11:30 〜 11:45
[SMP25-04] Geology and petrology of the Berrnabbane in the western Lützow-Holm Complex, East Antarctica: constraints on the tectonic setting of the Paleoproterozoic basement
キーワード：原岩、前期原生代、ベルナバネ、リュツォ・ホルム岩体、東南極
The Lützow-Holm Complex, situated over 400 km from 39 °E to 45 °E along the coast of the eastern Dronning Maud Land in East Antarctica, is one of the most critical areas for understanding the amalgamation of the Gondwana. The complex has long been investigated by members of the Japan Antarctic Research Expedition (JARE), and the results have expanded our knowledge on the Neoproterozoic–Cambrian collision. 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 whole Lützow-Holm Complex and the complex had been divided tectonically into several units (e.g., Takamura et al., 2018; Takahashi et al., 2018; Dunkley et al., 2020). However, these tectonic divisions proposed in several papers have not necessarily reached a consensus. The tectonic setting of each period is also still obscure, requiring more data acquisition, especially data from unexplored outcrops.
The Berrnabbane is a small outcrop (~1 km2) located in the western Lützow-Holm Complex that was first investigated by JARE 63rd expedition operated during the 2021–2022 season. The WNW–ESE trending hornblende–biotite felsic gneisses are widely exposed in the northern and southern parts. However, relatively thick pelitic gneiss layers are intercalated with felsic gneiss in the central part. The pelitic gneisses include garnet-bearing mafic granulite lenses and blocks. Metamorphic pyroxenite blocks are also observed only in felsic gneiss from the southern part. Some rocks record decompression texture, and multiple metamorphic ages of 600–580 Ma and c. 535 Ma were clearly identified by monazite and zircon dating from several samples in entire Berrnabbane.
The mafic granulite shows magmatic age of c. 1.9 Ga which is consistent with a maximum age of deposition obtained from surrounding pelitic gneisses. We obtained two magmatic ages from felsic gneisses; c. 1.0 Ga from the northern part, and c. 2.5 Ga from the central to the southern part. The latter felsic gneisses (SiO2 = 64–73 wt%) show a geochemical signature of high-Sr/Y (30–197), and their K2O/Na2O ratio is commonly lower than 1. These gneisses occasionally include metamorphic pyroxenite blocks containing high-Cr (up to 3000 ppm) and -Ni (up to 850 ppm). Some felsic gneisses are also rich in these components (Cr<142 ppm, Ni<65 ppm). Additionally, the epsilon Hf(t)values of these felsic gneisses range from +2.2 to +9.7, some of which correspond to the value of a depleted mantle. All these features strongly suggest melting of juvenile slab and subsequent adakitic magma formation and interaction with mantle in the oceanic arc tectonic setting at 2.5 Ga. This process might form the basement of the Lützow-Holm Complex. The result can be compared with neighboring terranes to understand the origin and/or extension of the Paleoproterozoic fragment at the Gondwana Suture zone.
The Berrnabbane is a small outcrop (~1 km2) located in the western Lützow-Holm Complex that was first investigated by JARE 63rd expedition operated during the 2021–2022 season. The WNW–ESE trending hornblende–biotite felsic gneisses are widely exposed in the northern and southern parts. However, relatively thick pelitic gneiss layers are intercalated with felsic gneiss in the central part. The pelitic gneisses include garnet-bearing mafic granulite lenses and blocks. Metamorphic pyroxenite blocks are also observed only in felsic gneiss from the southern part. Some rocks record decompression texture, and multiple metamorphic ages of 600–580 Ma and c. 535 Ma were clearly identified by monazite and zircon dating from several samples in entire Berrnabbane.
The mafic granulite shows magmatic age of c. 1.9 Ga which is consistent with a maximum age of deposition obtained from surrounding pelitic gneisses. We obtained two magmatic ages from felsic gneisses; c. 1.0 Ga from the northern part, and c. 2.5 Ga from the central to the southern part. The latter felsic gneisses (SiO2 = 64–73 wt%) show a geochemical signature of high-Sr/Y (30–197), and their K2O/Na2O ratio is commonly lower than 1. These gneisses occasionally include metamorphic pyroxenite blocks containing high-Cr (up to 3000 ppm) and -Ni (up to 850 ppm). Some felsic gneisses are also rich in these components (Cr<142 ppm, Ni<65 ppm). Additionally, the epsilon Hf(t)values of these felsic gneisses range from +2.2 to +9.7, some of which correspond to the value of a depleted mantle. All these features strongly suggest melting of juvenile slab and subsequent adakitic magma formation and interaction with mantle in the oceanic arc tectonic setting at 2.5 Ga. This process might form the basement of the Lützow-Holm Complex. The result can be compared with neighboring terranes to understand the origin and/or extension of the Paleoproterozoic fragment at the Gondwana Suture zone.