14:00 〜 14:15
[SMP24-02] Post-peak Cl- and CO2-bearing fluids in a Grt-Sil-Bt gneiss from southern Perlebandet, Sør Rondane Mountains, East Antarctica
キーワード：変成流体、高温変成岩、南極
Low H2O activity fluids (e.g., CO2-, NaCl-, and KCl-bearing fluids) are often recognized in the middle to lower crust (e.g., Newton et al., 1998; Touret and Huizenga, 2011). CO2-rich fluid in high-grade metamorphic rocks is usually observed as fluid inclusions, whereas Cl-bearing fluid is presumed by high-Cl concentration in hydrous minerals. In addition, some metamorphic minerals, such as cordierite, can incorporate molecular CO2 and H2O in the channel-cavities. In the H2O-CO2-NaCl ternary system, CO2- and NaCl-bearing fluids are extremely immiscible even at high-temperature (Shmulovich and Graham, 2004). In order to understand the metamorphic fluid evolution, therefore, it is important to examine whether both fluids coexisted simultaneously or not when both fluid compositions are presumed from a single sample.
In the Sør Rondane Mountains (SRM), East Antarctica, Late Proterozoic to Cambrian granulites are widely exposed (e.g., Jacobs and Thomas, 2004; Shiraishi et al., 2008; Osanai et al., 2013). Perlebandet is ~10 km long nunataks located at the westernmost part of the SRM. Perlebandet was categorized into NE-terrane which exhibits a clockwise P-T path, whereas SW-terrane exhibits a counter-clockwise P-T path (Osanai et al., 2013). However, Mieth et al. (2014) interpreted that Parlebandet is part of the SW-terrane on the basis of magnetic survey. The counter-clockwise P-T path estimated by Kawakami et al. (2017) supports this interpretation. So far, all previous studies in Perlebandet reporting the P-T conditions, zircon U-Pb ages, and CHIME ages in monazite dealt with samples from northern part of the nunataks (Asami et al., 2005; Shiraishi et al., 2008; Kawakami et al., 2017). This study deals with a Grt-Sil-Bt gneiss collected from the southern part of nunataks in Perlebandet.
The studied sample (sample FH19123001Q) mainly comprises garnet, sillimanite, biotite, cordierite, plagioclase, K-feldspar, and quartz. Black cracks filled by Cl-rich biotite (~0.5 wt% Cl) cut the gneissose structure. Cordierite is exclusively present as biotite-cordierite intergrowth around garnet porphyroblasts, suggesting the garnet breakdown reaction during retrograde metamorphism. Since sillimanite is included in cordierite within the intergrowth microstructure in rare cases, garnet breakdown reaction probably occurred under sillimanite stability field. Therefore, the reaction
Grt + Kfs + H2O --> Bt + Crd (1)
is likely to have occurred. Within the biotite-cordierite intergrowth microstructure, biotite has ~0.2-0.3 wt% Cl, whereas the CO2 and H2O peaks were obtained from cordierite by Raman spectrometer. Since both minerals within the intergrowth were formed simultaneously, the H2O-CO2-Cl- fluid infiltration is suggested to be triggered the reaction (1). The P-T conditions when the reaction (1) occurred are considered to be ~730 ºC, ~0.4 GPa in NaKFMASH system, because XFe value of garnet is ~0.8 (Spear et al., 1999). The dehydration reaction line and wet solidus shift to low-temperature and high-temperature side, respectively, due to the low H2O activity fluid (cf. Johannes and Holtz, 1996). This means that the isopleth of reaction (1) possibly shifts to high-temperature side, although H2O activity of the H2O-CO2-Cl- fluid is not estimated.
Further information is needed to constrain whether the reaction (1) is a part of counter-clockwise P-T path or not. The undated granite bodies are mapped in the southern part of nunataks in Perlebandet. Post-peak granitic intrusion can be a heat source and then the reaction (1) can occur, even following the counter-clockwise P-T path. To constrain the P-T path helps understand the origin and evolution of the H2O-CO2-Cl- fluid.
In the Sør Rondane Mountains (SRM), East Antarctica, Late Proterozoic to Cambrian granulites are widely exposed (e.g., Jacobs and Thomas, 2004; Shiraishi et al., 2008; Osanai et al., 2013). Perlebandet is ~10 km long nunataks located at the westernmost part of the SRM. Perlebandet was categorized into NE-terrane which exhibits a clockwise P-T path, whereas SW-terrane exhibits a counter-clockwise P-T path (Osanai et al., 2013). However, Mieth et al. (2014) interpreted that Parlebandet is part of the SW-terrane on the basis of magnetic survey. The counter-clockwise P-T path estimated by Kawakami et al. (2017) supports this interpretation. So far, all previous studies in Perlebandet reporting the P-T conditions, zircon U-Pb ages, and CHIME ages in monazite dealt with samples from northern part of the nunataks (Asami et al., 2005; Shiraishi et al., 2008; Kawakami et al., 2017). This study deals with a Grt-Sil-Bt gneiss collected from the southern part of nunataks in Perlebandet.
The studied sample (sample FH19123001Q) mainly comprises garnet, sillimanite, biotite, cordierite, plagioclase, K-feldspar, and quartz. Black cracks filled by Cl-rich biotite (~0.5 wt% Cl) cut the gneissose structure. Cordierite is exclusively present as biotite-cordierite intergrowth around garnet porphyroblasts, suggesting the garnet breakdown reaction during retrograde metamorphism. Since sillimanite is included in cordierite within the intergrowth microstructure in rare cases, garnet breakdown reaction probably occurred under sillimanite stability field. Therefore, the reaction
Grt + Kfs + H2O --> Bt + Crd (1)
is likely to have occurred. Within the biotite-cordierite intergrowth microstructure, biotite has ~0.2-0.3 wt% Cl, whereas the CO2 and H2O peaks were obtained from cordierite by Raman spectrometer. Since both minerals within the intergrowth were formed simultaneously, the H2O-CO2-Cl- fluid infiltration is suggested to be triggered the reaction (1). The P-T conditions when the reaction (1) occurred are considered to be ~730 ºC, ~0.4 GPa in NaKFMASH system, because XFe value of garnet is ~0.8 (Spear et al., 1999). The dehydration reaction line and wet solidus shift to low-temperature and high-temperature side, respectively, due to the low H2O activity fluid (cf. Johannes and Holtz, 1996). This means that the isopleth of reaction (1) possibly shifts to high-temperature side, although H2O activity of the H2O-CO2-Cl- fluid is not estimated.
Further information is needed to constrain whether the reaction (1) is a part of counter-clockwise P-T path or not. The undated granite bodies are mapped in the southern part of nunataks in Perlebandet. Post-peak granitic intrusion can be a heat source and then the reaction (1) can occur, even following the counter-clockwise P-T path. To constrain the P-T path helps understand the origin and evolution of the H2O-CO2-Cl- fluid.