17:15 〜 18:30
[SMP24-P01] Geochronology of the metamorphic rocks at the Oyayubi ridge of Brattnipene, Sør Rondane Mountains, East Antarctica.
キーワード：セールロンダーネ山地、東南極、変成岩、年代測定、ジルコン、モナザイト
The Sør Rondane Mountains (SRM) comprise medium to high-grade metamorphic rocks with granitic, syenitic and minor mafic dikes (e.g. Shiraishi et al., 1997, Antarctic Geological Map Series, Sheet 35). The SRM is divided into the NE terrane and the SW terrane by the Main Tectonic Boundary (MTB), and each terrane is defined by shape of P-T path and age distribution of detrital zircon (Osanai et al., 2013, Precambrian Research, 234)). The SRM is considered to be formed by collision of both terranes at 600-650Ma during which the NE terrane have thrusted up over the SW terrane (Osanai et al., 2013). This model also suggests that whole of the SW terrane behaved as a single geological body during and after the 650-600 Ma collision event. However, this model is not sufficiently correlated with the P-T conditions and ages, so it requires to be tested by conducting more detailed analyses widely in the SRM. This study reports geochronological result (LA-ICP-MS zircon U-Pb dating and EPMA monazite U-Th-Pb dating) of rocks from Brattnipene area in the SW terrane.
At the eastern slope of the Oyayubi ridge of Brattnipene, pelitic gneisses (garnet-sillimanite-biotite gneiss and garnet-biotite gneiss) and felsic gneisses (hornblende-biotite gneiss and garnet-clinopyroxene gneiss) are in contact with each other by the low angle ductile shear zone. The pelitic gneisses are distributed structurally upper than the felsic gneisses. The P-T conditions of these gneisses examined in this study were reported by Adachi et al. (2020, NIPR symposium abstract).
The garnet-sillimanite-biotite gneiss (sample No. TA19120703C) shows several textures indicating retrograde hydration. The texture replacing garnet by kyanite, sillimanite and biotite is interpreted as retrograde products which are reported from several localities in the Brattnipene area (e.g. Adachi et al., 2013a, Geological Society Special Publication, 383). Peak-T condition is estimated as ca. 800 °C. The garnet in garnet-biotite gneiss (TA19120703F) is extensively replaced by biotite and plagioclase, suggesting intense retrograde hydration. These pelitic rocks are located a few tens of cm away from each other.
Zircon grains in TA19120703C shows oscillatory- or planer-zoned cores truncated by vaguely zoned rims. The grains were dated using LA-ICP-MS U-Pb method and the results from the inherited core are grouped at 1100-900 Ma and 800-700 Ma (Th/U: 1.00-0.02), and those from the rim range from 600 to 540 Ma (Th/U: 0.31-0.02), Similar date distribution is also obtained from monazite EPMA dating. Monazite grains in TA19120703F yield date distribution with a large peak centered at ca. 550 Ma and without older date than 700 Ma. These data suggest that these pelitic gneisses reached peak condition at ca. 600 Ma and suffered retrograde hydration at ca. 550 Ma, which is consistent with the result reported from the central SRM in the previous studies (e.g. Adachi et al., 2013b, Precambrian Research, 234).
In contrast, the hornblende-biotite gneiss (TA19120703A) and garnet-clinopyroxene gneiss (TA19120701B) preserve information acquired under the peak metamorphic conditions in the matrix minerals. Especially plagioclase in hornblende-biotite gneiss shows increase in calcium concentration rimward, suggesting prograde process. No clear hydration texture can be recognized in these rocks. Peak P-T condition is estmated as 700-750 °C and 5-9 kbar. Zircons are present in the matrix whereas monazites are absent in these samples.
Zircon grains in TA19120701B shows oscillatory-zoned cores truncated by bright-CL mantles which are surrounded by dark-CL rims. Each domain yields different date cluster: the core at ca. 600 Ma (Th/U: 0.56-0.21, Ave. 0.43), the mantle at ca. 570 Ma (Th/U: 0.32-0.17, Ave. 0.23) and the rim at ca. 550 Ma (Th/U: 0.54-0.06, Ave. 0.16). Zircon grains in TA19120703A preserve oscillatory-zoned cores which are truncated and partially replaced by unzoned domains. The core dates cluster at ca. 600 Ma (Th/U: 0.75-0.12, Ave. 0.25), and the rim dates at ca. 550 Ma (Th/U: 0.20-0.06, Ave. 0.14). These data from the felsic gneisses indicate that igneous activity which formed the protolith of the felsic gneisses at ca. 600 Ma and the subsequent metamorphism which reached peak condition at ca. 550 Ma.
In summary, the pelitic gneisses experienced peak and subsequent retrograde metamorphism at around 600-540 Ma, whereas the felsic gneisses record prograde metamorphism after 600 Ma. This suggests that two distinct metamorphisms are recognized at the Oyayubi ridge of the Brattnipene in the SW terrane. If such a relationship can be observed widely in the SRM, the tectonic model for the SRM needs to be reconsidered.
At the eastern slope of the Oyayubi ridge of Brattnipene, pelitic gneisses (garnet-sillimanite-biotite gneiss and garnet-biotite gneiss) and felsic gneisses (hornblende-biotite gneiss and garnet-clinopyroxene gneiss) are in contact with each other by the low angle ductile shear zone. The pelitic gneisses are distributed structurally upper than the felsic gneisses. The P-T conditions of these gneisses examined in this study were reported by Adachi et al. (2020, NIPR symposium abstract).
The garnet-sillimanite-biotite gneiss (sample No. TA19120703C) shows several textures indicating retrograde hydration. The texture replacing garnet by kyanite, sillimanite and biotite is interpreted as retrograde products which are reported from several localities in the Brattnipene area (e.g. Adachi et al., 2013a, Geological Society Special Publication, 383). Peak-T condition is estimated as ca. 800 °C. The garnet in garnet-biotite gneiss (TA19120703F) is extensively replaced by biotite and plagioclase, suggesting intense retrograde hydration. These pelitic rocks are located a few tens of cm away from each other.
Zircon grains in TA19120703C shows oscillatory- or planer-zoned cores truncated by vaguely zoned rims. The grains were dated using LA-ICP-MS U-Pb method and the results from the inherited core are grouped at 1100-900 Ma and 800-700 Ma (Th/U: 1.00-0.02), and those from the rim range from 600 to 540 Ma (Th/U: 0.31-0.02), Similar date distribution is also obtained from monazite EPMA dating. Monazite grains in TA19120703F yield date distribution with a large peak centered at ca. 550 Ma and without older date than 700 Ma. These data suggest that these pelitic gneisses reached peak condition at ca. 600 Ma and suffered retrograde hydration at ca. 550 Ma, which is consistent with the result reported from the central SRM in the previous studies (e.g. Adachi et al., 2013b, Precambrian Research, 234).
In contrast, the hornblende-biotite gneiss (TA19120703A) and garnet-clinopyroxene gneiss (TA19120701B) preserve information acquired under the peak metamorphic conditions in the matrix minerals. Especially plagioclase in hornblende-biotite gneiss shows increase in calcium concentration rimward, suggesting prograde process. No clear hydration texture can be recognized in these rocks. Peak P-T condition is estmated as 700-750 °C and 5-9 kbar. Zircons are present in the matrix whereas monazites are absent in these samples.
Zircon grains in TA19120701B shows oscillatory-zoned cores truncated by bright-CL mantles which are surrounded by dark-CL rims. Each domain yields different date cluster: the core at ca. 600 Ma (Th/U: 0.56-0.21, Ave. 0.43), the mantle at ca. 570 Ma (Th/U: 0.32-0.17, Ave. 0.23) and the rim at ca. 550 Ma (Th/U: 0.54-0.06, Ave. 0.16). Zircon grains in TA19120703A preserve oscillatory-zoned cores which are truncated and partially replaced by unzoned domains. The core dates cluster at ca. 600 Ma (Th/U: 0.75-0.12, Ave. 0.25), and the rim dates at ca. 550 Ma (Th/U: 0.20-0.06, Ave. 0.14). These data from the felsic gneisses indicate that igneous activity which formed the protolith of the felsic gneisses at ca. 600 Ma and the subsequent metamorphism which reached peak condition at ca. 550 Ma.
In summary, the pelitic gneisses experienced peak and subsequent retrograde metamorphism at around 600-540 Ma, whereas the felsic gneisses record prograde metamorphism after 600 Ma. This suggests that two distinct metamorphisms are recognized at the Oyayubi ridge of the Brattnipene in the SW terrane. If such a relationship can be observed widely in the SRM, the tectonic model for the SRM needs to be reconsidered.