Nanogranitoid inclusions with grandidierite in mafic granulite from Austhovde, Lützow-Holm Complex, East Antarctica

Abstract

Grandidierite, (Mg,Fe)Al₃O₂(BO₃)SiO₄, was found in a garnet-clinopyroxene-ilmenite-rich mafic granulite from Austhovde in the Late Neoproterozoic to Early Cambrian Lützow-Holm Complex (LHC), East Antarctica, the first reported occurrence of this borosilicate in a mafic granulite. It occurs in one of the many nanogranitoid inclusions (NIs) in garnet. Quartz, sodic plagioclase, myrmekite, K-feldspar, epidote and biotite are also found only as inclusions in garnet. Garnet porphyroblasts show marked compositional zoning: Ca increases and Mg decreases from the core to rim with little change in Fe and Mn contents except for the outermost rim. Anorthite content of inclusion plagioclase increases from core to rim of host garnet in parallel with increase in garnet Ca towards the rim. This together with the distinctly different mineral assemblages within and exterior to garnet porphyroblasts suggests that partial melting took place and produced melts were extracted leaving a mafic and calcic restite. Partial melting also occurred locally in garnet porphyroblasts consuming different hydrous mineral inclusions to produce various NIs ranging from K-feldspar-rich to K-feldspar-free. Subsequent decompression at high temperatures resulted in breakdown of garnet to orthopyroxene + calcic plagioclase with further consumption of quartz, such that none remained in the matrix of the granulite. Grandidierite may have formed by a reaction between a trapped boron-bearing aluminous granitic melt and host garnet upon cooling.

INTRODUCTION

Grandidierite, (Mg,Fe)Al₃O₂(BO₃)SiO₄, has been reported in pegmatites, migmatites, pelitic hornfelses, and calc-silicate rocks (Grew, 1996). Its occurrence in mafic granulite is unexpected, because (1) boron is characteristically more abundant in sedimentary rocks than in mafic and ultramafic rocks (e.g., Leeman and Sisson, 1996; Grew, 2017; Grew et al., 2017) and (2) processes associated with metamorphism commonly lead to increasing loss of boron with increasing metamorphic grade (e.g., Kawakami et al., 2008). During the course of the study of NIs in various granulites we found a single grain of grandidierite in a garnet-clinopyroxene-ilmenite-rich mafic granulite (sample 84012223) from Austhovde in the Latest Proterozoic-Early Paleozoic Lützow-Holm Complex, East Antarctica. NIs are supercooled hydrous granitic melt inclusions in garnet most commonly found in pelitic and quartzo-feldspathic granulites from continental collision orogens worldwide (Cesare et al., 2009, 2011; Ferrero et al., 2012; Hiroi et al., 2014; Cesare et al., 2015; Ferrero et al., 2016; Ferrero and Angel, 2018; Ferrero et al., 2018; Hiroi et al., 2019; Hiroi, 2020; Hiroi et al., 2020; Nicoli and Ferrero, 2021). The purpose of the present study is to document the occurrences of various NIs and grandidierite in the mafic granulite and to demonstrate that partial melting took place in the mafic granulite at high-temperatures and boron is preferentially incorporated into melts, which were subsequently almost totally extracted.

GEOLOGIC SETTING

The Late Neoproterozoic to Early Cambrian LHC spans the Prince Olav, Sōya and Prince Harold coasts, between longitudes 39°E and 45°E (Figs. 1a and 1b). It is bounded by the Late Proterozoic Rayner Complex to the east and by the Late Neoproterozoic to Early Cambrian Yamato-Belgica Complex to the west. The LHC is characterized by progressive metamorphism from the upper amphibolite facies in the eastern part of the Prince Olav Coast to the upper granulite facies in the southern part of the Sōya Coast. Rocks of the LHC experienced a clockwise P-T path, as evidenced by the presence of relict prograde kyanite included in garnet at all grades (Hiroi et al., 1983; Motoyoshi et al., 1985; Hiroi et al., 1991). In addition, retrograde andalusite occurs in rocks that achieved sillimanite stability on the Prince Olav Coast (Hiroi et al., 1983, 1995; Kawakami et al., 2008). In pelitic and mafic granulites from the southern Sōya Coast, garnet is commonly partially replaced by cordierite ± orthopyroxene and orthopyroxene + anorthite symplectite, respectively, most likely formed during high-T decompression (Motoyoshi et al., 1989; Kawasaki et al., 1993; Ishikawa et al., 1994; Motoyoshi and Ishikawa, 1997; Fraser et al., 2000; Yoshimura et al., 2008; Kawakami et al., 2016; Tsunogae et al., 2016; Takahashi and Tsunogae, 2017). Hiroi et al. (2019) reported the common occurrence of andalusite in NIs in pelitic and quartzo-feldspathic granulites from the southern Sōya Coast.

Figure 1. (a) Map of Antarctica showing location of the Late Neoproterozoic to Early Cambrian Lützow-Holm Complex (LHC). (b) Map of Prince Harald and Sōya coasts, showing localities of high-grade metamorphic rocks containing nanogranitoid inclusions (NIs) in the LHC. Mafic granulite sample 84012223 of the present study is from Austhovde.

Shiraishi and Yoshida (1987) described general geology of Austhovde, which consists of three outcrops; Austhovde-kita Rock, Austhovde-naka Rock, and Austhovde-minami Rocks. Each outcrop is about 1.0 km in length. According to Shiraishi and Yoshida (1987), biotite gneiss and garnet-biotite gneiss are predominant, alternating with biotite-hornblende gneiss and biotite amphibolite in Austhovde-kita Rocks and Austhovde-naka Rocks. The foliation trends E-W and dips S in these outcrops. In contrast, Austhovde-minami Rocks is composed of the alternation of pyroxene-hornblende gneiss, siliceous garnet gneiss, quartzite, garnet-biotite gneiss, biotite gneiss, and marble. The general layering and foliation trend N-S and dip W, and the mineral lineation plunges gently toward W. Such a set of lithology and structure are similar to those of Skallen and Skallevikhalsen (Fig. 1). Sample 84012223 is from Austhovde-minami Rocks, which belong to the Rundvågshetta Suite of Dunkley et al. (2020), the protolith of which has a late Archaean age. This suite was metamorphosed at c. 580 Ma (Tsunogae et al., 2016).

PETROGRAPHY

Sample 84012223 is a medium-grained, massive, melanocratic rock, occurring as an isolated lens in pyroxene-hornblende gneiss like samples Ts11011405A and Ts11011601M studied by Tsunogae et al. (2016) and Takahashi and Tsunogae (2017). The sample is layered and subdivided into two parts; garnet-clinopyroxene-ilmenite-rich layer A and hornblende-rich amphibolitic layer B. The layer A was briefly described by Shiraishi and Yoshida (1987) as orthopyroxene ‘eclogite’. The amphibolitic layer B is composed mostly by hornblende and characteristically rich in large-ion lithophile elements.

The layer A of sample 84012223 is composed of >80 modal % garnet, clinopyroxene, and ilmenite with small amounts of orthopyroxene, hornblende, plagioclase, and magnetite. Garnet occurs as porphyroblasts up to 9 mm in diameter, containing numerous inclusions (Figs. 2-4): not only relatively common NIs (Figs. 3-5), but also small amounts of grandidierite, biotite, K-feldspar, plagioclase, quartz, zircon, ilmenite, magnetite, and fluorapatite (Fig. 2). Clinopyroxene is present mainly in the matrix, where it is intergrown with small amounts of hornblende and orthopyroxene. It commonly contains lamellae of orthopyroxene with or without ilmenite. Orthopyroxene occurs mainly in the matrix with plagioclase, where it has partially replaced garnet. It also appears as inclusions in the outer part of garnet porphyroblasts and rarely as a constituent of NIs. Hornblende appears both in the matrix and in garnet porphyroblasts. Plagioclase shows three distinct modes of occurrence: (1) as a constituent of symplectite with orthopyroxene replacing garnet, (2) as inclusions in garnet, and (3) as a constituent of NIs in garnet. Some grains of plagioclase included in garnet are not only antiperthitic but also myrmekitic, containing lamellar K-feldspar and vermicular quartz (Figs. 4a, 4c-1 to 4c-4). Biotite, K-feldspar, and quartz occur exclusively in garnet as isolated inclusions (Fig. 2) and as constituents of NIs. K-feldspar appears as lamella of antiperthitic plagioclase included in garnet and a constituent of NIs. Quartz inclusions tend to be present in the inner part of garnet porphyroblasts as euhedral to anhedral grains. Oxide minerals are ilmenite and magnetite. Ilmenite is present both in the matrix and in garnet as the predominant oxide mineral, whereas magnetite occurs mainly in the matrix and rarely occurs in NIs. Fluorapatite appears mainly as inclusions in garnet.

Figure 2. (a) Plane-polarized photomicrograph. Clinopyroxene is deep green because of its high X_Fe ratio. Compositional profile along line A-B is shown in Figure 6a. (b-1) to (b-6) X-ray elemental maps for Si, Na, K, Mg, Ca, and Ti of the same area in a. Increases in the contents of these elements are indicated by change in color from blue to green to yellow to red to white in all the X-ray elemental maps. Quartz, Na-rich plagioclase, and biotite are present exclusively in garnet, which is partially replaced by anorthite + orthopyroxene symplectite from outside and along fractures. (c) Backscattered electron (BSE) and (d) scanning electron microscope cathodoluminescence (SEM-CL) images of a NI composed of biotite, plagioclase and quartz in garnet. Note both quartz and plagioclase show euhedral-subhedral zoning with relatively sharp zone boundaries (red arrows). The mineral abbreviations are after Whitney and Evans (2010).

Figure 3. (a) BSE and (b) SEM-CL images of the area in the red rectangle in Figure 2a. Note many NIs and quartz grains with different CL brightness in garnet porphyroblast. Sharp zone boundary (red arrow) is seen in one of quartz grains. (c) BSE image and X-ray elemental maps for Si (d-1), Al (d-2), Mg (d-3), Na (d-4), and K (d-5) of the bottom-left NI-1 in a. Note skeletal quartz grains is surrounded by granophyric intergrowth of quartz + K-feldspar. (e) BSE image and X-ray elemental maps for Si (f-1), Al (f-2), Mg (F-3), Na (f-4), and K (f-5) of the bottom-right NI-2 in a. Note orthopyroxene is present whereas K-feldspar is absent in the NI.

Figure 4. (a) Plane-polarized photomicrograph of sample 84012223. (b) Plane-polarized photomicrograph of the rectangle (b) in (a). The garnet encloses many NIs in addition to euhedral quartz ± biotite inclusions. Elemental maps for Si (c-1), Ca (c-2), Na (c-3), and K (c-4) of plagioclase inclusion in the rectangle (c) in (a). It is myrmekitic and antiperthitic, being intergrown with quartz and containing lamellar K-feldspar. (d) Plane-polarized photomicrograph of the rectangle (d) in (a). BSE (e) and SEM-CL (f) images and elemental maps for Si (g-1), Al (g-2), Mg (g-3), Ca (g-4), and Na (g-5) of the rectangle in (d). One of NIs (NI-3) contains anorthite and epidote in addition to sodic plagioclase, quartz, and biotite. Note distinct Mg-depletion halo around the NI, indicating Fe-Mg exchange reaction between NI and host garnet.

Figure 5. Plane-polarized photomicrographs (a) and (c), BSE image (b), and elemental maps for Si (d-1), Al (d-2), Fe (d-3), Mg (d-4), Ca (d-5), Na (d-6), and K (d-7) of grandidierite-bearing NI-4 in Figure 4a. Note grandidierite occurs with biotite, ilmenite, and magnetite in the marginal part of the NI.

Nanogranitoid inclusions (NIs)

Numerous NIs are observed in garnet porphyroblasts (Figs. 3b, 4b, and 4d). Constituent minerals and textures differ from one NI to another even in the same garnet porphyroblast. Figure 3 shows two distinct NIs close to each other in the same garnet; one contains skeletal to dendritic quartz surrounded by granophyric intergrowths of K-feldspar and quartz in addition to biotite, sodic plagioclase, and K-feldspar; the other is composed of orthopyroxene, biotite, plagioclase, and quartz without K-feldspar. Figures 4d and 4e also show two distinct NIs; one is granitic, being composed of K-feldspar, sodic plagioclase, and quartz; and the other contains anorthite and epidote in addition to biotite, sodic plagioclase and quartz without K-feldspar. Thus, NIs are subdivided into two groups; K-feldspar-bearing and K-feldspar-free, although quartz/feldspars and K-feldspar/plagioclase ratios vary spatially even within the same K-feldspar-bearing NIs. These two groups of NIs are distributed randomly in garnet porphyroblasts. A grain of grandidierite, 20 µm × 5 µm in size, occurs with biotite, magnetite, and ilmenite in the marginal part of one of K-feldspar-bearing NIs (Fig. 5).

MINERAL CHEMISTRY

Minerals in sample 84012223 were analyzed with a JEOL JXA-8200 wavelength dispersive electron probe microanalyzer (EPMA) at the National Institute of Polar Research, Tokyo, Japan. Analyses were performed using a 15 kV accelerating voltage and 12 nA beam current, with a 2 µm beam diameter. Synthesized oxides and natural minerals were used as standards for major elements. The obtained data were corrected using a JEOL oxide-ZAF correction program. X-ray compositional maps were obtained on the same instruments using a 15 kV accelerating voltage, beam currents up to 300 nA, and a 1-2 µm beam diameter. Counting times were up to 50 milliseconds.

Backscattered electron (BSE) and cathodoluminescence (CL) images of NIs were obtained using a JEOL JSM-5600 scanning electron microscope (SEM) attached to a Link ISIS 300 energy dispersive X-ray spectrometer (EDS) system and an Oxford Instruments MiniCL cathodoluminescence detector at Chiba University, Chiba, Japan. X-ray compositional mapping was also performed with the SEM, using a 15 kV accelerating voltage and a 30 nA beam current. CL images were additionally obtained with a Gatan Chroma CL2 attached to JEOL JSM-7100F FE-SEM at the National Institute of Polar Research, Tokyo. The CL detector is equipped with red, green and blue filters to generate color Chroma SEM-CL images by combining multiple images. Grandidierite was identified with a Renishaw inVia Raman microscope Qontor at the National Institute of Polar Research, Tokyo. The sample was irradiated by laser with a wavelength of 532 nm.

Garnet

Garnet porphyroblasts show compositional zoning with broad Mg-rich cores (Prp₂₅Alm₅₉Sps₁Grs₁₄) surrounded by Ca-richer rims (Prp₂₀Alm₆₀Sps₂Grs₁₈) (Fig. 6). Fe and Mn contents are almost constant except for the outermost rims (Prp₁₈Alm₆₁Sps₂Grs₂₀), where Fe and Mn contents increase. The garnet surrounding NIs often shows local diffusion halo (e.g., Fig. 4g-3), indicating Fe-Mg exchange reaction between NIs and host garnet. The garnet adjacent to grandidierite (Fig. 5) is Prp₂₂Alm₅₇Sps₁Grs₂₀.

Figure 6. Compositional diagrams showing chemistry of garnet. (a) Line profile along line A-B in Figure 2a, showing compositional zoning of a porphyroblastic garnet. (b) Triangular diagram showing garnet composition and chemical zoning in terms of grossular, pyrope and almandine + spessartine.

Clinopyroxene

Clinopyroxene shows some compositional variation in X_Mg, which ranges from 0.59 to 0.64, with cores of large grains tending to be less magnesian. The Al₂O₃ content ranges from 0.85 to 2.94 wt%, whereas the Na₂O contents are relatively low, 0.27-0.32 wt%, corresponding to about 2 mol% jadeite. Representative core and rim are Di₅₀Hd₃₅CaTs₄En₄Jd₂ and Di₅₇Hd₃₁CaTs₂En₅Jd₂, respectively.

Orthopyroxene

Orthopyroxene in the matrix and included in the outer part of garnet porphyroblast has almost the same composition (En₄₈Fs₄₉MgTs₁CaTs₂). Orthopyroxene in NIs has the highest X_Mg (0.588) (En₅₇Fs₄₀MgTs₂CaTs₁), whereas orthopyroxene intergrown with myrmekitic plagioclase in the garnet mantle has intermediate X_Mg (0.52) (En₅₀Fs₄₇MgTs₁CaTs₂).

Hornblende

Composition of hornblende varies depending upon the mode of occurrence (Fig. 7). Matrix hornblende is ferropargasite, containing more than 2.0 wt% K₂O and 0.8 wt% Cl, respectively. In contrast, hornblende included in garnet is pargasite, and has a higher Ti content, up to 2.7 wt% TiO₂.

Figure 7. Diagrams showing compositions of biotite and hornblende. (a) Ti (atoms per formula unit = apfu) versus X_Mg = mol Mg/(Mg + Fe) diagram. (b) F (apfu) versus X_Mg diagram. (c) Cl (apfu) versus X_Mg diagram. Mineral compositions are variable, depending upon their modes of occurrence.

Biotite

Biotite occurring as inclusions in garnet and a constituent of NIs is variable in composition (Fig. 7). It is noteworthy that biotite in closely adjacent NIs (NI-1 and NI-2 in Fig. 3) in the same garnet has distinctly different Ti, Cl, and F contents in addition to different X_Mg. Biotite in NI-4 in Figure 5 is rather rich in Cl (0.77 wt% Cl), but extremely poor in Ti despite the coexistence with ilmenite (Fig. 7).

Plagioclase

Plagioclase intergrown with orthopyroxene and replacing garnet along its rim and farctures is anorthite (∼ An₉₀), whereas plagioclase inclusions in garnet have variable composition and in places show euhedral-subhedral zoning (Figs. 2c and 2d). Plagioclase in the Ca-poor inner part of garnet is relatively sodic (∼ An₅₀) and in places antiperthitic. Plagioclase in the garnet mantle is richer in Ca than that in the inner part of garnet (∼ An₈₆) and intergrown with vermicular quartz in addition to orthopyroxene. Moreover, such plagioclase is locally antiperthitic and contains relatively sodic (∼ An₅₈) patches. Thus, the An content of plagioclase inclusions in garnet systematically increases with increasing Ca content of host garnet. Plagioclase in NIs is usually sodic but highly variable from one NI to another. Plagioclase coexisting with dendritic quartz and K-feldspar in NI-1 in Figures 3a-3d is almost pure albite, while plagioclase coexisting with orthopyroxene in NI-2 in Figures 3a, 3b, 3e, and 3f is An₃₃. There are two distinct plagioclases in an epidote-bearing NI-3 in Figures 4f and 4g; acicular and almost pure anorthite is in direct contact with relatively sodic (∼ An₃₀) grains. Plagioclase in grandidierite-bearing and K-feldspar-rich Ni-4 in Figure 5 is much more calcic (∼ An₅₃).

Grandidierite

Analysis of grandidierite gave a formula of (Mg_1.06Fe_0.88Mn_0.02)_Σ=1.97Al_6.02Si_1.99B₂O₁₈ assuming boron content is ideal (Table 1), which is close to the ideal formula for grandidierite, (Mg, Fe, Mn)₂Al₆Si₂B₂O₁₈. The analytical total including ideal boron content is low, 98.19 wt%, which we attribute to the small size of the analyzed grain. X_Mg = 0.55, i.e., nearly midway between grandidierite and its Fe analogue, ominelite. Identification of grandidierite was confirmed by Raman spectroscopy. Figure 8 shows the Raman spectrum of the grandidierite grain in Figure 5. The spectrum is close to that posted for grandidierite at the RRUFF site despite the higher proportion of the ominelite component in sample 84012223 than in the reference sample at the RRUFF site, R050196, (X_Mg = 0.83) (LaFuente et al., 2015). Because the small grandidierite grain is sandwiched between quartz and garnet, Raman peaks from these minerals are also present in the spectrum.

Table 1. Representative compositions of minerals

Mineral	Grt			Cpx			Opx			Hbl
	Core	Mantle	Rim	Core	Rim	Matrix	in NI-1	in Grt	Matrix	in Grt	Matrix	Matrix
SiO2	38.99	38.93	38.79	50.80	51.21	51.61	52.60	51.55	51.11	40.62	39.90	40.06
TiO2	0.07	0.08	0.08	0.33	0.26	0.25	0.05	0.05	0.06	2.66	2.04	1.76
Al2O3	21.89	22.05	21.93	2.94	2.49	1.85	1.34	1.08	1.25	13.56	12.88	13.02
Cr2O3	0.03	0.00	0.00	0.07	0.03	0.04	0.03	0.03	0.01	0.06	0.15	0.07
FeO*	27.36	27.49	27.87	13.32	12.43	11.63	25.89	28.99	30.58	15.37	18.89	18.30
Fe2O3**
MnO	0.54	0.57	0.80	0.11	0.21	0.10	0.05	0.18	0.17	0.04	0.13	0.03
MgO	6.60	5.83	5.25	10.68	11.26	11.93	20.69	17.58	16.69	9.20	8.00	8.67
CaO	5.13	5.84	6.55	22.05	22.40	22.34	0.26	0.56	0.52	11.04	11.45	11.80
Na2O	0.00	0.00	0.01	0.32	0.27	0.30	0.01	0.00	0.00	1.92	1.40	1.28
K2O	0.00	0.02	0.00	0.00	0.02	0.00	0.00	0.00	0.01	1.62	2.00	2.28
B2O3#
P2O5
F										0.08	0.07	0.13
Cl										0.29	0.76	0.77
-O										−0.10	−0.20	−0.23
Total	100.61	100.81	101.28	100.62	100.58	100.05	100.92	100.02	100.40	96.36	97.47	97.94
O	12	12	12	6	6	6	6	6	6	23	23	23
Si	3.009	3.006	2.997	1.923	1.934	1.951	1.966	1.979	1.970	6.203	6.178	6.165
Ti	0.004	0.005	0.005	0.009	0.007	0.007	0.001	0.001	0.002	0.306	0.238	0.204
Al	1.991	2.007	1.997	0.131	0.111	0.082	0.059	0.049	0.057	2.440	2.350	2.361
Cr	0.002	0.000	0.000	0.002	0.001	0.001	0.001	0.001	0.000	0.007	0.018	0.009
Fe3+ #
Fe*	1.766	1.775	1.801	0.422	0.393	0.368	0.809	0.930	0.985	1.963	2.446	2.355
Mn	0.035	0.037	0.052	0.004	0.007	0.003	0.002	0.006	0.006	0.005	0.017	0.004
Mg	0.759	0.671	0.605	0.603	0.634	0.672	1.153	1.006	0.959	2.095	1.847	1.989
Ca	0.424	0.483	0.542	0.894	0.906	0.905	0.010	0.023	0.021	1.806	1.899	1.945
Na	0.000	0.000	0.001	0.023	0.020	0.022	0.001	0.000	0.000	0.568	0.420	0.382
K	0.000	0.002	0.000	0.000	0.001	0.000	0.000	0.000	0.000	0.316	0.395	0.448
B#
P
F										0.039	0.034	0.063
Cl										0.075	0.199	0.201
Total	7.990	7.987	8.000	4.011	4.014	4.011	4.003	3.995	4.000	15.709	15.808	15.861
XMg†	0.301	0.274	0.251	0.588	0.617	0.646	0.588	0.520	0.493	0.516	0.430	0.458
Alm	0.592	0.598	0.600
Prp	0.254	0.226	0.202
Sps	0.012	0.013	0.018
Grs	0.142	0.163	0.181

Bt				Pl							Kfs
in NI-1	in NI-2	in Grt	in Grt	in NI-1	in NI-2	in NI-3	in NI-4	in Grt	in Grt	Matrix	in NI-1
35.60	37.60	37.11	36.60	69.09	60.39	42.93	55.48	53.94	46.91	46.00	64.24
2.85	5.36	4.58	6.08	0.00	0.01	0.00	0.00	0.02	0.00	0.03	0.04
18.61	14.64	14.86	14.30	19.69	25.31	35.56	28.46	29.34	33.88	34.14	18.36
0.06	0.00	0.07	0.06	0.01	0.02	0.02	0.02	0.04	0.00	0.03	0.00
18.56	14.59	13.90	15.81	0.61	0.72	0.78	0.99	0.25	0.20	0.16	0.75
0.00	0.00	0.09	0.01	0.00	0.01	0.00	0.00	0.06	0.00	0.01	0.03
9.20	13.52	14.69	12.01	0.02	0.01	0.11	0.02	0.03	0.01	0.00	0.00
0.03	0.03	0.00	0.00	0.27	7.05	19.55	10.58	11.61	17.29	18.58	0.02
0.08	0.05	0.06	0.13	11.37	7.63	0.04	5.18	4.82	1.56	1.12	0.43
8.92	9.63	9.80	9.24	0.05	0.14	0.00	0.02	0.20	0.04	0.03	16.23
0.03	0.16	0.33	0.33
0.77	0.29	0.29	0.21
−0.19	−0.13	−0.20	−0.19
94.52	95.74	95.58	94.59	101.11	101.29	98.99	100.81	100.31	99.89	100.10	100.10
22	22	22	22	8	8	8	8	8	8	8	8
5.458	5.600	5.540	5.559	2.990	2.666	2.014	2.486	2.433	2.157	2.120	2.980
0.329	0.600	0.514	0.695	0.000	0.000	0.000	0.000	0.001	0.000	0.001	0.001
3.363	2.570	2.614	2.560	1.004	1.317	1.966	1.503	1.560	1.836	1.854	1.004
0.007	0.000	0.008	0.007	0.000	0.001	0.001	0.001	0.001	0.000	0.001	0.000
2.379	1.817	1.735	2.008	0.022	0.027	0.031	0.037	0.009	0.008	0.006	0.029
0.000	0.000	0.011	0.001	0.000	0.000	0.000	0.000	0.002	0.000	0.000	0.001
2.103	3.002	3.269	2.719	0.001	0.001	0.008	0.000	0.002	0.001	0.000	0.000
0.005	0.005	0.000	0.000	0.013	0.333	0.982	0.508	0.561	0.852	0.917	0.001
0.024	0.014	0.017	0.038	0.954	0.653	0.004	0.450	0.421	0.139	0.100	0.039
1.745	1.829	1.866	1.790	0.003	0.008	0.000	0.005	0.012	0.002	0.002	0.960
0.015	0.075	0.156	0.158
0.200	0.073	0.073	0.054
15.412	15.437	15.576	15.377	4.986	5.006	5.006	4.991	5.002	4.995	5.002	5.016
0.469	0.623	0.653	0.575
			An	0.013	0.335	0.996	0.528	0.564	0.858	0.900	0.001
			Ab	0.984	0.657	0.004	0.467	0.424	0.140	0.098	0.039
			Or	0.003	0.008	0.000	0.005	0.012	0.002	0.002	0.960

Gdd	Ep	Ilm		Mag	Ap
in NI-4	in NI-3	in NI-4	in Grt	Matrix	in Grt
19.44	37.97	0.21	0.02	0.06	0.25
0.03	0.08	50.74	50.61	0.07	0.02
49.80	25.43	0.05	0.04	0.25	0.00
0.02	0.04	0.07	0.05	0.33	0.00
10.25		47.43	47.27	89.73	0.08
	10.65
0.28	0.10	0.05	0.10	0.04	0.08
6.96	0.06	0.25	0.95	0.04	0.01
0.06	23.68	0.10	0.02	0.01	54.42
0.03	0.20	0.03	0.06	0.00	0.03
0.02	0.06	0.00	0.02	0.00	0.01
11.30
					42.44
					3.01
					1.02
					−1.50
98.19	98.09	99.00	99.14	90.53	99.98
cations=12	12.5	6	6	4	12.5
1.994	2.991	0.011	0.001	0.002	0.002
0.002	0.005	1.936	1.919	0.002	0.001
6.020	2.361	0.003	0.002	0.012	0.000
0.002	0.003	0.003	0.002	0.010	0.000
	0.631	0.110	0.162	1.969
0.879		1.902	1.831	1.000	0.006
0.024	0.007	0.002	0.004	0.001	0.006
1.064	0.007	0.019	0.071	0.002	0.001
0.007	1.998	0.005	0.001	0.000	4.897
0.006	0.003	0.007	0.006	0.000	0.005
0.003	0.006	0.002	0.000	0.000	0.001
2.000
					3.017
					0.799
					0.145
12.000	8.011	4.000	4.000	3.000	7.955
0.548		0.055	0.038

The mineral abbreviations are after Whitney and Evans (2010). NI, nanogranitoid inclusion.

^* Total Fe as FeO.

^** Total Fe as Fe₂O₃.

^# B and Fe³⁺ are estimated based on stoichiometry for grandidierite and ilmenite and magnetite, respectively; total Fe as Fe³⁺ for epidote.

† X_Mg = Mg/(Mg + Fe^*)

Figure 8. Raman spectrum of grandidierite in mafic granulite 84012223 from Austhovde, East Antarctica. Raman spectra of adjacent garnet (almandine) and quartz are also present. Reference Raman spectrum of grandidierite (R050196) is from the RRUFF project (Lafuente et al., 2015).

Epidote

Epidote occurring in NI-3 in Figures 4d-4g contains approximately 21 mol% pistacite component.

Ilmenite

Ilmenite contains about 4 mol% hematite component. It commonly shows partial decomposition to a fine-grained intergrowth of rutile and unknown hydrous silicate mineral, which is similar in bulk composition to the ‘ferropseudobrookite’ reported by Sakoma and Martin (2002) from an ilmenite-bearing aplitic syenite dyke in Nigeria. Both materials give some SiO₂ and low analytical totals, i.e., respectively, up to 3 wt% SiO₂ and totals of ∼ 90% from Austhovde. The Austhovde material resembles the ‘armalcolite pseudomorph’ in ilmenite from garnet-sillimanite gneiss from Skallevikhalsen in the Lützow-Holm Complex, which also gave low analytical totals (92.6 wt% on average, Kawasaki et al., 2013).

Flourapatite

Fluorapatite composition is variable; it contains up to 3.0 wt% F and 1.3 wt% Cl. Moreover, fluorapatite grains are commonly zoned, showing rimward decrease in Cl.

DISCUSSION AND CONCLUSIONS

P-T conditions

The layer A of sample 84012223 contains the peak metamorphic mineral assemblage garnet + clinopyroxene + orthopyroxene + hornblende + plagioclase + ilmenite ± magnetite. Garnet is partially replaced by the symplectite of anorthite and orthopyroxene, suggesting the following quartz-consuming reaction at high temperature and decreasing pressures (e.g., Harley, 1989) and therefore the former presence of quartz in the matrix.   

\begin{equation} \text{Grt} + \text{Qz} = \text{Opx} + \text{Pl} \end{equation} (1).

However, textures indicative of partial replacement and compositional zoning of garnet porphyroblasts, together with the compositional variation of other minerals, indicate some degree of disequilibrium, and P-T estimates may not be reliable. The peak temperature is estimated to be >800 °C at pressures >2 kbar by the garnet-clinopyroxene thermometers (Ellis and Green, 1979; Powell, 1985; Pattison and Newton, 1989), using both higher X_Mg garnet core-lower X_Mg clinopyroxene core pair and lower X_Mg garnet rim-higher X_Mg clinopyroxene rim pair. Pressure is inferred 6-7 kbar at temperatures > 800 °C by garnet-plagioclase-clinopyroxene-quartz barometer (Newton and Perkins, 1982; Eckert et al., 1991). Takahashi and Tsunogae (2017) and Tsunogae et al. (2016) estimated similar peak temperature of 800-850 °C but higher pressure of 8-9 kbar. The different pressure estimation may be due partly to the presence or absence of compositional zoning of garnet porphyroblasts. The inner part of garnet porphyroblasts in sample 84012223 of the present study may represent composition incompletely homogenized growth zoning. In general, the compositional zoning of garnet with Ca increasing towards the rim suggests an increase in pressure (e.g., Chu et al., 2016). However, the An content of inclusion plagioclase also increases with increasing Ca content of the host garnet, and thus the higher Ca content of garnet cannot be attributed to an increase in pressure alone. Instead, the concomitant increase in Ca content of garnet and plagioclase is attributed to partial melting, as discussed below.

Evidence for partial melting and melt extraction

The presence of NIs in refractory minerals in granulites may be a good indicator of partial melting of host rocks during high-temperature metamorphism (e.g., Cesare et al., 2009, 2011; Ferrero et al., 2012; Hiroi et al., 2014; Cesare et al., 2015; Ferrero et al., 2016, 2018; Ferrero and Angel, 2018; Hiroi et al., 2020). The garnet-clinopyroxene-ilmenite-rich layer A of sample 84012223 shows additional features indicative of partial melting and melt extraction. The most remarkable is the contrasting mineral assemblages within and exterior to garnet porphyroblasts. Small amounts of biotite, epidote, K-feldspar, relatively sodic plagioclase, and quartz occur only in garnet, suggesting that these minerals were consumed by partial melting reactions. Moreover, some calcic plagioclase grains included in the outer part of garnet porphyroblasts show antiperthitic and myrmekitic textures with relatively sodic patches. This suggests that the alkali feldspar component in plagioclase has preferentially entered melt, resulting in Ca-enrichment in both plagioclase and garnet. The parallel increase in Ca in garnet porphyroblast with An component of inclusion plagioclase in high-grade metamorphic rocks may be a good indicator of partial melting of host rocks as discussed by Hiroi et al. (1995, 1998).

Recent field and experimental studies have clarified that at relatively low pressures, below 8 kbar, amphibolites can melt at temperatures > 850 °C by the following reactions even in the absence of vapor (Percival, 1983; Ellis and Thompson, 1986; Beard and Lofgren, 1991; Rapp et al., 1991; Rushmer, 1991; Wolf and Wyllie, 1993, 1994; Pattison, 2003);   

\begin{equation} \text{Hbl} + \text{P1} \pm \text{Qz} = \text{Cpx} + \text{Opx} + \text{Melt} \end{equation} (2).

At higher pressures garnet becomes an important product phase, with the following melting reactions (Percival, 1983; Rapp et al., 1991; Wolf and Wyllie, 1994; Williams et al., 1995; Pattison, 2003);   

\begin{equation} \text{Hbl} + \text{P1} \pm \text{Qz} = \text{Grt} + \text{Cpx} + \text{Melt} \end{equation} (3).

The studied sample 84012223 is characterized by low X_Mg, which, together with Mn, may have a significant effect on the garnet-producing partial melting reaction to take place at lower pressure than 8 kbar (e.g., Pattison, 2003). In addition, the occurrence of biotite exclusively in garnet suggests it was also involved in melting reactions. The random distribution of NIs with various mineral assemblages in garnet porphyroblasts, unrelated to the compositional zoning, suggests local consumption of various hydrous mineral inclusions in melting as well as temporal overlapping of melting. The contrasting mineral assemblages within and exterior to garnet porphyroblasts suggest partial melts were extracted extensively, resulting in modification of bulk rock composition. The melt loss drives the residual mineralogy to more mafic and calcic and further melting results in a different melt composition. Estimation of the amount of melt loss is difficult because of the lack of original rock composition. Takahashi and Tsunogae (2017) reported that they failed to complete pseudosection calculation in the system Na₂O-CaO-K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O-TiO₂-Fe₂O₃ (NCKFMASHTO) for sample Ts11011601M with almost the same mineral assemblage and compositions with sample 84012223. Takahashi and Tsunogae (2017) concluded that the bulk chemistry of the analyzed sample might be significantly different from the bulk chemistry of the rock during the peak metamorphism because the rock had undergone partial melting followed by loss of melt.

Formation of grandidierite

A tiny grain of grandidierite occurs together with Ti-poor biotite and quartz in the marginal part of NI-4. Its mode of occurrence is similar to the fine-grained retrograde grandidierite reported by Grew (1996), though no relics of either borian sapphirine or of kornerupine-prismatine have been found in the present case. The following reaction may be most plausible to form grandidierite during cooling;   

\begin{equation} \text{Grt} + \text{Melt} = \text{Gdd} + \text{Bt} + \text{Qz} \end{equation} (4).

It is noted that plagioclase in grandidierite-containing NI-4 is Ca-rich (X_An = 53), suggesting a possibility of the local involvement of a Ca-rich borosilicate such as axinite in addition to epidote and tourmaline in partial melting reaction to form the NI. On the other hand, the grandidierite grain shows no sign of partial replacement by tourmaline, in contrast to the ominelite in a porphyritic granite from Japan (Hiroi et al., 2002). These observations suggest that water activities were low in the grandidierite-bearing NI, as was proposed for some of the similar occurrences of secondary grandidierite by Grew (1996).

Tectonic significance of nanogranite inclusions

Hiroi et al. (2019) mentioned the occurrence of NIs in mafic granulites from two other bedrock exposures in the LHC; Skallevikshalsen and Rundvågshetta in the southern part of Sōya Coast (Fig. 1). Figure 9 shows two notable features of NIs in sample YH05012104C from Skallevikshalsen (Supplementary Figure S1 shows similar features in sample YH05010902D from Rundvågshetta; available online from https://doi.org/10.2465/jmps.221209). The occurrences of dendritic-skeletal quartz and quartz grains with variable CL brightness with or without euhedral-subhedral CL zoning are common features to those observed in mafic sample 84012223 of the present study and other quartzo-feldspathic and pelitic granulites so far reported by Hiroi (2020) and Hiroi et al. (2014, 2019, 2020). On the other hand, the occurrences of epidote and Ti- and halogen-rich biotite are features that have not been observed in NIs in quartzo-feldspathic and pelitic granulites. The occurrences of grandidierite and plagioclase with >50 An component in the K-feldspar-bearing NI-4 is distinctive. Hiroi (2020) and Hiroi et al. (2014, 2019, 2020) claimed that some NIs, called felsite inclusions, show non-equilibrium textures such as skeletal, dendritic and spherulitic crystals of quartz and other minerals and porphyritic texture that are commonly seen in volcanic rocks. Quartz ‘phenocrysts’ often show simple and definite CL zoning with euhedral bright Ti-rich cores and dark Ti-poor rims and overgrowths as reported by Hiroi et al. (2020) and in the present study. The non-equilibrium mineral textures are consistent with the occurrences of glass and metastable phases such as cristobalite, kumdykolite and kokchetavite in NIs as reported by Cesare et al. (2009, 2015), Ferrero et al. (2016, 2018), and Ferrero and Angel (2018). More importantly, the preservation of such non-equilibrium fine textures with high surface/volume ratios and therefore high interfacial energy, metastable phases and relatively sharp CL zone boundaries (diffusion width <10 µm) in quartz grains needs rapid cooling, because such features are easily destroyed or modified by annealing and recrystallization during slow cooling from high temperatures over a period of millions of years or more. Thus, cooling rate of some granulites, regardless of bulk rock composition and mode of occurrence in the field, in the southern part of the Lützow-Holm Complex may be 1 to 2 orders of magnitude faster than believed up until now. Takahashi and Tsunogae (2017) inferred post-peak rapid decompression based on the study of carbonic fluid inclusions in sample Ts11011601M from Austhovde-minami Rocks. This is consistent with the occurrence of andalusite in NIs in pelitic and quartzo-feldspathic granulites from the southern Sōya Coast (Hiroi et al., 2019).

Figure 9. (a) Plane-polarized photomicrograph of mafic granulite sample YH05012104C from Skallevikshalsen. (b) Crossed-polarized photomicrograph of NI in the rectangle in (a). Note dendritic quartz and fine intergrowth of quartz and plagioclase. (c) Color Chroma SEM-CL image and (d) X-ray elemental map for Si of NI in (b). (e) BSE and (f) SEM-CL images of quartz and NIs in garnet. Note distinctly different CL brightness of different quartz grains. (g) and (h) SEM-CL images of quartz grains in (f). Note the narrow (<10 µm in width) boundaries between CL bright and dark areas (red arrows).

ACKNOWLEDGMENTS

We thank E. Tabata, A. Yanagi, M. Kato, Y. Sun, T. Kobayashi, and N. Furukawa for their cooperation and collaboration in the research of NIs at Chiba University. We also thank many collaborative researchers, in particular M. Satish-Kumar, T. Kawakami, N. Tsuchiya, M. Ishikawa, Y. Osanai, M. Owada, T. Tsunogae, A. Kamei, F. Higashino, K. Horie, M. Takehara, and M. Yoshida. We appreciate the constructive reviews of S.L. Harley and an anonymous referee. This work was supported by JSPS KAKENHI Grant Numbers 18K03789 to Y.H., 17H02976 and 21H01182 to T.H. and National Institute of Polar Research (General Collaboration Project no. 29-36), Japan.

SUPPLEMENTARY MATERIAL

Supplementary Figure S1 is available online from https://doi.org/10.2465/jmps.221209.

Note

Subsequent study using Raman spectroscopy of anorthite occurring in nanogranitoid inclusions in samples 84012223 and YH05012104C revealed that it is dmisteinbergite.

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