In Aegirine-Bearing Albitite from Iwagi Islet, SW Japan
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地学雑誌 Journal of Geography（Chigaku Zasshi） 130（3）369378 2021 doi:10.5026/jgeography.130.369 Dalyite（ K2ZrSi6O15） and Zektzerite（ LiNaZrSi6O15） in Aegirine-bearing Albitite from Iwagi Islet, SW Japan ＊ ＊＊ ＊ Teruyoshi IMAOKA , Sachiho AKITA and Mariko NAGASHIMA ［Received 16 October, 2020; Accepted 15 January, 2021］ Abstract The rare potassium zirconium silicate dalyite and lithium sodium zirconium silicate zektze- rite have been found in Cretaceous aegirine albitite from Iwagi Islet, Southwest Japan. Electron microprobe analyses of the dalyite and zektzerite yielded the following empirical formulas based on 15 oxygens: （K1.93Ba0.01）S1.94（Zr0.99Hf0.02）S1.01Si5.98O15 and Na1.03（Zr0.96Hf0.02Y0.01Sc0.01）S1.00Li1.02 （Si5.95Al0.04）S5.99O15, respectively. The occurrence of porous zircon, dalyite, and mantles of zektze- rite on resorbed zircon in the albitite suggests that those zirconosilicate minerals are the prod- ucts of metasomatic mineral replacement reactions by Na-, K-, and Li-rich hydrothermal fluids. This is the first documented occurrence of dalyite and zektzerite in the Japanese Islands. Key words： dalyite, zektzerite, albitite, Li-metasomatism, Iwagi Islet It also occurs in a block of peralkaline pegma- I. Introduction tite in the moraine of the Dara-i-Pioz glacier, Dalyite, K2ZrSi6O15, was first discovered in northern Tajikistan （Grew et al., 1993）, in peralkaline granite xenolith from Ascension the fluorite-bearing biotite syenogranite of the Island（ Van Tassel, 1952）. Since then, it has Cretaceous Del Salto pluton in the Patagonian been reported as an accessory mineral in a va- batholith, Aysén Province, Chile（ Welkner et al., riety of peralkaline silica-oversaturated rocks, 2002）, in the nepheline syenite pegmatite at including peralkaline granites, syenites, peg- Virikkollen, Haneholmveien, and Sandefjord in matites, charoitites, lamproites, lamprophyres, Norway, as euhedral crystals at Ampasibitika fenites, and carbonatites （see Jeffery et al., on the Ampasindava Peninsula, Madagascar, 2016 and references therein）. It is often asso- and as gemmy crystals up to 3 cm across at Mt ciated with chevkinite-（Ce）, aegirine, quartz, Malosa in Malawi1）. and britholite. The peculiar metasomatic rocks （albitites） Zektzerite, LiNa（Zr,Ti,Hf）Si6O15, is a very on Iwagi Islet（ 34°15¢47²N, 133°9¢39²E）, Ehime rare mineral that occurs in peralkaline granites Prefecture, Japan, have been studied by Sugi and syenites in association with arfvedsonite, and Kutsuna（ 1944）, Taneda（ 1950, 1952）, and ferrorichterite, astrophyllite, okanoganite, el- Murakami and Matsunaga（ 1966）. They con- pidite, aegirine, riebeckite, pyrochlore, sogdian- tain rare（ unusual） Li-rich silicates, including ite, and zircon. It was first found in miarolitic katayamalite（ KLi3Ca7Ti（2 SiO3）12（OH）2; Mura- cavities in the agpaitic granite of the Golden kami et al., 1983; Andrade et al., 2013）, sugilite 3+ 3+ Horn batholith, Washington（ Dunn et al., 1977）. （Na2K（Fe , Mn , Al）2Li3Si12O30; Kato et al., ＊ Graduate School of Science and Technology for Innovation, Yamaguchi University, Yamaguchi, 753-8512, Japan ＊＊ Asahi Consultant Co., Ltd., Tottori, 680-0911, Japan ― 369 ― 1976; Murakami et al., 1976; Armbruster around Mt Sekizen-san. Dikes of granophyre, and Oberhänsli, 1988）, and murakamiite（ Li- diorite porphyry, and granite porphyry cut these analogue of pectolite, LiCa2Si3O（8 OH）; Imaoka granites. et al., 2017）, and their mineralogical diversity The albitites occur as many small discrete is unique. During our chemical and mineralog- masses in the coarse-grained biotite granite ical studies of the albitites from Iwagi Islet, we around the summit of Mt Kuresaka in the east- found the rare Zr-bearing minerals dalyite and ern part of the islet（ Fig. 1c）. The largest body zektzerite. In this paper, we will describe the of albitite is several tens of meters in diameter, occurrence and chemical compositions of these and small albitite bodies, up to several tens minerals and discuss their metasomatic forma- of centimeters to meters wide are generally tion. oriented more-or-less NS, and irregularly distributed over an area of 1.5 ´ 0.8 km2. The II. Geological setting relationships between the albitites and the sur- Small bodies of metasomatites are distribut- rounding coarse-grained granite are transition- ed in the Setouchi area, SW Japan（ Fig. 1a）. al, so that albitized granite and quartz albitite They are known from seventeen areas and can be found between them. No intrusive con- occur in the granitoids of the Cretaceous Ryoke tact was found between them. and San-yo belts. The modes of occurrence, III. Petrography petrography, and major element chemistry of some of these metasomatites have been describ- Sample IWG-23, a dalyitezektzerite-bearing ed by Murakami（ 1976 and references therein）, albitite, was collected from the top of Mount Murakami and Matsunaga（ 1966）, and Mina- Kuresaka at the eastern end of Iwagi Islet. kawa et al. （1978）. According to Murakami Albite is the dominant mineral in the albitite, （1976）, the metasomatites commonly form and white to transparent albite and black aegi- small bodies less than 100 m across that are rine are visible to the naked eye. The minerals pipe-like, irregular sheets, or ovoid in shape. present include albite（ 99.7％）, alkali-feldspar The geology of Iwagi Islet and its peculiar （0.2％）, quartz（ 0.1％）, and aegirine（ < 0.1％）. albitites has recently been described in detail The albite（ 0.26 mm） is subhedralanhedral, by Imaoka et al.（ 2021）. Thus, only an outline and it forms aggregates of numerous, small, of the geology of the islet is provided in this granular crystals at the boundaries of large, paper. The Ryoke metamorphic rocks, made up broken, and deformed albite grains. Magmatic of schistose hornfelses derived from slates and quartz shows variable degree of dissolution sandstones, are the oldest rocks on the islet. and changed to secondary opal （SiO2∙nH2O） These occur in the central summit area of the （Imaoka et al., 2021）, which is termed “dequart- islet, and in the southwestern coastal area. zification” （e.g., Cathelineau, 1986; Boulvais Granitoids intrude the Ryoke rocks and are et al., 2007; Nishimoto et al., 2014; Figs. 2a, distributed extensively throughout the islet. b, and c）. They present in some vug and pore They can be divided into three types, coarse-, （Figs. 2a, b, and c）. Even with this sample, medium-, and fine-grained biotite granite, in quartz（ 0.51.7 mm） is opalized, and occurs as order of emplacement （Fig. 1b）. The coarse- prismatic and/or spherical crystals associated grained biotite granite occurs in the eastern with aegirine and an unknown SiFeKAl and northern parts of the islet as sheets in mineral. The aegirine（ 0.010.3 mm） occurs as the topographically lower part of the medium- anhedral aggregates and/or independent euhe- grained granite, and the sheets are accompa- dralanhedral grains, and it is often associated nied by pockets or dikes of aplite and pegma- with the opalized quartz and unknown SiFe tite. The fine-grained granite forms a sheet in KAl mineral（ Fig. 2a）. Small amounts of K- the upper part of the medium-grained granite feldspar occur as anhedral crystals included in ― 370 ― Fig. 1 （ a） Distribution of metasomatic rocks in Setouchi Province, SW Japan.（ b） Geological map of Iwagi Islet, Ehime Prefecture, Japan（ Imaoka et al., 2021）.（ c） Enlarged view of the area around Mount Kuresaka showing the occurrence of small masses of albitite, along with the location of the studied dalyitezektzerite-bearing albitite sample no. IWG-23. ― 371 ― Fig. 3 Backscattered electron images of an anhedral dalyite crystal filling an interstitial space （a）, and zektzerite including porous zircon（ b） from the albitite on Iwagi Islet, Ehime Prefecture, Japan. Abbreviations: Dal = dalyite, Zr = zircon, Zek = zektzerite, Ab = albite, Mnz = monazite. the quartz and albite. The accessory minerals present include a sogdianite-like mineral（ Fig. 2c）, zircon, fluora- patite, calcite, britholite-（Ce）, monazite-（Ce）, turkestanite, dalyite（ Fig. 3a）, zektzerite（ Fig. 3b）, and unknown SiFeKAl and SiThCa Fig. 2 Backscattered electron images showing the dis- minerals. The sogdianite-like mineral （4110 solution of magmatic quartz.（ a） Opalized quartz μm） is heterogeneous in appearance, and often filling a vug with aegirineaugite and a minute associated with the opalized quartz. The zircons SiFeKAl-mineral.（ b） Opal filling a vug that （340 μm） are euhedralanhedral, and often formed among albite crystals.（ c） Opalized quartz that exhibits a fine fibrous mesh texture and a enclosed by the sogdianite-like mineral. Porous sogdianite-like mineral. Abbreviations: Opl = zircon is commonly observed（ Fig. 3b）. The flu- opal, Ab = albite, Agt = aegirineaugite, Sog = orapatite（ 315 μm across） occurs as euhedral sogdianite-like mineral, SFK = SiFeKAl min- anhedral crystals in the albite, and it is often eral. associated with K-feldspar. The calcite（ 1230 μm across） occurs in the albite with the K-feld- spar. The britholite-（Ce）（ 222 μm） occurs as ― 372 ― anhedral grains at the grain boundaries of the 0.01 and 0.01 apfu, respectively） also indicate a quartz and K-feldspar. The monazite-（Ce）（ 4 limited substitution for K（ Table 1）. From the 11 μm across） is heterogeneous in appearance, analyses, the empirical formula based on 15 O and it occurs in the albite, often associated with apfu is（ K1.93Na0.01Ba0.01）S1.95（Zr0.99Hf0.02Ti0.01）S1.02 zircon. The dalyite was observed as an anhedral Si5.98O15, which is close to the ideal formula of colorless grain 14 ´ 29 μm in size enclosed by dalyite. Jeffery et al.（ 2016） classified dalyites the albite（ Fig. 3a）. The zektzerite（ 1065 μm） based on K ↔ Na substitution within the poly- occurs either as independent grains enclosed by hedral sites into Group 1, with an abundance of the albite or as grains within the albites that Na（ 0.120.19 apfu）, and Group 2, with signifi- enclose grains of zircon（ Fig.