10:15 AM - 10:30 AM
[SMP28-06] Influence of Fe3+ on the P-T-fluid evolution of calc-silicate granulite of Rundvågshetta, Lützow-Holm Complex, East Antarctica
Calc-silicate rocks are used for interpreting the temperature-pressure-fluid conditions and oxygen fugacity in regions that have been subjected to high-grade metamorphism (Satish-Kumar et al., 2006; Dasgupta and Pal, 2005). Information on fluid and oxygen fugacity are recorded in a mineral composition and are often expressed in terms of oxidation state of iron. While oxidation state of iron plays an important role in extracting geological information from rocks, most analytical instruments used in geology, such as XRF and EPMA, cannot distinguish between the oxidation state of iron. However, with the advancement of thermodynamic datasets for minerals and phase equilibrium modeling software, it has become possible to estimate the condition of iron and the amount of oxygen using these tools. In this study, we estimate the amounts of Fe3+ and amount of oxygen in the bulk rock compositions using phase diagram section (pseudosection) modeling and discuss the temperature-pressure-fluid evolution of calc-silicate granulite from Rundvågshetta, Lützow-Holm Complex, East Antarctica.
Lützow-Holm Complex, including Rundvågshetta, is a region that has been subjected to high- to ultrahigh-temperature metamorphism. Lützow-Holm Complex is characterized by an increase in metamorphic grade from amphibolite facies in northwestern region to granulite facies in the southeastern region, and thermal maximum is at the Rundvågshetta. Peak metamorphic conditions have already estimated from a metapelite from Rundvågshetta (Yoshimura et al., 2008). Calc-silicate granulite occurs as a block within the pyroxene granulite.
Thin section observation revealed that the calc-silicate rocks can be divided into five zones based on mineral assemblage and mineral modes. Zone Ⅰ and Ⅴ are characterized by the assemblage of Scp + Cpx + Qz + Pl, with no garnet. Zone Ⅱ and Ⅳ consist of Grt + Cpx + Scp + Pl + Qz, where significant mineral modes of scapolite and plagioclase were recognized. These zones were subdivided into two more domain; Scp-rich domain and Pl-rich domain respectively. Additionally, coronal garnet, which is considered to have crystalized during retrograde metamorphism, was observed. Zone Ⅲ is composed of Grt + Scp + Qz + Cpx, with garnet having a porphyroblastic texture. Garnet shows a solid solution between grossular and andradite. While the porphyroblastic and granular garnet shows composition closer to grossular, the coronal garnet is enriched in the andradite content.
A T-XO2 phase diagram sections were constructed for the both Scp-rich domain and Pl-rich domain to estimate the amount of O2. Using the estimated the value of O2, the corresponding P-T phase diagram sections were then constructed. The P-T range indicated by the mineral assemblage in this phase diagram section broadly agree with that of previous study, however, a discrepancy in temperature was observed. This discrepancy may be attributed to thermodynamic data uncertanty and/or it may represent partial re-equilibriation.
References
Dasgupta, S. and Pal, S., 2005. Origin of grandite garnet in calc-silicate granulites: Mineral–fluid equilibria and petrogenetic grids. Journal of Petrology, 46 (5), 1045–1076.
Satish-Kumar, M., Motoyoshi, Y., Suda, Y., Hiroi, Y. and Kagashima, S., 2006. Calc-silicate rocks and marbles from Lützow-Holm Complex, East Antarctica, with special reference to the mineralogy and geochemical characteristics of calc-silicate mega-boudins from Rundvågshetta. Polar Geoscience, 19, 37–61.
Yoshimura, Y., Motoyoshi, Y. and Miyamoto, T., 2008. Sapphirine + quartz association in garnet: implication for ultrahigh-temperature metamorphism at Rundvågshetta, Lützow-Holm Complex, East Antarctica. Geological Society, London, Special Publications, 308 (1), 377–390.
Lützow-Holm Complex, including Rundvågshetta, is a region that has been subjected to high- to ultrahigh-temperature metamorphism. Lützow-Holm Complex is characterized by an increase in metamorphic grade from amphibolite facies in northwestern region to granulite facies in the southeastern region, and thermal maximum is at the Rundvågshetta. Peak metamorphic conditions have already estimated from a metapelite from Rundvågshetta (Yoshimura et al., 2008). Calc-silicate granulite occurs as a block within the pyroxene granulite.
Thin section observation revealed that the calc-silicate rocks can be divided into five zones based on mineral assemblage and mineral modes. Zone Ⅰ and Ⅴ are characterized by the assemblage of Scp + Cpx + Qz + Pl, with no garnet. Zone Ⅱ and Ⅳ consist of Grt + Cpx + Scp + Pl + Qz, where significant mineral modes of scapolite and plagioclase were recognized. These zones were subdivided into two more domain; Scp-rich domain and Pl-rich domain respectively. Additionally, coronal garnet, which is considered to have crystalized during retrograde metamorphism, was observed. Zone Ⅲ is composed of Grt + Scp + Qz + Cpx, with garnet having a porphyroblastic texture. Garnet shows a solid solution between grossular and andradite. While the porphyroblastic and granular garnet shows composition closer to grossular, the coronal garnet is enriched in the andradite content.
A T-XO2 phase diagram sections were constructed for the both Scp-rich domain and Pl-rich domain to estimate the amount of O2. Using the estimated the value of O2, the corresponding P-T phase diagram sections were then constructed. The P-T range indicated by the mineral assemblage in this phase diagram section broadly agree with that of previous study, however, a discrepancy in temperature was observed. This discrepancy may be attributed to thermodynamic data uncertanty and/or it may represent partial re-equilibriation.
References
Dasgupta, S. and Pal, S., 2005. Origin of grandite garnet in calc-silicate granulites: Mineral–fluid equilibria and petrogenetic grids. Journal of Petrology, 46 (5), 1045–1076.
Satish-Kumar, M., Motoyoshi, Y., Suda, Y., Hiroi, Y. and Kagashima, S., 2006. Calc-silicate rocks and marbles from Lützow-Holm Complex, East Antarctica, with special reference to the mineralogy and geochemical characteristics of calc-silicate mega-boudins from Rundvågshetta. Polar Geoscience, 19, 37–61.
Yoshimura, Y., Motoyoshi, Y. and Miyamoto, T., 2008. Sapphirine + quartz association in garnet: implication for ultrahigh-temperature metamorphism at Rundvågshetta, Lützow-Holm Complex, East Antarctica. Geological Society, London, Special Publications, 308 (1), 377–390.