5:15 PM - 6:30 PM
[SMP25-P09] Petrography of a sulfide-rich garnet-sillimanite-biotite gneiss from Brattnipene, Sør Rondane Mountains, East Antarctica
Keywords:metamorphic rock, continental collision
The Sør Rondane Mountains (SRM) is located in eastern Dronning Maud Land, East Antarctica, where the lower crust of the East-West Gondwana collision zone is exposed. The SRM is divided into the NE terrane and the SW terrane by the Main Tectonic Boundary (MTB) (Osanai et al., 2013). The NE terrane is characterized by clockwise pressure-temperature (P-T) path of granulite-facies metamorphic rocks and inherited zircons older than 1200 Ma, whereas the SW terrane is characterized by the counter-clockwise P-T path and inherited zircons younger than 1200 Ma. In Brattnipene, two counter-clockwise P-T paths have been proposed previously (Adachi et al., 2013; Baba et al.,2013). However, these studies have weakness of constructing the P-T path using boulder samples (Baba et al., 2013) or using samples from distant outcrops (Adachi et al., 2013). In this study, we utilized a pelitic gneiss sample from the Nakayubi-ridge, Brattnipene to constrain the retrograde P-T path.
The sample used in this study is a sulfide-rich garnet-sillimanite-biotite gneiss (sample TK2009120403D2) mainly composed of garnet, sillimanite, cordierite, biotite, Fe-sulfides (pyrite and pyrrhotite), plagioclase, K-feldspar, and quartz. Quartz ribbons are developed parallel to the gneissosity. Accessory minerals are rutile, apatite, zircon, monazite and graphite. Garnet (XMg ~ 0.53) includes abundant sillimanite in the core. Garnet rim (XMg ~ 0.27) is replaced by secondary cordierite (XMg ~ 0.85) + Fe-sulfide + sillimanite + quartz or cordierite (XMg ~ 0.88) + Fe-sulfide + biotite (XMg = 0.77-0.81) + quartz. The former mineral assemblage tends to appear near the garnet when both assemblages are observed. Common sulfide occurrence in the assemblage implies that infiltration of sulfur-bearing fluid/melt triggered the breakdown of garnet. The presence of sillimanite in the replacement assemblage suggests that the garnet breakdown took place under the sillimanite stability field, and the formation of cordierite probably represents decompression process. Rutile enclosed in the secondary cordierite is finer-grained and shows higher Cr, V and Al concentrations compared to matrix rutile. Therefore, inclusion rutile is likely a product formed during the garnet breakdown. Zr-in-rutile thermometry (Tomkins et al., 2007) applied to the inclusion rutile gave ~700 oC for the pressure of the sillimanite stability field, and this probably represents the temperature of the garnet breakdown.
Although further detailed analysis is required, the observed decompression reaction texture in the sillimanite stability field contrasts with counter-clockwise P-T paths previously proposed for Brattnipene, and thus timing of the garnet breakdown reaction needs to be determined in order to understand its geological meaning in the tectonic framework proposed by Osanai et al. (2013).
The sample used in this study is a sulfide-rich garnet-sillimanite-biotite gneiss (sample TK2009120403D2) mainly composed of garnet, sillimanite, cordierite, biotite, Fe-sulfides (pyrite and pyrrhotite), plagioclase, K-feldspar, and quartz. Quartz ribbons are developed parallel to the gneissosity. Accessory minerals are rutile, apatite, zircon, monazite and graphite. Garnet (XMg ~ 0.53) includes abundant sillimanite in the core. Garnet rim (XMg ~ 0.27) is replaced by secondary cordierite (XMg ~ 0.85) + Fe-sulfide + sillimanite + quartz or cordierite (XMg ~ 0.88) + Fe-sulfide + biotite (XMg = 0.77-0.81) + quartz. The former mineral assemblage tends to appear near the garnet when both assemblages are observed. Common sulfide occurrence in the assemblage implies that infiltration of sulfur-bearing fluid/melt triggered the breakdown of garnet. The presence of sillimanite in the replacement assemblage suggests that the garnet breakdown took place under the sillimanite stability field, and the formation of cordierite probably represents decompression process. Rutile enclosed in the secondary cordierite is finer-grained and shows higher Cr, V and Al concentrations compared to matrix rutile. Therefore, inclusion rutile is likely a product formed during the garnet breakdown. Zr-in-rutile thermometry (Tomkins et al., 2007) applied to the inclusion rutile gave ~700 oC for the pressure of the sillimanite stability field, and this probably represents the temperature of the garnet breakdown.
Although further detailed analysis is required, the observed decompression reaction texture in the sillimanite stability field contrasts with counter-clockwise P-T paths previously proposed for Brattnipene, and thus timing of the garnet breakdown reaction needs to be determined in order to understand its geological meaning in the tectonic framework proposed by Osanai et al. (2013).