Sanukitoid
Encyclopedia
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.
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 LREE
s, 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 "adakite
s" (named for occurrences on the Adak Island in the Aluetian 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, a lithophone
invented in Japan.http://sciencelinks.jp/j-east/article/200121/000020012101A0603489.php
Granitoid
A granitoid or granitic rock is a variety of coarse grained plutonic rock similar to granite which mineralogically are composed predominately of feldspar and quartz. Examples of granitoid rocks include granite, quartz monzonite, quartz diorite, syenite, granodiorite and trondhjemite. Many are...
found in convergent margin settings. The term "sanukitoid" was originally used to define a variety of Archean
Archean
The Archean , also spelled Archeozoic or Archæozoic) is a geologic eon before the Paleoproterozoic Era of the Proterozoic Eon, before 2.5 Ga ago. Instead of being based on stratigraphy, this date is defined chronometrically...
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
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
andesite
Andesite
Andesite is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and dacite. The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende. Magnetite,...
from the Setouchi Peninsula of Japan, known as "sanukites" or "setouchites" (Tatsumi and Ishizaki 1982). Sanukite rocks are an andesite
Andesite
Andesite is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and dacite. The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende. Magnetite,...
characterized by orthopyroxene as the mafic
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
mineral, andesine
Andesine
Andesine is a silicate mineral, a member of the plagioclase feldspar solid solution series. Its chemical formula is 4O8, where Ca/ is between 30%-50%...
as the plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
, and a glassy groundmass. Rocks formed by processes similar to those of sanukite may have compositions outside the sanukitoid field.
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 LREE
Rare earth element
As defined by IUPAC, rare earth elements or rare earth metals are a set of seventeen chemical elements in the periodic table, specifically the fifteen lanthanides plus scandium and yttrium...
s, and no or minor Eu anomalies
Europium Anomaly
The Europium anomaly, in geochemistry, is the phenomenon whereby Europium concentration is either depleted or enriched in a rock relative to the other rare earth elements ....
. The term "sanukitoid suite" includes more evolved rocks derived from sanukitoid through fractional crystallization
Crystallization
Crystallization is the process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas. Crystallization is also a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid...
. Sanukitoids are similar in major and trace element composition to "adakite
Adakite
Adakite is a petrologic term for volcanic or intrusive igneous rocks that are interpreted to form in subduction zones from the mixing of mantle material with felsic partial melts of descending slabs of oceanic crust basalt...
s" (named for occurrences on the Adak Island in the Aluetian 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
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
, which has been previously metasomatised
Metasomatism
Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids.Metasomatism can occur via the action of hydrothermal fluids from an igneous or metamorphic source. In the igneous environment, metasomatism creates skarns, greisen, and may affect hornfels in the contact...
by silicate melts derived from the melting of a hot, young, subducting slab
Slab (geology)
In geology, a slab is the portion of a tectonic plate that is being subducted.Slabs constitute an important part of the global plate tectonic system. They drive plate tectonics both by pulling along the lithosphere to which they are attached in a processes known as slab pull and by inciting...
. 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
Boninite
Boninite is a mafic extrusive rock high in both magnesium and silica, formed in fore-arc environments, typically during the early stages of subduction. The rock is named for its occurrence in the Izu-Bonin arc south of Japan...
; 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, a lithophone
Lithophone
A lithophone is a musical instrument consisting of a rock or pieces of rock which are struck to produce musical notes. Notes may be sounded in combination or in succession...
invented in Japan.http://sciencelinks.jp/j-east/article/200121/000020012101A0603489.php