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[SMP25-11] Garnet–aluminosilicate–spinel–plagioclase geobarometer
Keywords:spinel, geobarometer, aluminosilicate, spinel + plagioclase symplectite
Spinel series mineral, (Mg, Fe, Zn, Mn) Al2O4 solid solution is one of the rock-forming minerals in pelitic metamorphic rocks. It mainly occurs in the sillimanite field, but also occurs in the kyanite field and andalusite field. Many authors studied spinel-forming reactions of pelitic metamorphic rocks. Some of these were solved as net-transfer reactions, geobarometers. Many geobarometers have been proposed for spinel-bearing pelitic metamorphic rocks (e.g. Harris, 1981; Perchuk et al., 1989; Nichols et al., 1992). For example, such net-transfer reactions are;
(1) Grt + 2Als = Spl + Crd,
(2) Crd = 2Spl + 5Qz,
(3) Grt + 2Als = 3Spl + 5Qz.
The reaction (1) means that the product minerals are spinel and cordierite. As a result, the spinel must be in contact with cordierite. Such textures are well-known in the Hidaka metamorphic belt and Ryoke metamorphic belt. On the other hand, the reactions (2) and (3) mean that the product minerals are spinel and quartz. As a result, the spinel must be in contact with quartz directly. This mineral assemblage is one of the diagnostic UHT metamorphic mineral assemblages (e.g. Waters, 1991; Harley, 1998; Ganguly et al., 2017). Therefore, net-transfer reactions (2) and (3) geobarometers can be used only for the UHT conditions. Generally, almost all pelitic metamorphic rocks contain quartz. However, spinel does not occur in contact with quartz in the amphibolite facies and/or in the granulite facies (lower than the UHT) conditions. Geobarometers (2) and (3) cannot be used for such metamorphic conditions.
Spinel + plagioclase symplectites in pelitic metamorphic rocks have been reported from many localities. We report occurrence of such texture, from Lützow–Holm Complex (LHC), East Antarctica, Hidaka Metamorphic Belt, Japan, Uetsu area, Japan, and Ryoke Metamorphic Belt, Japan. For example, Tenmondai Rock of LHC lies approximately mid-way along the amphibolite- to granulite-facies progression (Shiraishi et al., 1984). In this example, spinel occurs in a garnet-sillimanite gneiss. It is mainly composed of garnet, spinel, biotite, sillimanite, plagioclase, K-feldspar, quartz, ilmenite, and rutile. Dark green spinel (hercynite) occurs as spinel + plagioclase symplectite. Spinel and quartz are always bounded by plagioclase and spinel is not in direct contact with quartz. Garnet near this texture is enveloped by plagioclase. These textures are produced by the following metamorphic reaction; garnet + sillimanite = spinel + plagioclase. This can be described as the following two net-transfer reactions;
For CFAS-system: 5Grs + Alm + 12Als = 3Hc + 15An, and
For CMAS-system: 5Grs + Prp + 12Als = 3Spl + 15An.
We employed thermodynamic data from Holland and Powell (2011) for these reactions. Consequently, we propose the following geobarometers:
P [CFAS, sillimanite] = (-155761 + 587.4 T + RT lnKFe) / 29.378, and
P [CMAS, sillimanite] = (-132541 + 564.2 T + RT lnKMg) / 29.299.
Where P is in bar, T is in K. The equilibrium constant Keq is,
KFe = (aGrs5 aAlm) / (aHc3 aAn15) , and
KMg = (aGrs5 aPrp) / (aSpl3 aAn15) .
Additionally, we will propose for kyanite and andalusite equations. Our proposed geobarometers are free from quartz, corundum, orthopyroxene, and cordierite within these equations. These are useful to analyze pressure conditions for spinel-bearing pelitic metamorphic rocks within a wide pressure–temperature range (andalusite, kyanite, and sillimanite fields).
References
Ganguly et al. (2017) Jour. Petrol., 58, 1941–1974.
Harley (1998) Geol. Soc. London Spec. Publ., 138, 81-107.
Harris (1981) Contrib. Mineral. Petrol., 76, 229-233.
Holland & Powell (2011) Jour. Metamorphic Geol., 29, 333-383.
Nichols et al. (1992) Contrib. Mineral. Petrol., 111, 362- 377.
Perchuk et al. (1989) Jour. Metamorphic Geol., 7, 599-617.
Shiraishi et al. (1984) Mem. NIPR Spec. Issue, 33, 126-144.
Waters (1991) European Jour. Mineral., 3, 367-386.
(1) Grt + 2Als = Spl + Crd,
(2) Crd = 2Spl + 5Qz,
(3) Grt + 2Als = 3Spl + 5Qz.
The reaction (1) means that the product minerals are spinel and cordierite. As a result, the spinel must be in contact with cordierite. Such textures are well-known in the Hidaka metamorphic belt and Ryoke metamorphic belt. On the other hand, the reactions (2) and (3) mean that the product minerals are spinel and quartz. As a result, the spinel must be in contact with quartz directly. This mineral assemblage is one of the diagnostic UHT metamorphic mineral assemblages (e.g. Waters, 1991; Harley, 1998; Ganguly et al., 2017). Therefore, net-transfer reactions (2) and (3) geobarometers can be used only for the UHT conditions. Generally, almost all pelitic metamorphic rocks contain quartz. However, spinel does not occur in contact with quartz in the amphibolite facies and/or in the granulite facies (lower than the UHT) conditions. Geobarometers (2) and (3) cannot be used for such metamorphic conditions.
Spinel + plagioclase symplectites in pelitic metamorphic rocks have been reported from many localities. We report occurrence of such texture, from Lützow–Holm Complex (LHC), East Antarctica, Hidaka Metamorphic Belt, Japan, Uetsu area, Japan, and Ryoke Metamorphic Belt, Japan. For example, Tenmondai Rock of LHC lies approximately mid-way along the amphibolite- to granulite-facies progression (Shiraishi et al., 1984). In this example, spinel occurs in a garnet-sillimanite gneiss. It is mainly composed of garnet, spinel, biotite, sillimanite, plagioclase, K-feldspar, quartz, ilmenite, and rutile. Dark green spinel (hercynite) occurs as spinel + plagioclase symplectite. Spinel and quartz are always bounded by plagioclase and spinel is not in direct contact with quartz. Garnet near this texture is enveloped by plagioclase. These textures are produced by the following metamorphic reaction; garnet + sillimanite = spinel + plagioclase. This can be described as the following two net-transfer reactions;
For CFAS-system: 5Grs + Alm + 12Als = 3Hc + 15An, and
For CMAS-system: 5Grs + Prp + 12Als = 3Spl + 15An.
We employed thermodynamic data from Holland and Powell (2011) for these reactions. Consequently, we propose the following geobarometers:
P [CFAS, sillimanite] = (-155761 + 587.4 T + RT lnKFe) / 29.378, and
P [CMAS, sillimanite] = (-132541 + 564.2 T + RT lnKMg) / 29.299.
Where P is in bar, T is in K. The equilibrium constant Keq is,
KFe = (aGrs5 aAlm) / (aHc3 aAn15) , and
KMg = (aGrs5 aPrp) / (aSpl3 aAn15) .
Additionally, we will propose for kyanite and andalusite equations. Our proposed geobarometers are free from quartz, corundum, orthopyroxene, and cordierite within these equations. These are useful to analyze pressure conditions for spinel-bearing pelitic metamorphic rocks within a wide pressure–temperature range (andalusite, kyanite, and sillimanite fields).
References
Ganguly et al. (2017) Jour. Petrol., 58, 1941–1974.
Harley (1998) Geol. Soc. London Spec. Publ., 138, 81-107.
Harris (1981) Contrib. Mineral. Petrol., 76, 229-233.
Holland & Powell (2011) Jour. Metamorphic Geol., 29, 333-383.
Nichols et al. (1992) Contrib. Mineral. Petrol., 111, 362- 377.
Perchuk et al. (1989) Jour. Metamorphic Geol., 7, 599-617.
Shiraishi et al. (1984) Mem. NIPR Spec. Issue, 33, 126-144.
Waters (1991) European Jour. Mineral., 3, 367-386.