10:45 〜 11:00
[SMP22-07] Duration of metamorphism in the Berrnabbane from the Lützow-Holm Complex, East Antarctica; significance for microcontinental collision in the Gondwana suture zone
キーワード：変成作用継続時間、ベルナバネ、リュツォ・ホルム岩体、ゴンドワナ
The Lützow-Holm Complex of the eastern Dronning Maud Land in East Antarctica, is one of the most critical areas for understanding the amalgamation of the Gondwana. The main lithology is high-grade metamorphic rocks formed by the Gondwana orogeny. Based on the paleo-geographic positions at the Gondwana era, the orogen might have been formed by the collision between the Indo–Antarctic Craton and unidentified Craton covered by glaciers inland of the Antarctica. Recent zircon U–Pb ages proved the presence of various timing for the protolith formations (c. 2.5 Ga, 2.0–1.8 Ga, 1.0–0.8 Ga, and c. 0.6 Ga) in the entire Lützow-Holm Complex (e.g., Takamura et al., 2018; Takahashi et al., 2018; Dunkley et al., 2020). However, the geological meaning of these various protoliths is still obscure due to the lack of a direct relation between these protoliths.
The Berrnabbane is a small outcrop (~1 km2) located in the western Lützow-Holm Complex that was first investigated by the JARE 63rd expedition operated during the 2021–2022 season. Three different timings of protolith formation were recognized; 2.5 Ga, 1.9–1.8 Ga and 1.0 Ga. Rocks showing 2.5 Ga and 1.0 Ga protolith ages are felsic gneiss, whereas 1.9–1.8 Ga protolith ages were recognized from pelitic gneiss (as maximum depositional age) as well as from felsic and mafic blocks in the pelitic gneisses. The geological, geochemical, and isotopic features suggest that 1.9–1.8 Ga protolith corresponds to continental margin developed around the 2.5 Ga microcontinent. Consequently, the Berrnabbane was the most likely formed by collision between 2.5 Ga microcontinent (with 1.8 Ga reactivated continental margin) and 1.0 Ga microcontinent followed by main collision of the Indo–Antarctic Craton and unidentified craton.
Two different approaches were carried out to understand metamorphic ages of each rock type. U–Th–Pb monazite EMP dating was applied to four pelitic gneisses, and U–Pb zircon dating was applied to seven felsic gneisses, two pelitic gneisses, and three mafic granulites. The results revealed that the rocks having 2.5 Ga or 1.9–1.8 Ga protolith age show a wide range of metamorphic ages from c. 600 Ma to 500 Ma with a mean age of 554 ± 4 Ma [MSWD = 10] for zircon and 556 ± 5 Ma [MSWD = 5.3] for monazite, which strongly suggest multiple or long-term metamorphic duration in the Gondwana suture zone. However, only a normally distributed peak (527 ± 3 Ma; MSWD = 2.7) was obtained from the rocks having 1.0 Ga protolith age. The age is consistent with younger monazite and zircon age peaks (532 ± 4 Ma and 531 ± 3 Ma, respectively) obtained from 2.5 Ga and 1.9–1.8 Ga rocks. The present study strongly suggests that the duration of metamorphism varies between different microcontinents, which may constrain the microcontinental collision tectonics between large cratons in the Gondwana orogenic belt.
The Berrnabbane is a small outcrop (~1 km2) located in the western Lützow-Holm Complex that was first investigated by the JARE 63rd expedition operated during the 2021–2022 season. Three different timings of protolith formation were recognized; 2.5 Ga, 1.9–1.8 Ga and 1.0 Ga. Rocks showing 2.5 Ga and 1.0 Ga protolith ages are felsic gneiss, whereas 1.9–1.8 Ga protolith ages were recognized from pelitic gneiss (as maximum depositional age) as well as from felsic and mafic blocks in the pelitic gneisses. The geological, geochemical, and isotopic features suggest that 1.9–1.8 Ga protolith corresponds to continental margin developed around the 2.5 Ga microcontinent. Consequently, the Berrnabbane was the most likely formed by collision between 2.5 Ga microcontinent (with 1.8 Ga reactivated continental margin) and 1.0 Ga microcontinent followed by main collision of the Indo–Antarctic Craton and unidentified craton.
Two different approaches were carried out to understand metamorphic ages of each rock type. U–Th–Pb monazite EMP dating was applied to four pelitic gneisses, and U–Pb zircon dating was applied to seven felsic gneisses, two pelitic gneisses, and three mafic granulites. The results revealed that the rocks having 2.5 Ga or 1.9–1.8 Ga protolith age show a wide range of metamorphic ages from c. 600 Ma to 500 Ma with a mean age of 554 ± 4 Ma [MSWD = 10] for zircon and 556 ± 5 Ma [MSWD = 5.3] for monazite, which strongly suggest multiple or long-term metamorphic duration in the Gondwana suture zone. However, only a normally distributed peak (527 ± 3 Ma; MSWD = 2.7) was obtained from the rocks having 1.0 Ga protolith age. The age is consistent with younger monazite and zircon age peaks (532 ± 4 Ma and 531 ± 3 Ma, respectively) obtained from 2.5 Ga and 1.9–1.8 Ga rocks. The present study strongly suggests that the duration of metamorphism varies between different microcontinents, which may constrain the microcontinental collision tectonics between large cratons in the Gondwana orogenic belt.