill, Australia, the metamorphic temperature is evaluated to be 600•`700•KC from the composition of coexisting magnetite and titanite pairs (Buddington and Lindsley, 1964). Although there remain some problems about