森 祐紀

We determined the metamorphic age and pressure–temperature conditions recorded in a sillimanite-garnet-bearing pelitic gneiss from Niban-nishi Rock, which is part of Niban Rock, on the Prince Olav Coast, eastern Dronning Maud Land, East Antarctica. Niban-nishi Rock is recognized as a component of the Lützow-Holm Complex (LHC), which is characterized by metamorphic age of 600–520 Ma and metamorphic grade of amphibolite to granulite facies. Electron microprobe U–Th–Pb monazite dating of the examined gneiss revealed that, unlike the typical exposures of the LHC, Niban-nishi Rock experienced Tonian metamorphism at 940.1 ± 9.8 Ma (2σ level), and is thus more similar to the neighboring exposures of Cape Hinode and Akebono Rock. Small numbers of younger monazite ages of 827–531 Ma were also detected, and some of which might relate to the metamorphism of the LHC. Phase equilibrium modeling and geothermobarometry indicate metamorphic conditions of 690–730 °C/0.38–0.68 GPa and 620–670 °C/0.42–0.60 GPa for peak and retrograde stages, respectively. The obtained peak temperature is lower than that of typical exposures in transitional- and granulite-facies zones of the LHC, such as Akarui Point and Skallen. Metamorphic features of Niban-nishi Rock, such as the upper amphibolite-facies condition and occurrences of garnet with retrograde zoning and sillimanite in the matrix, differ from those of Cape Hinode and Akebono Rock, with the former belonging to granulite facies and the latter showing kyanite in the matrix and garnet with growth zoning. Investigating Niban-nishi Rock is key in revealing the tectonic relation between the LHC and Cape Hinode (the Hinode Block). Further field surveys are required to reveal the metamorphic variations and the relations among exposures on the Prince Olav Coast.

熊本県天草下島に産する高温の長崎変成岩類を対象にしています。Zr-in-rutile地質温度計とQuartz-in-garnetラマン地質圧力計，そしてメルトインクルージョンの存在から岩石が部分溶融を経験していたことを示しました。また，ジルコンU-Pb年代をもとに，これまでは120Maに始まったとされてきた現在の西南日本における白亜紀の高温変成作用が，130Maには既に始まっていたことを示しました。 (合計50回になるまではリンク先から無料でダウンロードできます。) (所属機関でこの論文を入手できない場合は，森までお気軽にお問い合わせください。)