Nd isotopic evolution of Archean tonalitic-granodioritic rocks of the Napier Complex, East Antarctica

Abstract

We examined Sm-Nd isotopes of samples from the Archean tonalitic-granodioritic orthogneiss mass of Mt. Riiser-Larsen in the Napier Complex, East Antarctica. Analytical data show εNd ≈ 0 of ∼ 3.27 and ∼ 3.07 Ga at the time of protolith formation, as determined by SHRIMP zircon analyses. This differs from previously reported Sm-Nd whole-rock isotope data from the oldest tonalitic orthogneisses of Mt. Sones in the Napier Complex, which show εNd = 0 at 3.87-3.80 Ga (T_CHUR), which is coincident with ∼ 3.8 Ga from SHRIMP zircon analyses. These data suggest that the voluminous and homogeneous tonalitic-granodioritic rocks retained the εNd ≈ 0 signal throughout the protolith-metamorphic process and that the source materials of the rocks showed εNd ≈ 0 at ∼ 3.80, 3.27, and 3.07 Ga. Tonalitic-granodioritic orthogneisses from the Napier Complex may contain genetic information regarding Nd isotopic evolution from the Eoarchean to Mesoarchean.

INTRODUCTION

The Napier Complex in Enderby Land, East Antarctica, comprises ancient continental components consisting predominantly of orthogneisses and paragneisses (Sheraton et al., 1987). Some orthogneiss blocks have been assigned ages of 4.0-3.8 Ga using zircon U-Pb dating, such as Mt. Sones, Gage Ridge, Mt. Jewell, and Budd Peaks in the Tula mountains (Williams et al., 1984; Black et al., 1986a; Harley and Black, 1997; Guitreau et al., 2019; Król et al., 2020) and Aker Peaks in Kemp Land (Kusiak et al., 2021), and using Pb-Pb, U-Th-Pb, Sm-Nd, and Rb-Sr whole-rock dating about Fyfe Hills (Sobotovich et al., 1976; DePaolo et al., 1982). Some studies have supported the possibility of inheritance or the very restricted distribution of a 4.0-3.8 Ga block in the Fyfe Hills (Compston and Williams, 1982; Black et al., 1983b; McCulloch and Black, 1984; Asami et al., 2002; Horie et al., 2012). Archean age data generated by various measurement methods, including zircon U-Pb dating and Sm-Nd whole-rock dating, have revealed four age clusters of ∼ 3.3, 3.1-2.9, ∼ 2.8, and 2.6-2.4 Ga (e.g., Sheraton et al., 1987; Suzuki et al., 2006). Black and James (1983) and Harley and Black (1997) described the tectonothermal events of D₁-M₁ at 3.1-2.9, D₂-M₂ at ∼ 2.8, and D₃-M₃ at 2.5-2.4 Ga. The best-distinguished age cluster of 2.6-2.4 Ga corresponds to high-temperature to ultrahigh-temperature (HT-UHT) metamorphism at ∼ 2.5 Ga (Grew, 1998; Harley et al., 2001; Crowe et al., 2002; Hokada and Harley, 2004; Kelly and Harley, 2005; Takehara et al., 2023). Various studies involving SHRIMP U-Pb zircon core dating have distinguished ages of ∼ 2.6 Ga from the cluster since the study of Shiraishi et al. (1997). Carson et al. (2002) proposed that orthogneisses from Tonagh Island crystallized at ∼ 2.63 Ga, pre-dating the peak of UHT metamorphism. Suzuki et al. (2006) interpreted ∼ 2.6 Ga as the emplacement age of A-type granite from Mt. Riiser-Larsen. In addition, recent precise zircon dating has allowed an age of ∼ 2.7 Ga to be distinguished from data yielding ages of ∼ 2.8 and ∼ 2.6 Ga and recognized as a cluster (Król et al., 2023).

Numerous studies have reported Rb-Sr, U-Pb, and Sm-Nd whole-rock isotope data for the Napier Complex (e.g., Black et al., 1983a; McCulloch and Black, 1984; Black et al., 1986b; Black, 1988; Owada et al., 1994; Tainosho et al., 1998; Miyamoto et al., 2004). Black et al. (1983a and 1983b) applied the Rb-Sr whole-rock isochron method to charnockitic gneisses to determine the timings of tectonothermal events of D₁-M₁ at 3.1-2.9, D₂-M₂ at ∼ 2.8, and D₃-M₃ at 2.5-2.4 Ga. Owada et al. (1994) obtained a ∼ 2.46 Ga Sm-Nd whole-rock isochron from orthopyroxene-bearing quartzofeldspathic gneisses with mylonitic texture and ultramafic blocks with deformation texture on Tonagh Island. Miyamoto et al. (2004) used Rb-Sr and Sm-Nd whole-rock isotope analyses to establish a peak UHT metamorphic age of ∼ 2.65 Ga for orthopyroxene-bearing quartzofeldspathic gneisses and an ultramafic block of Howard Hills.

In this study, Sm-Nd whole-rock isotope dating was conducted on samples reported previously by Suzuki et al. (1999). The samples comprise homogeneous and voluminous orthopyroxene-bearing quartzofeldspathic orthogneisses with tonalite-trondhjemite-granodiorite (TTG) affinity (tonalitic-granodioritic orthogneisses in Mt. Riiser-Larsen), for which SHRIMP zircon core age dating has yielded protolith formation ages of 3.3-3.0 Ga (Hokada et al., 2003). To focus on the Nd isotopic evolution of the early felsic crust, we compared our new data with results of previous Sm-Nd studies on tonalitic orthogneisses from Mt. Sones (Black and McCulloch, 1987; Guitreau et al., 2019) in the Napier Complex (Fig. 1).

Figure 1. Location of Mt. Riiser-Larsen in the Napier Complex, East Antarctica.

GEOLOGIC BACKGROUND OF MT. RIISER-LARSEN, NAPIER COMPLEX

Mt. Riiser-Larsen (66°47′S, 50°42′E) is located on the eastern coastline of Amundsen Bay, and its geology been divided into three lithological units: the Massive Gneiss Series (MGS), Layered Gneiss Series (LGS), and Transitional Gneiss Series (TGS) (Fig. 2; Ishizuka et al., 1998). The MGS is composed predominantly of TTG orthogneiss over an area of about 10 km × 5 km, comprising the so-called ‘Grey Gneisses’ (Fig. 3A). Hokada et al. (2008) lithologically classified the TTG orthogneisses in the MGS as the upper ‘massive (mainly tonalitic) orthogneiss’ and the lower ‘layered (mainly tonalitic) orthogneiss dominant’ (Fig. 2). The LGS is composed of metamorphosed supracrustal rocks, such as psammitic rocks with thin pelitic rock layers, basaltic rocks, A-type granitic rocks, and TTG rocks (Suzuki et al., 1999). The LGS exhibits nearly horizontal layers that are parallel to their gneissosity, which strikes NE-SW to E-W and dips at gentle angles (20°-40°) to the south-southeast (Ishikawa et al., 2000). The TGS between the LGS and MGS dips gently to the south-southeast and is widely exposed in the southwestern area (Fig. 2). The TGS consists mainly of quartzofeldspathic rocks that commonly show intense fabric and include various lithofacies and igneous rocks.

Figure 2. Simplified geologic map and cross-section of Mt. Riiser-Larsen, modified after Ishizuka et al. (1998) and Ishikawa et al. (2000), showing sample localities. Circles represent samples of tonalitic-granodioritic orthogneisses (TG) and granitic orthogneiss (G). White squares are samples of sills for quart-free tholeiite basaltic gneisses (S). Small M and L beside TG, G, and S and the circle’s color of green and pink present MGS and LGS, respectively. The values in parentheses associated with the sample numbers show the approximate elevations of localities. Instances of the letter ‘Z’ in circles indicate the localities of zircons analyzed using SHRIMP (Hokada et al., 2003). The dashed line on the cross-section divides the upper (‘massive orthogneiss dominant’) and lower (‘layered orthogneiss dominant’) parts of the TTG orthogneisses in the MGS (from Hokada et al., 2008).

Figure 3. Field photographs of the Mt. Riiser-Larsen study area. (A) Tonalitic-granodioritic orthogneisses of the MGS. The cliff height in the photograph is approximately 20 m. (B) Alternating domains of tonalitic-granodioritic orthogneisses and mafic granulites. (C) Mafic granulites appear as mafic mineral-rich layers and clumps in tonalitic-granodioritic orthogneisses. (D) Elongated, irregularly shaped mafic patches indicating partial melting.

Mt. Riiser-Larsen is recognized as the location recording the highest-temperature (>1100 °C) metamorphism in the Napier Complex at ∼ 2.5 Ga (e.g., Harley and Hensen, 1990; Harley and Motoyoshi, 2000; Hokada and Suzuki, 2006). Hokada et al. (2003) identified concordia ages of ∼ 3.27, 3.07, 2.85-2.79, and 2.52-2.45 Ga from tonalitic-granodioritic orthogneisses using SHRIMP U-Pb zircon analyses, with two TTG magmatic events being identified at 3.27 and 3.07 Ga and a high-grade (UHT) metamorphic event at ∼ 2.5 Ga. Suzuki et al. (2006) determined SHRIMP zircon ages of ∼ 2.8 and 2.6 Ga for A-type granitic gneisses from Mt. Riiser-Larsen, SHRIMP zircon and monazite ages of 2.51-2.48 Ga, and Sm-Nd mineral isochron ages of ∼ 2.38 Ga. They proposed an emplacement age of ∼ 2.6 Ga for the A-type granites, which contained inherited zircons with ages of ∼ 2.8 Ga, with subsequent cooling recorded during 2.51-2.38 Ga. Proterozoic dolerite dikes, the so-called ‘Amundsen Dikes’ (Sheraton et al., 1987), vertically cut basement rocks in Mt. Riiser-Larsen. These dikes comprise (i) tholeiite basalts with Sm-Nd and Rb-Sr isotopic whole-rock ages of ∼ 2.0 Ga and high-magnesian andesites of unclear age that strike NE-SW throughout the area; and (ii) alkali basalts with Rb-Sr isotopic whole-rock ages of ∼ 1.2 Ga and MORB-like tholeiite basalts of unclear age that strike N-S in the western area (Fig. 2; Suzuki et al., 2008).

SAMPLE DESCRIPTIONS

We examined six tonalitic-granodioritic orthogneisses (termed ‘TGs’ here), including five from the MGS (TG_M-1 to TG_M-5) and one from the LGS (TG_L-1); one granitic orthogneiss (termed ‘G’) from the northwestern MGS (G_M-1); and five mafic gneisses from sills in the LGS (S_L-1 to S_L-5) (Fig. 2 and Table 1).

Table 1. Sm and Nd contents and isotope ratios for whole-rock samples from Mt. Riiser-Larsen

Sample no.		No.*	SiO2	Sm	Nd	147Sm/144Nd	143Nd/144Nd	2s (±)	TCHUR	εNd
			(wt%)	(ppm)	(ppm)				(Ma)	(T Ga)
Tonalitic-granodioritic orthogneisses in MGS (TGM)
TGM-1	SS 011304	94	71.0	1.10	8.53	0.07792	0.510104	±0.000014	3230	0.6 (3.27)
TGM-2	SS 011306	96	67.8	2.36	14.1	0.1011	0.510569	±0.000013	3279	−0.1 (3.27)
TGM-3	SS 011505	103	64.2	6.15	32.3	0.1151	0.511002	±0.000012	3038	0.4 (3.07)
TGM-4	SS 012101	129	69.2	4.28	23.5	0.1101	0.511040	±0.000022	2798	0.0 (2.80)
TGM-5	SS 012401	153	70.6	1.85	14.3	0.07817	0.510113	±0.000014	3226	0.7 (3.27)
Granitic orthogneisses in the northwestern MGS (GM)
GM-1	SS 021102	269	73.0	0.87	12.1	0.04344	0.509668	±0.000014	2937	2.7 (3.07)
Granodioritic orthogneisses in LGS (TGL)
TGL-1	SS 020402A	209	71.4	0.33	2.57	0.07759	0.510263	±0.000014	3021	0.8 (3.07)
Sills of quartz-free mafic gneiss in LGS (SL)
SL-1	SS 012213	151	47.2	2.67	7.78	0.2075	0.512716	±0.000014	-	-
SL-2	SS 020401	208	48.8	2.96	7.84	0.2283	0.512919	±0.000012	-	-
SL-3	SS 020504	228	48.7	3.59	10.8	0.2010	0.512496	±0.000010	-	-
SL-4	SS 021303B	289	49.1	2.55	7.31	0.2109	0.512787	±0.000012	-	-
SL-5	HI 122705	601	48.4	2.86	8.94	0.1934	0.512589	±0.000013	-	-

No.* indicates sample numbers from Suzuki et al. (1999).

Sample T_CHUR and εNd values were calculated using Chondritic Uniform Reservoir (CHUR) values of ¹⁴³Nd/¹⁴⁴Nd = 0.512638 and ¹⁴⁷Sm/¹⁴⁴Nd = 0.1966 as the present CHUR (Goldstein et al., 1984).

The TGs are classified as calc-alkaline series and as tonalite-granodiorite, and their chondrite-normalized (Lu/Yb)_N/Yb_N-Yb_N relationships correspond to the Archean TTG suite after Martin (1987) (Suzuki et al., 1999). TG_M-1, 2, 4, and 5 are contained in the ‘layered orthogneiss dominant’ group of the MGS after Hokada et al. (2008). TGs commonly contain mafic granulites composed of mafic mineral-rich layers or masses (Figs. 3B and 3C) and meta-ultramafic components. TG_M-4 was obtained from the lower part of the MGS, and the surrounding rocks contain elongated irregular-shaped patches formed by partial melting (Fig. 3D) or rare occurrences of layers for meta-ultramafic (komatiitic) component (plate 3F of Suzuki et al., 1999). TG_M-3 was collected from the ‘massive orthogneiss dominant’ group of the upper MGS after Hokada et al. (2008). The field occurrence of TG_L-1 measures decacentimeters to decameters in width. TG_L-1 has a similar occurrence and mineral assemblage to those of the MGS. The modes of constituent minerals of the TGs have been described by Suzuki et al. (1999). In the northwestern area of Mt. Riiser-Larsen, a mass of orthopyroxene-bearing quartzofeldspathic orthogneisses appears as an enclave in the MGS (Fig. 2). Bulk-rock geochemical data for G_M-1 allow this sample to be classified as granite (Fig. 11 of Suzuki et al., 1999). The geochemical signatures of TG_M-1, 2, 5, TG_L-1, and G_M-1 indicate strong depletion in heavy rare earth elements (HREEs) with conspicuous positive Eu anomalies, whereas TG_M-3 and 4 are characterized by slight depletion of HREEs without Eu anomalies (Suzuki et al., 1999).

Mafic gneisses commonly occur as 2-4-m-wide sills in the LGS, displaying gneissic foliation that is locally oblique to the layering or foliation of neighboring gneisses (Ishikawa et al., 2000). These mafic gneisses are classified as tholeiite basalt and are further divided into quartz-free and quartz-bearing types (Suzuki et al., 1999). The analyzed samples S_L-1 to S_L-5 are quartz-free mafic gneisses (Table 1). The modes of constituent minerals of these samples have been reported by Suzuki et al. (1999).

ANALYTICAL PROCEDURE

Neodium isotope analyses were performed at the Graduate School of Science and Technology, Niigata University, Niigata, Japan. For whole-rock isotope analyses, TG rock samples of approximately 30³ cm³ volume were used. Powdered fractions of 100-200 mg were dissolved using an HF + HNO₃ + HClO₄ mixture in Teflon vials at 110 °C for 1 week. Details of the techniques of sample solution and extraction of Nd have been presented by Kagami et al. (1987). The isotope analyses were performed using multi-collector thermal ionization mass spectrometry (TIMS: MAT262). The isotope analytical method followed that described by Kagami et al. (1989). Correction of ¹⁴³Nd/¹⁴⁴Nd ratios for mass fractionation was normalized to ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219 (O’Nion et al., 1977). During the analyses, ¹⁴³Nd/¹⁴⁴Nd ratios were corrected to 0.512115 for JNdi-1 following the Nd isotope reference of the Geological Survey of Japan (Tanaka et al., 2000). ¹⁴⁷Sm/¹⁴⁴Nd ratios were calculated using the equation of York (1966), with a decay constant of λ¹⁴⁷Sm = 6.54 × 10⁻¹² yr⁻¹ (Lugmair and Marti, 1978), and using Sm and Nd contents determined by inductively coupled plasma-mass spectrometry (Suzuki et al., 1999).

RESULTS AND DISCUSSION

Table 1 and Figure 4 present Sm-Nd whole-rock isotopic data and ¹⁴³Nd/¹⁴⁴Nd versus ¹⁴⁷Sm/¹⁴⁴Nd diagrams, respectively. The Nd model ages for the Chondritic Uniform Reservoir (CHUR; T_CHUR) of TG_M-1, 2, and 5 lie within the range of 3.28-3.23 Ga, and those of TG_M-3, TG_L-1, and TG_M-4, are 3.04-3.02 and ∼ 2.80 Ga, respectively. These T_CHUR ages are mostly consistent with the ages of ∼ 3.27, 3.07, and 2.85-2.79 Ga derived from SHRIMP U-Pb zircon dating of tonalitic-granodioritic orthogneisses from Mt. Riiser-Larsen (Fig. 5; Hokada et al., 2003). In the study of Hokada et al. (2003), sample TH011302 from the eastern area of Mt. Riiser-Larsen yielded upper and lower concordia intercept ages of 3270 ± 12 and 2789 ± 35 Ma, whereas TH012816 sampled from the margin of the TGS yielded upper and lower concordia intercept ages of 3073 ± 12 and 2849 ± 9 Ma (Fig. 2). Hokada et al. (2003) proposed that these igneous TTG rocks formed at ∼ 3.27 Ga and then underwent extensive remelting or modification at ∼ 3.07 Ga, or that they formed at ∼ 3.07 Ga with some xenocrystic zircons with ages of ∼ 3.27 Ga. They interpreted the ∼ 2.80 Ga event inferred from 2.85-2.79 Ga zircon ages of the TTG rocks as a metamorphic event on the basis of zircon textures. TH012401 yielded a concordia age of 3267 ± 5 Ma, excluding ∼ 2.5 Ga age data, suggesting a UHT metamorphic event at that time. This older age was interpreted by Hokada et al. (2003) as representing the timing of TTG magmatism for the protolith of TH012401 on account of the lack of 3.1-2.6 Ga zircon age data. Sample TH012401 was collected approximately 30 m from TG_M-5 in a similar schistose layer, so these samples can be regarded as being from the same rock type (Fig. 2). The coincidence of the SHRIMP zircon core age of 3267 ± 5 Ma obtained from TH012401 and the T_CHUR age range of 3.28-3.23 Ga from TG_M-1, 2, and 5 signifies the following: 1) the timing of magma formation of these TGs was ∼ 3.27 Ga; 2) the UHT-metamorphosed TGs may have retained the Sm-Nd isotopic signature of the TG magma; and 3) the isotopic signature of the source materials from which the TG magmas were derived had similar ¹⁴³Nd/¹⁴⁴Nd ratios to the CHUR at that time.

Figure 4. (A)-(B) Plots of ¹⁴³Nd/¹⁴⁴Nd versus ¹⁴⁷Sm/¹⁴⁴Nd for the tonalitic-granodioritic orthogneisses from Mt. Riiser-Larsen and the tonalitic orthogneisses from Mt. Sones in the Napier Complex. The four lines in each of (A) and (B) extend through two points of the present CHUR and ¹⁴³Nd/¹⁴⁴Nd ratios for the CHUR of ¹⁴⁷Sm/¹⁴⁴Nd = 0 for ages of 2.80, 3.07, 3.27, and 3.80 Ga, respectively.

Figure 5. Nd isotopic evolution for the tonalitic-granodioritic orthogneisses from Mt. Riiser-Larsen and the tonalitic orthogneisses from Mt. Sones in the Napier Complex. Symbols are the same as those in Figure 4.

Samples TG_M-1, 2, and 5 (T_CHUR of 3.28-3.23 Ga) show an isochron age of 3015 ± 44 Ma, and samples TG_M-3, TG_L-1, and G_M-1 (T_CHUR of 3.04-2.94 Ga) show an isochron age of 2822 ± 108 Ma. Guitreau et al. (2019) presented an isochron age of 3273 ± 170 Ma for 3.8 Ga tonalitic orthogneisses from Mt. Sones, as a regression age in the supplement data. These isochron ages may record later geologic processes in closed systems, although it remains unclear what these geologic-isotopic processes might have been. It may be effective to consider Sm-Nd mineral isotope dating (Wang et al., 2022). Regardless, the results of T_CHUR = 3.28-3.23 Ga for TG_M-1, 2, and 5 suggest that the voluminous and homogeneous tonalitic-granodioritic orthogneisses barely retained the Sm-Nd isotopic signatures of the original magma. If T_CHUR ages of ∼ 3.0 and 2.8 Ga for the TG rocks also correspond to the age of magma formation, this implies that the MGS is a composite of ∼ 3.27, 3.07, and 2.80 Ga TG rocks.

Regarding point 3) above, there are three possibilities (Fig. 6): (A) TG magmas were derived directly from the mantle with the CHUR Sm/Nd ratio (εNd ≈ 0); (B) TG magmas were derived from mafic source materials and acquired a similar Sm/Nd ratio to that of the CHUR (εNd ≈ 0); and (C) TG magmas were formed immediately after mafic source materials were derived from mantle with the CHUR Sm/Nd ratio (εNd ≈ 0), that is, meaning that the TG magmas would not have formed by chance on the CHUR line after the mafic source materials formed from depleted mantle. The case of Figure 6B, for example, applies to the isotopic conditions of S_L-1 to 5 (Table 1 and Fig. 4A), although sills in the LGS may be younger and not related to the igneous event that generated the older TGs of the MGS. Proterozoic vertical tholeiite basaltic dikes (THB-m of Suzuki et al., 2008) in Mt. Riiser-Larsen also have similar Sm-Nd isotopic compositions to that of the CHUR (Fig. 4A). However, enriched MORB-like basaltic rocks have recently been excluded as a plausible source for TTG rocks on the basis of geochemical modeling and experiments (e.g., Martin et al., 2014). Although it is not possible to indicate which of Figures 6A, 6B, or 6C is more likely, the results of this study indirectly suggest that the mantle of 3.27-3.07 Ga did not deviate significantly from the CHUR line.

Figure 6. Changes in εNd values of tonalitic-granodioritic magmas for Mt. Riiser-Larsen and the timings of formation of their source materials: (A) TG magmas derived directly from mantle, (B) TG magmas derived from mafic source materials with a similar Sm/Nd ratio to that of the CHUR (εNd ≈ 0), and (C) TG magmas formed immediately after mafic source materials to be derived from mantle with the CHUR Sm/Nd ratio (εNd ≈ 0).

Orthogneisses of Mt. Sones are generally regarded as ancient rocks (Fig. 1). Geologically, Mt. Sones has been divided into massive orthogneiss consisting of orthogneisses and banded gneisses composed of ortho- and paragneisses (Black et al., 1986a). Williams et al. (1984) and Black et al. (1986a) reported SHRIMP zircon core ages of 3.95-3.93 Ga from HREE-depleted tonalitic orthogneiss (sample 78285007) from the massive orthogneiss of Mt. Sones. Harley and Black (1997) revised the SHRIMP zircon data from 78285007 by redetermining intercept ages with reference to Pb/U ratios and defined a concordia intercept age of 3800 + 50/− 100 Ma. Guitreau et al. (2019) obtained a laser ablation-inductively coupled plasma-mass spectrometry zircon U-Pb age for the same sample of 3794 ± 40 Ma. Black and McCulloch (1987) and Guitreau et al. (2019) analyzed 78285007 for Sm-Nd whole-rock isotope dating, the isotopic compositions for which are presented in Table 2. The data from Black and McCulloch (1987) were recalculated using the correction factors used to compare them with the data in this paper (Table 2). The T_CHUR ages are 3.87, 3.85, and 3.80 Ga and coincide with zircon core ages of ∼ 3.8 Ga from Harley and Black (1997) and 3794 ± 40 Ma from Guitreau et al. (2019) (Figs. 4 and 5; Table 2). The εNd values range from −2.0 to −0.3. Guitreau et al. (2019) further analyzed whole-rock ¹⁷⁶Lu-¹⁷⁶Hf and ^147,146Sm-^143,142Nd systematics and proposed that 78285007 orthogneisses were generated by reworking of source materials that formed as mafic protocrust at 4456-4356 Ma. The close coincidence of the T_CHUR and zircon core ages for the TTG orthogneisses (i.e., ∼ 3.27 and 3.07 Ga from Mt. Riiser-Larsen and ∼ 3.80-3.79 Ga from Mt. Sones; Fig. 5), reinforce the interpretation that the Napier Complex is a composite consisting of TTG protoliths that formed at different times. A dataset of zircon crystal ages and Sm-Nd whole-rock isotope data would be useful for placing constraints on the formation of the TTGs and source materials from the Eoarchean to Paleoarchean (or Mesoarchean; Fig. 6), although the dataset needs to be expanded to cover the whole of the Napier Complex.

Table 2. Sm and Nd contents and isotopic data for tonalitic orthogneisses from Mt. Sones

Sample no.	Sm	Nd	147Sm/144Nd	143Nd/144Nd	2s (±)	143Nd/144Nd*	TCHUR	εNd1	εNd1	εNd2
	(ppm)	(ppm)					(Ma)	(T Ga)	(T Ma)	(T Ma)
Black and McCulloch (1987)
78285007-A	3.49	24.42	0.0865	0.50901	±0.00002	0.50981	3880	−1.1 (3.80)	−1.2 (3794)	−1.3 (3794)
78285007-F	4.36	29.24	0.0901	0.50912	±0.00002	0.50992	3855	−0.8 (3.80)	−0.9 (3794)	−1.0 (3794)
78285007-J	3.97	30.64	0.0785	0.50886	±0.00002	0.50966	3810	0.0 (3.80)	−0.1 (3794)	−0.3 (3794)
Guitreau et al. (2019)
78285007	5.16	32.4	0.0970	0.510041	±0.000010	-	3947	−1.9 (3.80)	−2.0 (3794)	−2.0 (3794)

Black and McCulloch (1987) used ¹⁴⁶Nd/¹⁴²Nd = 0.636151 for mass fractionation correction (DePaolo and Wasserburg, 1976) and estimated the present (¹⁴³Nd/¹⁴⁴Nd)_CHUR as 0.511836 and the present (¹⁴⁷Sm/¹⁴⁴Nd)_CHUR as 0.1967 (Jacobsen and Wasserburg, 1980). For comparison with our isotopic data, we multiplied the ¹⁴³Nd/¹⁴⁴Nd value of each sample by 1.001569 as the correction factor, and these values are listed as ¹⁴³Nd/¹⁴⁴Nd*. T_CHUR and εNd¹ (T) were calculated using ¹⁴³Nd/¹⁴⁴Nd*. Values of εNd² (T) were then recalculated using the CHUR isotopic values (¹⁴³Nd/¹⁴⁴Nd = 0.512630 and ¹⁴⁷Sm/¹⁴⁴Nd = 0.1960) defined by Bouvier et al. (2008), used in Guitreau et al. (2019).

Archean orthogneiss masses are understood to be composites of micro-crustal blocks of TTGs and/or supracrustal blocks containing volcanic and/or sedimentary sequences (Roberts et al., 2016; Rollinson, 2016). Felsic magma would have been easily contaminated by the surrounding existing crust, which contained blocks of various lithofacies, and the felsic rocks would have been susceptible to contamination by subsequently generated magma and affected by later tectonothermal events (Ferreira et al., 2020; Kirkland et al., 2021). Nevertheless, parts of the tonalitic-granodioritic orthogneisses in the Napier Complex constitute a valuable record of the Nd isotopic signatures of juvenile TTG crusts.

CONCLUSIONS

1. 1.    This study presented Sm-Nd isotopic data for samples from the Archean tonalitic-granodioritic orthogneiss mass of Mt. Riiser-Larsen in the Napier Complex, East Antarctica. The isotopic data reveal εNd ≈ 0 at the time of protolith formation at ∼ 3.27 and ∼ 3.07 Ga, as determined by previous SHRIMP zircon analyses. These results mean that tonalitic-granodioritic orthogneisses of Mt. Riiser-Larsen record the Sm-Nd isotopic properties at the time of magma formation.
2. 2.    The mafic source materials of the tonalitic-granodioritic rocks with εNd ≈ 0 at the time of protolith formation suggest two possibilities: (i) the materials followed the Chondritic Uniform Reservoir (CHUR) line after derivation from mantle with the CHUR Sm/Nd ratio; or (ii) the materials were derived from mantle with the CHUR Sm/Nd ratio immediately prior to the extraction of TTG magmas at ∼ 3.27 and ∼ 3.07 Ga.
3. 3.    The Nd isotopic evolution along the CHUR line is consistent with previous Sm-Nd isotopic data for the oldest 3.80-3.79 Ga tonalitic orthogneisses of Mt. Sones. It is suggested that the voluminous TTG orthogneisses in the complex retained the εNd ≈ 0 signal throughout the protolith-metamorphic process and that the Napier Complex may be a composite of Eoarchean to Mesoarchean juvenile TTG blocks with supracrustal rocks in between.

ACKNOWLEDGMENTS

This study formed part of the Science Program of the Japanese Antarctic Research Expedition (JARE) supported by the National Institute of Polar Research (NIPR) under MEXT. We thank H. Kagami and K. Shiraishi for their fruitful discussions about isotopic data. We also thank H. Ishizuka for his support during fieldwork and for discussions about geology. M. Yuhara is acknowledged for his valuable assistance in Sm and Nd and thermal ionization mass spectrometry analyses. We also thank Y. Kawano for his techniques for calculating isochrons. Constructive reviews by two anonymous reviewers and editorial comments by M. Satish-Kumar helped to improve the manuscript. This work was supported by JSPS Research Fellowship for Young Scientists to SS while at the Graduate University for Advanced Studies and by JSPS KAKENHI Grant Number JP21H01182 to TH.

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