第3回極域科学シンポジウム/第32回極域地学シンポジウム 11月30日（金） 国立極地研究所 ３階ラウン

IKEDA TAKESHI

SHIMADA ASAMI

島田 麻美

池田 剛

National Institute of Polar Research Repository

リュッツホルム岩体オングル島に産するザクロ石角閃岩中の反応縁の微細組織  島田麻美、池田剛 九州大学  Microstructure of reaction rim of garnet amphibolites from the Lützow-Holm Complex at Ongul Island ASAMI SHIMADA & TAKESHI IKEDA Kyushu University       In the Lützow-Holm Complex, metamorphic grade increases from amphibolites facies in the northeast to granulite facies in the southwest [1]. The granulite facies metamorphic rocks are widely distributed throughout East Ongul Island [2]. The rock types are mainly garnet gneiss and hornblende gneiss [3]. Ultramafic rocks occur as thin layers in garnet gneiss. The ultramafic analyzed in this study rocks are composed mainly of hornblende and porphyroblasts of garnet. Corona structure occurs between garnet and hornblende in these rocks. In this study, we prepared thin sections of matrix and corona, and described the microstructure and crystal size distribution of biotite in the corona. Microstructure      In the matrix, hornblende-rich domain and plagioclase-rich domain occur. Both domains consist of hornblende, plagioclase, brown biotite, and orthopyroxene. Clinopyroxene occurs only in the plagioclase-rich domain. The corona consists mainly of plagioclase and green biotite, and occurs around the outer edge of garnet. Crystal size of green biotite and plagioclase increases as increasing the distance from garnet. Grains of plagioclase in the matrix and the corona have twin and compositional zoning. Garnet (about 15mm diameter) is roughly spherical but locally shows concavo-convex shape (around 0.5mm). Garnet (long axis 0.10-0.45mm) that is rounded and irregularly shaped locally occurs at the extension of embayed part. In immediate proximity to garnet, biotite of which the long axis orientates at right angles to garnet tends to occur at the embayed part of garnet.  Crystal Size Distribution      Measuring crystal size distribution for all biotite grains(about 3300 grains) in the corona shows that the frequency has mode in 0.10-0.15mm, and gently decrease on the coarse-grained side as compared with the fine-grained side. The average of the grain size is 0.23mm. The crystal size distribution is similar to lognormal distribution. However, spatial distribution of crystal size is not homogeneous, but mostly increases as increasing the distance from garnet (~3.67mm). Measuring the fine-grained part close to the garnet and the coarse-grained part far from the garnet separately shows that both have mode in 0.10-0.15mm, and are lognormal distribution. The coarse-grained part has high standard deviation relative to the fine-grained part.  Discussion      The mode of crystal size distribution is smaller than average and similar to lognormal distribution, suggesting that biotite maintained continuous nucleation and growth during corona formation. The crystal size distribution of fine and coarse grains parts is also lognormal. This can't be explained by the traditional idea of which thickness of the corona increases by formation of products in reaction front, but explained such that nucleation occurs everywhere in the corona not only in garnet interface.   References [1] Hiroi, Y., Shiraishi, K., Motoyoshi,Y. & Katsushima, T. (1987): Progressive metamorphism of calc-silicate rocks from the Prince Olav and Soya Coasts, East Antarctia. Proceedings of NIPR Symposium on Antarctica Geosciences, 1, 73-97. [2]Kawakami, T., & Ikeda, T. (2004): Structual evolution of the Ongul Islands, Lützow -Holm Complex, East Antarctica. Polar Geoscience, 17, 12-21 [3] Ishikawa, M., Shiraishi, K., Motoyoshi, Y., Tsuchiya, N., Shimura, T. & Yanai, K. (1994): Explanatory Text of Geological Map of Ongul Islands, Lutzow-Holm Bay, Antarctica. Antarctic Geological Map Series, Sheet 36. National Institute of Polar Research, Tokyo.    

Microstructure of reaction rim of garnet amphibolites from the Lützow-Holm Complex at Ongul Island

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Microstructure of reaction rim of garnet amphibolites from the Lützow-Holm Complex at Ongul Island

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