5:15 PM - 6:45 PM
[SMP24-P07] Chemical composition and microstructure of highest-grade gneisses of the Ryoke metamorphic complex in Yanai area, SW Japan
Keywords:Garnet, microstructure, nucleation and growth processes, Ryoke metamorphic complex
Metamorphic minerals nucleate and grow during prograde metamorphism. The growth process of the minerals would constrain their microstructural features such as grain size, spatial distribution and chemical composition. This study describes the microstructural features and the chemical compositions of constituent minerals in pelitic gneisses from the highest-grade garnet-cordierite (Grt-Crd) zone of the Ryoke metamorphic complex in Yanai area, SW Japan. The Grt-Crd zone shows an eastward increase in pressure from 250 to 700 MPa.
It is a common feature that garnet shows an outward decrease in Mg/(Fe+Mg) (hereafter #Mg). The P content has a maximum between the core and the rim. The Ca content is low at the rim and sometimes has several peaks in the interior. The #Mg values at the core of fine grains increases with increasing grain size, whereas those of coarse grains are almost constant. Biotite is mostly homogeneous except near the garnet grain where it is Ti-poor. Plagioclase and cordierite are homogeneous.
The #Mg values of biotite, cordierite and the core of coarse garnet in the west are lower than those in the east. The modal abundance and the mean size of garnet also increase whereas its number density decreases. The crystal size distribution (CSD) of garnet of all the samples has positive skewness, i.e., the tail is on the coarse-grained side. In the west, it resembles lognormal distribution. The positively skewed CSD is distinctly different from the CSD resulting from Ostwald ripening, suggesting that the textural modification during retrograde metamorphism may be negligible.
The phase diagram predicts that the values of #Mg of biotite, garnet and cordierite, increase with increasing pressure. This suggests that the modal abundance of garnet becomes higher at higher pressures provided that the bulk composition is constant.
The outward decrease in #Mg of garnet can be explained by a retrograde modification associated with the formation of Ti-poor biotite. The #Mg of the core of fine-grained garnet may be changed by this process. Therefore, the core of coarse-grained garnet likely preserves the #Mg value at the peak metamorphism.
The presence of plural peaks of the Ca content in one garnet grain suggests the coalescence took place during prograde metamorphism in the west. This may be responsible for the lognormal CSD.
The small modal abundance of garnet in the west reflects small mean grain size and large number density as compared with the garnet in the east. The phase diagram predicts that garnet is absent at low pressures and present at high pressures, under the conditions below the temperature of the univariant dehydration melting reaction, Sil+Bt+Qtz=Grt+Crd+Kfs+L. Therefore, the garnet grains in the west likely nucleated when the rock underwent the univariant reaction. The formation of a new mineral by an univariant reaction would cause a discontinuous increase in the modal abundance of the mineral. This may be responsible for the large number density of garnet in the west.
It is a common feature that garnet shows an outward decrease in Mg/(Fe+Mg) (hereafter #Mg). The P content has a maximum between the core and the rim. The Ca content is low at the rim and sometimes has several peaks in the interior. The #Mg values at the core of fine grains increases with increasing grain size, whereas those of coarse grains are almost constant. Biotite is mostly homogeneous except near the garnet grain where it is Ti-poor. Plagioclase and cordierite are homogeneous.
The #Mg values of biotite, cordierite and the core of coarse garnet in the west are lower than those in the east. The modal abundance and the mean size of garnet also increase whereas its number density decreases. The crystal size distribution (CSD) of garnet of all the samples has positive skewness, i.e., the tail is on the coarse-grained side. In the west, it resembles lognormal distribution. The positively skewed CSD is distinctly different from the CSD resulting from Ostwald ripening, suggesting that the textural modification during retrograde metamorphism may be negligible.
The phase diagram predicts that the values of #Mg of biotite, garnet and cordierite, increase with increasing pressure. This suggests that the modal abundance of garnet becomes higher at higher pressures provided that the bulk composition is constant.
The outward decrease in #Mg of garnet can be explained by a retrograde modification associated with the formation of Ti-poor biotite. The #Mg of the core of fine-grained garnet may be changed by this process. Therefore, the core of coarse-grained garnet likely preserves the #Mg value at the peak metamorphism.
The presence of plural peaks of the Ca content in one garnet grain suggests the coalescence took place during prograde metamorphism in the west. This may be responsible for the lognormal CSD.
The small modal abundance of garnet in the west reflects small mean grain size and large number density as compared with the garnet in the east. The phase diagram predicts that garnet is absent at low pressures and present at high pressures, under the conditions below the temperature of the univariant dehydration melting reaction, Sil+Bt+Qtz=Grt+Crd+Kfs+L. Therefore, the garnet grains in the west likely nucleated when the rock underwent the univariant reaction. The formation of a new mineral by an univariant reaction would cause a discontinuous increase in the modal abundance of the mineral. This may be responsible for the large number density of garnet in the west.