[S-MP25] Supercontinents and Crustal Evolution

[SMP25-07] U-Pb zircon chronology in the Napier Complex, East Antarctica: data from Mt. Reed, Mt. McMaster, and an unnamed nunatak

*Mami Takehara¹, Kenji Horie^1,2, Tomokazu Hokada^1,2 (1.National Institute of Polar Research, 2.The Graduate University for Advanced Studies, SOKENDAI)

The Napier Complex, East Antarctica, has attracted many scientists for a long time as a region that has both evidence of ultrahigh-temperature (UHT) metamorphism (Harley, 2016 and references therein) and long Archaean crustal history from 3800 Ma to 2500 Ma (e.g., Harley & Black 1997). The complex is where the regional UHT metamorphism was first recognized (Dallwitz, 1968), and it experienced extremely high temperatures (>1100 °C) based on the mineral assemblage of sapphirine + quartz (Harley, 2016, and reference therein). The duration and timing of the UHT metamorphism and the associated high-temperature metamorphism have been well investigated around 2550 to 2480 Ma (Harley, 2016, and reference therein). Moreover, the protoliths of the metamorphic rocks in the Napier Complex contain evidence of the Archean continental crusts. The previous studies reported Hadean to Eoarchean signatures from Mt. Sones and Gage Ridge regions (Black et al., 1986; Harley & Black, 1997), and for other regions, the possibility of the early Archean crust. Compston and Williams (1982) and Asami et al. (2002) reported old zircon ages of >3800-3600 Ma from the Fyfe Hills and Mt. Cronus regions in the western part of the Napier Complex, whereas 3000 Ma or younger protolith ages were also reported from the same regions (Horie et al., 2012). It suggests the importance of confirming the distribution of the early Archean ages and the protolith age to promote a discussion about the Archean crustal history in the Napier Complex.
To get new insights from the geochronological data of the Napier Complex, we focused on several outcrops in the Napier Complex and analyzed the tonalitic gneisses using a sensitive high-resolution ion-microprobe (SHRIMP IIe) at the National Institute of Polar Research, Japan. The samples were collected at Mt. Reed, Mt. McMaster, and an unnamed nunatak located between Mt. McMaster and Mt. Riiser-Larsen. The samples are collected during the fieldwork at the 2016-2017 Japanese Antarctic Research Expedition (JARE). In this study, we reported the compiled U-Pb zircon age data of the Napier Complex.
In the case of a tonalitic gneiss from the unnamed nunatak (Sample No. 170219-1A-05), most zircons have rounded habits and are typically over 200μm in size. Some grains show irregular shapes, but most grains have crystal faces. The dark cathodoluminescence-response (CL) domains contain mineral inclusions such as quartz, K-feldspar, albite, ilmenite, and rutile, whereas the mineral inclusions are absent in the bright CL-response domains. The U-Pb data of the zircons are scattered from 3388 to 2469 Ma and show several age peaks centered at 3225, 3175, 3144, 2910, 2800, 2572, and 2485 Ma. The components of the youngest age peak showed Th/U ratios lower than 0.1, which suggests that the last crystal growth occurred at ca. 2485 Ma regional UHT metamorphism. On the other hand, the older components show the protolith ages.
In the case of a tonalitic gneiss from Mt. Reed (Sample No. 170223-1A-06), U-Pb data of the zircons are scattered from 2858 to 2344 Ma and show several age peaks centered at 2798, 2712, 2660, 2478, and 2449 Ma. In the case of a tonalitic gneiss from Mt. McMaster (Sample No. 170219-1A-01), U-Pb data of the zircons are scattered from 2985 to 2462 Ma and show several age peaks centered at 2982, 2891, 2765, 2583, 2532, and 2485 Ma. In the presentation, further geochronological data and interpretation will be demonstrated.