Metamorphic P-T path
Sample BTN02A
Previous studies have described cordierite from Sinnan Rock and Cape Ryugu, which are located within the amphibolite-facies zone in the eastern part of the LHC (Shiraishi et al. 1989). Secondary cordierite has been identified in rocks along the southern part of the Soya Coast at Rundvågshetta, occurring as decompression-related corona textures consisting of cordierite + orthopyroxene and cordierite + sapphirine (Ishikawa et al. 1994; Motoyoshi and Ishikawa 1997; Yoshimura et al. 2008). However, no studies have reported large porphyroblasts of cordierite, and the sample described here is presumed to contain the largest cordierite grains known in the LHC.
Cordierite occurs as variably shaped, subhedral to irregular grains within the different textural domains of sample BTN02A. In cordierite-rich domains, large cordierite porphyroblasts contain biotite inclusions and are in contact with biotite within the matrix (Fig. 6b). The biotite inclusions within cordierite and biotite in the matrix have high TiO2 contents (~ 6.1 wt.%), suggesting crystallization under high-T conditions. Considering the estimated modal proportion (> 20 vol.%) and XMg values (~ 0.81) of cordierite, this phase was likely stable at the peak-T conditions of ~ 5.5 kbar and ~ 825°C. In contrast, the high spessartine content of the garnet in this domain suggests that the garnet formed under relatively low-T conditions.
Here, we analyze the mineral textures of the cordierite-rich domain to estimate the early P–T history of the sample. In the garnet-rich domain, cordierite replaces an embayed garnet margin (Fig. 6a) that has a high XMg value (~ 0.8), suggesting the partial breakdown of garnet and subsequent growth of cordierite. Conversely, post-cordierite garnet growth is evidenced by rounded cordierite inclusions within garnet grains (Fig. 6c), and by the replacement of garnet by cordierite (Fig. 6d). We interpret these contrasting textures as resulting from increasing pressure toward the garnet-in stability field and a subsequent cooling or drop in pressure leading out of the garnet stability field. Euhedral cordierite (Fig. 6b) potentially crystallized from partial melt during this process. However, the modeled pseudosection (Fig. 8) applies only to the cordierite-rich domains, as a high bulk XMg is assumed for the garnet-rich domain based on analyzed mineral compositions. Furthermore, we cannot exclude the possibility that local differences in bulk composition resulted in the breakdown of garnet in the garnet-rich domain, as both garnet and biotite have high XMg ratios. This possibility lies outside the scope of this study.
For sample BTN02A, pseudosection modeling was performed using whole-rock data from the cordierite-rich domains. In addition to the original modeling, we assessed the bulk-rock oxidation state using Fe2O3 estimated from average biotite and garnet compositions. A biotite Fe3+/(Fe3+ + Fe2+) value of 0.08 was applied, based on the composition of granulite-facies biotite (Stephenson 1977), and a garnet Fe3+/(Fe3+ + Fe2+) value of 0.02 was estimated from charge balance calculations. According to Palin et al. (2016), Fe2O3:FeO = 0.02:2.19, as obtained from modal mineral proportions and average mineral compositions. In Fig. 10a, a recalculated pseudosection incorporating the estimated O2 shows almost the same topology as the original model (Fig. 8a), except for the addition of spinel-stable fields. The location of the estimated P–T path (Fig. 10a) outside the stability field of spinel is consistent with this recalculation.
One of the main lines of evidence for a counter-clockwise P–T path for the Botnnuten gneiss is the growth of sillimanite and biotite at the margins of cordierite. The same texture, in which cordierite is overgrown by fine-grained biotite + sillimanite ± ilmenite, has been reported in granulite-facies metapelite and interpreted to indicate a counter-clockwise P–T trajectory involving isobaric cooling (Fig. 11; Clarke et al. 1989; Boger and White 2003; Halpin et al. 2007). Furthermore, based on our pseudosection modeling, the presence of garnet together with sillimanite and biotite suggests an increase in pressure.
Sample BTN02F
In the spinel–garnet-bearing gneiss (sample BTN02F), spinel is of consistent size and is abundant throughout the matrix. As the spinel does not contain significant ZnO, it is unlikely to be a product of the breakdown of staurolite; thus, the spinel-forming reaction remains unclear. Sillimanite and corundum occur together in the matrix with spinel, and garnet locally occurs as overgrowths on spinel rims (Fig. 7a). Corundum often coexists with biotite in the matrix (Fig. 7c), suggesting the low-temperature replacement of spinel. We did not observe corundum inclusions in garnet, which argues against early high-P conditions during prograde metamorphism. The consumption of spinel to form corundum, sillimanite, and biotite is consistent with the isobaric cooling and slight increase in pressure indicated by pseudosection modeling (Fig. 9b). However, the initial pseudosection modeling of the spinel-bearing gneiss is only semi-quantitative, as we did not consider the oxidation state of the sample. For the re-modeling of sample BNT02F, the modal proportions of spinel, garnet, and biotite were integrated with their average Fe3+/(Fe3+ + Fe2+) compositions to estimate Fe2O3 contents. The Fe3+/(Fe3+ + Fe2+) ratios for spinel and biotite were estimated to be 0.06 and 0.10, respectively, and the total Fe2O3 is inferred to be 0.57 wt.% (0.25 mol.%). Figure 10b shows a recalculated pseudosection for sample BNT02F incorporating O2, showing a different topology than the original model (Fig. 9a). Spinel stability is restricted at both low-P and high-T conditions (Fig. 10b), whereas the sillimanite stability field is expanded. The observed assemblage in sample BNT02F (Grt–Crn–Sil–Spl–Bt–Pl–Kfs–Ilm ± Melt in Fig. 10b) appears at ~ 6.2 kbar and 800–810°C. The stability fields for spinel, spinel + sillimanite, and sillimanite + corundum progress toward high-P and low-T conditions, which is consistent with the spinel inclusions we observe within both sillimanite and corundum and with the proposed counter-clockwise nature of the P–T path.
Boger et al. (2012) calculated a pseudosection considering Fe2O3 for spinel-bearing granulites, and their results showed the same phase relations as obtained in the present study, with spinel being stable under low-P and high-T conditions, and sillimanite under high-P and low-T conditions; therefore, we consider our P–T estimate to be reliable. White et al. (2002) interpreted the polyphase mineral growth texture of spinel inclusions within sillimanite and garnet rims to indicate minor decompression followed by isobaric cooling. However, polyphase spinel growth is unlikely to have occurred in our sample, as there is no evidence of spinel–symplectite and the spinel grains show a narrow range of compositions. Therefore, a predominantly isobaric cooling history, accompanying a slight increase in pressure, is more likely.
Comparison with previous works
Motoyoshi and Shiraishi (1985) used phase relations and geothermobarometry to infer metamorphic P–T conditions of 5–6 kbar and 750–800°C for samples collected from Botnnuten by the First Japanese Antarctic Research Expedition in 1957. Their geothermobarometric results (dashed gray square in Fig. 10a) are consistent with the results of our modeling, and they likely represent the conditions during peak metamorphism. Although these P–T conditions are similar to those of the Yamato–Belgica Complex, located ~ 250 km southwest of the LHC (Fig. 1b), Motoyoshi and Shiraishi (1985) concluded that Botnnuten is a low-pressure part of the Lützow–Holm Complex based on petrographical features. Despite the absence of staurolite, they assumed a staurolite break-down reaction and a prograde metamorphic history based on the occurrence of spinel. The ITD-related orthopyroxene–plagioclase symplectite textures that characterize the LHC have not been identified in the mafic gneisses at Botnnuten. More recent studies of the P–T conditions at Rundvågshetta, which is part of the high-grade section of the LHC, yielded maximum pressures of 13–15 kbar during early metamorphism (Kawasaki et al. 2011; Hiroi et al. 2019), much higher than those estimated for Botnnuten in the present study.
Tectonic significance
A counter-clockwise (CCW) P–T path and IBC have been reported from basement rocks of the Rayner Complex in McRobertson Land and the Northern Prince Charles Mountains (Boger and White 2003; Halpin et al. 2007; Morrissey et al. 2015). This metamorphism is thought to have been related to the emplacement of voluminous charnokite rocks at 990–900 Ma. In East Antarctica, a CCW P–T path has also been proposed for the SW terrane of the Sør Rondane Mountains in Eastern Dronning Maud Land (Baba et al. 2013). The SW terrane reached granulite-facies conditions of up to ~ 9 kbar and ~ 900°C (Baba et al. 2013) as a result of tectonic loading from over-thrusting of the NE terrane (Osanai et al. 2013). The timing of peak metamorphism has been constrained at 650–600 Ma (Osanai et al. 2013). Given that Botnnuten is an isolated inland nunatak, surrounded by an ice sheet, it is difficult to establish its tectonic relationship to surrounding lithologies. The Botnnuten gneisses did not reach high pressures (~ 6 kbar); therefore, the metamorphism was most likely caused by the emplacement of igneous rocks rather than tectonic loading associated with crustal thickening.
The basement rocks of the Yamato Mountains (Fig. 1b) are composed of granulite- to amphibolite-facies metamorphic rocks and granitoids. The estimated P–T conditions of granulite-facies metamorphism in the area are < 6 kbar and ~ 750°C (Asami and Shiraishi 1985). Asami and Shiraishi (1985) reported the development of garnet coronas and quartz between anorthosite and wollastonite in calc-silicate gneisses from this region, indicating high-temperature isobaric cooling (e.g., Fitzsimon and Harley 1994; Dasgupta and Pal 2001). The granitoids in the Yamato Mountains, which are mainly syenite, occur as lit-par-lit injection layers within the host gneiss (Asami and Shiraishi 1985). If these intrusions were the heat source for local metamorphism, an IBC path would have been likely. These results from previous studies indicate a metamorphic history marked by IBC.
Dunkley et al. (2020) summarized the available U–Pb age data for the LHC and proposed the following geological subdivisions based on protolith ages (from southwest to northeast): the Innhovde Suite (1070–1040 Ma); the Rundvågshetta Suite (2520–2470 Ma); the Skallevikshalsen Suite (1830–1790 Ma); the Langhovde Suite (1100–1050 Ma); the East Ongul Suite (630 Ma); and the Akarui Suite (970–800 Ma; Fig. 1a). These authors considered that Botnnuten may represent a klippe belonging to the Rundvågshetta Suite. However, we consider it unlikely that the low-pressure, shallow lithologies at Botnnuten correlate with the deep crustal lithologies of Rundvågshetta, given their different metamorphic histories. Instead, the Botnnuten rocks are more similar to the basement rocks of the Yamato Mountains. However, some uncertainties remain, as there is a clear NNE–SSW-trending discontinuity across the eastern margin of the Yamato Mountains, as seen in aeromagnetic maps (Golynsky et al. 2018; Fig. 12). This large-scale geological discontinuity suggests that a direct connection between Botnnuten and the Yamato Mountains remains problematic. A more detailed comparison of the two areas is necessary, focusing on a broader range of lithologies and locations.