Counter-clockwise P-T history deduced from kyanite-bearing pelitic gneiss in Tenmondai Rock, Lützow-Holm Complex, East Antarctica

Abstract

This paper first reports the counter-clockwise pressure-temperature (P-T) path for the Lützow-Holm Complex in East Antarctica. The metamorphic textures of kyanite-bearing pelitic gneisses from Tenmondai Rock including earlier spinel and ilmenite inclusions, geothermobarometric data, and pseudosection modeling indicate that the pressure increases prior to the peak metamorphic conditions at around 9 ± 0.5 kbar and 770-820 °C, followed by cooling to kyanite-stability field. We conclude that these gneisses underwent granulite-facies metamorphism with a counter-clockwise P-T path, but this contrasts with the widely recognized clockwise P-T path of the Lützow-Holm Complex basement rocks in general. One plausible hypothesis we proposed could be that this counter-clockwise P-T path originated from magmatism with late compression and the rocks of the different structural levels are juxtaposed, while acknowledging that this hypothesis conflicts with previous studies and that further work is needed to clarify these issues.

INTRODUCTION

It is generally assumed that the mineral assemblage in metamorphic rocks equilibrated at peak temperature, which may occur sometime after peak pressure and at lower pressure. In this regard, the occurrence of kyanite as inclusion within garnet porphyroblast is often interpreted to preserve evidence of higher-P and lower-T conditions in the early part of the metamorphic history (e.g., Spear, 1993).

The Lützow-Holm Complex (LHC), East Antarctica, is an Ediacaran to Cambrian age mobile belt (Fig. 1a) that resulted from the collision of East- and West-Gondwana (Hiroi et al., 1991; Shiraishi et al., 1994). Its clockwise (CW) P-T path is evidenced by the kyanite inclusions within garnets of pelitic gneisses and the subsequent development of orthopyroxene and plagioclase coronae around garnets in the mafic gneisses (Motoyoshi et al., 1989; Hiroi et al., 1991). It has long been accepted that Cambrian-aged metamorphism progressively increases in metamorphic grade from NE to SW across this region. However, Suzuki and Kawakami (2019) report peak temperature conditions of ∼ 830-850 °C from Akarui Point to Skallen, the pressure conditions for which would be within the kyanite stability field. This finding contrasts with the previous thought proposed by Hiroi et al. (1991). In addition, a recent U-Pb zircon study by Baba et al. (2022) suggests that the central Prince Olav coastal area experienced a Tonian (937 Ma)-aged, amphibolite-facies metamorphic event. These recent studies indicate to us that the previously accepted view of a progressive increase in metamorphic grade across this region needs to be reconsidered and that the metamorphic history of the LHC is almost certainly more complex than previously assumed.

Figure 1. (a) Geological setting of the southern part of the East African-Antarctic Orogen (EAAO), modified after Jacobs and Thomas (2004) and Meert (2003). The Lützow-Holm Complex (LHC) is situated adjacent to the East African-Antarctic Orogen (EAAO). Key sutures and Kuunga Orogenic belts are taken from Meert (2003). (b) Overview map of eastern Dronning Maud Land and Enderby Land, Antarctica. Terrane boundary is taken from Shiraishi et al. (2008). (c) Location map of Lützow-Holm Complex in Prince Olav Coast showing metamorphic zones after Hiroi et al. (1991). Ages obtained by U-Pb zircon and monazite CHIME are shown. Akarui Point (Shiraishi et al., 2003; Kazami et al., 2016). Tenmondai Rock (Takamura et al., 2020). Kasumi Rock (Tsunogae et al., 2015). Niban Rock (Dunkley et al., 2014). Cape Hinode (Shiraishi et al., 1994; Motoyoshi et al., 2005; Dunkley et al., 2014). Akebono Rock (Baba et al., 2022). Sinnan Rocks (Shiraishi et al., 1994). (d) Geological map of Tenmondai Rock (modified after Shiraishi et al. 1985).

During the 58th Japanese Antarctic Research Expedition, 2016-2017, we found kyanite as individual crystals in the matrix of pelitic gneisses from the Tenmondai Rock, rather than just as inclusions within other phases. The occurrence of kyanite within the matrix as opposed to only as inclusions conflicts with the previously reported clockwise P-T path. Below we provide details of the kyanite occurrences and mineral textures in pelitic gneisses from the Tenmondai Rock and based on our observations propose a new P-T path for this region. All sample numbers in this paper have been simplified from the collection list number of the JARE58 geology group by omitting the prefix ‘SB170110’. Mineral abbreviations follow Warr (2021), except for quartz (Qtz).

GEOLOGICAL BACKGROUND

Geological setting of the Lützow-Holm Complex

The Lützow-Holm Complex (LHC) is Ediacaran to Cambrian age orogenic belt that extends across an area of ∼ 500 km between the Western Rayner Complex (WRC) and the Yamato Mountains (Shiraishi et al., 1994, 1997). Many studies consider the LHC to be part of a collisional belt, formed ∼ 600-500 Ma, that extends through Sri Lanka, South India, and Madagascar (e.g., Shiraishi et al., 1994; Meert, 2003; Shiraishi et al., 2008; Boger et al., 2015; Osanai et al., 2016). The LHC consists of amphibolite- to granulite-facies metamorphic rocks and post-metamorphic igneous intrusions (Fig. 1b). These comprise quartzofeldspathic, intermediate to mafic, pelitic to psammitic gneisses, with calcsilicate, charnockites, and granitoids to ultramafic intrusions. Previous work indicates that the LHC is characterized by: 1) a progressive increase in metamorphic grade from amphibolite- to granulite-facies south-westwards from Shinnan Rocks to Rundvågshetta (Hiroi et al., 1991), 2) a CW metamorphic P-T path, as deduced from kyanite relics within garnet porphyroblasts, across the entire LHC (Hiroi et al., 1991; Yoshimura et al., 2008), and 3) metamorphic ages of 600-520 Ma, obtained by sensitive high-resolution ion microprobe (SHRIMP) and laser ablation-inductively coupled plasma-mass spectrometry U-Pb zircon dating (Shiraishi et al., 1994, 1997; Dunkley et al., 2014; Kawakami et al., 2016; Takahashi et al., 2018; Takamura et al., 2018; Dunkley et al., 2020; Takamura et al., 2020; Durgalakshmi et al., 2021). Based on the SHRIMP U-Pb zircon data, the LHC has been subdivided into three tectonic units (e.g., Tsunogae et al., 2016; Takahashi et al., 2018; Takamura et al., 2018). Recently summarized protolith age distributions indicate six geological provinces for the LHC (Dunkley et al., 2020), and the study area, Tenmondai Rock is located in the Akarui Suite of which age ranging from 970 to 800 Ma. Most of the metamorphic age estimations (600-520 Ma) has been conducted on rocks exposed around the Lützow-Holm Bay, the higher-grade zone. The actual ages of metamorphic events that affected localities further east in the Prince Olav Coast from Akarui Point remain unclear. Recent studies reveal that progressive increase in metamorphic grade is debatable (Iwamura et al., 2013; Suzuki and Kawakami, 2019).

Tenmondai Rock

Tenmondai Rock (68°24′S, 41°45′E) is a coastal exposure located 105 km NE of Syowa Station in East Antarctica (Fig. 1). It comprises layered gneisses, migmatitic gneisses, granites, and pegmatites. The layered gneisses consist of the biotite-hornblende gneiss, garnet-biotite gneiss, and amphibolite, which are dominant in northeast regions. The migmatitic gneisses are dominant in the central region together with granodioritic migmatite and gneissose granite. In the northeast and southwestern regions of Tenmondai Rock the gneissic foliation strikes NW-SE and dips at 50 to 70° to the E, whereas in the central region the strike of the foliation is highly variable, and it dips low to moderate angles in various directions. Shiraishi et al. (1985) proposed that the general structure is one of a dome, with granodioritic migmatite and migmatitic gneiss in the core. Shiraishi et al. (1985) reported that kyanite and spinel inclusions in both garnet and plagioclase, interpreting these to be metastable relics possibly formed by a staurolite-breakdown reaction. Hiroi et al. (1983) found andalusite-sillimanite-muscovite association in pelitic gneiss, and the andalusite is interpreted to be stable at post-peak conditions.

Takamura et al. (2020) investigated the metamorphic phase relations for a mafic granulite from Tenmondai Rock and conducted isochemical modeling which indicates that the rock reached P-T conditions of 7.8-8.4 kbar and 860-850 °C, with subsequent decrease in pressure to 2-3 kbar. They concluded that the P-T path of these gneisses was identical to that of the rest of the LHC. They also reported that mafic granulite from Tenmondai Rock yields magmatic ages of >808 Ma and metamorphic ages of 560-510 Ma. The mafic granulite they examined was collected from the same place with sample SB17011001A with Figure 1d. Recently, Kitano et al. (2021) also reported metamorphic ages of 620-530 Ma for pelitic gneiss.

In our study, samples of kyanite-bearing garnet-sillimanite-biotite gneisses (samples 01A, 02C, 02D, and 02K) were collected from the outcrop in the eastern area of Tenmondai Rock (Figs. 1d and 2a). These gneisses display weak gneissose banding, with leucocratic layers intercalated in places. The modal abundances of constituent minerals do not vary significantly between samples, with the assemblage being: quartz, plagioclase, K-feldspar, garnet, sillimanite, biotite, kyanite, muscovite and minor amounts of spinel, rutile, ilmenite, zircon, apatite, and monazite. Pale sky-blue kyanite can be seen clearly in the outcrop (Fig. 2b). Late-stage pegmatite and leucocratic discordant veins intrude these pelitic gneisses and cut the foliation with sharp boundaries (Fig. 2a). It is noted that a sillimanite-rich zone develops along the boundary between host gneiss and pegmatite.

Figure 2. (a) and (b) Photographs of field occurrences of garnet-sillimanite-kyanite gneisses and kyanite. (c) to (j) Photomicrographs (scale bar 1 mm) of garnet-sillimanite-kyanite gneisses. (c) Kyanite in matrix. Garnet occurs kyanite margins. Spinel is present at grain boundary between garnet and kyanite. (d) Garnet contains tiny quartz, apatite, and rutile. Kyanite is surrounded by garnet corona (Grt^c) which partially connects with garnet rim. (e) Sillimanite is surrounded by garnet corona. Kyanite is present in the matrix. (f) Garnet partially includes kyanite that contains spinel and rutile inclusions. (g) Garnet, sillimanite, and biotite association. Subhedral sillimanite has spinel and ilmenite inclusions. Sillimanite overgrows ilmenite. (h) Sillimanite and spinel surrounded by garnet. (i) Ilmenite and sillimanite surrounded by the sillimanite corona. (j) Garnet includes irregular shaped ilmenite and small spinel grains. Rutile showing euhedral shape is present in matrix. (k) and (l) Qualitative X-ray intensity maps of Mg and Ca for garnet in (d). (l) Ca content increase from core to mantle. Garnet corona have same intensity with garnet rim. Scale bars represent 1 mm.

ANALYTICAL METHODS

The chemical compositions of minerals in samples were analyzed with a wavelength-dispersive type electron microprobe (JEOL JXA-8200) at the National Institute of Polar Research (Japan), with natural silicates and synthetic oxides as standards, an accelerating voltage of 15 kV, a beam current of 12-15 nA, and a 2 µm probe diameter. ZAF correction was applied to the analysis and corrected element contents were recalculated to weight percent. Total iron is reported as FeO^T. Representative mineral compositions are listed in Table 1. Further detailed procedures of these analyses are described in Baba et al. (2019).

Table 1. Major element analyses for garnet (Grt), biotite (Bt), plagioclase (Pl) and spinel (Spl) in sample by EPMA

Sample No.	02K	02K	02K	02K	02K	02K	02C	02C	02C
Anal. No.	80/81	79	72	76	104	86	179	175	171
	Grt M	Grt R	Grt cor	Pl C	Spl in Ky	Bt mtx	Grt	Pl C	Spl in Sil
SiO2	39.38	39.79	39.08	61.89	0.02	37.10	39.14	63.22	0.02
TiO2	0.03	0.03	0.04	0.03	0.00	4.27	0.00	0.00	0.01
Al2O3	22.54	22.06	22.14	23.52	60.13	16.92	22.09	23.05	56.34
Cr2O3	0.01	0.09	0.00	0.00	0.70	0.00	0.09	0.00	1.82
FeOT	27.55	28.48	31.03	0.07	25.65	13.14	31.69	0.01	33.96
MnO	0.34	0.41	0.42	0.00	0.03	0.13	0.39	0.00	0.15
MgO	9.13	9.48	8.15	0.01	8.84	14.53	7.37	0.00	5.34
CaO	1.79	1.03	0.82	4.97	0.00	0.01	0.75	4.56	0.02
Na2O	0.00	0.00	0.00	8.87	0.11	0.16	0.02	8.89	0.02
K2O	0.00	0.00	0.03	0.17	0.00	9.76	0.01	0.35	0.01
ZnO	-	-	-	-	4.93	-	-	-	3.06
Total	100.83	101.38	101.71	99.53	100.40	96.01	101.55	100.08	100.73
O	12	12	12	8	4	22	12	8	4
Si	3.000	3.019	2.992	2.758	0.001	5.453	3.009	2.795	0.001
Ti	0.002	0.002	0.002	0.001	0.000	0.472	0.000	0.000	0.000
Al	2.024	1.973	1.998	1.236	1.947	2.931	2.002	1.201	1.889
Cr	0.001	0.005	0.000	0.000	0.015	0.000	0.006	0.000	0.041
Fe	1.754	1.807	1.987	0.003	0.589	1.615	2.038	0.000	0.808
Mn	0.022	0.026	0.027	0.000	0.001	0.016	0.026	0.000	0.003
Mg	1.036	1.072	0.930	0.001	0.362	3.182	0.844	0.000	0.226
Ca	0.146	0.084	0.067	0.238	0.000	0.001	0.062	0.216	0.000
Na	0.000	0.000	0.000	0.766	0.006	0.045	0.002	0.762	0.001
K	0.000	0.000	0.003	0.010	0.000	1.831	0.001	0.020	0.000
Zn	-	-	-	-	0.100				0.064
Total cation	7.984	7.990	8.008	5.011	3.021	15.547	7.989	4.995	3.035
XMg	0.372	0.372	0.319	-	0.380	0.663	0.293	-	0.219
Xgrs	0.049	0.028	0.022	-	-	-	0.021	-	-
Xprp	0.350	0.359	0.309	-	-	-	0.284	-	-
Xsps	0.007	0.009	0.009	-	-	-	0.009	-	-
Xan	-	-		0.237	-	-	-	0.221	-

FeO^T represents total iron oxide. C, core; M, mantle; R, rim; cor, corona; mtx, in matrix.

Whole-rock chemical composition of the sample rock was determined using the about 50 × 50 × 10 mm (∼ 100 g) billet that remained after preparing a thin section. A powdered sample weighting 1.8000 ± 0.0005 g was thoroughly mixed with twice excess Li-metaborate and fused to form a glass bead in an induction furnace using a NT-2000 Bead Sampler (Tokyo-Kagaku Co., Ltd.). Whole-rock element contents were determined in a RIGAKU RIX2000 X-ray fluorescence (XRF) spectrometer instrument at Shimane University, equipped with an Rh-tube operated at 50 kV and 50 mA. The loss on ignition was determined gravimetrically by placing the sample powder in a furnace at 110 and 1000 °C. Instrument operating conditions of XRF spectrometers are given in Kimura and Yamada (1996). Detailed procedures of these analyses were described in Baba et al. (2022).

Kyanite was identified with JASCO NRS-1000 laser-Raman spectrometer at the National Institute of Polar Research, Tokyo. A green laser (532.02 nm stroke) with an intensity of 10 mW and beam diameter of 1 µm was used for illumination, with the Raman spectrum characteristic of kyanite (Fig. 3).

Figure 3. Raman spectrum of kyanite. R02-02K in Figure 2d, R03-02K in Figure 2e, R04-02K in Figure 2f, R08-01A in Figure 2c.

PETROGRAPHY AND MINERAL CHEMISTRY

Kyanite occurs as a distinct crystal in the matrix of all samples (Figs. 2c and 2e) and is surrounded by plagioclase and partially enclosed in garnet (Figs. 2d-2f). The secondary garnet corona around kyanite can sometimes be seen to be connected with the rim of a nearby garnet (Fig. 2d). Kyanite often contains spinel and rutile inclusions and is partially enclosed by the garnet (Fig. 2f). Sillimanite is also mantled by the garnet corona (Figs. 2e and 2h). It contains spinel and ilmenite inclusions (Figs. 2g-2i). Spinel occurs at the boundary between sillimanite and garnet corona (Fig. 2h).

Garnet is medium-grained (0.5-1.5 mm) and is not large enough to be considered a porphyroblastic phase in the samples collected. Relatively large garnet grain in sample 02K, ∼ 3 mm in diameter (Figs. 2d, 2k, and 2l), shows weak compositional zoning with an increase in grossular content from the core (0.040) to mantle (0.050) (Fig. 2l), and decrease to the rim (Fig. 4a). The garnet corona that surrounds an adjacent kyanite (Figs. 2d, 2k, and 2l) has the same composition as the rim of the garnet (Fig. 4a). Garnets in other samples 02C and 02D have homogenous composition, except for their Mg/(Fe + Mg) ratios (Fig. 4a).

Figure 4. (a) Compositional variations of garnet in terms of Mg/(Mg + Fe_total) versus grossular (Grs) and Mg/(Mg + Fe_total) versus spessartine (Sps). (b) Compositional variations of biotite in terms of Mg/(Mg + Fe_total) versus TiO₂ (wt%). (c) Compositional variations of spinel in terms of Zn-Fe²⁺-Mg and Cr-Fe²⁺-Mg ternary diagrams. (d) Compositional variations of feldspar in terms of An-Ab-Or ternary diagrams. ovg., plagioclase overgrowth on garnet corona.

Biotite occurs as flakes (0.5 mm) without preferred orientation in the matrix. Secondary biotite occurs in various textural settings (mainly the marginal part of garnet and aluminosilicate). The matrix biotite grains have TiO₂ contents of 3.8-5.6 wt% and Mg/(Fe + Mg) ratios of 0.66-0.71 in sample 02K (Fig. 4b). Biotite in sample 02C has a homogeneous TiO₂ and Mg/(Fe + Mg) composition. Biotite in sample 02D has slightly higher TiO₂ and lower Mg/(Fe + Mg) compared to sample 02K.

Spinel occurs as small crystals, mainly as inclusions within sillimanite, garnet, and kyanite, or rarely in the matrix surrounded by plagioclase. Spinel inclusions within sillimanite and kyanite have lower Mg compositions than those within garnet (Fig. 4c). All analyzed spinel contains Zn (ZnO = 2.5-6.3 wt%). Spinel inclusions within sillimanite have Cr₂O₃ up to 2.7 wt% in sample 02C (Fig. 4c).

Plagioclase is present in the matrix and around the thin garnet corona surrounding kyanite and sillimanite grains (Figs. 2d and 2f). There are no significant differences in compositions between these textural settings (Fig. 4d) [X_an = 0.22-0.25(02D), 0.18-0.23(02C), and 0.23-0.27(02K)].

Ilmenite and rutile occur in similar amounts and are included in sillimanite and garnet (Figs. 2g and 2j). In places, ilmenite is partially enclosed by thin sillimanite corona (Fig. 2g). The garnet contains fine-grained spinel and ilmenite inclusions, whereas rutile is present in the matrix in sample 2C (Fig. 2j).

Quartz occurs as irregular shaped, medium sized crystals (0.5-2.0 mm). Occasionally, large grains of up to 3 mm are found in the leucocratic domain. Modal amounts of K-feldspar are variable between samples. Most K-feldspar is perthite to perthitic orthoclase (X_or = 0.72-0.90), but mesoperthite with fine lamellae (X_or = 0.59 as mixed composition) is also present in sample 02K. Irregular shaped fine muscovite occurs as a secondary mineral adjacent to biotite and K-feldspar. Secondary sillimanite needles with radial orientation replace earlier large sillimanite crystals in the leucocratic domain of sample 01A. This leucocratic domain is relatively coarse-grained and interpretated to represent a separated partial melt.

P-T ESTIMATIONS

Phase equilibria modeling

In order to investigate the metamorphic conditions, an NCKFMASTOH P-T and NCKMnFMASTOHZn P-T isochemical phase diagrams were computed based on free-energy minimization, using the Perple_X 6.8.7 software (Connolly, 2005) and end-member thermodynamic data from Holland and Powell (2011) (filename: hp62ver.dat). The activity models of White et al. (2014) were used for orthopyroxene, biotite, melt, garnet, cordierite, chlorite, mica, staurolite and chloritoid, Wheller and Powell (2014) for sapphirine, White et al. (2002) and Holland and Powell (2011) for spinel and GaHcSp (for Zn-bearing sysytem), Holland and Powell (2011) for osumilite, Fuhrman and Lindsley (1988) for feldspar, and ideal solution models for ilmenite (IlGkPy). Details of these models and any modifications are given in the ‘Perple_X solution model glossary’ and ‘Perple_X Updates’ (http://www.perplex.ethz.ch/perplex_updates.html). Calculations were based on assumptions inherent in two types of models: (1) a model that uses whole-rock compositions with loss on ignition assumed to be total H₂O for NCKFMASTOH system and (2) a model that used compositions based on mineral proportions and average mineral compositions (mineral modes + Zn) for NCKMnFMASTOHZn system according to the calculation proposed by Palin et al. (2016).

Based on 6685 point counts from five thin sections, sample 02K contains 35.0 vol% quartz, 5.8% plagioclase, 22.2% K-feldspar, 20.3% garnet, 6.3% biotite, 7.4% sillimanite, 0.4% kyanite, 0.7% ilmenite, 0.5% rutile, 0.4% spinel, and 0.6% muscovite. The oxidation state was estimated using Fe₂O₃ from average biotite, garnet, spinel, and ilmenite compositions. A biotite Fe³⁺/(Fe³⁺ + Fe²⁺) value of 0.11 was applied, based on the assumption by Holdaway et al. (1997), and a garnet, spinel, and ilmenite Fe³⁺/(Fe³⁺ + Fe²⁺) values were estimated from charge balance calculations. Fe₂O₃:FeO = 0.10:8.00 was obtained from modal mineral proportions and average mineral Fe³⁺ composition (see Palin et al., 2016) and adjusted for both the calculation models. The results of this modeling are shown in Figures 5a and 5b. In Figure 5a (whole-rock model), spinel is absent in the resultant pseudosection, whereas for the mineral modes + Zn model (Fig. 5b) spinel (as GaHcSp) is present in all P-T fields. Stability fields of peak associations of garnet-plagioclase-K-feldspar-sillimanite/kyanite-quartz-rutile-melt-biotite (no. 9 of Figs. 5a and 5b) are almost identical in both diagrams (>800 °C and >6 kbar).

Figure 5. (a) NCKFMASTOH P-T pseudosection for the sample 02K using the whole-rock composition. Na₂O = 1.987, MgO = 4.753, Al₂O₃ = 9.253, SiO₂ = 71.475, K₂O = 1.453, CaO = 1.443, TiO₂ = 0.961, FeO = 8.290, O₂ = 0.020, H₂O = 0.365 (mol%). (b) NCKMnFMASTOHZn P-T pseudosection for the sample 02K using the estimated rock composition based on modal amount of constitute minerals with its average composition. Na₂O = 0.95, MgO = 5.29, Al₂O₃ = 11.35, SiO₂ = 69.15, K₂O = 2.31, CaO = 0.67, TiO₂ = 1.08, MnO = 0.08, FeO = 8.18, O₂ = 0.10, H₂O = 0.49, ZnO = 0.02 (mol%). (c) Isopleths of modal amount of garnet and spinel (GaHcSpl) for result (b) are shown. Colored fields represent a observed modal amount of garnet (20 vol%) and spinel (0.40 vol%), and inferred stable mineral assemblage of melt-garnet-plagioclase-K-feldspar-sillimanite-quartz-rutile±biotite. (d) P-T diagram showing geothermobarometric results by garnet-biotite geothermometer and GASP (garnet-kyanite-quartz-plagioclase), and GRISP (garnet-rutile-ilmenite-quartz-plagioclase) geobarometer. Vapor-absent melting reaction curves are taken from Spear (1993). Spl in, see text in detail. H&C, Hodges and Crowley. N&H, Newton and Haselton (1981). B&L, Bohlen and Liotta (1986). H97, Holdaway et al. (1997). H&S, Hodges and Spear (1982).

Geothermobarometry

The temperatures and pressures calculated from several sets of analyses for samples 02K and 02C are given in Table 2. PTQuick software (Simakov and Dolivo-Dobrovolsky, 2009) and GTB program 2.0 (Spear and Kohn, 1999) were used for these calculations. To assess the early metamorphic conditions, we used the composition of the low-Ca garnet core and high-Ca garnet mantle together with matrix plagioclase core and matrix biotite core. Matrix plagioclases in sample 02K have similar compositions from core to rim (Fig. 4d). Here we used a plagioclase core which is assumed to be in equilibrium with the garnet core and mantle. Matrix biotite contains high-TiO₂ (4.34-4.27 wt%) and is assumed to be in equilibrium with garnet produced at peak P-T conditions. Temperatures calculated using the garnet-biotite Fe-Mg exchange thermometer (Hodges and Spear, 1982; Holdaway et al., 1997; Holdaway, 2000) range from 709 to 824 °C at 10 kbar. Pressure conditions calculated for temperatures of 800 °C by means of GASP (Grt-Ky-Pl-Qtz) barometry (Newton and Haselton, 1981; Hodges and Crowley, 1985) range from 8.8 to 10.1 kbar. As for sample 02C, the pressure condition estimated by means of GRISP (Grt-Rt-Ilm-Qtz-Pl) barometry (Bohlen and Liotta, 1986) yields 9.0 kbar. Pressure results obtained by spot 84-86-87 (a) and 80/81-86-87 (b) in Table 2 and GRISP results and temperatures by spot (b) (Hodges and Spear, 1982; Holdaway et al., 1997) are plotted in Figure 5d.

Table 2. Thermobarometric results for Grt-Bt, GASP and GRISP associations of kyanite-bearing pelitic gneisses

Sample		02K	02K	02K	02C
Spot		84-86-87 (a)	80/81-86-87 (b)	106-110-114 (c)	146-147-153 (d)
Mineral		Grt-Bt-Pl	Grt-Bt-Pl	Grt-Bt-Pl	Grt-Ilm-Pl
Domain		C-mtx-mtx	M-mtx-mtx	C-mtx-mtx	M-mtx-mtx
Grt	XMg	0.360	0.372	0.362	0.305
	Xgrs	0.040	0.049	0.035	0.021
	Xalm	0.605	0.539	0.611	0.674
	Xpyp	0.349	0.350	0.346	0.296
Bt	XMg	0.663	0.663	0.672	-
	Ti	0.470	0.470	0.510	-
Pl	Xan	0.243	0.243	0.235	0.172
Ilm	Ti	-	-	-	0.959
T (°C)	H97	770	768	746	-
(at 6 kbar)	H&S	810	806	781	-
	H00	715	718	698	-
T (°C)	H97	789	786	763	-
(at 10 kbar)	H&S	819	824	800	-
	H00	727	730	709	-
P(kbar)	H&C	5.9	6.6	5.6	-
(at 600 °C)	N&H	5.5 [5.5]	6.2 [6.3]	5.3 [5.2]	-
	B&L	-	-	-	6.7
P(kbar)	H&C	9.1	10.0	8.8	-
(at 800 °C)	N&H	9.3 [9.2]	10.1 [10.2]	8.9 [8.8]	-
	B&L	-	-	-	9.0

C, core; M, mantle; mtx, matrix grain core; inc, inclusion in Grt; X_Mg, Mg/(Mg + Fe_total); H97, Holdaway et al. (1997); H00, Holdaway (2000); H&S, Hodges and Spear (1982); H&C, Hodges and Crowley (1985); N&H, Newton and Haselton (1981); B&L, Bohlen and Liotta (1986).

Pressuures in [ ] represent Sil-bearing calibrations.

DISCUSSION

Interpretation of metamorphic evolution

The P-T conditions estimated from pseudosection diagrams are consistent with the result by geothermobarometry that yield around 9 ± 0.5 kbar and 770-820 °C (Fig. 5d). The estimated conditions are assumed to represent close to peak conditions. We can constrain other metamorphic processes based on the calculated pseudosection diagrams involving isopleths of modal amounts of minerals (Figs. 5c and 5d) and observed mineral textures.

1. 1)    Cordierite is not present in analyzed samples. This indicates that metamorphic pressures should have been over 5 kbar.
2. 2)    Spinel occurs as inclusions. This suggests it was an early phase which was probably consumed to form sillimanite/kyanite and garnet (Figs. 2f-2j). Although GaHcSp is stable in all P-T fields in Figure 5b, the GaHcSp volume decreases toward low-T and high-P conditions (Fig. 5c).
3. 3)    The modeled volume of sillimanite increases towards high-P condition under 6 kbar (Fig. 5d) and decreases towards high-P and high-T conditions (10 to 9 vol%). Since the change in garnet volume is inversely correlated, at >6 kbar and >750 °C, this suggests that the observed garnet corona replacing sillimanite (Figs. 2e and 2h) was formed in response to metamorphic conditions changing to high-T and high-P.
4. 4)    Garnet includes both ilmenite and spinel, whereas rutile and kyanite occur in the matrix. This suggests that ilmenite was an early stable phase and rutile become stable later at high-P conditions.
5. 5)    The observed modal proportions of 0.40 vol% spinel (GaHcSp) and 7.4 vol% sillimanite suggest low-P (<5.5 kbar) and high-T conditions (>850 °C) from the isopleths (Figs. 5c and 5d). However, these conditions are inconsistent with the absence of cordierite in the samples.
6. 6)    The stability field of spinel shown Figure 5d (Spl in) is calculated in the NCKMnFMASTOH system using the spinel activity model of White et al. (2002) instead of GaHcSp. Spinel appearance is limited in the low-P together with cordierite in the model ignoring Zn.

Based on the above summary of the textural and modeling evidence, we propose an estimated counter-clockwise (CCW) metamorphic P-T path shown in Figure 5d. Kyanite and sillimanite in the matrix suggest that retrograde metamorphism has progressed close to the sillimanite-kyanite phase transition curve. As garnet volume isopleths show no particular changes along the retrograde P-T path (Fig. 5d), garnet corona on the kyanite margin (Fig. 2d) may have formed at this stage.

Comparison with previous works and tectonic significance

This is the first report of a CCW P-T path from the LHC. It has long been accepted that the LHC is a typical continental collision metamorphic terrane with the widespread CW P-T path. In Tenmondai Rock, the decompression P-T path proposed by Takamura et al. (2020) contrasts with our results. Peak metamorphic conditions around 8 kbar and 800 °C estimated for mafic granulite by Takamura et al. (2020) are consistent with our estimation of peak condition. They demonstrated the CW P-T path with decompression texture of mafic granulite close to our sample locality. Thus, two contrasting CW and CCW P-T paths are obtained in a narrow area. They also proposed the retrograde stage of the low-P condition (2-3 kbar), which is inconsistent with our samples lacking cordierite in pelitic gneisses.

What cause these contrasting rocks with both CW and CCW signatures in almost the same place in high-grade metamorphic rocks? One plausible hypothesis is that the rocks of the different structural levels and timing are juxtaposed. The CW P-T path is expected in the LHC, and our finding of the rocks with CCW P-T path could be that the proposed CCW path resulted from crustal thickening caused by compression postdating magmatism (Bohlen, 1987, 1991; Baba, 1998). The domal structure of Temnondai Rock is consistent with a compressive deformation event concurrent with or closely postdating gneiss formation. The presence of granitoids and late pegmatite indicates an active magmatic regime. However, it needs to be determined if the known granitoid intrusions could have provided sufficient thermal input to reach the peak-T metamorphic condition.

The conflict between our proposed P-T path and that of Takamura et al. (2020) could be resolved if the pelitic gneiss reported here and the mafic granulite examined by Takamura et al. (2020) were metamorphosed at different crustal levels and timing, were subsequently juxtaposed at the same crustal levels by some tectonic process, such as emplacement of granite before migmatization. Alternatively, the CCW P-T path was followed by a decompression P-T path (after 9 kbar and 720 °C), the products of which were preserved only in mafic granulite. All the rocks in Tenmondai Rock were later exhumed and cooled to the andalusite-stability field (Hiroi et al., 1983). The late-stage lower-pressure condition estimated by Takamura et al. (2020) might correspond to this final stage. Resolving the apparent conflict between the two proposed P-T paths will be a focus of future research.

ACKNOWLEDGMENTS

We would like to thank the members of the 58th Japan Antarctic Research Expedition and the crew of the icebreaker SHIRASE. SB acknowledges C. Kitano for support of sample preparations at the University of the Ryukyus. We thank T. Tsunogae and anonymous reviewers for constructive comments and T. Kawakami for editorial handling. SB and PN thank J. Booth for his improvement of the English and comments. This work was partly supported by the National Institute of Polar Research [General Collaboration Projects 25-17 and 2-20], the Research Organization of Information and Systems [ROIS-DS-JOINT 004RP2018].

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