[S-MP46_28PM2] Deformed rocks, Metamorphic rocks and Tectonics

[SMP46-P11_PG] P-T estimates of a metapelite containig garnet zoning from Mefjell, Sør Rondane Mountain, East Antarctica

3-min talk in an oral session

*Yumiko TSUBOKAWA¹, Masahiro ISHIKAWA¹, Takashi ICHIKI¹, Tetsuo KAWAKAMI², Satish-kumar MADHUSOODHAN³, Noriyoshi TSUCHIYA⁴, Grantham GEOFF⁵ (1.Yokohama National University, 2.Kyoto University, 3.Niigata University, 4.Tohoku University, 5.Council for geoscience, South Africa)

Keywords:East Antarctica, Sør Rondane Mountain, pressure and temperature conditions

The Sør Rondane Mountains, East Antarctica have been considered to be situated in the Gondwana suture zones. Therefore the mountains have attracted interest as a key area for understanding amalgamation process of the supercontinent. The mountains consist of amphibolite- to granulite-facies metamorphic rocks with granitic intrusions, and the timings of the main metamorphism are interpreted as c. 640-600 Ma and c. 550-500 Ma. Metamorphic rocks from northern and eastern part of the mountains (Balchenfjella and northern part of Austkampane) record a clockwise P-T path, on the other hand, metamorphic rocks from central part of the mountains (Brattnipene and eastern Menipa) record anti-clockwise P-T path. This suggests each area records a different P-T path. However, pre-peak P-T conditions of southwestern part of the mountain such as Mefjell have been still not clear.In this study, we report a garnet porphyroblast with a prograde zoning in a metapelite from Mefjell. The St-bearing Grt-Sil-Bt gneiss mainly consists of garnet, biotite, sillimanite, quartz and plagioclase, with minor K-feldspar, staurolite, apatite, monazite, ilmenite and magnetite. The garnet grain is 12 mm in diameter, with the change of color from reddish in the core to transparent in the rim. The garnet has core-rim boundary defined by Mn-zoning. The garnet is typically almandine-rich, and shows compositional zoning with decrease in spessartine content from the core (Alm₆₃Sps₂₄Prp₁₄Grs₆) to the rim (Alm₇₄Sps₂Prp₂₀Grs₄), and spessartine content increase again towards the outer-rim (Alm₇₃Sps₁₁Prp₂₀Grs₆). The garnet includes staurolite, sillimanite, biotite, chlorite, plagioclase, K-feldspar, quartz, apatite and ilmenite. Garnet-ilmenite and staurolite-garnet geothermometers yield a temperature increase towards rim from 350-400 to 630-700℃. Garnet-Al₂SiO₅-quartz-plagioclase geobarometer applied to rim inclusions yields 7.2kbar±0.9kbar for an assumed temperature of 650℃.