13:45 〜 14:00
[SMP24-01] Decompression P-T evolution recorded in a pelitic gneiss from Tangarden, Sør Rondane Mountains, East Antarctica
キーワード：大陸衝突帯、変成作用、南極、温度-圧力履歴
The Sør Rondane Mountains (SRM), East Antarctica exposes a collision zone lower to middle crust recording tectono- and plutono-metamorphic events at ca. 650-500 Ma that possibly correspond to the EAAO and Kuunga orogeny. The SRM is divided into the NE and SW terranes bounded by the Main Tectonic Boundary (MTB), along which the NE terrane is considered to have thrusted up onto the SW terrane (Osanai et al., 2013). This model was built on the observation that the metamorphic rocks of the NE terrane (with inherited zircons >1200 Ma present) record clockwise P-T paths whereas the SW terrane rocks (without inherited zircons >1200 Ma) record counter-clockwise P-T paths. However, this tectonic model is only build on small number of P-T path constraints and needs to be tested from more P-T path constraints from wide area of the SRM. This study aims to constrain the P-T path from Tangarden located in the SW terrane.
In Tangarden, intercalations of intermediate, mafic, calcsilicate and pelitic gneisses are dominated. Mylonite is developed parallel to the gneissose structure. Where mylonitization is weak, mafic and calcsilicate gneisses are boudinaged to form lenses within the host gneisses. The sample used in this study is a garnet-sillimanite-biotite gneiss (sample TK2020010101H) collected from the intercalation dominated by mafic, felsic and intermediate bulk compositions. Pelitic lithology is locally affected by retrograde hydration that replaced garnet with Ti-poor green biotite aggregate, but the sample TK2020010101H is only weakly affected by this hydration. In this sample, biotite (mostly TiO2 > 0.5 wt%) and sillimanite are arranged along the gneissosity, which is overgrown by cordierite. Garnet (XMg = 0.21-0.38) commonly include biotite (mostly TiO2 > 1.0 wt%) throughout the grain, and rarely includes sillimanite at the rim. Garnet is partly surrounded by cordierite (XMg = 0.76-0.81) and Ti-poor biotite (mostly TiO2 < 0.5 wt%), and these minerals also form pressure shadows developed on garnet. Isolated cordierite in the matrix (XMg = 0.77-0.80) exhibit elongated shape with biotite inclusions arranged parallel to the gneissosity. Cordierite (XMg = 0.79) and hercynite (XMg = 0.28-0.31, ZnO = 3.5-3.7 wt%) are formed between garnet and sillimanite. This sillimanite grain is most likely a former inclusion enclosed in the garnet rim before the development of cordierite + spinel between garnet and the sillimanite grain. Cracks developed in garnet are filled with cordierite + biotite ± quartz. Anthophyllite (XMg = 0.60-0.68) is common in the cracks developed in garnet, and is intergrown with crack-filling cordierite. These pieces of observation suggest that following reactions took place:
(1) Bt + Sil + Qtz = Grt +Kfs + melt
(2) Bt + Sil + Qtz = Grt + Crd + Kfs + melt
(3) Grt+ Kfs + melt/H2O = Crd + Bt
(4) Grt + Sil = Crd + Spl
(5) Grt + Qtz + H2O = Ath + Crd
Based on petrogenetic grid for the NaKFMASH system (Spear et al., 1999), reaction sequence from (1) to (4) suggests nearly isothermal decompression P-T path from ~7 kbar, 800 oC to ~3 kbar, 750oC. This result is not consistent with the shape of counter-clockwise P-T paths reported from the SRM, and rather mimics to clockwise P-T paths. More examples of P-T paths from wide area of the SRM are needed to understand the tectonic history of the SRM.
In Tangarden, intercalations of intermediate, mafic, calcsilicate and pelitic gneisses are dominated. Mylonite is developed parallel to the gneissose structure. Where mylonitization is weak, mafic and calcsilicate gneisses are boudinaged to form lenses within the host gneisses. The sample used in this study is a garnet-sillimanite-biotite gneiss (sample TK2020010101H) collected from the intercalation dominated by mafic, felsic and intermediate bulk compositions. Pelitic lithology is locally affected by retrograde hydration that replaced garnet with Ti-poor green biotite aggregate, but the sample TK2020010101H is only weakly affected by this hydration. In this sample, biotite (mostly TiO2 > 0.5 wt%) and sillimanite are arranged along the gneissosity, which is overgrown by cordierite. Garnet (XMg = 0.21-0.38) commonly include biotite (mostly TiO2 > 1.0 wt%) throughout the grain, and rarely includes sillimanite at the rim. Garnet is partly surrounded by cordierite (XMg = 0.76-0.81) and Ti-poor biotite (mostly TiO2 < 0.5 wt%), and these minerals also form pressure shadows developed on garnet. Isolated cordierite in the matrix (XMg = 0.77-0.80) exhibit elongated shape with biotite inclusions arranged parallel to the gneissosity. Cordierite (XMg = 0.79) and hercynite (XMg = 0.28-0.31, ZnO = 3.5-3.7 wt%) are formed between garnet and sillimanite. This sillimanite grain is most likely a former inclusion enclosed in the garnet rim before the development of cordierite + spinel between garnet and the sillimanite grain. Cracks developed in garnet are filled with cordierite + biotite ± quartz. Anthophyllite (XMg = 0.60-0.68) is common in the cracks developed in garnet, and is intergrown with crack-filling cordierite. These pieces of observation suggest that following reactions took place:
(1) Bt + Sil + Qtz = Grt +Kfs + melt
(2) Bt + Sil + Qtz = Grt + Crd + Kfs + melt
(3) Grt+ Kfs + melt/H2O = Crd + Bt
(4) Grt + Sil = Crd + Spl
(5) Grt + Qtz + H2O = Ath + Crd
Based on petrogenetic grid for the NaKFMASH system (Spear et al., 1999), reaction sequence from (1) to (4) suggests nearly isothermal decompression P-T path from ~7 kbar, 800 oC to ~3 kbar, 750oC. This result is not consistent with the shape of counter-clockwise P-T paths reported from the SRM, and rather mimics to clockwise P-T paths. More examples of P-T paths from wide area of the SRM are needed to understand the tectonic history of the SRM.