328 Journal of Mineralogical andM. PetrologicalSatish－Kumar, Sciences, Y. Yoshida Volume and 99I. ,Kusachi page 328─ 338, 2004 Special Issue High temperature contact metamorphism from Fuka contact aureole, Okayama, Japan 329 The role of aqueous silica concentration in controlling the mineralogy during high temperature contact metamorphism: A case study from Fuka contact aureole, Okayama, Japan * * ** Madhusoodhan SATISH－KUMAR , Yasuhito YOSHIDA and Isao KUSACHI *Department of Biology and Geosciences, Faculty of Science, Shizuoka University, Shizuoka 422－8529, Japan **Department of Earth Sciences, Faculty of Education, Okayama University, Okayama 700－8530, Japan The contact aureole at Fuka, Okayama, Japan is peculiar for the occurrence of extensive high－temperature skarn resulting from the intrusion of Mesozoic monzodiorite into Paleozoic marine limestone. The occurrence is also notable for the finding of ten new minerals, of which five are calcium－boron－bearing minerals, and scores of other rare minerals. Skarn formation at Fuka can be classified into three major types 1. Grossular－vesuvianite－ wollastonite endoskarn, 2. Gehlenite－exoskarns, and 3. Spurrite－exoskarns. Grossular－vesuvianite－wollas- tonite endoskarn forms a narrow zone (few centimeter width) separating the exoskarn and the igneous intrusion. It is also found, developed independently, along contacts of the younger basic intrusive dykes and limestone in the region. The gehlenite －exoskarns, in most cases, are spatially associated with igneous intrusion and are extensive (decimeter to meter thick). However, exceptions of independent gehlenite dikes are also observed. Retrogression of the gehlenite endoskarns results in the formation of hydrogrossular and/or vesuvianite. Accessory phases include schrolomite and perovskite. The predominantly monomineralic spurrite －exoskarn was formed in the outer zone of the gehlenite－skarn parallel to the contact as well as independent veins, dikes and tongues. The spurrite －exoskarn may extend tens of meters. Spurrite coexists with tilleyite or rankinite, although larnite is absent. Idiomorphic gehlenite and vesuvianite are the most common accessory phases observed. Retrograde hydration of spurrite to foshagite, scawtite and hillebrandite is commonly observed as veins and alteration zones within the spurrite exoskarn. Petrogenetic grids were constructed using “THERMOCALC” for the observed mineral assemblage in the － － － spurrite skarn. Mineral fluid equilibrium in the CaO SiO2 CO2 system was computed, considering the metaso- － matic input of aqueous silica. Phase diagram analysis in the form of T XCO2 grids with varying silica activity indicated that the stability field of spurrite is strongly controlled by the activity of silica in the fluid. Optimum silica concentration in the fluid was between 9.1 × 10−4 and 4.5 × 10−3 mol/liter, above which wollastonite becomes stable, whereas further reduced silica activity will generate larnite. Appropriate temperature condition for the formation of spurrite is between 850°C and 1080°C at an XCO2 fluid composition of 0.05 to 0.42. At tem- perature conditions lower than 850°C, the spurrite stability field becomes narrow, with low CO2 activity. The formation of extensive spurrite－exoskarn suggests that the silica activity, temperature and fluid composition remained within the spurrite stability field. Petrogenetic analysis of phase diagrams suggests that the exoskarn formation at Fuka contact aureole was robustly controlled by the activity of silica in the high temperature meta- somatic fluid. Introduction skarn formation resulting from hydrothermal fluids ema- nating from granitic intrusions or those resulting from Skarn formation at igneous－limestone contact metamor- basic intrusions at high temperature but with less fluid phic environment is most suited for studies on metasoma- activity (see Kerrick, 1991 and references therein). How- tism, fluid flow and diffusion transport of elements. Most ever, there is still ambiguity in skarn formation mecha- studies till date focused on low to medium temperature nism and related processes at high temperature conditions M. Satish －Kumar, [email protected] Corresponding (>800°C) accompanied by copious fluid flow at contact author aureoles. Temperature gradient in the crust and perme- I. Kusachi, [email protected]－u.ac.jp ability are crucial factors controlling reactive transport of 328 M. Satish－Kumar, Y. Yoshida and I. Kusachi High temperature contact metamorphism from Fuka contact aureole, Okayama, Japan 329 fluids as well as fluid flow. Further, higher temperature (Henmi and Kusachi, 1992) morimotoite (Henmi et al., conditions enhance element mobility. Understanding 1995), kusachiite (Henmi, 1995), takedite (Kusachi et al., metasomatism associated with high temperature skarn 1995), parasibirskite (Kusachi et al., 1998), and formation has wide implications on element recycling in okayamalite (Matsubara et al., 1998). Five of these new the crust, especially in volcanic arc provinces, wherein minerals are calcium boro －silicates. Preliminary extensive magmatic activity plays a key role in crustal geochemical investigations by Kusachi (1975) and growth. Further, skarn formations are often associated Kusachi et al. (1978) emphasized lattice diffusion of ele- with major economic mineral deposits. ments as the principal skarn forming mechanism. How- CaO–SiO2–Al2O3–vapor system forms the simplest ever, not much is known about the physical conditions, chemical system at siliceous igneous rock －limestone fluid composition, distance and quantity of material input intrusive contacts. Wollastonite and grossular garnet are accompanying the skarn formation. Here we present the most common minerals in this system observed in field, mineralogical and petrogenetic analysis of the high－ contact aureoles of moderate temperature conditions and temperature skarn. The formation of high temperature high fluid activity. However, at high temperature condi- skarn minerals is evaluated using petrogenitic grids tions (>800°C) and silica under saturated conditions, min- involving aqueous silica with varying fluid composition erals such as spurrite [Ca5Si2O8(CO3)], tilleyite and temperature conditions constructed with the aid of [Ca5Si2O7(CO3)2] and rankinite [Ca3Si2O7] are expected in “THERMOCALC” (Powell and Holland, 2001) place of wollastonite, whereas gehlenite forms instead of grossular garnet. However, spurrite－gehlenite skarn has Geology around Fuka only limited occurrence in the world, e.g. Kilchoan, Scot- land (Agrell, 1965), Christmas Mountains, North America Fuka is located about 40 km west －northwest from (Joesten, 1974; 1976), Fuka, Japan (Kusachi, 1975; Kusa- Okayama city, southwest Japan. Geographically, Fuka is chi et al., 1978) and Apuseni Mountains, Romania (Pascal situated in the uplifted Kibi－pleateau with an elevation of et al., 2001; Marincea et al., 2001). By the rarity of natu- about 500 m, and the Nariwa River runs from west to east ral occurrence, studies on high temperature skarn has not and makes V shaped valley. The area that yields skarn is － － much progressed, although the CaO SiO2 vapour system located between the slope of the south bank of the Nariwa triggers the transport of elements by means of decarbon- river and plateau face (Kusachi, 1975; Omae et al., 2002). ation－dehydration reactions in the crust. The basement rocks of Paleozoic age in this area com- Earlier studies on skarn formation at Fuka focused prises of pelitic rocks, psammitic rocks and chert, overlain on the mineralogic and geochemical aspects (e.g. Kusachi, by calcareous rock of the Nakamura Formation (Fig. 1). 1975). Ten new minerals were discovered from this Volcanic rocks of Mesozoic age covers the sedimentary occurrence. They are bicchuite (Henmi et al., 1973), rocks in the western part of the Fuka region. Several gen- fukalite (Henmi et al., 1977), oyalite (Kusachi et al., erations of younger andesitic dikes are found puncturing 1984), henmilite (Nakai et al., 1986), clinotobermerite the basement rocks and form few centimeter thick grossu- Figure 1. Geological map around Fuka (simplified after Teraoka et al., 1996). Inset show the study area in western Japan. 330 M. Satish－Kumar, Y. Yoshida and I. Kusachi High temperature contact metamorphism from Fuka contact aureole, Okayama, Japan 331 lar – vesuvianite –wollastonite skarn assemblages. primary minerals observed across fractures, evidencing the retrogression. Unaltered spurrite is purple to grey in Field relations color. Four classic skarn outcrops, where typical high－tempera- Fuka north outcrop ture skarn formation could be observed, are described briefly here. Skarn formation in this outcrop has been documented by Fuka west outcrop This outcrop is located at the western extremity of this metamorphic zone (location 3 in Figure 1). Here skarn crops out at the slope of a hill. The outcrop sketch is given in Figure 2a (after Kusachi et al., 1978). The skarn formation at this outcrop shows zonal pattern starting with quartz monzonite at the igneous side to gehlenite zone and then to spurrite zone. There is also a transition zone between gehlenite and spurrite zones, where both minerals coexist. Kusachi et al. (1978) observed that the skarn at this outcrop is formed by the intrusion of quartz monzonite and the primary minerals (gehlenite and spur- rite) produced. At later stage, the skarn was altered by the intrusion of younger andesitic dike acted as a source of heat and fluids during retrogression. The gehlenite