12:00 〜 12:15
[SMP26-12] Tracing the Latest Archean adakitic protolith in the western Lützow-Holm Complex, East Antarctica
キーワード：珪長質片麻岩、アダカイト、リュツォ・ホルム岩体、東南極
The Lützow-Holm Complex in eastern Dronning Maud Land, East Antarctica, is one of the most crucial areas for understanding the amalgamation of Gondwana. The major lithology consists of high-grade metamorphic rocks formed during the Gondwana orogeny. Zircon U–Pb ages have revealed various timings for the protolith formations (approximately 2.5 Ga, 2.0–1.8 Ga, 1.0–0.8 Ga, and 0.6 Ga) throughout the entire Lützow-Holm Complex. (e.g., Dunkley et al., 2020, Polar Sci). The geological significance of these various protoliths was unclear due to the absence of a direct relationship among them. Recently, based on the results from the outcrop “Berrnabbane”, Nakano et al. (2023, JpGU abst) suggested that the 2.5 Ga protolith represented a microcontinent formed in an oceanic arc or initial island arc setting. They also proposed its continental margin was reactivated at 1.9–1.8 Ga and subsequently collided with a 1.0 Ga continent during the amalgamation of Gondwana. Although the distribution and origin of the microcontinent are significant issues in considering the assembly of Gondwana, the chemical, chronological, and isotopic data are quite limited. Since the 2.5 Ga protolith is characterized by an adakitic composition, we analyzed the whole-rock chemistry of 54 felsic gneisses and 37 mafic/ultramafic blocks and layers from Berrnabbane, Strandnibba, Rundvågshetta, Instekleppane, Vesthovde, and Kista in this study to investigate the distribution, origin, and tectonic significance of the oldest protolith in the Lützow-Holm Complex. In this presentation, we focus exclusively on felsic gneiss.
Based on the mineral assemblages, the felsic gneisses are divided into Hbl+Bt (Berrnabbane), Opx±Cpx±Hbl±Bt (Strandnibba, Rundvågshetta, Vesthovde, Kista), Grt+Opx (Rundvågshetta), Grt+Bt (Rundvågshetta, Instekleppane, Vesthovde), and Bt (Rundvågshetta, Vesthovde) gneisses, as well as leucocratic gneiss (Strandnibba, Rundvågshetta, Instekleppane) that lacks mafic minerals. Our chemical analyses indicate that all outcrops, except Kista, contain low-Y and high-Sr adakitic protolith. However, some felsic gneisses, such as the Hbl–Bt gneiss from Berrnabbane with protolith ages of 1.9 Ga and 1.0 Ga (Nakano et al., 2023), Grt–Opx and Bt gneisses from Rundvågshetta, as well as Opx–Cpx gneiss from Vesthovde, do not show this characteristic. The compositions of adakitic protolith are quite varied; for instance, SiO2 mainly ranges from 53 wt% to 77 wt% and K2O ranges from 0.5 wt% to 8 wt%. The significant compositional variation may be caused by both original chemical differences and chemical modifications during later high-grade metamorphism.
Our preliminary analyses from adakitic Opx–Cpx gneisses at Rundvågshetta and Strandnibba yielded upper intercept ages of 2499±28 Ma and 2502±24 Ma, respectively. Their εHf values range from +3 to +6, similar to data from Berrnabbane. In contrast, oscillatory-zoned zircon in leucocratic gneiss with an adakitic composition at Strandnibba showed a mean age of 570 ± 4 Ma, and the replaced rim yielded approximately 530 Ma. The oscillatory-zoned zircon displays bimodal extremely low εHf values of -23 to -21 and -30 to -28. The former value corresponds to a Hf model age (3.0–2.8 Ga) similar to the felsic gneisses with a 2.5 Ga protolith age (2.9–2.5 Ga), suggesting that the protolith of leucocratic gneiss formed from the partial melting of a 2.5 Ga adakitic protolith during an earlier stage of collision orogeny. The concentration of trace and rare earth elements in felsic gneisses also supports this concept. Adakitic Grt–Bt gneiss from Instekleppane also shows a similar upper intercept age of 2543±30 Ma to Opx–Cpx gneiss from the Strandnibba and Rundvågshetta, while it includes some older zircon grains. Additionally, the εHf values exhibit a bimodal signature of approximately +6 and -4 to -2. While the positive values align with 2.5 Ga protoliths from other outcrops, the negative values correspond to the Hf model age between 3.3 and 3.2 Ga, similar to those derived from zircons with lower εHf values from leucocratic gneiss at Strandnibba (3.4 to 3.2 Ga). The above-mentioned preliminary results suggest that the latest Archean (~2.5 Ga) adakitic protolith may be widely distributed in the western Lützow-Holm Complex, and they potentially accompany a much older basement formed at ~3.0 Ga or older. In the presentation, we will include additional U–Pb and Hf isotopic data for discussion.
Based on the mineral assemblages, the felsic gneisses are divided into Hbl+Bt (Berrnabbane), Opx±Cpx±Hbl±Bt (Strandnibba, Rundvågshetta, Vesthovde, Kista), Grt+Opx (Rundvågshetta), Grt+Bt (Rundvågshetta, Instekleppane, Vesthovde), and Bt (Rundvågshetta, Vesthovde) gneisses, as well as leucocratic gneiss (Strandnibba, Rundvågshetta, Instekleppane) that lacks mafic minerals. Our chemical analyses indicate that all outcrops, except Kista, contain low-Y and high-Sr adakitic protolith. However, some felsic gneisses, such as the Hbl–Bt gneiss from Berrnabbane with protolith ages of 1.9 Ga and 1.0 Ga (Nakano et al., 2023), Grt–Opx and Bt gneisses from Rundvågshetta, as well as Opx–Cpx gneiss from Vesthovde, do not show this characteristic. The compositions of adakitic protolith are quite varied; for instance, SiO2 mainly ranges from 53 wt% to 77 wt% and K2O ranges from 0.5 wt% to 8 wt%. The significant compositional variation may be caused by both original chemical differences and chemical modifications during later high-grade metamorphism.
Our preliminary analyses from adakitic Opx–Cpx gneisses at Rundvågshetta and Strandnibba yielded upper intercept ages of 2499±28 Ma and 2502±24 Ma, respectively. Their εHf values range from +3 to +6, similar to data from Berrnabbane. In contrast, oscillatory-zoned zircon in leucocratic gneiss with an adakitic composition at Strandnibba showed a mean age of 570 ± 4 Ma, and the replaced rim yielded approximately 530 Ma. The oscillatory-zoned zircon displays bimodal extremely low εHf values of -23 to -21 and -30 to -28. The former value corresponds to a Hf model age (3.0–2.8 Ga) similar to the felsic gneisses with a 2.5 Ga protolith age (2.9–2.5 Ga), suggesting that the protolith of leucocratic gneiss formed from the partial melting of a 2.5 Ga adakitic protolith during an earlier stage of collision orogeny. The concentration of trace and rare earth elements in felsic gneisses also supports this concept. Adakitic Grt–Bt gneiss from Instekleppane also shows a similar upper intercept age of 2543±30 Ma to Opx–Cpx gneiss from the Strandnibba and Rundvågshetta, while it includes some older zircon grains. Additionally, the εHf values exhibit a bimodal signature of approximately +6 and -4 to -2. While the positive values align with 2.5 Ga protoliths from other outcrops, the negative values correspond to the Hf model age between 3.3 and 3.2 Ga, similar to those derived from zircons with lower εHf values from leucocratic gneiss at Strandnibba (3.4 to 3.2 Ga). The above-mentioned preliminary results suggest that the latest Archean (~2.5 Ga) adakitic protolith may be widely distributed in the western Lützow-Holm Complex, and they potentially accompany a much older basement formed at ~3.0 Ga or older. In the presentation, we will include additional U–Pb and Hf isotopic data for discussion.