Sanukitoid

Sanukitoids are a variety of high-Mg granitoid found in convergent margin settings. The term "sanukitoid" was originally used to define a variety of Archean plutonic rock, but now also includes younger rocks with similar geochemical characteristics (Shirey & Hanson 1984; Rogers et al. 1985; Stern et al. 1989; Kelemen et al. 2004). They are called "sanukitoid" because of their similarity in bulk chemical composition to high-magnesium andesite from the Setouchi Peninsula of Japan, known as "sanukites" or "setouchites" (Tatsumi and Ishizaki 1982). Sanukite rocks are an andesite characterized by orthopyroxene as the mafic mineral, andesine as the plagioclase, and a glassy groundmass. Rocks formed by processes similar to those of sanukite may have compositions outside the sanukitoid field.

Sanukite is a black hard rock produced in Mt. Nijo on the boundary of Osaka and Nara prefectures

The term was originally defined by Stern et al. (1989) to refer to plutonic rocks containing between 55 and 60 weight percent SiO₂, with Mg# >0.6, Ni >100 ppm, Cr >200 ppm, K₂O >1 weight percent, Rb/Sr <0.1, Ba >500 ppm, Sr >500 ppm, enrichment in LREEs, and no or minor Eu anomalies. The term "sanukitoid suite" includes more evolved rocks derived from sanukitoid through fractional crystallization. Sanukitoids are similar in major and trace element composition to "adakites" (named for occurrences on the Adak Island in the Aleutian island arc). Both suites are thought to form by melting of a mafic igneous rock protolith that has been metamorphosed to garnet-pyroxene (eclogite) or garnet-amphibole assemblages (Rapp et al. 1991; Thorkelson & Breitsprecher 2005).

The most common source for sanukitoids is probably the mantle, which has been previously metasomatised by silicate melts derived from the melting of a hot, young, subducting slab. When the oceanic crust is subducted and metamorphosed, it is close to its melting point and a slight increase in temperature may cause melting. These melts are initially high in silica at low melt fractions, and decrease in silica as melting proceeds. Melts derived from the eclogite or garnet-amphibole slab are strongly enriched in Sr (no plagioclase in residue) and depleted in HREE and Y (abundant garnet in residue). This melt reacts with the mantle to create the characteristic high Sr, low Y, and high LREE/HREE ratios (Drummond & Defant 1990). Some adakites may form by melting of thick crustal roots of island arcs, but these cannot assimilate mantle wedge components so sanukitoids will not form in this setting.

Sanukitoids and adakites are distinct from another variety of high-Mg andesite called boninite; boninites have major element concentrations similar to sanukitoids, but they are extremely depleted in incompatible trace elements (e.g., LREE) despite their relatively high silica contents. Thus there is no evidence that the mantle wedge that melts to form a sanukitoid has experienced previous extensive melt extraction (Martin et al. 2005).

Sanukite has been used as the material for the bars of the hōkyō (磬石), a lithophone invented in Japan.

References

• Barager, W.R.A and T.N. Irvine. (1971) "A Guide to the Chemical classification of the Common Volcanic Rocks." Canadian Journal of Earth Sciences, Vol. 8, pp. 523–548.
• Drummond, M. S., and Defant, M. J., 1990, A model for trondhjemite-tonalite-dacite genesis and crustal growth via slab melting: Archean to modern comparisons: Journal of Geophysical Research, v. 95, no. B13, p. 21503-21521.
• Kelemen, P. B., Yogodzinski, G. M., and Scholl, D. W., 2004, Along-strike Variation in Lavas of the Aleutian Island Arc: Genesis of High Mg# Andesite and Implications for Continental Crust, in Eiler, J., ed., Inside the Subduction Factory: Washington DC, American Geophyical Union, Geophysical Monograph Series, Volume 138.
• Martin, H., Smithies, R. H., Rapp, R., Moyen, J.-F., and Champion, D., 2005, An overview of adakite, tonalite-trondhjemite-granodiorite (TTG), and sanukitoid: relationships and some implications for crustal evolution: Lithos, v. 79, no. 1-2, p. 1-24.
• Rapp, R. P., Watson, E. B., and Miller, C. F., 1991, Partial melting of amphibolite/eclogite and the origin of Archean trondhjemites and tonalites: Precambrian Research, v. 51, no. 1-4, p. 1-25.
• Rogers, G., Saunders, A. D., Terrell, D. J., Verma, S. P., and Marriner, G. F., 1985, Geochemistry of Holocene volcanic rocks associated with ridge subduction in Baja California, Mexico: Nature, v. 315, no. 6018, p. 389-392.
• Shirey, S. B., and Hanson, G. N., 1984, Mantle-derived Archaean monzodiorites and trachyandesites.: Nature, v. 310, no. 5974, p. 222-224.
• Stern, R., G.N. Hanson and S.B. Shirey (1989) "Petrogenisis of Mantle derived LILE-enriched Archaean Monzodiorite, Trackyandesites (Sanukitoids) in southern Superior Province." Canadian Journal of Earth Sciences, Vol. 26, pp. 1688–1712.
• Tatsumi, Y., and Ishizaka, K., 1982, Origin of high-magnesian andesites in the Setouchi volcanic belt, southwest Japan, I. Petrographical and chemical characteristics: Earth and Planetary Science Letters, v. 60, no. 2, p. 293-304.
• Thorkelson, D. J., and Breitsprecher, K., 2005, Partial melting of slab window margins: genesis of adakitic and non-adakitic magmas: Lithos, v. 79, no. 1-2, p. 25-41.